Severe Weather Forecast Checklist from Springfield MO NWS

Bill Hark

EF5
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A COMPREHENSIVE SEVERE WEATHER FORECAST CHECKLIST AND REFERENCE GUIDE

http://www.crh.noaa.gov/techpapers/service...0/tbl-cnts.html


I found this checklist/summary of factors for severe storm and tornado forecasting on the Nashville, TN website (originally from the Springfield NWS). Although it appears slightly out of date, it is a nice summary of factors to watch for while forecasting severe storms.

Bill Hark
 
I gravitated pretty quickly to this section:


1. Will the CAP Break?

One of the most challenging parts of severe weather forecasting is determining when, and if, the cap will break. Most forecasters have experienced a busted forecast where they had a very unstable environment, with a cap in the low levels, and a convective temperature that was never reached. Conversely, situations such as the Plainfield, IL 1990 tornado, can occur when you have a strong cap with prodigious CAPEs and helicities. Temperatures > or = 12°C at 700 mb and/or CIN < -50 j/kg normally inhibits thunderstorms, however, severe storms can develop regardless of CAP strength, if low level forcing exists. Typically, the cap will be eroded by surface heating and/or large scale lifting.

Emlaw (1991) has the one of the best operational paper we've come across on this subject to date. He offers two techniques designed to be used 4 to 8 hours in advance of late afternoon convection to determine the probability of strong convection in a capped environment. Forecasters should also utilize data from special and modified soundings, pireps, mesonet hourly data (where available), LAPS and MSAS based skew-t's , and GOES soundings.

I have found the technical memorandum (NWS SR 134) and the page with information on purchasing it here, but was wondering if any of you operational wizards are familiar with Emlaw's ideas or if they're something that, by now, we're all more or less aware of.

This document doesn't go into any more detail about this other than listing the author and the citation in the back.
 
Originally posted by Amos Magliocco


I have found the technical memorandum (NWS SR 134) and the page with information on purchasing it here, but was wondering if any of you operational wizards are familiar with Emlaw's ideas or if they're something that, by now, we're all more or less aware of.

This document doesn't go into any more detail about this other than listing the author and the citation in the back.

I haven't seen this document before, but it reminds me that Davies has a page on cap strength estimation that I know I have found useful for the seasonal aspects of estimating cap strength.

http://members.cox.net/jondavies1/700mbTca...p/700mbTcap.htm

It also reminds me that I recently saw a thread somewhere where cap strength was estimated with 850 mb temps - which in my view at least can be pretty dangerous, particularly during the warm season when the boundary layer depth can be higher than 850 mb.

Glen
 
Yeah, seems like it's so precarious to use any rules of thumb or hard and fast thresholds for cap strength that it's good to know them ALL in a way, to try and compose a better mental conceptual model. So many factors influencing cap strength through the vertical depth that it seems like the single hardest thing to nail down.

But I also agree with some other chaser friends of mine who say that if all the other 'ingredients' are in place for tornadic storms and the cap looks even modestly stout, you still have to go for it. Maybe that's a reflection of a sort of acknowledged lack of skill with forecasting inhibition, I don't know, (I acknowledge it on my part! LOL) but I agree with it. Unless it's thermonuclear, you pretty much go if that's the only problem.
 
Agreed - easy to get burned on caps. Always tough to forecast, both for people and models. Then, you are stuck with making a mental decision on the strength of the surface convergence needed to overcome the cap if heating isn't likely to cut it. That said - I think models perform better at defining the axis of the 700 mb thermal ridge - so that can be used to make sure you are at least on the correct side of the ridge for the best chance of breaking the cap when picking a target. A lot of amateur forecasters I've noted don't even look for this in their forecast - a big mistake IMO, like not being on the cyclonic side of the LLJ (of course there can be exceptions, before someone chimes in.....).

Glen
 
I finally finished reading through this resource. One point that I found interesting was about northwest flow spawning severe weather outbreaks in summer months.

"77% of all NWF outbreaks include at least one tornado (Johns 1982). Outbreaks usually last 8 to 10 hours."

This reminded me of the July 13 Derecho event of last year that dropped the Roanoke F4. I remember going out to chase large hail, mainly because the tornado parameters weren't very favorable. It would have been good to have this knowledge at the time though. I would have probably targeted tail end charlie instead of the huge bow. I remember looking at SPC's tornado forecast which was relatively low compared to the hail and wind probabilities and thinking that this was going to be a tornado less day. However, SPC did a pretty good job. With only one tornado dropping they didnt need a huge swath with high percentages.

It seems like this note is especially useful for midwest chasers. It seems like every year we get at least one good summer derecho/severe squall event and west/northwest flow is usually what brings it on.
 
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