Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu May 08 2025 07:34:26 FOUS30 KWBC 080733 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 333 AM EDT Thu May 8 2025 Day 1 Valid 12Z Thu May 08 2025 - 12Z Fri May 09 2025 ....THERE ARE A SLIGHT RISK OF EXCESSIVE RAINFALL OVER SOUTH TEXAS=20 & PORTIONS OF THE NORTHEAST... ....Deep South Texas... A strong cold front pushing south out of central Texas will meet=20 up with Gulf moisture, resulting in organized thunderstorms that are likely to form across the mountains of northeast Mexico and move across the area. With 700 hPa temperatures around 9C, the environment shouldn't be prohibitively capped, so there's little reason to expect the storms to fade as they move through South TX. Hourly rain amounts to 2.5" with local amounts to 5" are expected. After coordination with the CRP/Corpus Christi TX and BRO/=20 Brownsville TX forecast offices, raised a Slight Risk for the=20 region. ....Central Gulf Coast & Southeast... The stalled front draped north of the Gulf Coast will continue to=20 act as a trigger for additional thunderstorms on Thursday. In=20 addition to what is still a moist air mass, there will be the=20 added assistance of an approaching upper level trough to provide=20 additional divergence/support. The central Gulf Coast remains most susceptible to flash flooding given their closer positioning to the precipitable water values of 1.5-1.75". These favorable=20 atmospheric parameters are on top of what are now highly sensitive=20 soils following multiple days worth of heavy rainfall. The Slight=20 Risk was removed from southeast Louisiana where the QPF/recent=20 heavy rainfall pattern no longer overlaps, while the Marginal Risk=20 remains -- which shows some contraction -- given the precipitable water values of 1.25-1.5" ahead of the approaching cold front and=20 sufficient instability (MLCAPE between 500-1,500 J/kg). The FL=20 Space Coast is also at risk for flash flooding given the similar=20 setup to Wednesday and potential for more saturated soils following any thunderstorm activity on Wednesday. Localized flash flooding=20 is possible, most notably in the region's more urban areas. ....Northeast... A complex upper-level evolution will begin on Thursday over the=20 Northeast, portending a wet and stormy setup. An anticyclonic wave break over southeast Canada is splitting a lobe of the positively- tilted upper trough over southern Ontario into a cold low by=20 Thursday night. Guidance remains split on how far west the=20 blossoming shield of precipitation will advance, but most guidance=20 is coming into a consensus on a swath of the Northeast that stems=20 from eastern PA into southern NY. The greatest concern lies near=20 the PA/NJ border per the 00z HREF guidance. Soils have grown=20 exceptionally sensitive over the past several days thanks to=20 multiple rounds of heavy rainfall for portions of the Northeast.=20 The Slight Risk was shifted southwest from continuity to the NJ/PA border in deference to the 00z HREF, though model spread in=20 amounts and locations remains. Roth/Mullinax Day 2 Valid 12Z Fri May 09 2025 - 12Z Sat May 10 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE NORTHEAST & SOUTHEAST... ....Gulf Coast & Southeast... A large upper low positioned over the Lower MS Valley will linger=20 over the region through Friday night with heavy rainfall possible=20 from the central Gulf Coast to the Southeast coast. Precipitable=20 values of 1.5-1.75" are forecast, implying nearly saturated=20 soundings. Effective bulk shear could be sufficient to help=20 sustain areas of organized thunderstorms. There is wide variance in the QPF output from the various pieces of guidance, and most of=20 the guidance isn't terribly wet, so have kept the excessive=20 rainfall risk level at Marginal. ....Northeast... Out ahead of the upper low, guidance is showing strengthening=20 onshore flow which would transport copious amounts of moisture into the Northeast. Moisture will wrap around the northern flank of the 500 hPa low into a potential comma head pattern across northern NY and central New England. Farther east, the warm sector will=20 feature highly saturated and deep warm cloud layers that could=20 contain some weak elevated instability. There remains some spread=20 in where the axis of heaviest rainfall sets up, which is why a=20 Slight Risk remains unadded. However, given trends in guidance are=20 all pointing towards a cut-off low over the Northeast and soils=20 throughout the region are highly sensitive, there may be the need=20 for a Slight Risk upgrade in future forecast updates once=20 confidence increases in where the heaviest rainfall takes shape. Roth/Mullinax Day 3 Valid 12Z Sat May 10 2025 - 12Z Sun May 11 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE SOUTHEAST & NEW ENGLAND... Southeast... A cold low drops down to the Gulf coast with time, spurring a development of a low vaguely near the Loop Current. Despite good agreement aloft, there is a definite difference in the guidance in lower levels, with the 00z NAM a bit north of the 00z GFS at 850 hPa. This sort of pattern usually leads to a semi-convective low=20 near the Loop Current which then moves northeast towards the=20 Florida Panhandle or Big Bend, though the low's approach to the=20 region looks more evident on Sunday or so/beyond the day 3 period.=20 Some of the guidance has a decent QPF signal near the FL Big Bend,=20 generally agreeing on 2-3" areal average, but it there is a bit of=20 dispersion. It appears the 850 hPa boundary is still down in the=20 Gulf much of the day. However, there could be enough moisture, instability, and effective bulk shear for issues in northern FL.=20 For the moment, the Marginal Risk area remains in place due to the=20 dispersion seen in the guidance. Northeast... The guidance has a signal for moderate to heavy rainfall across portions of New England over an area of relatively low flash flood guidance values. This is near and ahead of a cold low racing through the area. Precipitable water values rise to 1-1.25", which given the cool 1000-500 hPa thickness values, should lead to saturation. Given the strong 500 hPa height falls during the afternoon hours, some instability is bound to be available. Added a Marginal Risk area per the above. Per coordination with CAR/the Caribou ME forecast office. left northern ME out as the=20 seven day rainfall across the state shows a fairly strong gradient, with northern ME generally left out of the recent rainfall. Roth Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!7v9JPvkIyyRTy6XHpLEbvDs5_GWZR82gqjJuoFfttlxx= dkYmLfy9OaedTOk7Fb9ykPYpxcB23V4AInY2IggOQ6Ybs78$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!7v9JPvkIyyRTy6XHpLEbvDs5_GWZR82gqjJuoFfttlxx= dkYmLfy9OaedTOk7Fb9ykPYpxcB23V4AInY2IggO7RvSGOw$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!7v9JPvkIyyRTy6XHpLEbvDs5_GWZR82gqjJuoFfttlxx= dkYmLfy9OaedTOk7Fb9ykPYpxcB23V4AInY2IggOxT5cyFk$=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) .