Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Apr 12 2024 00:57:21 FOUS30 KWBC 120057 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 857 PM EDT Thu Apr 11 2024 Day 1 Valid 01Z Fri Apr 12 2024 - 12Z Fri Apr 12 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM PART OF THE OHIO VALLEY EASTWARD INTO PORTIONS OF THE CENTRAL APPALACHIANS,=20 MID-ATLANTIC, AND THE NORTHEAST... ....01Z Update... Adjustments made reflect current observation trends and recent runs of the hi-res guidance.=20=20 While some of the Slight Risk was removed from the Ohio Valley, ongoing moderate to heavy rain and flash flooding will remain a concern=20 through the evening from West Virginia northward into western=20 Pennsylvania. Deep, moist southerly flow ahead of a mid-level wave will continue to support south-north training of storms across=20 this region for at least the next few hours. PWs around 1.3 inches, along with modest instability, continue to support rainfall rates=20 of 0.25-1 inch/hr across this region, with only a slow eastward=20 progression of the ongoing band. Consensus of the CAMs guidance=20 shows additional amounts of 0.5-1 inch for most areas, however=20 given the already saturated conditions, any additional rainfall may raise additional runoff concerns. Refer to WPC MPD #0153 (valid=20 through ~0540Z) for more detailed information concerning near-term=20 heavy rainfall and flash flooding concerns in this area. Further east, maintained a Slight Risk extending through northeastern Pennsylvania into the Catskills. Guidance shows multiple rounds of showers and storms lifting north across the region. While these storms are expected to be much more progressive, HREF guidance shows some potential for combined total amounts of 1-3 inches across this area overnight, especially in the Poconos-Catskills region. These may may cause localized runoff concerns as well, given the relatively moist antecedent conditions reflected by low flash flood guidance values. Pereira Day 2 Valid 12Z Fri Apr 12 2024 - 12Z Sat Apr 13 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF NORTHERN NEW ENGLAND... ....20Z Update... There were no changes made to the SLGT and MRGL risk areas inherited for D2 across the Northeastern US. Both global and hi-res deterministic maintain a QPF maximum in-of of the White Mountains in NH up through the northern Appalachians in western ME due to primed upslope enhancement from a strong 85H LLJ aligned well with the south to southeast facing terrain. Modest SWE located over Northern New England coupled with locally heavy rainfall will cause some flood concerns for the interior where snowpack has held firm. Heaviest precip will fall between 12-20z Friday with a high probability of at least 1" of total precip over the aforementioned areas with EAS signals between 40-50% in NH with a focal point within the peaks of the White's. Signal is a bit stronger into west-central ME where probabilities peak near 70-80%, the same locations with the deepest remaining snowpack. Given the trends remaining favorable, and with local WFO headlines highlighting the flood risk from the tandem rainfall/snowmelt, maintained continuity from previous forecast. Kleebauer ....Previous Discussion... Made only minor modifications to the previously-issued Slight and Marginal Risk area. There was a bit of an eastward expansion based on the latest WPC deterministic QPF and the NBM. But the northern and western periphery remained in place without any real changes where the risk of flooding was due to a combination of rainfall and snow melt. There has been some run to run variation but the overall idea of strong moisture transport of deep moisture and higher dewpoints at low levels from the Atlantic that gets directed normally to the terrain has remained fairly consistent. Thus saw little or no reason to make any meaningful removal from the Slight/Marginal Risk areas due to the low-end excessive rainfall threat across northern New England. Bann Day 3 Valid 12Z Sat Apr 13 2024 - 12Z Sun Apr 14 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ALONG A PORTION OF THE CALIFORNIA COAST... ....20Z Update... Very little change in regards to the overall synoptic forecast for the incoming upper-low off the Pacific. Fairly consistent presentation within all global deterministic and ensembles with only some minor shifts of the total rainfall depictions that are within tolerance to avoid any necessary risk upgrades. NBM probabilities are fairly bullish within the zones encompassing Big Sur and the Transverse Range with probabilities around 40-50% for at least 2" in the vicinity of both zones. A general 1-2" is expected for the area just south of San Jose, down the coast into the north side of Los Angeles. The highest totals are within the coastal areas with emphasis to the terrain focused areas just inland of the coastline. This is fairly typical with these setups and the flood threat remains marginal in the grand scheme with rates being capped closer to the 0.25-0.5" marker with perhaps a few convective cells overachieving late-Saturday into Sunday morning as the upper-low moves ashore and the height falls help steepen lapse rates and promote a burst of heavier rainfall over the central CA coast. Maintained continuity from the previous forecast issuance with a MRGL risk over portions of the CA coast and terrain just inland. Kleebauer ....Previous Discussion... Generally light to locally moderate rainfall should gradually be working its way southward from southwest Oregon into northern and central California by the time the Day 3 period begins on Saturday morning as low pressure over the eastern North Pacific eases southward. On-shore flow ahead of the low and associated upper level trough will support some one quarter- to isolated on half- inch per 6 hours along the coastal ranges. Thinking is that the on-shore flow will be shunted southward with time, but there could be an up-tick in rainfall rates as cold mid-level temperatures near the center of the upper low results due to steepening low level lapse rates at time of maximum daytime heating. Bann Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!9OwlBtAFCykoDRFMq88piEei_W4fL0mIkynuUcyqJad8= dF4CFk5HdoGdsiifTw-5rwlXFbDytUeqqfnOryp6XqQ_ywo$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!9OwlBtAFCykoDRFMq88piEei_W4fL0mIkynuUcyqJad8= dF4CFk5HdoGdsiifTw-5rwlXFbDytUeqqfnOryp6OTEWWts$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!9OwlBtAFCykoDRFMq88piEei_W4fL0mIkynuUcyqJad8= dF4CFk5HdoGdsiifTw-5rwlXFbDytUeqqfnOryp6y3iRv7w$=20 =3D =3D =3D To unsubscribe from WX-STORM and you already have a login, go to https://lists.illinois.edu and use the "Unsubscribe" link. Otherwise email Chris Novy at cnovy@cox.net and ask to be removed from WX-STORM. --- SBBSecho 3.20-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .