Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri May 09 2025 19:35:48 FOUS30 KWBC 091935 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 335 PM EDT Fri May 9 2025 Day 1 Valid 16Z 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... 16Z Update: The general consensus from guidance this morning was to maintain continuity with the broad MRGL in place across the Southeastern U.S. The corridor of greatest focus will lie between Baton Rouge/New Orleans over towards Mobile during the evening and overnight hrs. as a surface low initiates along the Central Gulf Coast with a strengthening 925-700mb convergence layer on the east and northeast flank of the cyclone. CAMs this morning were pretty bullish on a narrow axis of heavier precipitation within that eastern flank of the low, likely in due part to the convergent pattern interacting with what is leftover of a quasi-stationary front situated along the Gulf Coast. Areal PWATs are still within that +1/+2 deviation marker with the 12z KLIX sounding coming in ~1.75" of atmospheric moisture. Anticipating PWATs to stay below the usual 2" marker that is customary for higher impact probs with regards to flash flooding. The previous discussion touched on the expected amounts pretty well with a general 1-3" anticipated with some localized prospects of 4-8" within a small zone encompassing Southeast LA over to Southern MS/AL. The main concerns will be any of the urban zones where impervious surfaces provide minimal absorption with higher run off potential. This area will be monitored closely for a targeted upgrade if conditions warrant, but for now, the lack of heavier rates (12z HREF probs for >2"/hr running between 10-30% anywhere during the forecast cycle), as well as neighborhood probs for >5" signaling 50% or less across the I-10 corridor between New Orleans to Mobile with the highest probs right along the coast. The previous MRGL was kept with perhaps a targeted SLGT risk in a small zone where urbanization factors could enhance the prospects. The MRGL risk also aligns with the current ML First Guess Fields which have been pretty consistent the past several iterations. Kleebauer ...Previous Discussion.. A cutoff upper low is forecast to stall over the Central Gulf Coast today, accompanied by a nearly stationary frontal boundary upon which a wave of low pressure will form this afternoon and evening. Convection moving onshore on the northern and eastern side of the low will likely result in locally heavy rainfall from southeastern Louisiana through southern Mississippi and Alabama to the western Florida Panhandle. Plentiful moisture will be in place with perceptible water values greater than 1.5 inches, and there should be just enough CAPE and effective bulk shear along the immediate coast to support some sustained/organized convection. Model guidance has come into good agreement on the overall QPF footprint with some variances on where the highest values are along the coast. The general consensus among the hi-res CAMs is for the higher amounts (2-3 inches with locally higher amounts possible) to fall between far southeastern Louisiana and Destin, Florida. Even with those amounts, the FFGs in these areas remain fairly high with 3-4+ inches needed to pose flash flooding concerns. The main concerns with activity this evening will be thunderstorms with efficient rain rates tracking over any urban or poor drainage areas. There is a broad Marginal Risk area in place that includes the Central Gulf Coast and extends into southern Georgia and portions of North and Central Florida where localized thunderstorms with heavy rain may pop up this afternoon. ....Northeast... 16Z Update: The main change from the previous forecast was the removal of areas within the southern bounds of the MRGL risk across Southeast PA into NJ. With a steady progression of the low to the northeast, a cold front will drag through the Mid Atlantic with the final rain threat exiting by later this afternoon. The overall heavy rain prospects have shifted north towards LI and Southern New England leaving the threat close to, if not already a nil for flash flooding the remainder of the period in those southern zones, so the risk was removed for the above areas. Further north, rainfall will continue with a narrow corridor for elevated convection across the eastern half of LI up through the eastern half of Southern New England where a tongue of higher 850-500mb will be present this afternoon leading to some chances for elevated thunder with slightly better rates. Outside that, the primary precip scheme will be stratiform leading to capped rates and potential for flash flooding residing in the lower end of the inherited MRGL risk threshold. The main reason for the MRGL extension to the west is the very moist antecedent soils where NASA SPoRT is consistently depicting the top soil layer running >90% saturation meaning small stream and river flooding will exacerbate runoff concerns through the period. Totals between 1-2" will be the most common across the Hudson Valley to points east with rates sufficiently capped <1"/hr with low probs of even reaching 2"/3-hrs (20-30%) anywhere with the best threat over LI. The MRGL was maintained for the low-end threat across New England, Northeast PA, and Eastern NY State. Kleebauer ...Previous Discussion.. A slow-moving coastal low will bring a prolonged period of onshore flow to portions of the Northeast, which will produce some decent rainfall totals across the region. Most of the precipitation on the northern side of the system will be in the form of stratiform rain with modest rain rates at best, but rainfall totals will add up today into tonight, resulting in widespread amounts of 1-2 inches, locally higher, in the 24 hour period. Rain rates will likely fall short of the 1/3/6 hr FFGs in most locations. However, soils across the region are already saturated and there will likely be enough runoff to flood rivers, creeks, streams, and even some low- lying areas. The better chances of localized flash flooding will be to the south over Long Island and southern Connecticut. Hi-res CAMs have been consistently showing a higher stripe of QPF across Long Island where convection will move onshore ahead of the surface low. Given ample moisture from the Atlantic and a modest amount of CAPE, showers and storms that develop will likely be able to produce rain rates greater than 1 inch per hour. A Marginal Risk area is in place across portions of the Northeast to account for the localized flash flood threat across the region. Dolan Day 2 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 previous forecast remains on track for both the Southeast and Northeastern U.S MRGL risk areas as the overall run to run consistency in the synoptic pattern has yielded similar QPF outputs within the recent 12z guidance. The heaviest rain across the Southeast will translate eastward due to the mean trough pattern over the Southern Plains shifting more towards the Mississippi Valley longitude. Surface low across the Gulf will=20 occlude and wander north and northwest during the course of the=20 period with deep layer flow basically aligned south to north from=20 the FL Panhandle up through AL/GA. Surface and upper lows will=20 become vertically stacked by Saturday afternoon leading to slow=20 storm motions under the parent circulation with a steady push south to north within the eastern flank of the circulations. Scattered=20 to numerous thunderstorms will accompany both areas of interest=20 leading to the risk area encompassing a sprawling zone with the=20 cyclonic arc back towards the Mississippi Valley on the northwest=20 side of the circulation.=20 Stalled front along the Southeast Atlantic coast extending through Southern GA will act as an inflection point for convection with=20 mean storm motions likely aligned with the boundary as you work=20 from southwest to northeast (Tallahassee to Charleston, SC line).=20 This will lead to a secondary maxima developing over the course of=20 Saturday morning and afternoon, a consistent signature within the=20 latest CAMs. The primary axis of interest will reside over the=20 Western FL Peninsula up through the AL/GA line till about the I-20=20 corridor. Despite the overall setup indicating 1-3" of rainfall=20 with locally higher in that vicinity, the area in question is under the influence of a D0-2 drought so the threat of flash flooding with the current forecasted rates should help curb the threat initially (See D3 discussion below). The MRGL was maintained outside some expansions on the northern periphery of the risk area to align with current QPF.=20 Northeast U.S. saw the western edge towards VT brought back further east as the forecast surface low progression and subsequent dry air advection pattern behind the vacating low will yield less of a risk of persistent rainfall in that portion of New England. There was enough consensus to trim part of the risk area to account for the changes leading to NH and ME as the only areas where the risk was maintained. QPF details remain steadfast from previous forecast with locally moderate to heavy rainfall leading to an areal=20 average of 0.75-1.5" of rainfall with upwards of 2" plausible over=20 Central ME down towards the Southern ME coast. Kleebauer ...Previous Discussions.. ....Southeast... A stationary boundary will remain draped across the Southeast, with several waves of low pressure expected to form along the boundary over the weekend as upper level short waves pivot around the upper low over the region. Compared to Friday, the QPF footprint will translate east into areas with relatively drier antecedent conditions. Models are agreeable that widespread rainfall totals of 1-2 inches are expected across southern and central Alabama and Georgia and North Florida. Locally higher amounts above 2 inches will also be possible, but exactly where may vary within the region. The highest QPF will likely be along the immediate coast in the Florida panhandle as convection moves onshore, and training onshore convection will likely pose the highest threat for flash flooding. Efficient rain rates (2+ inches per hour) are expected due to a saturated atmosphere with ample CAPE and shear to support thunderstorms. Flash flooding may be fairly limited to urban and poor drainage areas given the higher FFGs (3-5 inches). To account for this, a Marginal Risk area is in place across much of the Southeast. ....New England... As a coastal low tracks south of the coast, moderate precipitation will focus over New England on the northern side of the system. The low is expected to be fairly progressive, which might help to limit rainfall totals, but a decent swatch of rainfall is forecast from Vermont and New Hampshire through southern Maine. Soils in Vermont and New Hampshire are expected to be somewhat sensitive given heavy rains expected today. Rainfall will be mostly stratiform across New England, limiting rain rates, but there will likely be enough moisture and instability to produce localized rates of 1+ inches over the course of a few hours, which may come close to 3/6 hr FFGs. A Marginal Risk area remains in place across Vermont, New Hampshire, and most of Maine, but northwestern Maine was removed from the Marginal as model QPF has trended downwards. Dolan Day 3 Valid 12Z Sun May 11 2025 - 12Z Mon May 12 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE SOUTHEAST... 20Z Update: The primary change for the forecast was a general expansion of the SLGT risk area further northeast into SC to account for the heavier rainfall threat protruding inland given=20 the strong uni-direction component advecting rich Gulf moisture=20 poleward between the closed upper reflection over the Mississippi=20 Valley and the surface ridge over the Western Atlantic. Cold front from the north will also hit a roadblock as it motions south, likely becoming more quasi-stationary across North GA and the SC Piedmont before being pushed back as a warm front. This will likely coincide with a stronger low-level convergence signature up into those areas by the afternoon and beyond on Sunday into Monday. Considering the nature of the persistent moisture advection regime and enhanced theta_E pattern, expecting widespread thunderstorm development with rates likely reaching between 1-3"/hr within the heaviest cores. Antecedent soil conditions should moisten with the day prior, so the overlap will create a better threat for natural runoff, not including the urban threat that is customary in these setups. The previous SLGT risk was maintained with those expansions to reflect the latest trends.=20 Kleebauer=20 ...Previous Discussion.. A wave of upper level energy will pivot around the upper low across the Southeast, sparking another wave of convection from the Gulf Coast to the Southeast Atlantic Coast. Models are showing a more organized area of surface low pressure moving into the Southeast with significant rainfall totals on the eastern side. Widespread totals of 1.5-2.5 inches are expected with locally high amounts above 3 inches. Considering this will be the second day in a row with heavy rainfall in the Southeast, areas with saturated soils from the day before may be slightly more prone to flood impacts with additional heavy rain on Sunday. The most favorable flash flood conditions will be across North Florida and southern and central Georgia where 1000+ J/kg MUCAPE and 30+ knots of effective bulk shear with support organized convection. Additionally, precipitable water values in excess of 1.5 inches will support efficient rain rates in strong thunderstorms. A Marginal Risk area is in effect for much of Alabama, Georgia, South Carolina, and North and Central Florida, with an embedded Slight Risk area over North Florida and southern and central Georgia. Dolan Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!9rD_5i-iyGzgfR4v-WYGKqvtpXE31WtMihPS5pCGDdXQ= hTiNvX9HhviDG9M6hXsjhE1bqrOR3FEf4Km5MU8sWMNj0jg$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!9rD_5i-iyGzgfR4v-WYGKqvtpXE31WtMihPS5pCGDdXQ= hTiNvX9HhviDG9M6hXsjhE1bqrOR3FEf4Km5MU8sTa9QYJs$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!9rD_5i-iyGzgfR4v-WYGKqvtpXE31WtMihPS5pCGDdXQ= hTiNvX9HhviDG9M6hXsjhE1bqrOR3FEf4Km5MU8saOoNDwE$=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. 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