The purpose of this blog isn’t to get into big forecast related discussions, there are already too many blogs out there that attempt to do such things. That said, I do have the fortunate advantage of explaining severe weather and forecasting information to common everyday people all the time who travel the highways with us at StormTours.com, and I know there are many of you read through this site and you might not understand exactly why we (storm chasers in general) have had such a terrible time in May. Perhaps you’ve seen The Weather Channel and their live crew traveling with the large VORTEX2 armada and you’ve noticed – “Hey, these guys are NOT showing any severe weather on here – whats up with that?”

There were many unfortunate meteorological occurances that happened during May 2009 that all played little roles in basically destroying our chances for severe weather. Now if you don’t want severe weather – that’s a great thing I’m sure. But for people like us – people that live, eat and breathe the thought of viewing incredibly structured supercell thunderstorms over the Great Plains – well, we’ve been left high and dry lately. In short – the jet stream, which helps produces a strong vertical sheer element in the atmosphere, it went to Canada during May.

Perhaps it heard that it was going to need a new passport to cross that border so it wanted to get up there why it had the chance. Or, perhaps it was due to a major low pressure system that came in across the eastern Gulf of Mexico, nearly becoming a tropical depression and combined with a massive dominating high pressure system over extreme southwest Texas – all these things added up and left us with partly cloudy skies, cooler than seasonal average temperatures and just really yucky weather in general.

High pressure as you probably know circulates in a clockwise fasion. Look at a map of Texas and put a big “H” over the southwestern portion, and then draw a line starting below that “H” and go clockwise around the “H” until you get to the right of it. What is happening? The direction that the air is traveling at the surface to the east of the high pressure is generally south – and when this happens, it not only blocks any deep, rich moisture coming up from the Gulf of Mexico, but it also helps push any moisture that might be in the plains back to the south. Naturally we have to have moisture in the plains to get any really good extreme weather, so you can imagine as long as that big “H” is in south Texas, it doesn’t help matters much.

Lets take a look at some forecast guidance, namely the WRF model in its current 72-hour run for 7pm on Saturday (0Z SUN).

Here is the game changer! A low pressure system starts to dig into the plains during this model run, the wind will start shifing from the south in about 36 hours prior to this event and increasing in speed from Friday night into Saturday. This in turn will of course carry much needed (for tornadic storms) deep, rich moisture from the Gulf of Mexico northward into Nebraska.

Wow – that almost looks like a real dry line that might develop from northeastern Colorado (aka: The Cheyenne Ridge) back towards Wichita and then towards the southwest. For those who are trying to learn – notice the area near Wichita where the line bows out? That’s called a dry punch, generally it means that a low pressure system is located to the west, while the best moisture will probably located just to the east, or southeast.

At the surface things are looking pretty good so far, but there are still a few things that we need to know. But, before we get into that – let’s see what will be happening aloft. Will the jet stream be there to give us the boost of flow needed for good horizontal vorticity?

An image is worth a thousand words – so I’ll make this part brief. “Big Trough”. This is what will be happening at 300 mb, or about 30,000 feet. If you’re flying from Chicago to L.A. on this day, you’re going to be fighting some stout winds most of your trip! If you’re storm chasing on this day, you’re going to have an exit region of the jet stream located over northern Oklahoma and southern Kansas! It’s all still good so far!

Let’s come down to about 500 mb, or about 18,000 feet and take a look:

I’m still impressed, but the jet stream and low pressure trough isn’t considered “ideal”. This system has been sitting over the Pacific Ocean just off the shores of California for a few days and the low pressure system has already pretty much reached its mature point. As it moves across the United States the center of the upper-low is still over Nevada while the exit region gets spilled out over the Rockies and into the plains. This creates a trough axis from SW to NE, or a positive tilted trough. While the trough itself is very good news, if it were negatively tilted our chances of extreme weather would be taken up a few notches. Still yet though, its been a rough year and its good to see some flow returning to the plains so that we can refresh the atmosphere with needed moisture!

There is one more thing that I want to talk about and especially since it’s June. The “CAP”, this is a layer of warm air that will sit on top of the surface layer and may prevent storms from actually developing at all. Typically, in June the CAP is pretty strong in the southern plains, and often in the central plains. This is because this layer of warm air gets “pushed” into the plains from higher desert elevations, such as the New Mexico plateau. By June the air over New Mexico is extremely warm – thus when it moves into the plains it creates a strong CAP. In order to get storms to develop we’ll need some stong instability so that as the heat rises from the surfaces it has enough momentum to break through this warm layer. The CAP strentgh can usually be measure in the plains by examining the air temperature at 700 mb (just about 10,000 feet). But we can cheat a little bit and look at this model graphic:

This shows us basically two different things – the colored gradients indicate the CAPE (Convective Available Potential Energy) and the hatched areas indicated the CAP or lid strength. In order to get tornadic supercells to develop we’ll generally need at least 1500 j/kg of CAPE, which it appears that we’ll have in the central plains. However there is a problem – there is a hatched area on top of it! This might mean that any instability that develops won’t be enough to punch through the CAP. Except for maybe a tiny area from about Altus, Oklahoma back towards Abilene, TX – but down here we won’t have the upper-level flow that we’ll have in the central plains for great rotating updrafts. Let’s examine the CAP strength using a SKEW-T diagram with the model forecasting data.

If you’re not familiar with this funny looking chart, it’s called a Skew-T/Log-P diagram and its the most useful tool you can take with you when you’re storm chasing (in my personal opinion). Using this chart you can easily intrepret just about anything you want to know about the atmosphere as long as you know a few basic pieces of information to start off with such as the surface temperate and the dewpoint, as well as the general “profile” of each throughout the atmosphere. There is a great PDF file online from Texas A&M University that can help you get up to speed located here:http://www.met.tamu.edu/class/atmo251/book05soundings.pdf

This particular diagram above is a forecast skew-t based on the latest run of the WRF model for Wichita, KS. So in short, it’s attempting to forecast the atmospheric profile for Wichita in 72 hours. Naturally, its not exact, but for our purposes of trying to estimate the cap strength its a good start.

In order to estimate the cap using a Skew-T is extremely simple – locate the height level for 700 mb and follow it across until you intersect the temperature profile. You’ll notice in the diagram above there is a dash line at this location, its because this graphic has already done most of the hard work for you (making it very easy to use when you understand the diagram itself). What this is telling us is that the temperature at about 10,000 feet is going to be about 12 degrees C, or (if you’re an American) about 54 degree F. That doesn’t sound to warm to you and I, but if you’re a parcel of air leaving the surface and cooling off as you rise up – it’s pretty darn warm! In order for that parcel to break through this warm air the surface dew point temperature is going to need to be at least 26 degrees C, or (American) 79 degrees F – that’s a pretty tall order. And from looking at the diagram it is indicating a dewpoint temp of about 19 degrees C, so it’s not quite there. Which means that of course – the CAP may be, and probably will be the big spoiler on Saturday afternoon.

I’m going to pretty much leave it at that, but the gist of this is that unless this information changes – we’re going to likely be digging south along the dryline where the convective temperature (and closer proximity to the Gulf of Mexico) will help the temp/moisture profile to the point where the CAP won’t be such a limiting factor – but the trade off will be less upper-level flow.

UPDATE (23Z June 07): Just wanted to post this radar imagery to show current severe warned cells in the southern target area along the dryline. Not a bad pick from a 72-hour old forecast!

Tags: forecasting, storm chasing, tornadoes, weather