Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri May 09 2025 00:59:15 FOUS30 KWBC 090059 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 859 PM EDT Thu May 8 2025 Day 1 Valid 01Z Fri May 09 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 ....Portions Lower Ohio Valley, Tennessee Valley, Southeast, Mid=20 Atlantic, and Northeast... Trimmed quite a bit of the Marginal Risk area across these areas, given the weak mid-level lapse rates (~6-6.5 C/Km) and thus diminishing CAPE trends following sunset. Slow-moving, favorable area of large-scale forcing (DPVA/upper divergence) ahead of the=20 upper trough will be aided across the Mid Atlantic overnight due to some left-exit region upper jet contribution via the 90-100kt upper jet streak that pushes into the Southeast. This will lead to slightly more favorable low-mid layer moisture transport, with 850-700 mb moisture flux anomalies closer to 1 standard deviation above normal. Recent HRRR and other CAM runs show isolated pockets of 3+ inches of rain overnight, especially across Upstate SC into=20 central NC, with 1-2+ inch totals over parts of VA/MD and the northern Mid Atlantic. However despite the favorable forcing and slow storm motions, expect the increasingly marginal instability to limit the coverage and intensity of the stronger cells, especially after 03-04Z per the 18Z HREF exceedance probabilities. Thus the Marginal Risk will continue, with the expectation of mainly isolated/localized short-term runoff issues, again aided by the slow storm motions and especially where any west-east bands=20 set up and train. Hurley 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 Southeastern U.S. still anticipated with the main changes being the positioning of local maxima. 12z HREF remained steadfast on its probabilistic outputs with an elevated EAS probs for >1" located across Southern MS/AL with lower probs elsewhere. The signal for >2" is much lower (<10-20%) for anywhere within the area of interest meaning guidance still has a wide variance in where the strongest cells will materialize during the forecast cycle. The antecedent conditions are most favorable across Southeast LA where heavy rains in the prior periods have decreased areal FFG's to more attainable exceedance values. The rest of the Southeast is pretty much near normal for the 1/3/6 hr. FFG exceedance intervals leading to a higher rate threshold necessary to exhibit flash flooding. The best opportunity for a targeted upgrade will likely be within that corridor from Southeast LA through Southern MS/AL just given the location of highest theta_E's correlated with better upper jet dynamics in the form of a jet coupling between the mid-latitude jet trailing the cyclonic flow to the north and the southern jet streak focused to the south. 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 lower risk threshold and has merit with the D2 ML First Guess Fields. The MRGL was relatively unchanged with a small expansion into the Philadelphia metro to cover for recent QPF trends. 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 leading to confidence in pulling the southern portion of the risk area further north. As for the general pattern, little has changed otherwise with the core of heaviest precip focused across ME where precip is very much welcomed in all locales given current drought. The MRGL risk was maintained due to the forecasted 1-2" rainfall forecast with some terrain focused areas and urbanized zones the most likely to see any hydro impacts. 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 outside significant convective training and/or tropical influence. Current moisture advection regime off the Gulf is situated for a prolonged training threat within the eastern flank of the maturing surface low positioned directly over the Central Gulf Coast. Deep layer mean flow is progged to be due southerly across the entire Western FL Panhandle which could ultimately lead to a targeted SLGT risk issuance if the current setup stands. There's still a bit of discrepancy on specifics with where the most prolific moisture transport will end up, but ensemble and ML depictions are close to that aforementioned area, but either a west or east displacement would cause differences in potential impacts as some larger population centers would get involved if the setup takes aim a bit more upstream. The MRGL is in place currently, but would not be surprised if a targeted upgrade is necessitated in future forecasts, especially once in range of the CAMs. 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!-ltrrhKPHzwz449P7MeapPVWVLDEXbpckBye_mL99wSn= 0tZYqHOPbEBnzhQhcFMFCf_czOFeTQJxr9J8HkDRi1JKOz8$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-ltrrhKPHzwz449P7MeapPVWVLDEXbpckBye_mL99wSn= 0tZYqHOPbEBnzhQhcFMFCf_czOFeTQJxr9J8HkDRJQpmyv0$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-ltrrhKPHzwz449P7MeapPVWVLDEXbpckBye_mL99wSn= 0tZYqHOPbEBnzhQhcFMFCf_czOFeTQJxr9J8HkDRFyU_kX0$=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) .