Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu May 08 2025 19:37:28 FOUS30 KWBC 081937 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 337 PM EDT Thu May 8 2025 Day 1 Valid 16Z Thu May 08 2025 - 12Z Fri May 09 2025 ....THERE ARE A SLIGHT RISK OF EXCESSIVE RAINFALL OVER SOUTH TEXAS... ....Deep South Texas... 16Z Update: Little change with the overall expectations across Deep South TX. Still looking at primary impacts from two rounds of convection. The first is already occurring with a strong thunderstorm slowly progressing eastward off South Padre as the updraft and primary mesocyclone matured enough to drop a few inches of rainfall in short succession along the South TX coast plain past few hrs. 12z KBRO sounding came in with a deep moist profile with PWATs settled at 2.15" putting it at the new 12z Daily Max for the date. This is a testament to the environment available for any convective regimes whether that be from more pulse variety convection this morning and afternoon, and eventual MCS progression as the potent shortwave ejects southeast out of MX generating a more organized heavy rain prospect from the Big Bend, southeast. 12z HREF neighborhood probs for >3" are between 60-90% over the area extending from CRP down to BRO to about 40 miles inland along that stripe. Neighborhood >5" are highest near Brownsville to McAllen (30-60%) lending credence to the higher flash flood threat in the region, especially in those more urbanized corridors along the Rio Grande Valley. The previous SLGT was relatively unchanged considering the setup, in agreement with the local WFO's. Kleebauer ...Previous Discussion.. A strong cold front pushing south out of central Texas will meet 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/ Brownsville TX forecast offices, raised a Slight Risk for the region. Roth ....Central Gulf Coast & Southeast... 16Z Update: Only minor changes were made for the inherited MRGL risk across the Gulf Coast to the eastern FL Peninsula. Convective pattern across the Central Gulf Coast will likely be tied to the immediate coastal areas where the greatest convergence will align with a frontal approach from the north coinciding with the remnant stationary front positioned just off the LA/MS coast. The previous MRGL was removed out of LA due to the threat likely to remain either off the coast, or fall over the Southern Parishes south of the urban corridor a bit further north. Southern Parishes are much less prone to flash flood threats due to soil types and swamp environments. Trends have focused away from the area of most concern, so the MRGL risk was removed due to a non-zero, but sub- threshold threat. Across FL, thunderstorms will develop this afternoon along the sea breeze with some anchoring influence over the Northeast FL coast due to the presence of the stationary front. Rates will peak at 2-3"/hr max leading to more isolated flash flood concerns mainly within the urban corridor along the Space Coast. Coverage of thunderstorms will be scattered in nature leading to a more MRGL risk for flash flooding when coupled with the expected magnitudes, thus the previous risk was generally maintained. Kleebauer ...Previous Discussion.. The stalled front draped north of the Gulf Coast will continue to act as a trigger for additional thunderstorms on Thursday. In addition to what is still a moist air mass, there will be the added assistance of an approaching upper level trough to provide 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 atmospheric parameters are on top of what are now highly sensitive soils following multiple days worth of heavy rainfall. The Slight Risk was removed from southeast Louisiana where the QPF/recent heavy rainfall pattern no longer overlaps, while the Marginal Risk remains -- which shows some contraction -- given the precipitable water values of 1.25-1.5" ahead of the approaching cold front and sufficient instability (MLCAPE between 500-1,500 J/kg). The FL Space Coast is also at risk for flash flooding given the similar setup to Wednesday and potential for more saturated soils following any thunderstorm activity on Wednesday. Localized flash flooding is possible, most notably in the region's more urban areas. Roth ....Northeast... 16Z Update: The previous SLGT risk inherited was downgraded due to a bit of a degraded convective output from 12z CAMs and associated HREF signaling a more isolated flash flood concern this afternoon with the primary focus within the urban zones of Northeast PA and Northwest NJ. Cold front progression is slowly sinking south with small theta_E ribbon bisecting the above area aligning with a stalled boundary in place. Once the cold front approaches and mid- level ascent pattern builds within the LER of an approaching speed max to the south, expecting scattered convection to initiate across Northeast PA with mean storm motions pushing any activity in Northern NJ and the Lower Hudson of NY. Activity will be slower moving in general which does favor the flash flood prospects in one regard, but the deterrent for anything appreciable is the limited deep layer moisture to work with as the PWAT anomalies are running pretty normal for the time of year. HREF hourly probs for >1"/hr are highest (30-50%) over a short window between 18-23z before the setup shifts and we begin to focus more on the evolving surface cyclogenesis pattern to the south that will usher more precipitation into the region. This scheme will be more stratiform in nature with some embedded thunder possible across the Mid- Atlantic up to around I-80 latitude. Some areas that see convective threat earlier in the day will see more rain overnight, so some initial priming could allow for a localized flood threat overnight, but the lack of a true convective element will likely yield more low-end potential than anything else. In coordination with the local WFO's that were previously within the SLGT, the risk was dropped with a solid MRGL remaining for the dual threat this period with emphasis on what occurs this afternoon and early evening. Kleebauer ....Ohio Valley... Slow-moving shortwave trough over the Mid-Mississippi Valley will wander eastward through the Ohio Valley promoting scattered showers and thunderstorms from MO all the way towards WV through the period. 12z HREF neighborhood probs for >2" of rainfall is actually pretty robust (>60%) in a corridor extending from St Louis to Louisville down to around Paducah. The key in all this is the matter of timing for all of this precip to fall with the majority of hourly rates likely capped ~1"/hr at peak intensity. Normally a MRGL risk wouldn't be considered for this type of threat, but moist antecedent soils for a large part of the Ohio Valley lean this closer to the MRGL risk. This helped maintain continuity with the isolated threat encompassing the area between I-70 to I-40 between the Mississippi River, east into WV. Kleebauer 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... 20Z Update: The period remains pretty consistent in run to run QPF variance with the general convective threat across the=20 Southeastern U.S. still anticipated with the main changes being the positioning of local maxima. 12z HREF remained steadfast on its=20 probabilistic outputs with an elevated EAS probs for >1" located=20 across Southern MS/AL with lower probs elsewhere. The signal for=20 >2" is much lower (<10-20%) for anywhere within the area of=20 interest meaning guidance still has a wide variance in where the=20 strongest cells will materialize during the forecast cycle. The=20 antecedent conditions are most favorable across Southeast LA where=20 heavy rains in the prior periods have decreased areal FFG's to more attainable exceedance values. The rest of the Southeast is pretty=20 much near normal for the 1/3/6 hr. FFG exceedance intervals leading to a higher rate threshold necessary to exhibit flash flooding.=20 The best opportunity for a targeted upgrade will likely be within=20 that corridor from Southeast LA through Southern MS/AL just given=20 the location of highest theta_E's correlated with better upper jet=20 dynamics in the form of a jet coupling between the mid-latitude jet trailing the cyclonic flow to the north and the southern jet=20 streak focused to the south.=20 Across the Northeast, total precip will remain firmly between 1-3" thanks to maturing mid-latitude cyclone moving northward off the Northeast coast with much of the Northern Mid Atlantic through New England well positioned within the LER of a strengthening upper speed max juxtaposed over the Central Mid Atlantic around the trough base. The total precip and intra-hour rates ~1"/hr will be the greatest factor for any flash flood prospects since the majority of the precip will likely be within a stratiform scheme during the peak of the event. Hi-res ensembles and overwhelming global deterministic outputs are generally modest with the anticipated rates over the most impacted areas. This is a stronger case for river and small stream flooding which is the reasoning for the widespread Flood Watch issuances and not so much the flash flood variety. If there was an upgrade, it would likely be very targeted within Southern New England where there is a better case for elevated instability within the WCB process generally over more urbanized zones. In any case, the threat is still within the=20 lower risk threshold and has merit with the D2 ML First Guess=20 Fields. The MRGL was relatively unchanged with a small expansion=20 into the Philadelphia metro to cover for recent QPF trends.=20 Kleebauer ...Previous Discussions.. ....Gulf Coast & Southeast... A large upper low positioned over the Lower MS Valley will linger over the region through Friday night with heavy rainfall possible from the central Gulf Coast to the Southeast coast. Precipitable values of 1.5-1.75" are forecast, implying nearly saturated soundings. Effective bulk shear could be sufficient to help sustain areas of organized thunderstorms. There is wide variance in the QPF output from the various pieces of guidance, and most of the guidance isn't terribly wet, so have kept the excessive rainfall risk level at Marginal. ....Northeast... Out ahead of the upper low, guidance is showing strengthening 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 feature highly saturated and deep warm cloud layers that could contain some weak elevated instability. There remains some spread in where the axis of heaviest rainfall sets up, which is why a Slight Risk remains un-added. However, given trends in guidance are all pointing towards a cut-off low over the Northeast and soils throughout the region are highly sensitive, there may be the need for a Slight Risk upgrade in future forecast updates once 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... 20Z Update: The main change this period was over the Northeastern U.S. MRGL risk were the southern edge was pulled a bit northward given the trends in QPF distribution. The ECMWF was one of the furthest south in terms of the heavier QPF placement over the past cycle of runs, however the ML disagreed with the assessment and had the heavier precip further north as the the low will move steadily northeast before occlusion. 12z ECMWF is now in line with the rest of the deterministic and subsequent ensembles and ML output=20 leading to confidence in pulling the southern portion of the risk=20 area further north. As for the general pattern, little has changed=20 otherwise with the core of heaviest precip focused across ME where=20 precip is very much welcomed in all locales given current drought.=20 The MRGL risk was maintained due to the forecasted 1-2" rainfall=20 forecast with some terrain focused areas and urbanized zones the=20 most likely to see any hydro impacts.=20 Across the Southeast, beginning to see a greater consensus on heavier precip entering near the Big Bend over Apalachicola National Forecast, an area that is notoriously hard to flood=20 outside significant convective training and/or tropical influence.=20 Current moisture advection regime off the Gulf is situated for a=20 prolonged training threat within the eastern flank of the maturing=20 surface low positioned directly over the Central Gulf Coast. Deep=20 layer mean flow is progged to be due southerly across the entire=20 Western FL Panhandle which could ultimately lead to a targeted SLGT risk issuance if the current setup stands. There's still a bit of=20 discrepancy on specifics with where the most prolific moisture=20 transport will end up, but ensemble and ML depictions are close to=20 that aforementioned area, but either a west or east displacement=20 would cause differences in potential impacts as some larger=20 population centers would get involved if the setup takes aim a bit=20 more upstream. The MRGL is in place currently, but would not be=20 surprised if a targeted upgrade is necessitated in future=20 forecasts, especially once in range of the CAMs.=20 Kleebauer ...Previous Discussion.. 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 near the Loop Current which then moves northeast towards the Florida Panhandle or Big Bend, though the low's approach to the region looks more evident on Sunday or so/beyond the day 3 period. Some of the guidance has a decent QPF signal near the FL Big Bend, generally agreeing on 2-3" areal average, but it there is a bit of dispersion. It appears the 850 hPa boundary is still down in the Gulf much of the day. However, there could be enough moisture, instability, and effective bulk shear for issues in northern FL. For the moment, the Marginal Risk area remains in place due to the 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 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!9e0ivgDUQSmpeiHsC9Z4ao0Odhe0PPodDpnWDaAA71Nn= pBC4m93kIIEt_KgtYI2cTaCMOCoxnv_p-VN_KN2xykOWe1s$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!9e0ivgDUQSmpeiHsC9Z4ao0Odhe0PPodDpnWDaAA71Nn= pBC4m93kIIEt_KgtYI2cTaCMOCoxnv_p-VN_KN2xHkBDkNc$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!9e0ivgDUQSmpeiHsC9Z4ao0Odhe0PPodDpnWDaAA71Nn= pBC4m93kIIEt_KgtYI2cTaCMOCoxnv_p-VN_KN2xSTUwr-Y$=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) .