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Mike Powell | All | HVYRAIN: Excessive Rainfa |
March 21, 2025 9:30 AM * |
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FOUS30 KWBC 210737 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 337 AM EDT Fri Mar 21 2025 Day 1 Valid 12Z Fri Mar 21 2025 - 12Z Sat Mar 22 2025 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Wegman Day 2 Valid 12Z Sat Mar 22 2025 - 12Z Sun Mar 23 2025 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Wegman Day 3 Valid 12Z Sun Mar 23 2025 - 12Z Mon Mar 24 2025 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE LOWER MISSISSIPPI AND TENNESSEE VALLEYS AS WELL AS FOR THE WESTERN OLYMPIC PENINSULA OF WASHINGTON STATE... ...Lower Mississippi and Tennessee Valleys... A strong cold front trailing behind a deep low tracking across the Upper Midwest will be the primary forcing supporting widespread shower and thunderstorm activity across portions of the Mississippi and Tennessee Valleys on Sunday. The front will be driven with a robust longwave trough moving across the Midwest. Much of the Marginal Risk area will be in the favorable RER of the jet. As is typical with thunderstorm outbreaks, there will be an LLJ out of the Gulf that will be in place ahead of the front. The southwesterly LLJ will be parallel to the orientation of the front (southwest to northeast), which will support training of the leading convection. 5-10 kt southwesterly Corfidi Vectors further support the potential for training. Thus, the forward speed of the front, which should be slowing...but should be fast enough to limit training potential...will be the deciding factor as to how widespread the instances of flash flooding will be. FFGs decrease markedly towards the north into the Ohio Valley, but the lack of atmospheric instability in that region will keep most of the rainfall in this area to stratiform rain. Further, when the front is moving southeastward across the Ohio Valley, the forcing and moisture availability will not be as optimized as a bit later when the front is further south. Thus, the inherited Marginal Risk over much of Kentucky and southern Indiana was trimmed with this update. The greater instability and Gulf moisture will be closer to the Gulf, where the Marginal Risk remains with this update. The flooding threat will be partially offset by the higher FFGs in the lower Mississippi Valley, so between that and the forward speed of the front, instances of flash flooding should remain isolated, favoring a continuation of the Marginal Risk. As always, a slowing of the front or additional moisture/instability with future updates could require a Slight Risk upgrade. Should the front become more progressive then it's possible the Marginal Risk could be downgraded or shrunk further. ...Olympic Peninsula of Washington... A strong trailing cold front will move into the coast of the Pacific Northwest on Sunday. IVT values will peak around 600-650 kg/ms during this time. The western Olympic Peninsula's soils are well saturated from repeated rounds of rain over the past few weeks, so this round could result in isolated flash flooding on the smaller streams and creeks that drain the Olympic Mountains. There were no changes to the inherited Marginal Risk. Expect 2 to 4 inches of new rainfall in the area through the day on Sunday. Wegman $$ --- SBBSecho 3.20-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (618:250/1) |
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