Thursday, March 1, 2012

Dangerous Severe Weather Outbreak Possible Tomorrow

I am closely monitoring the potential for extreme severe weather tomorrow, March 2, 2012.

Have noted Storm Prediction Center's issuance of a moderate risk for Ohio Valley into the nation's Midsection and South Central US.

Model Used for Discussion: NAM due to recent issues with GFS

NAM has been putting out uncomfortably high EHI forecast values for South Illinois into Missouri, Tennessee and Kentucky. This is where the threat should start off with.

Deep layer shear from that point on increases to over 100 knots out west to over 70 knots in the area being monitored for severe weather. This deep layer shear is more than supportive for super cells, and should easily assist any possible tornado threats. Lower Level Jet stream will be quickly moving along with speeds upwards of 50 knots in the areas mentioned above. Jet stream at 300 mb level will be screaming at over 100 knots. Additionally, I have noted the NAM's split flow regime over Tennessee and Kentucky, only further enhancing my fears for this potential outbreak.

Progressing several hours further, the risk begins to shift east. Deep layer shear increases with this shift eastward. Lifted Index will be as low as -8, a very low number to see for early March. This intense lifting is signaling the deep layer shear to also begin to influence the rising air and may begin to spin that rising air. Split flow mentioned previously begins to wear off but is still very much present in the Tennessee/Kentucky region. At this point, surface winds should begin to increase beyond 20 knots.
High theta-e at this point marks the high instability that will be present in the region, even though CAPE measurements reflect a somewhat substantial 2500 j/kg.

Storms should be moving pretty fast, with some exceeding 40 knots in spots, which equates out to 46 MPH. And then there's the issue of storm relative winds. SR Winds at 850mb are going directly opposite of the forecast storm motions. As for exactly what this implies remains to be seen, but it is never a good thing to see two levels in the atmosphere going different directions.

My Thoughts

I'm thinking that this won't be a terrible outbreak situation, but it will be pretty rough. This intense wind shear will do its job on updrafts that will be persuaded to rise by 1000 - 2000 j/kg of instability. As the shear takes hold and the updrafts strengthen in association with the moderate instability that will be on the increase, I am thinking that any storms that form will start out with a moderate risk for tornadoes that will be on the rise fairly quickly into the afternoon hours.
Here's my thinking for the tornado risk.

It is next to impossible (if not impossible) to predict tornadoes.
Models are still having disagreements (GFS is a tad less energetic).

Any questions may be asked below.


Joshua Steiner said...

Andrew, have you seen the 18z NAM model? It's showing high EHI values (values over 1) as far north as the Michigan/Ohio border. Highest EHI and low level storm relative helicity is indicated to be located from Cincinnati down southern Kentucky and as far north as Lima, Ohio. If the storm becomes stronger and more negatively tilted than what is currently forecasted, I bet that the tornado threat will extend much farther north than what is anticipated now. (It may not seem like the upper level system is negatively tilted, but on the 500 millibar progs, a well defined negatively wave is shown on the NAM and GFS.

Anonymous said...

You do realize that a 60% chance is an extreme high risk for SPC, right?

Anonymous said...

severe weather outbreaks 2 day apart back to back...This is going to be a stressful spring/summer for many =(
Question: Is this weather due to the mild winter everyone had/the pretty darn warm GOM?

I was just thinking too, the -PNA is sending the cold air/fronts into the country and the +NAO is providing the warm air to completely dominate the other 1/2 of the country. Right? Thanks, too, Andrew.

- Reid

Andrew said...

Joshua: Yes- I based my forecast on the 18z NAM. This will be a horrible day for many tomorrow.

Anonymous: I use a different system than the SPC. I can't exactly put it into words, but 60% in my eyes is like 60 out of 100- the odds are more favorable, but details have to be worked out to be confident.

Reid: I haven't been monitoring the winter aspect as this severe weather has been so back to back. I will try and answer it on this post in a couple days after the severe weather dies down for a bit.