Tuesday, March 31, 2015

Multi-Day Severe Weather Threat for Midwest, Ohio Valley

A multi-day severe weather event will unfold Wednesday and Thursday across the Midwest and Ohio Valley, respectively.

Tropical Tidbits (click to enlarge)
Thunderstorm development is expected over southern Minnesota, northern Iowa, and eastern Nebraska by Wednesday evening, with the most intense convection being simulated over northwest Iowa. The image above shows forecasted radar reflectivity over the Midwest area at 7PM Wednesday. In other words, this is what one forecast model thinks the radars will see unfolding at 7PM Wednesday. If it does verify, this could be a rather intense severe weather event for anyone caught in the line of fire of a particularly nasty line segment of storms, such as the one shows in northwest Iowa.

The primary threats here will be damaging winds and hail; the linear shape of these storms should cut down on the tornado threat.

Tropical Tidbits
By Thursday afternoon, thunderstorm development is renewed in Indiana, Michigan, Kentucky, and Ohio. These cells should be more individualized, raising the hail and tornado threats, but still keeping the tornado threat at a rather low level.


Monday, March 23, 2015

Long Range - Monthly Outlook: April 2015

This is the long range/Monthly outlook for late March into April 2015.

We'll first begin with a diagnosis of the atmosphere. For those who find the technical jargon a little overwhelming, summaries to the post are provided at the bottom of the page.

The above image shows relative atmospheric angular momentum anomalies on the top panel, with the global anomaly on the bottom panel. According to the latest data, we are in a globally positive-AAM (+AAM) state, mainly due to a nice surge of +AAM to the equator. This has pushed the atmosphere into an El Nino mindset, which has shown up in our atmosphere. We'll go more in-depth to that later on.

The graphic above shows the tendency anomalies of the atmospheric angular momentum on the top panel, with the net tendency on the bottom panel. Notice how we're currently recovering from a massive +AAM tendency spike, and are now in a net negative AAM tendency. The cause of this tendency is not the thing we will analyze; right now, we are noting that we're in a +AAM state with -AAM tendencies.
This fits well into the four stage descriptions of the Global Wind Oscillation (GWO), which takes those above graphics and makes them into useful data. Here's the descriptions of all four stages of the GWO; pay careful attention to the first and last sentences of each stage.

The four primary phases of the GWO are described below, along with generally cold season (November-March) probable weather impacts for the USA. The GWO recurrence interval, or "time it takes to make a circuit", ranges from a broad 15-80 days. Two of the stages project strongly on El Nino and La Nina circulation states, which are also characterized by positive (Stage 3) and negative (Stage 1) global AAM anomalies, respectively.  Stages 2 and 4 are transitional.

Stage 1 (La-Nina like) – the global relative AAM anomaly is negative. The negative anomaly is primarily due to easterly upper level wind anomalies that extend from the Eastern Hemisphere tropics to the Western Hemisphere mid-latitudes. A retracted Pacific Ocean jet stream is a key feature in the total field.  Troughs are probable across the western USA with a ridge over the southeast.  High impact weather is favored across the Plains.

Stage 2 – the global relative AAM tendency is positive. This means that negative AAM is being removed from the atmosphere by surface friction and mountains. At the same time, westerly wind anomalies are intensifying in equatorial regions of the Western Hemisphere. Fast Rossby wave dispersion events in both hemispheres are a coherent feature of this stage and Stage 4.  A cold regime is probable across the central USA.

Stage 3 (El-Nino like) – the global relative AAM anomaly is positive. Westerly wind anomalies move into the Eastern Hemisphere, broaden in latitudinal extent and link up with deep westerly flow anomalies over the mid-latitude Western Hemisphere. An extended Pacific Ocean jet stream and southward shifted storm track is observed  favoring high impact weather events along the USA west coast.

Stage 4 – the global relative AAM tendency is negative. Positive (westerly) AAM anomalies are being removed by surface friction in the Western Hemisphere mid-latitudes and through mountain torques across the Northern Hemisphere topography. The next phase of the oscillation (if there is one) is represented by easterly wind anomalies intensifying over equatorial regions of the Western Hemisphere. This stage has enhanced subtropical jets and closed lows in the subtropics favoring rainfall events over the southwestern USA.
What did you notice in each stage description? Did you notice how each description began with details on how the net (global relative) AAM or AAM tendency was positive or negative? Let's apply that here.
We saw earlier how we are in a +AAM / -AAM tendency phase right now. Using those particular sentences in each description, it appears we're in Stage 3 or Stage 4 of the Global Wind Oscillation. Let's investigate each description closer. In Stage 3, among other details, the Pacific jet stream is extended (can stretch across the Pacific), and active weather impacts the West US. In Stage 4, the subtropical jet stream is strengthened, and closed lows in the subtropics can permit heavy rainfall over the Southwest.

Do we have any of those factors?

The answer is yes. The graphic above shows 200 hPa heights and wind speeds in ~4 day periods, with the least recent panel in the top left and the most recent panel in the bottom left. Notice how the Pacific jet stream is clearly extended into the North Pacific, fulfilling our Stage 3 ideals. If you'll look back at the stage description for Stage 3, you'll notice how it says 'El-Nino Like'. This means that conditions in this stage are most likely to provoke an El Nino-like response in the atmosphere. We're already seeing that response, namely in the extended Pacific jet and the AAM charts. I expect this to continue throughout April due to ensemble agreement.

The temperature anomaly charts in the US came back very cold for the next week or so. Each individual member is shown on those smaller boxes, while the large box contains the mean temperature departure. There is very good agreement on this cold, as well, valid on March 28th.

Lastly, let's go over the tropics. We're currently seeing a very strong tropical forcing episode over Phase 8 of the Madden-Julian Oscillation, a signal that has been created thanks to a powerful convectively coupled Kelvin Wave (CCKW). The long range ECMWF ensembles believe we see this current wave dissipate before we dip back into the circle of the unknown, and this proposal is backed up by the GFS model. This renders tropical forcing useful until just about March 28th, before the wave weakens into the Circle of Death on the MJO chart. Phase 8 and Phase 1 events in this time of year routinely create warm and cold conditions, respectively, in the Midwest, Central US, and East US. I have a good feeling that the Phase 8 activity will far outpace the Phase 1 activity, keeping us in the El Nino mindset well into April.

To summarize the forecast:

- The last days of March (24-30) may see sustained cooler than average temperatures. This may persist even longer in the Northeast.
- Early April will see continued threats to the West for heavy rainfall and moisture. A warm-up is expected for this time period.
- Mid and Late April can expect more El Nino conditions, with warm weather in the Morth and stormy/cool weather in the South.


Sunday, March 22, 2015

Sunspot Numbers Plummeting, Stratosphere Warming

The number of sunspots has been anomalously low as of late, the lowest trend since roughly July 2014.

The above image shows sunspot numbers from NOAA over the past many months. Higher sunspot numbers (red) indicate that the sun is more active than usual. In the last couple of months, the number of sunspots has been dropping steadily from average values earlier in the year into 2014, interestingly enough in the midst of the strongest solar storm unleashed on the Earth in this solar cycle, per some reports.

What does this mean for our weather? The influence of the sun on our weather is still somewhat murky, but one big derivation we can make is that the stratosphere tends to cool down when the sun is active, and warm when the sun is quieter.

We are already seeing the stratosphere warm up, with more warming expecting in the future. The panels above show observed temperature values at different parts of the stratosphere in color, with the forecasted values in dashed lines. Notice how the ECMWF model expects the 10hPa level (top panel) to heat up notably, with a similar story in the 30hPa level (2nd panel from top). Although this warming won't be sustained, it's quite possible we see additional warming as the stratosphere exits its winter phase and enters its summer phase.

To summarize:

- Sunspot numbers have been steadily decreasing in the last few months, as we begin to end the current solar cycle.
- Stratospheric temperatures are responding by warming up, signaling and end to the winter phase.


March 24, 2015 Severe Weather Outlook

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

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.

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?

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.


Wednesday, March 18, 2015

Warmer Weather Returns After Early April Cold Blast

It looks as if warm weather will make a comeback after a stark cold shot hits to close March and open April.

Tropical Tidbits
In the image above, we see 500mb geopotential heights over the West Pacific, where reds indicate above normal values typically associated with high pressure and calm weather. Blues indicate toughing, symptomatic of colder and stormier weather on the surface. In this chart, valid on March 27th, we can see the strong ridge over Japan and areas just off to the north, with a slight trough to the south.

Using this, we can predict what the first week or so of April will be like. The Typhoon Rule tells us we can expect weather phenomena in Japan to 'happen again' here in the US about 6-10 days later. If the image above is valid on the 27th, we should expect some warmer weather around the first couple days of April. However, it's very possible this warmth is actually delayed, due to the aforementioned cold weather hitting the country at the end of March and start of April.

To summarize:

- After cold weather hitting in late March into the first couple days of April, warmer weather should make a comeback.


Sunday, March 15, 2015

Wintry Blast Expected to Close March, Open April

After a warmer ending to March, it is expected that a cold air mass will deliver one of the last punches of wintry weather to the country.

Tropical Tidbits
The image above shows 500mb geopotential height anomalies across the West Pacific. This forecast comes off the GFS model, valid for Wednesday, March 18th. We see a strong ridge located over Japan in this image, likely a result of the deep toughing further east in the North Pacific, as well as some more activity further west. When we apply the Typhoon Rule, stating that weather events occurring over Japan will 'come back' and 're-appear' in the US 6-10 days later, we could expect a generally warmer pattern persisting through about March 26th-ish, probably a bit longer.

Tropical Tidbits
The picture becomes much more ominous by March 23rd. By that time, the GFS model expects a rather large upper level low to drop into Japan, as shown by the deep negative anomalies over the island nation. In a nutshell, this means very cold weather is expected about 6-10 days later, likely in the very final days of March and the first week or so of April. If this happened in the middle of winter, we would likely be looking at record-breaking cold. However, due to the significant loss of snow-cover as of late, as well as climatological numbers telling us how quickly our average high and low temperatures are rising, this will likely be more of a strong cold blast, as opposed to a deep freeze.

To summarize:

- Predominantly warm weather is expected to persist through the end of March.
- The last few days of March and first week of April could see a shot of rather strong cold weather.


Wednesday, March 11, 2015

Colder, Stormier Weather Returns for Late March

Model guidance is indicating we're likely to see another stormy and cool period to end March.

Tropical Tidbits
The image above shows 500mb geopotential height anomalies over the West Pacific and eastern Asia. In this chart, oranges depict ridging aloft, a signal for warmer and generally quiet weather. Similarly blues depict toughing/storminess, and thus brings about cooler weather. In the graphic above, we see a persistent trough sitting over Japan and the Koreas, as signified by the plethora of blues. This is actually a mess of storm systems and cool weather combined.

When we apply the Typhoon Rule, which states weather phenomena occurring in Japan is reciprocated here in the US about 6-10 days later, we should expect a generally stormy and cool pattern around March 18-24 or so. This includes a stormy end to spring break for those who vacation next week, and a similarly cool/wet pattern for those who begin their vacations the week after next.

Tropical Tidbits
Models are already honing in on that colder weather. The above graphic again shows 500mb geopotential height anomalies, but this time over the United States and valid on March 18th. Here, we see prevalent ridging over the western half of the country, leading to pleasant conditions in the Rockies and Plains. However, a deep upper level low is forecast to traverse the Northeast and southern Canada, as the deep blues show. This means cold weather, possibly bringing about snowfall in the process for some.

To summarize:

- A generally stormy and cool pattern is expected for the second-to-last week or so of March, primarily around March 18-24.


Monday, March 9, 2015

March 9-11 Severe Weather Potential

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

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.

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.


Sunday, March 8, 2015

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.

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.

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.


Saturday, March 7, 2015

Long Range Climate Models Forecast El Nino Development in Summer, Fall

After a failed El Nino prediction for this winter, despite the NOAA declaration of the phenomenon in the last few days, model guidance is once again forecasting an El Nino developing in the summer and fall.

The above image shows the sea surface temperature deviation from normal, centered over the central Equatorial Pacific, forecasted until late 2015. The red line shows observed anomalies to date, while the yellow boxes portray the range of SST anomalies in the forecast period. This particular forecast sees water temperatures warming steadily from late spring into the fall, when the average of the boxes reaches 1.0º C at the very end of the forecast period.

The next forecast, from the Australian Bureau of Meteorology, shows temperatures quickly warming up from mid-spring onwards, solidly in El Nino territory by the early winter of 2015-2016. This was another model that consistently forecasted an El Nino this past winter, but never verified. It's quite possible we see this happen again this summer and fall.

The atmosphere is, right now, quite conditioned for an El Nino to develop. We have been seeing sustained negative values from the Southern Oscillation Index (SOI), an indicator of El Nino conditions in the atmosphere. This winter, it didn't come out as expected. However, if we keep a similar SST set-up across the Pacific, as well as our pre-conditioned environment for the El Nino, we could see that develop. I remain pretty skeptical, however.

To summarize:

- Model guidance is projecting an El Nino to form for the summer and fall months.
- Because the majority of model guidance was incorrect in their forecasts this past winter, uncertainty is extremely high.


Wednesday, March 4, 2015

Long Range Outlook for March, Early April 2015

This is the Long Range Outlook for the period of March 2015 into early April 2015.

Paul Roundy
The above image is pretty complicated, but bear with me and it won't be so bad. This chart shows a longitude-by-time forecast for tropical convection anomalies (colored shadings) between 7.5º North and 7.5º South latitude, centered on the Equator. Blue colors correspond to enhanced tropical convection (thunderstorms), while yellows are indicative of suppressed convection. There are a lot of lines here, but the only ones we need to worry about are the red lines, which show the active phase of the Madden-Julian Oscillation in solid (dashed is the suppressed 'phase'), as well as the Kelvin Wave, which is in thin pink.

We currently see an MJO wave evolving from about 75 degrees East, as the red lines in that longitudinal sector show. It is expected that this wave will strengthen in the next week or two as an Equatorial Rossby Wave moves over the region, shown as a light-green delineation. The placement of tropical convection in this part of the world is equivalent to a Phase 4 MJO event. As shown in the image below, temperatures during a Phase 4 event will tend to lean warmer than normal for much of the country, but a deep snowpack across much of the North and Central parts of the US tells me this warm-up could very well be muted around March 12th.

A Kelvin Wave then forms around that March 12th period, and this convectively-coupled Kelvin Wave traverses the Pacific in time for the final days of the month. This very progressive tropical forcing theme tells me the country is likely in for some roller coaster weather patterns for the remainder of the month, despite the MJO wave signal only shifting eastward slowly in contrast to the Kelvin Wave movement. This particular chart has us reaching Phase 5 of the Madden-Julian Oscillation by the final days of March, which would typically favor a cooler than normal pattern for the Northern US.

However, this cold outlook for the end of March is working in contrast to a projected pattern shift in the Pacific, in which ensemble guidance sees a trough positioning itself along the west coast of North America to permit warmer weather in the Central and East US. With warm signals coming from the Typhoon Rule as Japan undergoes a generally-warm pattern in the next two weeks-plus, and tropical forcing not really being effective this past month due to such dominance from the Pacific pattern, I'll opt for a warmer outlook to close March.

To summarize:

- An overall cool pattern should remain in place through about mid-March.
- Model guidance is then indicating, at the very least, the synoptic (overall) weather pattern will undergo some changes for the last half of March. I see this as being a good shot for a warmer pattern.
- This warmer pattern could continue into early April.


Tuesday, March 3, 2015

March 4-5 Potential Ice Storm

There is an increasing likelihood that accumulating ice will pose a threat to residents in the Southern US.

High resolution precipitation-type forecasts show rain impacting the states of Mississippi, Georgia, and Alabama into the Carolinas late on March 4th into the 5th, before widespread icing begins to take hold. Areas affected include northern Mississippi, northern Alabama, central Tennessee, eastern Kentucky, and portions of the Mid-Atlantic. Snowfall may then be expected to the north.

Ice accumulations from the GFS model lay down pockets of close to 0.50" in portions of Arkansas and Tennessee, as well as Texas and Louisiana. Model guidance usually doesn't do well with freezing rain forecasts, but amounts over 0.10" are very possible, if not likely, in northern Mississippi into Tennessee.

To summarize:

- An ice storm may impact the South US on March 4th and 5th.
- Accumulations could reach as high as 0.25" of ice in many areas, with isolated higher amounts.