Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu May 29 2025 21:30:26 FOUS30 KWBC 292130 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 530 PM EDT Thu May 29 2025 Day 1 Valid 2124Z Fri May 30 2025 - 12Z Fri May 30 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF=20 CENTRAL TEXAS... 21z Update: A Slight risk was added to much of central TX with this update. Convection should expand in coverage and grow upscale into an MCS or convective cluster or two tonight. Generally looking like a forward propagating convective system, which should limit the extent/magnitude of flash flooding. However a few cell mergers and high rainfall rates should be enough for isolated to scattered FFG exceedance. This evolution is supported by both the latest HREF and REFS ensemble systems, as well as recent runs of the HRRR and RRFS. Both the HREF and REFS show neighborhood probabilities of exceeding 3" in the 40-60% range over much of the Slight risk area...with much of this rain falling in just an hour or two. These rainfall=20 rates should be high enough to produce isolated to scattered flash=20 flooding...especially over an more sensitive urban or low lying=20 areas. Chenard ....16Z Update... Some reshuffling of the large Marginal Risk across a large section of the country was done with this update. The inherited Marginal was removed from the Texas Gulf Coast and most of Louisiana and Mississippi with this update, as the area behind the ongoing convection tracking along the Gulf Coast should stay dry and relatively storm-free this afternoon with northerly flow hampering the moisture in this area somewhat. There is also good signal among the CAMs for a minimum of precipitation in this area through 12Z. However, the Marginal was expanded to include much of the southern Appalachians and much of the Florida Peninsula with this update; both for the same reason. Afternoon convection is likely to develop in these areas, which while unlikely to organize, could be slow- moving, tied to the sea breeze in FL, or the terrain in the Apps, which could lead to a localized prolonged period of heavy rain. Some sensitivity to flooding in these areas from moist soils would also support an isolated flash flooding threat. Wegman ....Previous Discussion... ....Mid Mississippi and Ohio Valleys... A shortwave currently analyzed across the Central Plains will continue to progress eastward overnight under the base of a broad ULL situated over the Midwest. Increased mid and upper forcing will aid in scattered convection through the course of the period in the area spanning from Eastern KS through the Central Ohio Valley. Some of the stronger convective cores will be capable of locally enhanced rainfall rates between 1-2"/hr at peak intensity leading to isolated flash flood prospects in-of the above zone. The positive news in the setup is the moisture anomalies across the region of interest remain near to just slightly above normal with NAEFS anomalies signaling less than +1 deviation for the area. This will limit the upper threshold of flood concerns for the setup, but some of the heavier cores could still cause some flash flood concerns within more urbanized settings and over some of the terrain focused areas of Southern MO, as well as Eastern KY/TN. The previous MRGL risk was generally maintained with only some minor adjustments on the northern periphery of the risk area. ....Southeast U.S... A shortwave trough over the Southern Plains will roll eastward overnight and through the period with focused ascent within the confines of the Southeastern U.S. Weak jet coupling from a vacating jet over the Tennessee Valley and approaching jet from the west will create a period of stronger forcing in-of the Central Gulf Coast early this morning with convective activity rooted in a narrow corridor of elevated theta_E's stemming from southerly flow near the coast. Expectation is for scattered heavy convection to initiate across Southern LA and migrate eastward with the stronger forcing advancing across the Southeast. Scattered thunderstorms will be found across off the Southeast by late-morning and beyond, but the progressive nature of the threat should limit the chances for widespread flash flooding. QPF mean between 1-2" is forecast along the Central Gulf Coast with some deterministic maxima between 4-5" showing up within the 00z CAMs. This put modest (30-50%) HREF neighborhood probs for >3" within that zone between New Orleans to Mobile, but even still that signature is not going to be enough to move the needle outside a MRGL threat. A cold front will sweep through the region from northwest to southeast leading to a sharp shift in the environmental moisture availability and limiting instability region-wide. This will ultimately end the threat for any convection and put an end to any risk in place. For now, the MRGL risk remains across the Southeastern U.S. with emphasis on the Central Gulf Coast for best threat in the period. Further east over Southeastern SC, another round of strong thunderstorms will likely occur late-afternoon with the best flash flood risk likely within more urban zones like Charleston and small areas like Hilton Head. This actually the zone with the highest >3" probs in the country, but again, the threat is very localized for flash flood concerns considering the sandier soils prompting higher FFG's. In this case, the threat will continue to be more in-line with a MRGL risk leading to general maintenance of the previous forecast. ....Southern Plains... Increased ascent within the confines of a cold front pressing south through the Southern Plains will lead to a corridor of heavy rainfall developing upstream across the Caprock, moving southeast within the mean flow and expected upwind trajectory of any cold pool generation. Regional instability parameters point to a sufficiently buoyant environment along and ahead of the advancing cold front leading to a suitable MCS maintenance regime that will begin over Eastern NM and eventually work its way southeast through the rest of the Texas Caprock, Permian Basin, and Concho Valley by nightfall. HREF EAS probs for >1" are modest (20-40%) for the probability scheme, but a stronger neighborhood prob for >2" (50-80%) exists over that area referenced above. Progressive nature of the eventual complex should curb some of the threat, but scattered instances of flash flooding will be plausible where the MCS migrates overhead. The highest threat will likely be within more urban settings and over the Concho Valley where 3-hr FFG exceedance probs are highest (30-40%) due to antecedent soil moisture. Kleebauer Day 2 Valid 12Z Fri May 30 2025 - 12Z Sat May 31 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE OHIO VALLEY AND MID ATLANTIC... ....20Z Update... Continued increases in the forecast rainfall from the 12Z guidance suite, sufficient instability for storms, ample upper level forcing from a strong shortwave that rapidly turns negatively tilted, and soil sensitivity due to recent heavy rainfall have all worked to increase the concern for flash flooding across portions of the Ohio Valley and Mid-Atlantic Friday and Friday night. Generally, the convection will track from west to east...so the portion of the Slight in Kentucky really focuses on Friday morning, whereas the portion in northern New Jersey is primarily focused on after midnight Friday night. Soil sensitivity to flash flooding is high as soil moisture levels remain well above the normal for this time of year due to rain as recently as yesterday, and not including any shower and isolated thunderstorm activity probable around the DMV region the afternoon and this evening. Further, possible repeating rounds of heavy rain on Friday interacting with the terrain and full rivers should cause onset of flash flooding to occur sooner...closer to the rainfall start time...in these areas, making any potential flooding more hazardous in West Virginia. Further east, urban concerns will increase the potential impacts from flash flooding. The storms will also occur in the afternoon and evening, during the diurnal warmest time of day, which will add more instability for the storms to feed on. Given all of the above, have introduced a higher end Slight from eastern Kentucky to the Baltimore metro. The 12Z HREF suite shows an over 50% chance of exceeding 6-hour FFGs across the DMV. The Slight Risk area was expanded northeast across the Philadelphia Metro and much of northern New Jersey with this update. While the storms will impact this region Friday evening and into the overnight, here too recent heavy rainfall will make flash flooding more common compared to if the soils were dry. By Friday evening a coastal low will be rapidly forming, and so instability will be waning as the precipitation shield evolves into more of a comma shape, with the heaviest rains along the Delaware River on the cold/more stable side of the low. Nevertheless, remnant instability and long duration of rainfall will still cause widely scattered instances of flash flooding. Since the low will have a comma-shape Friday night, the dry slot will end the rainfall sooner in some areas. While there is some uncertainty where it will be, it looks to focus along the Jersey Shore. Thus the Slight was kept from the coast a bit expecting the rain to end there sooner than areas along the Delaware River. The surrounding Marginal Risk was also expanded into NYC and southern New England for Friday night. Here too the dry slot could limit rainfall amounts, especially the further east you go. Upslope into the terrain of Connecticut and recent rains around NYC could still cause locally heavy rain before the dry slot moves in. Wegman ....Previous Discussion... A potent upper trough will slowly pivot eastward while taking on a more neutral tilt as it swings through the Great Lakes and Ohio Valley on Friday. At the surface, low pressure will develop within the middle of an evolving jet couplet in-of the eastern portion of the Ohio Valley with deterministic output placing the low nearby Eastern KY. The entire dynamic between sfc-500mb will play an integral role in the potential for heavy rainfall in areas of KY/WV the first half of the period, shifting eastward through the Shenandoah of VA by the afternoon and evening Friday before providing quite the convective posture over the DMV and points northeast. 00z suite has come in much more aggressive in the presentation of heavy rainfall across the above areas with a dual maxima signature present within the ensemble means. The first max is over Eastern KY into Southern WV thanks to intense large scale ascent being driven by a powerful shortwave rotating into the region along the base of the mean trough as it approaches the region. At the surface, a narrow tongue of surface based instability will be co-located within the mid and upper level ascent max being delivered by the trough, a product of a warm front lifting north ahead of the aforementioned surface low off to the west. A line of thunderstorm activity will likely transpire within proxy of the warm front leading to enhanced rainfall rates likely to spur totals between 1.5-3" with potentially higher over a span of 3-6 hrs. before a cold front sweeps through the area in wake of the lows progression to the east-northeast. Current FFG exceedance probs for both 3 and 6hr increments are pretty robust (>50%) within the corridor referenced above, along with some hourly rate exceedance probs between 50-80% between the 12-18z time frame on Friday. The signature typically correlates to a better than average threat for flash flooding, especially when you account for the antecedent soil moisture and the complex topography situated over Eastern KY and Southern WV. This allowed for an addition of a SLGT risk across these two areas. Further east, soil moisture anomalies will remain elevated thanks to a barrage of rainfall the past 24 hrs. and very wet conditions overall the past 2 weeks. As the surface low matures on its trek to the east and the trough begins a more negative tilt to the west, expectation is the for the low to begin intensifying steadily with a sharper deformation axis beginning to take shape across the Central Mid Atlantic. Heavy rains on the western flank of the low will likely occur across the Northern and Central Shenandoah, an area that has been hit hard over the course of May and is already experiencing relatively higher stream flows. Current HREF depiction for 6hr exceedance probs over Northwestern VA are between 50-80% over the Blue Ridge and adjacent valley areas from Harrisonburg to just north of Winchester. A smaller max is co-located with the more urban centers of Northern VA towards the DC metro, but that is at the end of the hi-res temporal window. The assessment of the final 6 hr period from the CAMs and the global deterministic and ensemble outputs in the final 12 hr time frame indicate a mass of heavy rainfall being depicted within the MD Piedmont to the Chesapeake. NAM Nest is incredibly bullish in its interpretation, likely due to the strong low to mid-level lapse rate signature across the area looking at forecast soundings, as well as persistent 250-750 J/kg MUCAPE still present prior to the cold front arrival. QPF output is around 1-2" currently for the DC/Balt Metro and surrounds for the period, a signal strong enough to warrant a targeted upgrade in the forecast risk. As a result and with coordination from the local Sterling, VA WFO, a SLGT risk was issued for the period across the above area. Heavy rainfall will occur further north as the low matures, but the instability gradient is fairly tight on the northwest periphery of the low as we move closer to Saturday morning. It's plausible places like Philadelphia and areas of Southern NJ and Southeast PA could see a targeted upgrade as well for the D2, but the signal is more borderline for those locales. The MRGL risk was maintained for those areas up to Northern NJ, but will be something to monitor as we move closer. Kleebauer Day 3 Valid 12Z Sat May 31 2025 - 12Z Sun Jun 01 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF NORTHERN NEW ENGLAND... ....20Z Update... Very few changes were needed to the inherited Marginal Risk, with maybe a small southward adjustment across Massachusetts, where rainfall occurring just before the start of the period 12Z Saturday will be ongoing into the start of the D3 period. The surface low will be well-formed by 12Z Saturday, with surface pressures dropping into the 980 mb range. However, instability will be lacking as New England is on the "cold" side of the low...so almost all of the rainfall will be stratiform. Terrain interactions will locally increase rainfall rates, so the Marginal, while low-end and low-confidence, remains in place. Wegman ....Previous Discussion... Powerful surface low across the Mid Atlantic on D2 will motion to the northeast, eventually dropping down into the 980s as it occludes and drifts north over New England. Residual heavy rains will occur across parts of Northern New England with the heaviest rainfall likely over Northern NH and ME, especially within the northeastern flank of the low where WCB process will advect deeper moisture poleward with weak buoyancy likely remaining over the region during the first 12 hrs of the D3 period. Totals of 1-2" with locally higher are forecast over the above zones leading to localized flash flood potential given the complexity of the terrain impacted and relatively good dynamics at play. The MRGL risk inherited was only adjusted on the western half of the risk as trends have pushed the heavier rain a little further east compared to previous forecasts. The area over Central and Western ME is the most susceptible in this pattern with potentially greater impact into Southern New England if the low slows down and brings a stronger convective output towards Eastern MA and RI the front end of D3. Will be an area to assess closely in the coming updates, but maintained the nil there for now. Kleebauer Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!-1QWFYdJTb4Tz0bFfyHRNUoruZ87lfhHvG__4A2bPwjo= X7DRH7F3ruHugBQeHQFK0E2zMuhFPHe67GZANi1vOmN5VYE$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-1QWFYdJTb4Tz0bFfyHRNUoruZ87lfhHvG__4A2bPwjo= X7DRH7F3ruHugBQeHQFK0E2zMuhFPHe67GZANi1vZahGEYw$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-1QWFYdJTb4Tz0bFfyHRNUoruZ87lfhHvG__4A2bPwjo= X7DRH7F3ruHugBQeHQFK0E2zMuhFPHe67GZANi1vs9-e-eY$=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) .