KSStorm Blog

Monday Scenario: Preparation Urged

09 May 2010 @ 8:44 AM

Pretty much in line with what I expected, the wording of various weather service forecast products has taken another step up in tone, and I’m going to echo the step-up. Tomorrow is a potentially dangerous setup for a lot of people. Today is the day to make plans about shelter — to consider the places you plan to go tomorrow and find out, if you don’t know, where shelter locations are in those places. A hand-held or desktop weather radio is a good investment — and of course in Kansas and Wichita, you can rely on KWCH and KFDI to have it very well covered (gratuitous plug now finished).

There are still some difference in model timing and the resulting feature placement. They’re gradually coming closer, though. The way I see it now, the differences are between the worst weather being from just west to just east of Wichita and it being in eastern Butler/Greenwood counties. There are also some differences with the southern extent of the storms. I’m not sure whether I’m being chaser-selfish or more concerned about protecting life and property where I live — but I hope the scenario plays out closer to the NAM solution in Oklahoma and closer to the GFS solution in Kansas. That has the dual benefits of keeping the Oklahoma chasers off southern Kansas roads and in their own back yard and shifting the worst weather to less-populated areas all around.

Should the opposite happen, we may be looking at a May 3, 1999 scenario displaced a bit to the north and east.

Overview: Storm Prediction Center Graphics

Categorical Risk

Total Probability of any Severe Weather

First change to note since yesterday: the Moderate risk area is slightly east of where the risk area was painted yesterday. Wichita is now on the west edge instead of near the middle. Most NWS forecasters have been buying the GFS model throughout the ramp-up to this event. It has been the most consistent throughout. The NAM (or now that we’re in the 2-day range, the NAM/WRF) model is a bit slower evolving the features and leaves the risk area back to the west, basically what’s outlined by the Slight risk above.

I would not focus so much on the 45% probability of severe weather for the Moderate Risk area as I would on the hatched part of the graphic on the right. That indicates the expectation among forecasters that pretty much whatever goes up tomorrow will be capable of high-end severe weather (EF2-EF5 tornadoes, 2” + hail within 25 miles of any point in that hatched area).

Why is this? Here are graphics showing the factors I find important. The values I list underneath them are my estimates for Wichita.

Surface Temperature: 78-82
Surface Wind: SE gusting to 30

700mb Temperature
(an indication of cap strength): +5 C.
700mb Wind: SW @ 50 kts

Biggest takeaway at this point: in the bottom portion of the atmosphere, wind shifts from SE at 30mph to SW at ~55mph. That’s more then enough twist to have some significant storms if they fire.

GFS CAPE: 1000

GFS CIN: 0 to-5

NAM CAPE: about 2000

NAM CIN: 0

Here you can see the two model differences. The best CAPE (a measure of the potential energy in the air) has already shifted east of Wichita by 7pm Monday according tot he GFS, while the NAM paints Wichita between two areas of storms. If that plays out, my feel is that the eastern one will be more supercellular in nature, while the west one will be a line of storms. CIN, a measure of how well the atmosphere is overcoming the potential energy in the air, is an important indicator of “if” storms will go. the takeaway is that capping is weak no matter which model you look at. I like the GFS better in that parameter – I like to see a –5 cap eroding between 4 and 4pm. The absence of cap in the NAM might not give storms enough chance to bubble up and gather strength.

GFS LI – best just east of Wichita

NAM LI – best just west of Wichita, but a respectable –7 overhead

I may be totally wrong in my thinking, but I like to see Lifted Index (LI) at or just above the CIN value. It seems like if the LI is too high, things go up to fast, but if there’s just a little more than the value of the CIN, (note the signs are opposite), it seems the storms boil and strengthen a bit more.

Just one derived product to show today…but it’s a biggie:

NAM-WRF STP

This is the Significant Tornado Parameter for 7pm tomorrow according to the NAM/WRF. I don’t claim to understand what all goes into it, or what a value of 7-10 means, but that dark red area over Sedgwick and southern Butler counties is ominous. That is the maximum value the parameter has. Note, too, this being the NAM, that another similar area is right along I-44 in Oklahoma. That’s what leads to my earlier comment concerning a repeat of May 3, 1999 slightly displaced.Other derived parameters based on both models paint a similar picture.

In the words of NWS forecasters:

Storm Prediction Center, Norman, OK:

LONG-TRACK STRONG TORNADOES WILL BE POSSIBLE...
PARTICULARLY FROM OK PORTION OF I-40/44 NWD
INTO SCNTRL/SERN KS DURING THE EARLY EVENING.

NWS Wichita:

POTENTIALLY LONG-TRACK TORNADOES DURING THE AFTERNOON AND EARLY
EVENING ON MONDAY. SUPERCELL THUNDERSTORMS ARE EXPECTED TO DEVELOP
DURING THE MID TO LATE AFTERNOON JUST AHEAD OF THE DRYLINE AND RIGHT
MOVING STORMS WILL QUICKLY MOVE OFF TO THE NORTHEAST AT 40 TO 50KNOTS.

NWS Norman (emphasis mine):

SEVERE THUNDERSTORMS ON MONDAY AFTERNOON AND EVENING...MAY MOVE AT
SPEEDS GREATER THAN 45 MPH...LEAVING LITTLE TIME FOR PEOPLE TO REACT
TO WARNINGS. YOU ARE ENCOURAGED TO THINK AHEAD TO WHERE YOU
WILL BE ON MONDAY...HOW YOU WILL RECEIVE WEATHER WARNINGS...
AND WHERE YOU WILL SHELTER IF A STORM APPROACHES YOUR LOCATION.

My target for tomorrow stands at this point. Essentially a back porch chase.

Posted By: sroberts
Last Edit: 09 May 2010 @ 09:03 AM

Email • Permalink