Very Wet Week Ahead

A very wet 10 days is ahead for the eastern half of the nation.

Multiple model forecasts from multiple forecast systems are projecting the next 10 days to be full of water for the eastern half of the nation. Shown above is the predicted total rainfall from the European model system. Past forecasts have been even wetter than this, but the latest forecast predicts many states east of the Plains receiving over 3 inches of rain. If you're looking for a mind-blowing statistic, look no further: in this forecast, each state on this map (excluding New Mexico and Arizona) is projected to receive over an inch of rain in the next 10 days. That may not seem like a lot, but when you remember that this forecast image accounts for all but a few states in the US Mainland, you get the idea of how big this wet trend is. A few smaller states in the Mid-Atlantic may be added to that exclusion list, as they are just so small it really is hit-or-miss as far as if they will get in on that 1 inch mark. The highest totals print out in Arkansas, Louisiana and Mississippi, where a whopping 5 inches of rain could hit multiple, small-scale areas in those states. Indiana and Michigan could also get in on those high totals. The top of the image indicates the highest total on this graphic is 6.6 inches, which appears to be forecasted to hit northern Arkansas. Get the rowboats out!

Andrew

Tornado Warning - Tornado on the Ground - North of Clinton, AR

Tornado is ON THE GROUND north of Clinton, Arkansas. Life and property is threatened as this storm moves northeast.

TIMES OF ARRIVAL:

Fifty-Six: 5:20 PM CT
Calico Rock, Pineville: 5:30 PM CT

Take shelter now. Please share to get the word out about this very dangerous situation.

April 9th Severe Weather Outbreak

Have demoted post title as a result of no upgrade to 'Moderate Risk' of severe weather by Storm Prediction Center.

Predicted Exclusive Risk Index (ERI) for April 9th, 2013

Discussion: After a quick vacation day, I am back to analyze the threat for this severe weather event on April 9th.

High resolution model guidance is concerning and unsupportive of widespread tornado development. Current solution appears to be isolated thunderstorm development quickly congealing into a squall line. My thinking is these cells are only briefly able to mature into supercells before coming together, greatly constricting tornado development. Storm Prediction Center continues to hint at the potential for an upgrade to a Moderate Risk for an undetermined area, but with that upgrade not coming today, the chances for the earlier-predicted tornado outbreak have lowered.

Have pulled out the blue 'Severe' risk area from the ERI to reflect lowering chances for a big severe weather event, but maintained a solid area of 'Enhanced' severe weather. I placed this Enhanced area from Oklahoma and southeast Kansas into Missouri and northwest Arkansas. The angle of this risk area was made to coincide with expected northeast storm motions , but may have to be expanded a bit north into Kansas if later model forecasts take the storm into a more north-northeast motion.

Will update as needed tomorrow afternoon, but here's a few notes on the situation:

•Storm Action Day will be formally declared tomorrow.
•Storm Action Day will LIKELY be declared on April 10th.
•Critical Storm Action Day is UNLIKELY on April 9th and 10th.

Andrew

Special Discussion: April 9th Tornado Outbreak

Storm Prediction Center has outlined two areas of severe weather on April 9th (red) and April 10th (purple). It's looking like the April 9th outline will be more enhanced as far as the tornado threat goes, while precipitation forecasts from the American GFS model suggest April 10th will be more of a squall line event, spanning multiple states.

Multiple sounding forecasts suggest this event will be held down by a slight capping inversion. As you may recall, a capping inversion involves a situation where the air at the surface is unable to rise due to a layer of warmer air just above the surface. However, as soon as that capping inversion breaks (which is expected to occur in the evening hours of April 9th), air attempting to rise and create thunderstorms will have over 1500 j/kg of instability available. From here, this rising air will form updrafts, which will then turn into weak (and likely rotating) thunderstorms. These initial storms will feed on upper air support and continuing availability of moderate instability to mature into supercells. With elevated helicity and high amounts of atmospheric spinning in the immediate vicinity of the mature supercells, tornadoes should begin to become a real concern as the 7:00 PM (central) hour comes to a close.

Main concern for tornadoes is in much of Arkansas and into Missouri. Storms will begin in Oklahoma and quickly turn supercellular. I believe the storms will quickly allow National Weather Service offices to issue severe thunderstorm warnings, but it may take another while before tornado warnings will be needed. I expect most of the supercells to become tornadic in Arkansas before transitioning into southern Missouri as the storms quickly move off to the northeast. Louisiana will also get into the severe weather, and they have been included in the greatest risk. It is these areas that are most likely to experience tornadoes on the evening of April 9th and overnight. I only accounted for the evening portion of this event, so areas into Illinois, Tennessee and Mississippi have not been completely accounted for going into April 10th.

I outlined a forecasted sounding from the most recent American GFS model, and denoted a few select factors. The first thing to note is the presence of a loaded gun sounding. The loaded gun sounding is signified by a slight capping inversion (yellow line to the left of the red line) just above the surface, with high instability above that capping inversion (yellow line far to the right of the red line). The name 'loaded gun' comes from the idea that the 'gun' (severe weather) could fire at any time- the trigger (capping inversion) just has to be pulled (erased). As soon as that capping inversion is erased, the severe weather gets going. Typically, you will see a loaded gun sounding that is much stronger, where the yellow line is much farther away from the red line than what you see above. What you see above is an 'okay' loaded gun sounding. Regardless, the mere presence of the loaded gun sounding indicates that severe weather is targeting this area. We'll start with the PW. The PW index is precipitable water, where values above 1.00 inch mean a humid atmosphere, and 2.00 inches or more is almost like you're breathing in water because it's so humid. The projected precipitable water values for Little Rock in this sounding is at 1.63 inches, which means Little Rock will be smack dab in the middle of the warm sector. If you recall, a warm sector is the warmth and humidity pulled north from the Gulf of Mexico by a storm system's warm front, and it is in here where the cold front thrives and creates severe thunderstorms. Continuing on, we see the CAPE index just above 1500 units. The elevated values of CAPE (instability) means that thunderstorms are likely to be able to access this instability and create severe thunderstorms. The amount of instability isn't as great as it could be, but you get what you get. The index right below the CAPE value is CIN, which is stability in the atmosphere. Instability creates thunderstorms, stability suppresses them. As a part of the typical loaded gun sounding, there is a small amount of stability just above the surface. The sounding lists the amount of CIN as 7, which is essentially no stability whatsoever. That's a good sign for storms that could have trouble breaking the layer of stability, as they will have little trouble breaking the stability. The last value outlined on the sounding is something called EHI, which is the Energy-Helicity Index. This index combines instability and pure spinning in the atmosphere to create a value that is one of the best predictors of tornadoes to date. As a general rule of thumb, elevated EHI values above 1.0 tend to indicate the presence of at least slight tornado dynamics. Forecaster Jon Davies indicates values above 2.5 are conducive for supercells and are favorable for tornadoes. That said, the projected EHI value of 3.6 is quite high and great for tornado development and propagation. There are several other parameters we could look at for guidance into forecasting this event, but from the aforementioned factors, I have a pretty good idea of what to expect.

Summary
A tornado outbreak is expected over eastern Oklahoma, Arkansas, portions of northeast Texas and southern Missouri. The cells should initiate in the evening hours and mature to supercell status in the later hours of the night. Tornadoes are likely in the aforementioned areas, and more than one of them could be long-tracked and strong. Further clarification is anticipated by tomorrow's new Storm Prediction Center outlook

Things to note:

-Storm Action Day has been declared for April 9th. Upgrade to a Critical Storm Action Day may be needed.
-Storm Action Day is possible on April 10th. Decision will be made in coming days.

Andrew

April 8-10 Significant Tornado Outbreak

Critical Storm Action Days may be needed for this event. For those who don't know, a Critical Storm Action Day (CSAD) means all resources here at The Weather Centre are devoted to the severe weather event. No other unrelated posts will be published.

I have decided to go ahead and change the title of this event to a 'Significant Tornado Outbreak' in light of strong Storm Prediction Center discussion wording, very favorable projected soundings, and the wind fields from different levels of the atmosphere.

The Storm Prediction Center in Norman, Oklahoma has released three areas highlighted for a severe weather event in the long range forecast. The three areas are highlighted for April 8th, 9th and 10th, and I have used those dates to make the appropriate dates for this post. First area of interest is in northwest Kansas, where the storm system may put out an area of severe weather the day before the big show begins. The big story is late in the evening on April 9th into the overnight hours and morning of April 10th.

Strong jet stream will punch into the Plains with this system to allow upper air support to a severe weather event. Wind speeds well over 100 knots will be focused in the immediate vicinity of this severe weather event for April 9th. Jet streak of 120 knots appears to be pushing north and east with the movement of the storm that will most likely aid in more enhanced tornado development in Arkansas and portions of MO/OK. Extrapolation of this image would have the jet stream slide up the stream and eventually depart overnight to possibly aid in thunderstorm development further north into the Midwest;  however the extent of that potential is yet to be seen.

Lower level winds will be howling above 50 knots in the areas where severe weather development is expected. Weaker winds in the 40-50 knot range are encompassing nearly the entire area outlined in purple by the Storm Prediction Center for this outbreak on April 9th. Main concern rests in areas where lower level winds are maximized- I feel it is these areas that will have the most support for tornadoes and general severe weather. Certainly cannot count out a strong, long-tracked tornado with this event. Wind shear will be enhanced from border to border to allow for the thumbs-up for tornado potential. The more wind shear is present, the more favorable the environment becomes for tornadoes. These favorable upper and lower level wind fields, combined with the widespread elevated wind shear tells me many are in for a rough ride come Tuesday.

Sounding from central Arkansas valid for the evening of April 9th unveils a loaded gun sounding. The loaded gun sounding is signified by a slight capping inversion (yellow line to the left of the red line) just above the surface, with high instability above that capping inversion (yellow line far to the right of the red line). The name 'loaded gun' comes from the idea that the 'gun' (severe weather) could fire at any time- the trigger (capping inversion) just has to be pulled (erased). As soon as that capping inversion is erased, the severe weather gets going. Typically, you will see a loaded gun sounding that is much stronger, where the yellow line is much farther away from the red line than what you see above. What you see above is an 'okay' loaded gun sounding. Regardless, the mere presence of the loaded gun sounding indicates that severe weather is targeting this area. Looking at a few choice parameters on the righthand side of the sounding, we see just over 1300 units of CAPE (instability). That number isn't too high compared to what you can see in spring, but considering this is the first outbreak of the season, it isn't too shabby. Just below the 'Wind' section, the STM (storm motion) parameter is at 237/37. This means the storms will be moving from the southwest and to the northeast (237 degrees on a compass points towards the southwest), while the number 37 refers to how fast the storm will be going in knots. The last value of the numbers is BSHR, which indicates bulk shearing. We see that at a solid 96, meaning there will be quite a bit of shearing in the area- a big tornado factor. PW (precipitable water) will be at 1.66 inches, a very humid atmosphere thanks to the flow from the Gulf of Mexico. The BRN (Bulk Richardson Number) is in the teens, which is the prime range for severe thunderstorms. The EHI value is at 3- the higher the number, the more rotation and energy there is in the atmosphere (thus, the higher chance of a tornado). 3 is considered a solid amount of rotation and energy.  HEL shows helicity, the pure spinning of the atmosphere. That value rests at 365, quite a high number and conducive for tornadoes.

Have finally added a 'Severe' demarcation to the graphic to highlight where the highest chance of overall severe weather is. Also added a red delineation to highlight where I believe the best chance of tornadoes will be. I am concerned for the Arkansas area and surrounding regions as far as tornado potential goes, I fear that lives may be lost if enough rotating thunderstorms can form and mature to become supercells. Other than those two additions, the ERI graphic was not changed. Will likely have a special graphic out tomorrow to highlight the specific tornado threat.

Please share this with friends, coworkers and family- this is the first severe weather outbreak of the season, and it could be a tornado outbreak. Many are likely unprepared.

Andrew