Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Oct 08 2023 19:42:40 FOUS30 KWBC 081942 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 341 PM EDT Sun Oct 08 2023 Day 1 Valid 16Z Sun Oct 08 2023 - 12Z Mon Oct 09 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR THE ATLANTIC COAST OF SOUTHEAST FLORIDA AND PORTIONS OF CENTRAL NEW YORK EAST OF LAKE ONTARIO... ....16Z Update.... Only minor shifts in the Marginals across NY and FL. Long axis band has been situated over northern Oswego county into southern Jefferson, extending out into Lewis county over the northern and central Tug Hill area. Persistent moderate to heavy rainfall with embedded thunderstorms will remain the norm through the course of the morning and afternoon before a slow migration to the north with the main band as winds back and allow the orientation to shift up into northern Jefferson by the end of the period. Local precip amounts of 1-3" will be possible along and east of the I-81 corridor out into the Tug Hill Plateau where upslope enhancement could bring maximum totals over 3" by the time the band shifts north. The threat for flash flooding is on the lower end of MRGL considering the lack of a strong convective signal, but enough precip could cause some localized issues for places that receive 1"/hr, thus maintained the MRGL risk. Across FL, there's still some modest signals for locally heavy rainfall somewhere across southeast FL with the main concern being the population center located from Miami up through West Palm Beach. 12z HREF probability for 1-2"/hr rainfall rates is still on the high side with 70-80% for at least 1"/hr and 40-50% for 2"/hr which is typically the threshold for flooding in that area of the country. 40 dbz Paintball reflectivity indicates the best chance for heavy rain will occur between 18-23z before rapidly falling off with the convective threat shifting off the coast. With local maximum QPF from hi-res deterministic and LPPM around 4-4.5", this pins a low end MRGL risk for local flash flooding concerns within the urban corridor, thus maintaining the previous forecast. Kleebauer ....Central New York... No significant changes were made to the Marginal Risk areas in NY and FL. In NY, in collaboration with BUF/Buffalo, NY forecast office, the Marginal was left in place east of Lake Ontario. As the extratropical remnants of Philippe track northwestward into Quebec, it will draw cold air with 850 temps to -3C over the Great Lakes. The Lake Ontario water temperature is 68F, or 20C. This 23C temperature difference will result in extreme instability developing as the cold air moves over the warm waters of the lake. Lake enhancement has already started across Oswego County, and will intensify today as the low center approaching the Maine coastline tracks into central and western Quebec. The band will gradually drift northward today from Oswego and Lewis counties up to Jefferson county tonight. The extreme instability will allow the lake enhancement to grow convectively, resulting in localized heavy rain, which as is the nature of lake-effect may repeat over the same areas. For the Tug Hill Plateau region, upslope enhancement may locally further increase rainfall rates. Except in a small swath of Lewis and Oneida counties, soils in this area are still drier than normal despite recent rainfall. Thus, at first the ongoing rainfall will be beneficial, but in those areas where the heaviest rainfall lingers longest, isolated flash flooding is possible. ....Southeast Florida... A front currently nearly stalled out over central FL will act as a conveyor belt for deep Gulf moisture to ride northeastward along and south of the frontal interface today. Low level easterly flow will create an area of frictional convergence along the coast in the Miami-Ft. Lauderdale area. CAMs guidance shows that there's potential for strong thunderstorms that may linger over those urban areas, resulting in multiple inches of rain in a short period over a flood-sensitive area, though they have backed off on the potential in the latest runs. The Marginal Risk area remains unchanged with this update, and is low-confidence. Wegman Day 2 Valid 12Z Mon Oct 09 2023 - 12Z Tue Oct 10 2023 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Wegman/Kleebauer Day 3 Valid 12Z Tue Oct 10 2023 - 12Z Wed Oct 11 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR SOUTH TEXAS... The overall synoptic pattern remains steadfast with the latest guidance, but an uptick in the overall QPF field was the biggest change from the previous forecast cycle. The magnitude of rainfall is still within the premise of 3-5" with locally higher amounts, but the spatial coverage of the heavier QPF footprint has expanded further inland, back towards the Rio Grande. GEFS ensemble guidance has become increasingly bullish on the higher totals based off the increase in forecast ensemble precip, as well as associated probabilities for both >2" (20-30%), and >3" (10-15%) totals within the 24 hr period. When you factor in the main time frame for precipitation is back-loaded to the second half of D3, that creates a reasonable forecast for heightened flash flood concerns given the short time frame the heavy precip will fall. PWATs continue to be impressive within the means showing a 2.25-2.5" PWAT outline from KCRP to Eagle Pass and points south. This environment is of tropical origin which creates a higher potential for efficient warm processes and higher QPF probabilities. Would anticipate a further increase in the QPF footprint so long as the synoptic scale setup remains the same. Considering the shift in the heavier precip field further inland, have expanded the SLGT risk out to the RGV and will now encompass most of WFO Corpus Christi's CWA and all of WFO Brownsville's CWA. The SLGT was outlined using a blend of the NBM/GEFS/Ensemble bias-corrected QPF field with the edges defined with the 1.25" QPF contour. Kleebauer ....South Texas... A surge of moisture associated with Potential Tropical Cyclone 16-E in the eastern Pacific will track northward across Mexico into the western Gulf, and into the Texas Gulf Coast by Tuesday night. Meanwhile a strong and nearly stationary upper level jet will be centered over the Tennessee Valley, which will put south Texas in the favorable right entrance region. Thus, rain along the TX Gulf Coast may begin in Deep South Texas as early as Monday night/very early Tuesday morning, and will gradually intensify as it lifts north closer to the favorable jet dynamics through the day Tuesday. Finally, by Tuesday night, the aforementioned remnants of PTC 16E will reach the northwestern Gulf/Texas Gulf Coast. This influx of moisture will greatly enhance rainfall rates as PWATs skyrocket to as high as 2.5 inches Tuesday night. There remains significant uncertainty as to the track of the low, and how far west the associated heavy rainfall can move inland from the Gulf Coast. Since confidence has increased markedly as to the rainfall threat along the immediate Gulf Coast, a Slight Risk area representing a double upgrade was introduced for that area in coordination with CRP/Corpus Christi, TX forecast office. The surrounding Marginal represents the considerable uncertainty as to how far north and west the heavier rains will get, and is likely to change with future forecast updates. Soil moisture across Deep South Texas is above normal due to recent rainfall, so the addition of 3 to 5 inches of rain with higher local totals is likely to result in widely scattered instances of flash flooding, especially along the Gulf Coast and any urbanized areas, such as Corpus Christi and Brownsville. This increased the confidence enough to warrant the Slight Risk upgrade for this area. There is good agreement that this should be a one-day event for the Texas Gulf Coast, as the greatest moisture associated with the low quickly shifts east into the eastern Gulf on Day 4/Wednesday. That said, the moisture associated with the other eastern Pacific tropical cyclone, T.S. Lidia will be following on the heels of PTC 16E, so that will need to be monitored for the Wednesday/Wednesday night time frame. Wegman Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!-ESgxGmyDuamWu-eKJ9WmBYOnIXIwVbpXeTgUalCg-eQ= bhza5hGAMZDCk1kGxk4jbHY2k8nwzUZK7gLC-Y2bQInHCE4$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-ESgxGmyDuamWu-eKJ9WmBYOnIXIwVbpXeTgUalCg-eQ= bhza5hGAMZDCk1kGxk4jbHY2k8nwzUZK7gLC-Y2bWCQ4BqE$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-ESgxGmyDuamWu-eKJ9WmBYOnIXIwVbpXeTgUalCg-eQ= bhza5hGAMZDCk1kGxk4jbHY2k8nwzUZK7gLC-Y2bgPBrDns$=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 3.14-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .