Subj : HVYRAIN: Excessive Rainfa To : All From : Mike Powell Date : Tue Nov 21 2023 09:24:00 FOUS30 KWBC 210730 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 229 AM EST Tue Nov 21 2023 Day 1 Valid 12Z Tue Nov 21 2023 - 12Z Wed Nov 22 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FROM PORTIONS OF THE SOUTHEAST TO MID-ATLANTIC... Only very minor changes were made to the Marginal Risk area on the Day 1 outlook. Approaching longwave upper trough and attendant 130-150 kt jet streak on the lee side will favor a robust, elongated warm conveyor belt (WCB) with upper level moisture origins from the eastern Pacific south of Baja. Favorable along-stream upper level forcing (deep-layer Qs convergence and ascent) will get a boost from shortwave energy pivoting through the trough base, which will allow the trough to amplify further through Wed and Thu. As noted in the previous discussions, instability will be very limited, especially across the northern half of the outlook area. However, PWs averaging 2 standard deviations above normal (1.0-1.5" northern portion of the outlook area...1.5-1.75"+ south), along with 850-700 mb moisture flux anomalies between +3 and +4, will foster widespread rainfall totals between 1-3+", highest along the eastern slopes of the southern-central Appalachians give the terrain enhancement. If any area has slightly elevated chances for 4"+ amounts and increased isolated flash flood/rapid runoff concerns, it's along the Blue Ridge Mountains of central/northern VA. Here, the 00Z HREF depicts high probs (60-80%) for 24-hr QPF to exceed 5 year ARI values. Much of the region is experiencing very dry ground conditions and drought, with relative soil moisture within the top 40cm layer currently below the 20th percentile, and in many areas below the 10th percentile. Therefore much of this rainfall is expected to be beneficial, while the lack of even elevated instability should limit the short-term rainfall rates. However, isolated areas of flash flooding will be possible where drainage is impaired, especially with the fallen foliage or urban influence, and/or where the soils have become more hydrophobic given the recent dry spell (30 day rainfall <25% of normal across most of the outlook area per AHPS). Hurley Day 2 Valid 12Z Wed Nov 22 2023 - 12Z Thu Nov 23 2023 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Hurley Day 3 Valid 12Z Thu Nov 23 2023 - 12Z Fri Nov 24 2023 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Hurley $$ --- SBBSecho 3.14-Linux * Origin: Ilink: CCO - capitolcityonline.net (454:1/105) .