Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri May 30 2025 01:00:19 FOUS30 KWBC 300052 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 852 PM EDT Thu May 29 2025 Day 1 Valid 01Z Fri May 30 2025 - 12Z Fri May 30 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF CENTRAL TEXAS... A Slight risk was maintained across central TX. Convection is expanding in coverage this evening along a southward shifting cold front. This activity should continue to expand in coverage and=20 grow upscale into an MCS or convective cluster or two tonight.=20 Generally looking like a forward propagating convective system,=20 which should limit the extent/magnitude of flash flooding. However=20 a few cell mergers this evening as convection organizes will=20 likely lead to some flash flood risk. Even once activity becomes=20 more progressive high rainfall rates should be enough for isolated to scattered FFG exceedance, especially where cell mergers briefly extend rainfall duration. Recent HRRR runs have indicated some=20 upstream development overnight near the Rio Grande, which does seem plausible given forecast increase in low level moisture transport=20 and instability around 4000 j/kg. If this does occur, and then=20 merges with the approaching MCS (as the HRRR indicates) then a more substantial flash flood risk could evolve later tonight near the=20 Rio Grande in south central TX. A Marginal risk was maintained across portions of the eastern=20 Carolinas into far southeastern VA. Convection will continue to=20 pose a localized flash flood risk this evening, although the=20 activity over NC and VA is expected to weaken fairly rapidly this=20 evening, diminishing the risk. The approach of a strong shortwave will potentially bring a=20 localized flash flood risk to portions of KY/TN and southern IL/IN later tonight as well. Limited instability will keep rainfall=20 rates in check...however the very dynamic nature of the approaching shortwave should still allow for some heavier convective elements=20 within the broader rain shield. Rainfall could locally approach or=20 exceed 2", potentially enough for some minor flood concerns,=20 especially over any more sensitive urban or low lying areas. Chenard 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!63NFVABJhmYrOUOd7zHTjMpH23W1fpdAmZB-r34mWoFI= 8_hdUvBtofODMyaA5o22SjbSif2G4QmL3LIc-85W2uV-3hg$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!63NFVABJhmYrOUOd7zHTjMpH23W1fpdAmZB-r34mWoFI= 8_hdUvBtofODMyaA5o22SjbSif2G4QmL3LIc-85WNS55CCc$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!63NFVABJhmYrOUOd7zHTjMpH23W1fpdAmZB-r34mWoFI= 8_hdUvBtofODMyaA5o22SjbSif2G4QmL3LIc-85WhL3XEGo$=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) .