Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Dec 09 2024 19:55:34 FOUS30 KWBC 091955 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 255 PM EST Mon Dec 9 2024 Day 1 Valid 16Z Mon Dec 09 2024 - 12Z Tue Dec 10 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE CENTRAL GULF COAST... Current radar and surface observations indicate the ongoing forecast on track with southwest-east oriented bands of showers and storms impacting the central Gulf Coast with brief periods of training and hourly rainfall rates up to 2". Overall the pattern evolution will continue to point to a benign instability axis (MUCAPE ~500-1000 J/kg) being confined closer to the Gulf with sufficient deep layer shear and moisture presence (PWATs of 1.5-2.0" and b/w +1 to +2 deviations) allowing for a small area of convective development capable of locally enhanced rainfall rates for flash flood potential. Given the overall antecedent dry conditions prior, the prospects of flash flooding will likely be relegated to more urban settings along the Central Gulf coast, including places such as New Orleans, Biloxi, and Mobile. A few other Parishes within Southeastern LA also contain areas that experience a bit easier ability to flash flood given some of the above urbanization factors, so the previous SLGT risk was maintained with only some minor modifications made to the overall risk area. Latest 12z HREF neighborhood probabilities for >3" of rainfall are in the 20-40% range with highest prospects likely between 1-2" when taking relevant CAMs into account. With coordination from the local WFOs along the Gulf Coast earlier for the SLGT risk, there was no need to deviate much from what was proposed in the previous forecasts. Best chances will reside from training echoes on persistent west- southwest flow during the diurnal instability maximum, carrying into the evening before the setup decays prior to the next period of relevant convection for D2. Snell/Kleebauer Day 2 Valid 12Z Tue Dec 10 2024 - 12Z Wed Dec 11 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM SOUTHEASTERN ALABAMA TO UPSTATE SOUTH CAROLINA... ....1930 UTC Update... Minimal (cosmetic) changes made to the Day 2 ERO based on the latest guidance suite. Within the Slight Risk area, blend of operational guidance continues to show areal average totals between 2-3 inches, with locally higher totals per the CAMs. Continues to look like a low-end Slight however (neighborhood exceedance probabilities closer to 15% vs. 40%), considering the lack of deep-layer instability which will limit the short-term rainfall rates. Hurley ....Previous Discussion... As the previous shortwave trough kicks to the northeast, a stronger mid-level trough will dig quickly out of the Rockies into the=20 Southern Plains by Tuesday morning with sights downstream on areas=20 east of the Mississippi. Increasing meridional flow ahead of the=20 digging trough will aid in the advection of deep moisture poleward=20 with the initial surge propagating north out of the Gulf through=20 the Southeast by Tuesday afternoon, eventually spreading northward=20 into the Southern and Central Mid Atlantic the second half of the=20 period. The advection pattern will be sufficient in increasing=20 regional mid-level instability as profile moisture saturates deeper into the profile across the areas above. PWAT anomalies on the=20 order of 2-3 deviations above normal will be common over parts of=20 GA spreading up through the neighboring Carolinas, eventually=20 surging into the Central Mid Atlantic and Northeastern U.S by the=20 end of the D2 cycle. This will prime several areas for heavy=20 rainfall prospects as the final ingredients come to together for a=20 locally enhanced precip pattern. The trough to the west will begin tilting more neutral with a strengthening upper jet positioning allowing for large scale ascent to increase in earnest as we move into Tuesday afternoon and beyond. The main trigger for a line of enhanced convection will form at the hands of a strong surface cold front that will shove eastward beneath an intensifying surface low that develops in conjunction to the favorable upper level evolution. Current deterministic sfc to 850mb wind field indicates a corridor of backed flow across much of GA into the Carolinas during Tuesday evening ahead of the cold front allowing for widespread rainfall to develop downstream of the approaching front. As the cold front swings eastward by late Tuesday night through early Wednesday, the peak rainfall rates will be occurring across the Carolina Piedmont down through Central GA, aligning with the cold front as it moves through the region. The eventual heavy rain footprint will spread northeastward into the Southern Mid Atlantic with locally heavy rainfall impacting areas as far north as Virginia with some lighter precip making headway into areas further north. Places across Southwestern VA down to the I-40 corridor in NC will see anywhere from 1-2" of rainfall with locally higher further south through the zone. 2-3" with local totals to 4" will be plausible for the area extending from Southeastern AL through Central GA into Upstate SC, an area more prone to flash flood prospects thanks to the complexity of the local terrain. This signal has been prevalent over the past series of deterministic model outputs and is now firmly within the ensemble mean as bias corrected ENS now supports 2.5-3" through much of the area outlined above in the 2-3" forecast. The saving grace in this setup is the time of year allowing for lower surface based instability parameters and very dry antecedent conditions leading into the event. This allowed for a SLGT risk to be sufficient for the setup, a carry over from the previous forecast. The MRGL risk still extends into VA where the northern periphery of the heavy rainfall is expected. Kleebauer Day 3 Valid 12Z Wed Dec 11 2024 - 12Z Thu Dec 12 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM CENTRAL AND EASTERN LONG ISLAND UP THROUGH MUCH OF NEW ENGLAND... ....1930 UTC Update... Minimal changes made to the Day 3 ERO based on the latest guidance suite. One of the more notable tweaks was to extend the Marginal Risk a little farther s-sw to include more of the NC Piedmont and the Charlotte metro region. This is because the event (heavier rainfall) will be ongoing Wed morning in that area. TPW anomalies across New England during Day 3 peak between 4-4.5 standard deviations above normal per the NAEFS, where areal-average totals between 2-3" are still expected within the Slight Risk area. Once again, as with the Slight Risk area on Day 2, the main inhibitor for flash flooding will be the lack of elevated instability and thus cap on short-term rainfall rates.=20 Hurley ....Previous Discussion... As the longwave pattern evolves further, the neutral-tilted trough from previous D2 time frame will enhance further with the mean=20 trough taking on a more negative tilt allowing for enhanced surface low strengthening as the surface low progresses northward into New England and eventually Quebec. PWAT anomalies on the order of 2-3=20 deviations above normal will quickly progress northward=20 encompassing much of New England by Wednesday afternoon, even=20 getting up towards +4 deviations as we move into Wednesday night=20 over Northern New England from NH through ME. This pattern is=20 textbook for a corridor of heavy rainfall to spread from the Mid=20 Atlantic through all of the Northeast with the primary areas of=20 interest location along and ahead of the cold front that will sweep eastward during the forecast period. Portions of Western NY state=20 and Northern PA will eventually change to winter precipitation=20 allowing for a decay in flash flood concerns, however areas=20 downstream will be under a respectable deluge of rainfall prior to=20 the rain threat ceasing in wake of the front. Totals of 1.5-2.5" of rain are forecast across much of Long Island up through Southern New England, eventually moving up through ME as we close out the forecast period. 1-2" is forecast for points further west, but those areas also have the added snow melt factors that will play into the flooding concerns as high dewpoints and heavy rain will promote rapid snow melt for places in the interior over VT/NH and the neighboring Berkshires in MA. The expectations are for some creeks and smaller streams to become problematic in the setup and could cause localized flood concerns during the peak of the heavy rainfall. The rivers across New England can thankfully take a decent surge of moisture after a very dry fall, so the prospects of significant flooding are very low at this point. The setup is still worthy of a large SLGT risk encompassing much of New England with the best threats likely over urbanized areas in LI and Southern New England and over those zones in the interior where rapid snow melt has historical precedence for localized flash flood concerns. A MRGL risk will encompass the SLGT risk across Northeast with an extension down into Central and Eastern NC as the early portion of the forecast period will still see moderate to heavy rainfall Wednesday morning before pulling northward. Kleebauer Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!5p4ZIKz1GdsVc9JBavSpwL6p9Cu5wM9ksJBBkdB2iBDr= PFEy4To3GNv9tTLjwx8o3Nb9MZSQAQ1FRtmUQN1rgdB-8hA$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!5p4ZIKz1GdsVc9JBavSpwL6p9Cu5wM9ksJBBkdB2iBDr= PFEy4To3GNv9tTLjwx8o3Nb9MZSQAQ1FRtmUQN1r3TR_tT0$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!5p4ZIKz1GdsVc9JBavSpwL6p9Cu5wM9ksJBBkdB2iBDr= PFEy4To3GNv9tTLjwx8o3Nb9MZSQAQ1FRtmUQN1rp4IniKA$=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) .