Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Apr 11 2024 01:02:24 FOUS30 KWBC 110102 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 902 PM EDT Wed Apr 10 2024 Day 1 Valid 01Z Thu Apr 11 2024 - 12Z Thu Apr 11 2024 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE FLORIDA PANHANDLE, SOUTHERN ALABAMA, AND SOUTHERN GEORGIA... ....01Z Update... Changes to the Day 1 ERO reflect recent observation trends, as well as guidance from the latest CAMs. The convective system that is now moving across southeastern=20 Alabama, southwestern Georgia, and the Florida Panhandle continues to produce some heavy rainfall rates on the order of 1-2 in/hr=20 across portions of the Florida Panhandle northward to the southern=20 Alabama-Georgia border. While recent MRMS data shows rates have=20 been coming down over the past hour as the system interacts with a more stable environment, renewed convective development, supported by increasing low level inflow and upper level divergence ahead of an amplified trough, is expected to carry the threat for heavy rainfall and flash flooding into the overnight hours. A low level=20 jet increasing to 50-60 kts will support deepening moisture (PWs=20 1.75-2 inches) and instability across the Florida Panhandle into=20 southern Georgia and southeastern Alabama along with the threat for increasing rainfall rates. Consensus of the CAMs shows widespread=20 2-4 inches, with locally heavier amounts overnight within the area highlighted by the Slight Risk. The heaviest amounts are more=20 likely to center with the deepest moisture and greater instability=20 over the Florida Panhandle into the neighboring counties of=20 southwestern Georgia, and southeastern Alabama, where a Moderate=20 Risk was maintained. Further west, a Slight Risk was maintained over Arkansas where strong synoptic forcing and modest instability are supporting some locally heavier rainfall rates. This is expected to continue=20 through the evening, with the CAMs guidance showing some potential=20 for locally heavy amounts of 1-3 inches. Fortunately, guidance=20 suggests most of this will fall outside of the areas hit with the=20 heaviest rainfall earlier today. While a broad Marginal Risk was=20 maintained further to the northeast into the Ohio Valley, a lack of instability is expected to limit the threat for widespread heavy=20 rainfall rates. Pereira Day 2 Valid 12Z Thu Apr 11 2024 - 12Z Fri Apr 12 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PARTS OF THE CENTRAL APPALACHIANS AND MID ATLANTIC, ALONG WITH PORTIONS OF THE OHIO VALLEY... ....Summary... Trimmed backed the eastern fringes of the previous Slight Risk across NJ and NY state. Extended the SLGT risk through eastern OH, western PA, and WV given the increasing threat of convection within an area of lower FFGs. ....20Z Update... Recent trends in the synoptic evolution of our broad mid-latitude cyclone have allowed for some additions to the area SLGT risk from the previous forecast period. Large scale ascent on the eastern flank of the strengthening SLP over the central Midwest will increase drastically during peak diurnal instability across portions of OH/PA/WV with scattered to widespread thunderstorms anticipated after 18z Thursday. 12z HREF probability fields indicate a decent threat for rates to reach upwards of 1"/hr (25-40%) with a low-end potential for 2"/3-hrs (5-10%) which would meet the current FFG exceedance intervals located over eastern OH and western PA, west of the Laurel Highlands. Surface low will be pretty robust with very efficient moisture transport all the way into the Ohio Valley and Northeastern US with regional environment solidly within a bound of 1-2 deviations above normal for PWATs. This is attributed to the strong low-level moisture advection flanking into the eastern side of the low within the warm-conveyor belt. This is not a textbook flash flood threat, but one that is coincident with stronger surface cyclones (<990mb). HREF EAS probabilities for at least 1" in precip are between 45-65% over the outlined area in eastern OH with a precipitous drop in probabilities for at least 2". This caps the higher end outcomes necessary for anything higher than a SLGT risk, but was warranted given the setup and lower FFG intervals in place after last weeks heavy rain episode. Further east, very low FFG intervals for all 1/3/6 hr periods will lead to a heightened risk of flash flooding anywhere there a decent convective coverage. The urban corridor from NoVA up through DC/Balt into the eastern half of PA is the most susceptible to convective impacts. Guidance has somewhat trended away from the "worst case scenarios" as the primary 85H LLJ screams into the area, but quickly vacates to the northeast with best moisture flux aimed more at the adjacent Delmarva and shores of NJ up into LI/CT. Those areas have higher FFG intervals given the sandier soils, thus the threat for flash flooding is low enough to maintain just a MRGL risk for the area. As a result for the above, maintained mostly continuity from the previous forecast cycle with the largest shift being the addition of a SLGT out across OH, western PA, and northern WV. Kleebauer ....Previous Discussion... ....Synopsis... As the mid-upper level trough becomes more negatively tilted, large-scale forcing and deep-layer moisture transport will remain favorable up the Eastern Seaboard. The main differences however include the increased forward speed of the upper trough and surface low/associated fronts, along with weaker deep-layer instability the atmosphere will have to work with. ....Parts of the Central Appalachians-Mid Atlantic... Per collaboration with WFOs RNK and AKQ, have removed the Slight Risk across much of central and southern VA. Rather, have confined the Slight Risk where the guidance means indicate greater coverage of 1-3 inches of rainfall in areas of decidedly lower FFG values (widespread 1hr FFGs of inch or less, 3hr FFGs between 1-1.5"). ....Elsewhere across the Eastern CONUS... Maintained the Marginal Risk area outside of the areas with lower FFGs, as 1-3" of rainfall within 2-3 hours would more likely lead to localized/isolated areas of flash flooding. Day 3 Valid 12Z Fri Apr 12 2024 - 12Z Sat Apr 13 2024 ....THERE IS A SLIGHT RISK OVER PORTIONS OF NORTHERN NEW ENGLAND... ....Summary... Based on the latest guidance trends and collaboration with the Gray WFO, have expanded the Slight Risk area northwest across western ME and northern NH. This matches well with the Flood headlines in place due to the incoming heavier rain and snowmelt combination that will result in localized flooding. ....20Z Update... In coordination with the Gray/Portland WFO in Maine, have expanded the SLGT risk from the previous forecast to the Canadian border to align with current Flood headlines in preparation for the incoming moderate to heavy rain threat that will couple with rapid snowmelt along the higher terrain of NH and western ME. Current forecast remains steadfast from previous runs within the ensemble suite with deterministic only showing minor shifts in the precipitation maximum located over the White Mountains into western ME. A solid SWE of 1-3" is positioned within the southern and southeast slopes of the northern Appalachians which is subject to rapid snowmelt due to the flux of higher dew points and expected heavier rains from our sprawling surface cyclone to the southwest. Forecast precip of 0.75-1.5" when combined with the snowmelt will generate local flood concerns within the terrain as stream flows will take in a large quantity of water over the 24 hour period. The MRGL risk area was unchanged from previous forecast as the threat of locally heavy rain from the setup will allow for a low-end flash flood threat over New England. Kleebauer ....Previous Discussion... ....Northeast... Anticipated mainly (at most) localized instances of flash flooding as the low lifts northward across western Quebec and the cold and occluded surface fronts push through. A big factor to the excessive rainfall outlook areas is the aforementioned wet antecedent soils, exacerbated somewhat by the ongoing snowmelt. Maintained the Slight Risk across eastern parts of NH into ME, given the heavier rainfall footprint (1-2") per the ECMWF and GEM. Hurley Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!95xlAajFOMF88QVKi2-FNI6tF9Ke2bfL7ilLGI0O27t0= MW4KD59mVKNCgahllI6Vwd56Na3fjh-X4lM6dD2QbCBBh0o$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!95xlAajFOMF88QVKi2-FNI6tF9Ke2bfL7ilLGI0O27t0= MW4KD59mVKNCgahllI6Vwd56Na3fjh-X4lM6dD2Q2d7VFz8$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!95xlAajFOMF88QVKi2-FNI6tF9Ke2bfL7ilLGI0O27t0= MW4KD59mVKNCgahllI6Vwd56Na3fjh-X4lM6dD2Qoed095w$=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) .