Prognosis on Feb. 28-29 Severe Weather Event

Prognosis--

Latest short range models I am seeing are developing a fair squall line starting tomorrow around 5:00 PM CST, with the squall one forming later in the evening. The line looks to be separate storm cells congealed into a weak line formation. This would be okay for supercell formation, but radar returns really are not impressive. Some strong storms certainly look possible from this, but a devastating event is not in the books here.
Just glancing at radar returns from the latest short range models tells me that the least held together storm cells look to be present in east Oklahoma into Kansas and Missouri. After that, the tornado threat looks to weaken in Arkansas as the storms turn more linear and less of individual cells.

Opinion on SPC Day 2 outlook...
The SPC appears to be accurately portraying this threat, but I am not thinking that a 'moderate risk' level is necessary for tomorrow. The way the NAM is playing out things tells me that Arkansas will probably not get anything in a moderate risk atmosphere, but if the SPC does decide it is necessary, I think it would be a low level moderate risk.
However, the NAM is having surface winds at over 20 MPH across the entire potentially affected area, leading the way for possible wind shearing which would easily make the way for the threat of tornadoes.

February 28-29 Storm Discussion (2/26/12)

Prognosis-

Storm Prediction Center has issued a Slight Risk for severe weather on 'Day 3' (2 days away, because Day 1 is today, day 2 is tomorrow, etc.). This Slight Risk includes a 30% risk of severe weather in Arkansas, Southwest Missouri and East Oklahoma, with formidable risks still present in Northeast Texas and the rest of the aforementioned states.

A worry I have to start with is the tilt of the storm. Every storm has a tilt. There is a positive tilt, in which 500mb winds (seen above) will be strongest to the southwest of the system, or a negative tilt, where the strongest winds will be to the southeast of the system. This situation involves a negative tilt storm, and let me tell you why that is bad news.

Negative tilted troughs indicates that the storm has reached maturity and is at maximum strength. Additionally, negative tilted troughs will have cold air running over the warm air, which thus provides additional instability and makes for more severe thunderstorms.

These 500mb winds are also concerning, as the top wind speeds appear to be up to 110 knots, which equates to about 130 MPH. This raises the concern for wind shearing. With high winds at different height levels in the atmosphere, wind shearing can take place. This shearing makes the environment for tornadoes much more conductive than without shearing. Unfortunately, that appears to be what will be happening in this situation.

This is what's called a hodograph. Hodographs measure where the radiosonde is being taken by winds in the atmosphere, which are then used to tell if there is rotation in the atmosphere. A radiosonde is what is sent up in those big weather balloons by the National Weather Service. This hodograph is forecast for the time storms are to be present in Arkansas. This hodograph was based near Hughes, Arkansas.

In this hodograph, we see forecast strong lower level winds as the lines are very long from one number to the next. Notice how it begins to form a half circle shape, meaning that there is some form of rotation being detected at that point in Arkansas. Suddenly, the half circle stops and makes a full circle/square shape. This is exactly what no one wants to see.

What this complete shape means is that there is suddenly a very tight area of rotation in the atmosphere that could cause a tornado. No meteorologist ever wants to see that complete shape.

Moving on, the line then goes off the chart, but likely makes another half circle shape, again indicating some form of rotation may be present.

This hodograph tells us that there may be some tornadic activity in the area come February 28-29. I am concerned about that complete circle that was shown in the hodograph, and I believe that will be something to seriously watch.

The NAM is showing that there will be a lifting mechanism in place to get the storms going. This image portrays the Lifted Index, or LI. When the LI goes negative, it means that stronger storms are likely. When it is positive, it means that the atmosphere is more stable than not.

The Lifted Index here calculates to about -5 in Arkansas and Texas, which is some pretty good instability that could very well get those storms rocking.

The final index is the Energy Helicity Index (EHI). It calculates rotation + energy (CAPE) in the atmosphere to make one of the better indicators for potential tornadoes. Values over 1 are considered apt for tornadoes.

In this image, we see values of 1 to 1.25 in some scattered spots in Arkansas, leading me to believe that Arkansas will be where the strongest of storms happen and where the best chance for a tornado will be. I find it hard to believe that people in the Lower Great Lakes will get anything big out of this as there will be more limited moisture and atmospheric instability.

All in all, I suggest you keep a sharp eye on this. It could get interesting.

-Andrew

Brief Look into Winter 2012-2013

Here are forecasted precipitation and temperature anomalies (respectfully) for DJF (December, January, February). These forecasts are from the Ensemble Canonical Correlation Analysis (ECCA) forecast system at the Climate Prediction Center.

For precipitation, the ECCA is projecting above normal precipitation across the northern US, but also majorly in the West US, where a 40% chance is present. For the North US, it looks like widespread areas of up to 10% are present. The same is present in the Northeast.

As for temperatures, it appears a major area of below normal temperatures are forecasted in the Plains, Midwest, Ohio Valley, South Plains, Southeast, Northwest, Southwest. The High Plains and Northeast are slightly above normal, though.

This is simply a look into next winter and not a forecast.

February 28-29 Storm Discussion

"Dynamic System Signals Spring's Commencement"

A dynamic system is poised to strike the US and bring everything from snow to possible tornadoes. Here's the latest:

Snowfall

The strong system that comes through will be cold on the north part, indicating that the event will be quite a snowy one. Accumulations at this point look to be pretty substantial, with this strong a system not keeping 12 inches+ out of the question. Then again, it is a bit too far out to forecast snowfall. Any snowfall should be wind driven, with this strong a system making for possible blizzard conditions in the Dakotas into Minnesota.

Severe Weather

There does look to be some severe weather possible with this system. Lower, middle, and upper level winds look to be pretty strong during this event, with 700mb winds possible reaching above 70 knots (80 MPH). A strong jet stream (over 130 knots (150 MPH) are possible) combined with a strong lower level jet stream (70 knots + possible) makes for a very potent tornado set-up in the spring and summer. However, it is late February. Thus comes the issue of lack of instability. Without instability and lifting mechanisms, it looks like this event will cause some possible brief spin-ups in north parts of the rain area, with some more potent rotation in the severe weather area.

Questions can be asked below.

-Andrew

Potential Severe Weather Event Feb. 28 (Issued 2/25/12)

Top left image: 700mb winds
Top right image: 300mb winds
Bottom left image: 850mb winds
Bottom right image: Precipitation

The set-up for severe weather continues to intensify with major wind speeds at heights that are monitored for tornado development. If winds at any of the 3 wind images are high, it usually indicates a risk for some spinning motion in the air which could then cause tornadoes.
What the GFS is currently showing is a few clusters of storms that would then evolve into a squall line. Squall lines are traditionally unfavorable for tornadoes as they are cells in a linear formation, like trying to have car wheels turning inside a box. It just doesn't work.

It looks like this level of wind shear would be favorable for some tornadic activity, but it is impossible to predict where tornadoes will occur. These are just guesses.

There is the issue on how there is virtually no CAPE, or instability, forecast for the storms to use. Additionally, the Lifted Index (lift) and CINH (Cap that stops instability) are next to nothing, if nothing. The non-presence of those indices pretty much cancels any potential of severe weather. However, the way that this is mapped out tells me that a brief, weak tornado is possible. Thunderstorms will be likely as this is a squall line when you come down to it.