Friday, May 21, 2010

Where's all the rain that was promised?

I know there are very few complaints that the forecasted rain and thunderstorms (Flash Flood Watches/Severe Thunderstorm Watches) didn't materialize yesterday and overnight after record flooding earlier in the month that many are still recovering from. However, you still may be curious about how all the forecasters seemingly missed the boat on this one (pardon the pun).

Below are the 24-hour forecast rainfall totals from several computer models, all valid from 7am Thursday to 7am Friday (this morning). You'll recall this was roughly the period of the Flash Flood Watch that was issued on Wednesday. Generally, Memphis-area forecasters were expecting anywhere from 1-3" of widespread rain. The primary computer models used for this area - the GFS and NAM (the first two images below, respectively) - differed widely in their handling of the approaching system up until the night before, lowering the forecast confidence a great deal. Thumbnails of the model forecasts are below and can be clicked for larger views. The high-res WRF model (last two images) actually did a pretty good job showing the heavier precip east of the Memphis area.

What actually happened was this: a mesoscale convective system (MCS, or large area of thunderstorms) moved out of OK and across AR during the overnight hours. Typically ahead of a complex of storms like this, an outflow boundary exists which acts kind of like a mini-cold front. It shifts the wind and ushers in cooler air that has been produced by the thunderstorms' downdrafts. As has happened many times in the past, the approaching thunderstorms were moving away from their energy source at the coolest time of day (dawn) and began to fall apart in eastern AR. The outflow boundary continued east and moved across the metro area. Scattered morning showers resulted from the decaying MCS and outflow boundary, but the "punch" was gone to produce stronger weather. (By the way, this phenomena happens so often that many Memphians like to blame the storms decay on the bluff or the "Power of the Pyramid." I don't think so...)

Behind the boundary, most of the metro was left with a southwest wind, lower dewpoints (drier air), cloud cover, and generally more stable conditions - not favorable for the development of severe weather. During the afternoon, the airmass that would have been over us had it not been for the boundary that pushed it east was ripe for storms and they formed to the east and south of the city. Hence, we miss the morning storms as they decay before reaching the Mississippi River, then we miss the afternoon storms (outside of a few over the eastern metro during the late morning) thanks to the airmass modification that took place behind the boundary. Radar-estimated rain totals from 11:30pm Wednesday night through 1:25pm Friday is shown in the image below, which clearly shows the relative dearth of precip in east AR and the metro area. Again, I don't think anyone is complaining...

As this system departs the region today, we are in for another pattern shift, this one fairly significant, as high pressure typical of summertime in the Mid-South builds in overhead. Dewpoints are not dropping a lot behind the departing front, so there will be some humidity in the air, but the big story will be widespread 90+ degree temperatures as we head into the weekend and next week. By early next week, some additional Gulf moisture will work its way in and we will likely see some scattered afternoon and evening showers and thunderstorms for the mid-week time frame, though temperatures will continue to run near 90 in the afternoons through the week. Details can always be found in the MWN Forecast.

Finally, a word on (the site). A new layout is coming very soon! Content will change very little, however the menus will move to the left side of the screen, the format will be wider, and you'll be able to get to the most frequently-used site content more quickly from the front page. I think you'll like it! For more information on this, and other items related to MWN, including sneak peeks at new features before they are released to the general public, sign up for the MWN Newsletter!

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