Storm Highway by Dan Robinson
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Wednesday, April 18, 2007 - 3:09AM

Going on alert for the weekend

By DAN ROBINSON
Editor/Photographer
25 Years of Storm Observing
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Now that the new Tuesday evening run of the GFS is out, I'm finding it harder to ignore the upcoming troughs forecast to impact the Plains this weekend and next week. Here is the new 500mb GFS forecast for Friday morning. By the way, the colors and format of these GFS model images look a little different from the ones I posted yesterday, because I'm using the UCAR model outputs rather then the NCEP versions. I like the UCAR presentation better, but their images only go out to day 7. To get images for days 7-14, I have to use the NCEP maps.

It is hard for any observer to look at that and say "I am definately not going to chase". If it was mid-May and we were looking at images like this, we'd probably be leaving here headed west within 24 hours. But, as I mentioned yesterday, there are two factors that make me hesitate in driving what will end up being a 3,000 mile plus round trip to chase this setup. First is the storm motion, which should be seasonably fast. While one can intercept and document a tornado with fast storm motions (see our September 22, 2006 expedition), their forward speed makes an intercept more difficult for a variety of reasons. Second, the moisture forecasts are good, but not ideal:

Widespread dewpoints in the 50s are shown across the Plains, with a broad, peculiar axis of 60s nosing in. As a side note, there is an extreme drop-off of dewpoints parallel to the Texas/New Mexico border - this is the dryline, an important meteorological feature for observing. I'll talk more in detail about the dryline later. The 50s dewpoints shown here are half-decent, but not optimal for tornadoes. Again, if that image showed a more prominent pool of 60s right up against the dryline, I'd be more excited.

There are two kickers for the dewpoint issue, however. One is that the GFS has been erring on the side of caution recently in the realm of dewpoints. That is, the actual dewpoints have been slightly higher than what the GFS has forecast. The other issue is that on the high Plains of western Kansas and Texas, the high elevation (from 3,000 to 4,000 feet) means that storms can still accomplish great feats with the lower moisture.

The next trough forecast to impact the Plains comes on Tuesday. This trough is smaller, but more intense - and moisture is forecast to be much better with this second system.

The main problem I see with trough #2 is that it may end up being too far south in less-than-ideal chase terrain, mainly south of the Dallas-Fort Worth area.

So what do we do? I'm convinced that we should watch these upcoming setups very closely. I'm not ready to say we're thinking of going - yet. There are some caveats, but it looks to possibly offer a few good expedition days for the money. Our funding is limited, however, and any trip we make now will shorten the time we can stay out later in May and June. If we use a chunk of our funds now, we'll be in a bind if the Plains really gets rocking in mid to late May. We'll just have to wait and see what the model runs in the coming days show us. For the probability table, I'll give some weight back to our pre-standby time block of April 21-30 to reflect this weekend's prospects.

Based on the current outlook, this probability table charts the chance of our trip starting on a particular date:

2007 Weather Expedition - Departure Date Probability as of April 18
April 21-3010%
May 1-1022%
May 11-2022%
May 21-3122%
June 1-1522%
No trip2%

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