March 24, 2015 Severe Weather Outlook

The Storm Prediction Center has issued a severe weather outlook for March 24th, 2015.

SPC

The Storm Prediction Center is expecting potential severe weather to develop in much of Missouri, northeast Oklahoma, northwest Arkansas, eastern Kansas, southern Iowa, and extreme western Illinois. The current risk is listed as a Slight Risk, which is not a significant threat outlook, but should be monitored.

Analysis

WPC forecast, valid Wednesday, March 24th

The severe weather set-up will include a strong low pressure system moving into the Upper Midwest. A warm front well displaced from the severe weather outlook area (shown in Michigan and Ohio above) will allow for a narrow corridor of potentially severe weather to develop. These storms should develop along the cold front that will shift eastward through the area. But is this risk actually legitimate?

COD

The above image shows projected radar reflectivity at 7 AM Central Time on Tuesday. We see a large complex of showers and thunderstorms moving across the corridor highlighted for severe weather on this day. This ongoing complex likely will hinder any severe weather prospects for later on in the day, which would end up ruining this severe weather chance. The best risk for severe weather is likely with that morning's band of showers and storms, portrayed in Missouri and southern Iowa on this NAM model forecast.

To summarize:

- Missouri, Iowa, Illinois, and Oklahoma are at risk for severe weather on March 24, 2015.
- This severe weather potential could be ruined by ongoing showers and storms early in the morning.

Andrew

March 9-11 Severe Weather Potential

The Storm Prediction Center has highlighted portions of the Gulf Coast for a severe weather threat.

SPC

Today's severe weather threat is focused in southeastern Louisiana and extreme southern Mississippi, where a Marginal risk of severe weather has been posted. Radar imagery shows a broad swath of moderate to heavy precipitation across the south-central Plains, feeding on moisture from the Gulf of Mexico. Thunderstorms appear to be moving inland over the Marginal risk area, and it looks as though the main threats will be tornadoes and wind.

SPC

Tomorrow's severe weather threat, for March 10th, is more expansive than today's. We see a Marginal risk of severe weather across eastern Louisiana, southern and central Mississippi, and southwestern Alabama. A general risk of thunderstorms surrounds this more elevated threat delineation. A similar set-up to today should evolve over the central Gulf Coast, as that rich Gulf of Mexico moisture continues to allow for thunderstorms to flow onshore, possibly even training in some spots to lay down heavier totals than forecast.

Andrew

Anomalously Warm Caribbean May Foreshadow Severe Weather Season

The current state of the Caribbean waters into the southern Gulf of Mexico could foretell our upcoming severe weather season.

NOAA

The above image shows sea surface temperature anomalies over the Gulf of Mexico, Caribbean, and southern Atlantic for early March. In this image, we notice a generally average to slightly warmer than average trend for the Caribbean waters, with spots of even warmer waters. Temperatures in the Gulf of Mexico are running above-normal to well-above-normal, particularly south of Louisiana.

As was stated by Storm Prediction Center forecaster Rich Thompson in his Tornado Forecasting Workshop sessions, a more plentiful severe weather season could be in store if you begin with a moist environment down in the Caribbean during the winter months, which can then transfer into the Gulf of Mexico for spring, to provide for a rich moisture source. Going solely by that, we're in a good position for a pre-established moisture source to kick off the severe weather season, best shown by the warm water temperatures from the Gulf Coast straight down to South America. It isn't the warmest water temperature swath we've ever seen, but it's notable.

NOAA

Also an interesting feature is the rock-steady pool of well above-normal water temperatures in the northeast Pacific and Gulf of Alaska. If this can stay in place for the spring season, and we're able to maintain cyclogenesis in the North Pacific to carry storms into North America, it's possible we see a northwest flow-dominated severe weather season, which can feature intense events in the Midwest, Great Lakes, Ohio Valley, and Mid-Atlantic.

I'll have much more on this in my 2015 Severe Weather Season Outlook on Wednesday, March 18th.

To summarize:

- A warm Caribbean and Gulf of Mexico may be setting the stage for at least an average severe weather season.
- SST patterns off the West Coast could also allow for a severe weather focus in the North-Central and Northeast US.
- All of this will be re-evaluated in the severe weather seasonal outlook on March 18.

Andrew 

Severe Weather Discussion for June 17 - June 20

This is a severe weather discussion focusing on the time period of June 17 to June 20.

Long range forecasts from the Weather Prediction Center show an upper level low entering the Pacific Northwest, provoking ridging in the northern Plains, which is further amplified by an additional trough in the Great Lakes. This ridge then progresses eastward and strengthens, as the upper level low in the Northwest also shifts east, and the Great Lakes trough recedes into Canada. This opens the door for a substantial ridge to push northeast into the Southeast US, as the forecast for early next week shows. At that point, warm and unstable air will flow north into the Central and East US, which will eventually be utilized by an approaching weak trough seen best in Montana by the Day 7 forecast (bottom middle panel).

Projections of convective available potential energy (CAPE), a crucial ingredient for thunderstorms, put instability values over 5000 j/kg. When one considers that 2000 j/kg of CAPE is sufficient for the development and propagation for potentially severe thunderstorms, concern quickly rises over this forecast of incredibly high instability.

Lower level wind speed projections for Wednesday, the same time period as the above image, show a band of enhanced winds extending from Oklahoma into southern Canada due to influence from a storm system draped across the North Plains. Wind speeds inside this band look to meet and exceed 50 knots, meaning any thunderstorms that may form on Wednesday have great potential to be sustained. If there is little wind shear in an environment, a thunderstorm updraft can become entangled with the downdraft, which then weakens the storm. This phenomenon is what creates 'pulse storms', or those small-celled thunderstorms that form and then die off just as fast. However, with forecasts showing substantial lower level winds for Wednesday, more organized, sustained convection may be anticipated.

Additional updates concerning this situation will be published in the near future.

Andrew

Active Severe Weather Potential Looming for Mid-June

I am still monitoring the potential for a period of active weather in mid-June.

The image above shows a long range forecast of outgoing longwave radiation (OLR) anomalies over the equatorial region of the globe, valid on June 14th. In both of these images, the yellows represent positive OLR, or suppressed tropical convection, while the blues denote negative OLR, or enhanced tropical convection. We can see a strong active Madden-Julian Oscillation (MJO) signal on the bottom panel, with deep blues indicating abnormal convection occurring over southern India into the waters east of the subcontinent. The placement of this enhanced convection tells me we are looking at a Phase 2 MJO, as the OLR composite shows below.

OLR anomalies for each MJO phase

Now, outside of the fact that Phase 2 MJO events have been known to increase the risk of violent tornado outbreaks in the spring months, the mid-level pattern during a June Phase 2 MJO event also signals the potential for severe weather.

The image above, produced by Americanwx, shows 500mb height anomalies over the northern hemisphere when the Madden-Julian Oscillation is in Phase 2 during the month of June. In this image, warm colors indicate high pressure/calm and warm weather, while cool colors denote stormy and cooler weather. During a typical MJO Phase 2 event in June, we tend to see ridging set up from the Bering Sea to the Great Lakes, enhanced slightly by weak troughing in the West. Because of this weak West US troughing, warmth and moisture are pumped north into the Central and East US from the Gulf of Mexico. This action then feeds into the increased potential for severe weather. Based on this composite, I do feel this time period may be defined by some active severe weather.

More evidence for an active severe weather period in mid-June arises when we take a look at the long range CFS severe weather forecast. The dates on the right display the date the forecast was made on, and the dates on the bottom legend indicate when this forecast is valid for. Warmer colors indicate the higher potential for severe weather. For instance, if I want to know the severe weather potential for May 30th on a forecast made on May 5th, I go up the legend on the left to find May 5th, then scroll on the corresponding horizontal line to find the May 30th box. In this case, we have been seeing the majority of recent forecasts highlighting the June 10-25 period as the one to watch out for, as the varying-colored boxes show on the upper-right corner of this image. Sometimes, there is safety in numbers in the weather world. In this case, with the relative consistency of these forecasts for the mid-June timeframe, I feel comfortable raising the concern of a potential active period for the aforementioned timeframe.

Andrew

Potential Severe Weather Outbreak Projected for Mid-June

I'm seeing an increased probability for a severe weather event, potentially an outbreak, for the middle of June.

The image above shows a long range forecast of outgoing longwave radiation (OLR) anomalies over the equatorial region of the globe. In both of these images, the yellows represent positive OLR, or suppressed tropical convection, while the blues denote negative OLR, or enhanced tropical convection. We can see a strong active Madden-Julian Oscillation (MJO) signal on the bottom panel, with deep blues indicating abnormal convection occurring over southern India into the waters east of the subcontinent.

If we take a look at MJO composites for each phase, with blues once again defining enhanced convection and oranges signifying suppressed convection, it would seem that storms just east of India fit in best with a Phase 2 or 3 MJO event, seen on the left panels of this graphic above. Considering there has been a link observed between the MJO being in Phase 2 and violent tornado outbreaks occurring consequentially, there is reason to monitor this period for potential severe weather.

The image above shows projected 500mb height anomalies over the northern hemisphere. We can signify the blues with negative height anomalies, which indicate the presence of cooler and stormier weather, while reds denote positive height anomalies, which allow for warmer than normal and calm weather. In the image above, if we look towards the Bering Sea, we can see a very strong ridge of high pressure, seen by the splotch of red around the Aleutian Islands. This image is valid for May 13-14, and if we extrapolate it out 2-3 weeks using the Bering Sea Rule, we can expect high pressure and warm weather to appear in the East US. This extrapolated timeframe looks to fall around the time when this potential severe weather outbreak may strike, and the presence of strong high pressure migrating across the Eastern and Central US may amplify the potential for active weather.

Another device we can use to predict potentially active weather is this grid chart. The dates on the right display the date the forecast was made on, and the dates on the bottom legend indicate when this forecast is valid for. Warmer colors indicate the higher potential for severe weather. For instance, if I want to know the severe weather potential for May 20th on a forecast made on May 5th, I go up the legend on the left to find May 5th, then scroll on the corresponding horizontal line to find the May 20th box. In this case, we have been seeing elevated supercell composite values around the June 15-20 period for some time now, as the red box indicates. This is a long range forecast, but as you can see with severe weather events further to the left on the image, we can get a feel for severe weather potential a long way out, with more accurate forecasts usually coming a few days to a week prior to the event.

This potential event will be updated in coming days as more information becomes available.

Andrew

May 21-25 Potential Storm System & Severe Weather

I'm analyzing the May 21-25 time period for a substantial storm system to bring cool weather and possibly some severe weather to the nation.

Tropical Tidbits

The image above shows projected 500mb height anomalies over the western Pacific. We can signify the blues with negative height anomalies, which indicate the presence of cooler and stormier weather, while reds denote positive height anomalies, which allow for warmer than normal and calm weather. Looking towards Japan on this forecast graphic, valid on May 15th, we see a substantial closed low approaching Japan, bringing in cooler than normal and unsettled weather for the area. If we utilize the Typhoon Rule, we can expect this storm system to impact the United States 6-10 days after this storm affects Japan, which would give us a May 21-25 time period.

Indications I am getting from this graphic tell me much of the United States may be impacted by this cooler than normal weather, and while I do believe severe weather is possible, I'm currently thinking any severe weather will be limited due to the lack of a powerful ridge advecting north over Japan prior to this closed low hitting the country.

Andrew

May 7th-8th Potential Severe Weather Threat

It's looking more like we can expect a severe weather threat around the May 7th-8th period.

Tropical Tidbits

(The following is copied from a May 1st post) On the afternoon of May 2nd, we see a small but still-substantial ridge of high pressure pushing over Japan, as a strong storm system to the west begins pushing in the direction of the island nation. There is a rule, well explained by Joe Renken, that states a weather phenomenon in East Asia will be reciprocated in the United States 6-10 days later. This means that if there is a storm system in Japan on a certain day, we can expect a storm in the US 6-10 days after that. The same goes for high pressure and warm weather. Seeing as this ridge appears on May 2nd in Japan, we can expect some warm weather around May 8-12th.

Tropical Tidbits

By May 3rd, we see a strong storm pushing into Japan in the wake of the ridge we saw in the image above. Looking at the trough, it looks like the system is negatively tilted, as seen by the pressure contours pushing in a south-east direction. If this trough reflects on the United States in about a week, which it very well may, the negative tilt may result in a pretty substantial severe weather threat. Locations of this threat are unknown at this time, but be cautious for some severe weather in this timeframe. (End copied portion)

The Storm Prediction Center has outlined an area of enhanced severe weather risk on May 8th from Oklahoma into southern Wisconsin. This threat area includes cities such as Chicago IL, Milwaukee WI, and Wichita KS, just to name a few. 

The surface analysis forecast from the Weather Prediction Center for May 8th shows a strong low pressure system moving northeast into Minnesota, dragging a warm front to the north, which should allow for a warm, moist and unstable atmosphere to flow over the area. To the west, we see a cold front draped across the Central and Southern Plains, with a dryline denotation in western Oklahoma and Texas. I expect that we will see the cold front be the main cause for this severe weather event, while the dryline to the south may provoke additional activity in the southern Plains. I anticipate the highest tornado threat to remain in the southern Plains, as dryline features typically can retain more tornadic cells than cold fronts, but we will analyze that idea later on. The Storm Prediction Center indicates we can expect the main threats to include damaging winds and hail, with a tornado threat still present, as is indicated below.

AS MODELS BRING THE UPPER TROUGH EWD INTO THE PLAINS DAY 6 /THU 5-8/
   HOWEVER...MORE COMPLETELY MODIFIED GULF MOISTURE APPEARS LIKELY TO
   HAVE SPREAD INTO THE CENTRAL U.S. PERMITTING DEVELOPMENT OF MORE
   SUBSTANTIAL CAPE.  AS THE SURFACE LOW SHIFTS NEWD ALLOWING A
   TRAILING FRONTAL ADVANCE ACROSS THE CENTRAL PLAINS
   VICINITY...DEVELOPMENT OF AFTERNOON SURFACE-BASED STORMS APPEARS
   LIKELY.  WITH THE GREATEST JUXTAPOSITION OF FAVORABLE SHEAR AND
   MODERATE CAPE LIKELY TO EXIST FROM ROUGHLY OK NEWD ACROSS KS/MO/IA
   AND INTO IL AND SRN WI...IT APPEARS THAT INCLUSION OF A 30
   PERCENT-EQUIVALENT RISK AREA CAN BE INTRODUCED ATTM WITH MODERATE
   CONFIDENCE...WITH HAIL/DAMAGING WINDS LIKELY ALONG WITH SOME TORNADO
   POTENTIAL.

I do believe we may see another severe weather outlook posted for May 7th for areas further to the west, as we take a look at rank-analog tornado probabilities off of the GEFS modeling system. Out of the four panels, the one which highlights the overall tornado probability is in the top-left corner, and we can see in that panel that Nebraska and Kansas may be in line for as high as a 10% chance of a tornado, which would certainly warrant a severe weather outlook.

If we take a look at the same four-panel forecast image, now valid on May 8th, we can see how the Storm Prediction Center is highlighting areas in Oklahoma into Wisconsin. It appears that the tornado threat here is weaker and displaced farther west than the SPC outlook for this day, but as was mentioned earlier, the main threats should be damaging winds and hail. I find that the threat for this day is more accurately highlighted in the other three panels, which calculate different values for ingredients of severe weather and general thunderstorms.

Overall, we are looking at a severe weather threat on May 8th (and potentially May 7th) over the Plains and Midwest regions.

Andrew

Long Range Severe Weather Threat Looming

I'm watching for a severe weather threat in about a week's time.

Tropical Tidbits

On the afternoon of May 2nd, we see a small but still-substantial ridge of high pressure pushing over Japan, as a strong storm system to the west begins pushing in the direction of the island nation. There is a rule, well explained by Joe Renken, that states a weather phenomenon in East Asia will be reciprocated in the United States 6-10 days later. This means that if there is a storm system in Japan on a certain day, we can expect a storm in the US 6-10 days after that. The same goes for high pressure and warm weather. Seeing as this ridge appears on May 2nd in Japan, we can expect some warm weather around May 8-12th.

Tropical Tidbits

By May 3rd, we see a strong storm pushing into Japan in the wake of the ridge we saw in the image above. Looking at the trough, it looks like the system is negatively tilted, as seen by the pressure contours pushing in a south-east direction. If this trough reflects on the United States in about a week, which it very well may, the negative tilt may result in a pretty substantial severe weather threat. Locations of this threat are unknown at this time, but be cautious for some severe weather in this timeframe.

Andrew

Significant Tornado Outbreak Expected Sunday, Monday

A significant tornado outbreak is expected on Sunday and Monday, with strong to violent tornadoes expected on these two days.

The Storm Prediction Center has issued a moderate risk of severe weather for northeast Texas, northern Louisiana, eastern Oklahoma, southern Missouri, and most of the state of Arkansas. This moderate risk comes as a strong upper level low brings moisture and other destabilizing factors up to the Plains, likely resulting in one of the more significant tornado events in recent memory. The Storm Prediction Center has indicated that long-tracked and strong tornadoes are possible if this event comes to fruition, and an upgrade to the agency's highest risk level, the High Risk, will come tomorrow if model guidance continues to hone in on this severe weather event.

The image above shows the East WRF-ARW model forecast for maximum updraft helicity over its entire 48 hour forecast period. The idea of highlighting the maximum updraft helicity values is to identify where potentially tornadic supercells may strike for this severe weather event. A look at this image, showing maximum updraft helicity for the Sunday event, gives a clear indication of what we could be facing with this situation. This particular model shows multiple long-tracked, rotating supercells moving northeast from Texas into Arkansas. Based on the high helicity values, it looks like some of the most intense cells may be able to put down some strong tornadoes, but it will take another round of model guidance to confirm or reject this idea.

A look at the Storm Prediction Center's SREF (short range ensemble model guidance) projection for the probability of strong tornadoes only worsens this threat. We see the chance of a significant tornado at or above 75% in Arkansas, but there's a very small speck in the middle of Arkansas, where it seems to increase another contour level. The next contour level above 75% is 90%, meaning that this model run indicates there is a 90% chance of a significant tornado in central Arkansas. Predicting tornadoes is a very difficult endeavor, but this graphic gives us an idea of where the worst tornado threat may be.

On Monday, we see another moderate risk area, now located just to the east over northeastern Louisiana, much of Mississippi, and western Tennessee. This moderate risk area also has the potential to be upgraded to a High Risk, as many of the same dynamics that look to produce a potentially deadly environment on Sunday will be in place again on Monday. If you or someone you know resides in the moderate risk area for either Sunday or Monday, please alert them to this severe weather. This outbreak may be one of the more significant ones of the last few years, and should be dealt with with extreme caution.

The SPC SREF projection for the probability of significant tornadoes is also startlingly high for the Monday evening event, as the graphic above shows. We see the highest probabilities placed in far northeastern Mississippi into northwest Alabama, located a bit east of the current Monday moderate risk area. While this forecast may see slight adjustments in the near future, the trend of a significant tornado event on Sunday and into Monday remains clear.

To summarize:
• A significant tornado event is expected Sunday and Monday.
• Long-tracked, violent tornadoes may occur on either of those days.
• Anyone in either of the moderate risk areas on Sunday or Monday should immediately prepare for potentially extreme weather, which may threaten lives and property.

Andrew

April 26-30 Severe Weather Outbreak: Monday Outlook

This forecast is solely for the severe weather on Monday, April 28th. For the Sunday, Monday and Tuesday outlooks, please look at the bottom of this post for links.

The Storm Prediction Center has outlined a rather large area of potential severe weather to kick off the workweek on Monday, April 28th. This is a continuation of the severe weather events on Saturday and Sunday, as a strong upper level low continues to ravage the country. When the SPC issues these sorts of long range outlooks, they highlight areas with a 30% chance or more of observing severe weather within 25 miles of any given point. This means that any of the areas highlighted in Arkansas, Louisiana, Mississippi, Alabama, Tennessee, Kentucky, Indiana, Illinois and Missouri may see some formidable severe weather, with a lesser-but-still-present severe weather threat surrounding this circular shape.

A look at the projected 500mb isobars and wind speeds for late Monday evening continues to exemplify why this event will be a multi-day, potentially significant severe weather outbreak. We see a long strand of anomalously high wind speeds extending from Oregon, rounding the closed low and punching into the area we're watching for Monday's severe weather risk around Missouri and Illinois. Although the closed low does not have a negative tilt that could then intensity the situation further, it does look like this piece of the Pacific jet stream will be what keeps the severe weather going, even though the closed low won't be doing much, other than continuing to pull moisture up from the Gulf of Mexico.

A look at 700mb winds, just a few thousand feet off the ground for the same time as the 500mb image shows strong lower-level winds over the area we're watching for the severe weather threat Monday evening. Considering the streak of 40 to 60 knot winds in the Midwest and southern Plains will only continue to intensity during Monday evening, as it is the nocturnal Lower Level Jet stream (LLJ), the severe weather threat will likely continue as well, at least through Monday evening.

A look at projected dewpoints and the Lifted Index for Monday evening really emphasizes the moisture this system has to work with. We see the projected dewpoints in shaded colors, and the dark blues represent the highest dewpoints, and thus some pretty muggy air. This all bodes well for severe weather potential, which is confirmed by the very low Lifted Index values, some as low as -9. The Lifted Index, also called the LI, is a measure of buoyancy of the air- in other words, when the LI is negative, air tends to rise and thus create thunderstorms. When the LI is positive, it indicates a stable atmosphere, suppressing thunderstorms. Seeing numbers close to double-digit negatives tells me this closed low will be keeping the severe weather fireworks going well into the workweek.

To summarize:
• A potentially significant severe weather event is possible on Monday.
• Those in the outlined area may want to review severe weather preparations and watch for further updates on this situation.

Other posts pertaining to the April 26-30 Severe Weather Outbreak:
• Saturday Severe Weather Outlook: Click Here 
• Sunday Severe Weather Outlook: Click Here
• Tuesday Severe Weather Outlook: Click Here (Coming soon)

Andrew

April 26-30 Severe Weather Outbreak: Sunday Outlook

This forecast is solely for the severe weather on Sunday, April 27th. For the Sunday, Monday and Tuesday outlooks, please look at the bottom of this post for links.

The Storm Prediction Center has issued a severe weather outlook for Sunday, April 27th, indicating the continuance of Saturday's severe weather. In the graphical representation of the severe weather risk above, we see there is a large 'slight risk' of severe weather covering Nebraska, Kansas, Oklahoma, Texas, Louisiana, Mississippi, Tennessee, Kentucky, Arkansas, Indiana, Iowa and Missouri. We also see a much more alarming 'Moderate risk' of severe weather, covering extreme northeast Texas, southwest Arkansas, northwest Louisiana and southeast Oklahoma. This moderate risk is particularly alarming, as it appears to be only the 9th time in Storm Prediction Center history that a moderate risk has been issued two days away from the predicted event. 

The image above shows the Storm Prediction Center's graphical representation of the percentage chance of severe weather within 25 miles of any given point. We see the 'slight risk' area as defined by the 15% chance swath, with an enhanced severe weather chance in the 30% red region. But it's the 'moderate risk' area that wins the contest, clocking in at a 45% chance of severe weather within 25 miles of any point. Putting that in simpler terms, you could flip a coin, and there's just about a 50/50 chance of severe weather within 25 miles of that purple area. The black hatched area signifies a 10% or greater chance of significant severe weather (i.e. very large hail, very strong winds or strong tornadoes).

The image above shows isobars (contour lines) and wind speeds (colored) at the 500mb level of the atmosphere on Sunday evening. Here, we see our strong trough that we went over in the Saturday Outlook post has now become a closed low, as seen by the closed isobars in Kansas and Nebraska. This indicates that the storm has passed maturity, and is now on the path to weakening. Despite this, a screaming mid-level Pacific jet stream, clocking in at nearly 100 knots, will be feeding this severe weather threat for days to come. We see the closed low wrapping those strong mid-level winds into the warm sector, enhanced right over that meeting point of Texas, Arkansas and Oklahoma, and right over the Moderate Risk outline. These strong winds indicate the enhanced risk of severe weather.

If we look at wind speeds at the 700mb level, just a few thousand feet off the ground, we see wind speeds of close to 60 knots enhanced over our severe weather area, and especially over that Moderate Risk outline of Texas, Arkansas and Oklahoma. Seeing as this 700mb wind component is also known as the lower level jet stream (LLJ), and is a nocturnal jet stream, we can expect these winds to sustain themselves as Sunday wears on, thus continuing the severe weather threat. These high wind speeds combined with the high mid-level winds we just went over make me more and more concerned for Sunday.

According to model guidance, there is every right to be concerned. The image above shows the forecasted supercell composite for Sunday night. In other words, the higher colors on the legend indicate the higher likelihood for a supercell, which is defined as a rotating thunderstorm. Seeing as the supercell composite is enhanced right over that moderate risk area again, it's clear that this focal point of Louisiana, Arkansas, Texas and Oklahoma will be the bullseye for Sunday's severe weather threat.

Interestingly enough, the forecasted significant tornado composite, which is an index that can help determine the likelihood of tornadoes in a given area, goes off the scale right over the Moderate Risk area, but in my opinion, it's best to wait another day or so before jumping on this train of violent tornadoes (though I will agree that strong tornadoes are possible here).

To summarize:
• A potentially significant severe weather event is expected to occur Sunday.
• A Moderate Risk has been issued for Oklahoma, Texas, Louisiana, and Arkansas.
• Those in the Moderate Risk area may want to prepare for potentially strong tornadoes on this day.

Other posts pertaining to the April 26-30 Severe Weather Outbreak:

• Saturday Severe Weather Outlook: Click Here
• Monday Severe Weather Outlook: Click Here
• Tuesday Severe Weather Outlook: Click Here (Coming soon)

Links labeled Coming Soon will be out in the next few hours.

Andrew

April 26-30 Severe Weather Outbreak: Saturday Forecast

This forecast is solely for the severe weather on Saturday, April 26th. For the Sunday, Monday and Tuesday outlooks, please look at the bottom of this post for links.

The Storm Prediction Center is highlighting an enhanced severe weather risk over the Southern Plains on Saturday, April 26th. The image above shows a graphical representation of the SPC's thoughts for Saturday's severe threat, with the percentages showing the likelihood of severe weather within 25 miles of any given point. In this graphic, we see that the states of Nebraska, South Dakota, Iowa, Kansas, Oklahoma, Missouri and Texas are in the general 'slight risk' categorical severe weather outlook, indicated by the yellow 15% shading, while south-central Kansas, central Oklahoma and north Texas are in a 30% swath of enhanced severe weather chances. We also see the black hatched denotation, meaning areas within that have a 10% or greater chance of seeing significant severe weather within 25 miles of any point (think extremely large hail, very high winds, and strong tornadoes instead of "tamer" severe weather). Let's go over how this will evolve.

The projected surface analysis on Saturday evening from the Weather Prediction Center shows our strong storm located in northeast Colorado, with a warm front extending through the Central Plains and snaking south into Arkansas and Louisiana. We then identify our cold front way back west in Colorado and New Mexico, but we also see an orange boundary in western Texas, Oklahoma and Kansas. What is that boundary? That is a dryline. In the same sense that warm and cold frontal boundaries separate warm and cold air, a dryline boundary separates dry and moist air, sometimes creating a substantial temperature gradient in the process. Drylines are the source of numerous storms in the Plains, and look to be the source for Saturday's severe weather, as well.

On Saturday evening, we will begin seeing a strong trough (storm system) in the Southwest begin to attain a negative tilt. A negative tilt is visually seen as the isobars trying to 'push' towards the south-east direction, indicating the storm system has reached maturity, and we see that phenomenon occurring in the 500mb vorticity forecast map above. This negative tilt looks to be attained in the late evening hours of Saturday, which brings up some timing issues for this severe weather event on Saturday, which we will discuss more next. The general thing to take away here, though, is that this will be a very strong storm system surging eastward to kick off this multi-day significant severe weather outbreak.

There does look to be some timing issues on Saturday, not just with respect to how fast the storm can acquire a negative tilt, but also with the presence of a capping inversion Saturday evening. Shown above is the Saturday evening forecast for instability, marked in j/kg by contour lines, and stability, shown by the presence of blue shading; the darker the blue shading, the higher the stability. Note that instability means air can rise because the air at the surface is warmer than the air above the surface, and stability means air cannot rise, as the air above the surface is warmer than the air at the surface. If we recall that air can only rise if the surrounding air is colder than the surface, the presence of stability means thunderstorms cannot form. In this forecast image, we see that there is a lot of projected instability over the Southern Plains, over 3000 j/kg in some places, which is a very high amount of instability. In some spots, we see white, which means no capping inversion, but across the remainder of the SPC-outlined severe weather risk, we see a significant capping inversion in place, as the dark shading shows. I'm worried that this cap will be too strong to break through, and the trough will be too slow attaining that negative tilt and moving closer to the Southern Plains leading to a bust in the forecast.

Let's hypothesize for a moment that storms do form on Saturday. A weather model named the WRF-ARW can predict the maximum updraft helicity for the entire 48 hours it forecasts for. In this instance, it means we can see how strong the storms that form rotate and spin from the morning of April 25th to the morning of April 27th, encompassing Saturday's severe weather in the process. We see on this image that the WRF-ARW model projects multiple tornadic storms to develop across the Central and Southern Plains, with two notably strong supercells violently rotating from southwest Oklahoma to the central portion of that state, and another from western to eastern Kansas. So while the environment may not be too favorable right now, any storms that do form and can sustain themselves look to be rather significant.

To summarize:
• A potentially significant severe weather event may occur on Saturday.
• The Central and Southern Plains would be affected.
• Model guidance is forecasting an inhospitable environment for storms, which may greatly hamper the severe weather threat.
• Any storms that do form have the potential to be significant and potentially tornadic.

Other posts pertaining to the April 26-30 Severe Weather Outbreak:

• Sunday Severe Weather Outlook: Click Here
• Monday Severe Weather Outlook: Click Here
• Tuesday Severe Weather Outlook: Click Here (Coming soon)

Links labeled Coming Soon will be out in the next few hours.

Andrew

**Significant Severe Weather Outbreak Expected This Weekend**

A significant severe weather outbreak, possibly including strong to violent tornadoes, is expected Saturday, Sunday and Monday.

The Storm Prediction Center has outlined three days of potential severe weather in their long range outlook. In these outlooks, the SPC will mark regions it believes are at risk for enhanced severe weather. Typically, one or two days may be outlined at a time, but it is uncommon (though not unheard of) to see three different areas outlined, especially considering how much area they cover. By SPC estimates, over 47 million people may be affected by this potential severe weather outbreak.

The wording by the Storm Prediction Center is very strong, as this excerpt below shows:

..SIGNIFICANT MULTI-DAY SEVERE EVENT EXPECTED ACROSS PORTIONS OF THE
   CENTRAL AND SOUTHERN PLAINS SATURDAY AND SUNDAY AND ACROSS THE MS
   VALLEY AND MID SOUTH ON MONDAY...STRONG TORNADOES ARE POSSIBLE...

Jet Stream Forecast for Saturday

The whole situation looks to unfold as a strong trough digs into the Southwest United States, developing a negative tilt in the process. A negative tilt, commonly seen how the isobars seem to push towards the southeast like you see above over Arizona, indicates the maturity of the trough in question, and thus means weather of any kind is generally stronger than it would be if the storm were not fully mature yet, or if it was beginning to occlude and weaken. In this forecast of the jet stream, valid Saturday night, we see a strong Pacific jet stream powering the trough as it digs east, and the divergence over the South Central Plains, shown as how the two jet streams (subtropical jet stream, seen along the bottom of the image, and the Pacific jet stream) separate just west of Texas. Divergence in this sense means the air is rising in monumental fashion, also a big signal for severe weather potential.

Lifted Index and Dewpoints over the Southern Plains, valid Saturday

The Lifted Index (LI), a measurement of how unstable the atmosphere is, looks to be plummeting as low as -11 or -12, indicating extreme amounts of instability. Worse yet, the sharp gradient in dewpoints (shown by the shaded colors) indicates the presence of a dryline, literally meaning a line where the air is dry to the west and very humid to the east. These drylines almost always originate and stick around in the Plains, and are a big reason why we see major severe weather down there. I have little doubt based on the projected Lifted Index that if/when a storm does break the capping inversion predicted to be in place (a capping inversion hinders thunderstorm development, and is the opposite of instability), we may very well see violent thunderstorms capable of strong tornadoes and potentially extreme severe weather. The rhetoric here is elevated, but there is quite a bit of concern here on how this will evolve.

By Sunday, the trough looks to continue to mature and eventually close off, as the full-circle contours in the 500mb wind speed image above show. That negative tilt is also evident, as we see the contours tilting towards the south-east direction. In response to this strong trough, as well as the intense Pacific jet stream we discussed above which is also evident in the image above, the mid-level jet stream will be absolutely howling over the Plains, right over the area projected to be hit on Sunday. We could see mid-level winds of over 80 knots, an astounding strength for this event.

Projected Energy-Helicity Index (EHI) values look to be heading towards double-digit territory on Saturday, a major concern for tornadic activity that evening. The EHI combines instability and helicity (spinning) to make an index that tries to predict situations where tornadoes are likely to occur. Values as elevated as the ones shown above confirm my concern for this event, which won't be just one day, but possibly three days of continuous severe weather problems.

To summarize:
• A potentially significant severe weather event is expected Saturday, Sunday and Monday.
• Potentially strong tornadoes are anticipated Saturday evening.
• Risks beyond Saturday look to focus more on other modes of severe weather.
• Those in the risk areas outlined by the SPC should review severe weather guidelines and preparations.

Andrew