Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Oct 01 2025 15:29:55 FOUS30 KWBC 011529 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1129 AM EDT Wed Oct 1 2025 Day 1 Valid 16Z Wed Oct 01 2025 - 12Z Thu Oct 02 2025 ....THERE'S A MARGINAL RISK OF EXCESSIVE RAINFALL FOR MUCH OF THE=20=20 EASTERN FLORIDA PENINSULA... 16Z Update: After coordination with local Seattle WFO and observational/short term trends in guidance, have opted to remove the MRGL risk in place across the Olympic's for the period. Locally heavy rainfall will be expected with the current pattern, however low probs for >2"/6-hrs and generally sub <1"/hr rainfall rates expected outside a few elevated convective signals later in the period over the highest terrain warrant a non-zero, but sub-MRGL risk forecast for flash flood prospects.=20 Across eastern FL, the threat for locally heavy rainfall continues for the afternoon and evening with a favoring setup closer to the end of the forecast when surface trough alignment along the eastern peninsula generates a stronger low-lvl convergence signature in proximity to the trough. HREF blended mean QPF still distributes between 1-1.5" in the mean for spots along the eastern coast, however HREF neighborhood probs for >5" are between 15-35% along the same area of the QPF mean. This is grounds for maintaining a forecast of locally enhanced rainfall rates that could overperform the mean quite easily if they setup in a given area. For this alone, the risk area was relatively maintained with the biggest change being the addition of all of the Miami metro included in the latest update.=20 Kleebauer ...Previous Discussion.. ....Eastern Florida... A positively tilted trough will swing through the Southeast today. CAA over warm Gulf/Atlantic waters should generate convection along a surface convergence zone over the Florida Coast today. MUCAPE between 500-1000 J/Kg within a moist environment (1.75-2.25 in. PWATs) should produce scattered thunderstorms with efficient rain rates. Moist easterly flow over the inverted coastal surface trough may generate some backbuilding storms this afternoon/evening. HREF 5" neighborhood exceedance probabilities are between 20-30% across the entire risk area, so there's potential for overperformance of rainfall. Urban areas are of particular concern to flash flooding. Kebede Day 2 Valid 12Z Thu Oct 02 2025 - 12Z Fri Oct 03 2025 ....THERE'S A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE OLYMPIC PENINSULA, THE SIERRA, THE GREAT BASIN AND EASTERN FLORIDA... ....Olympic Mountains... The excessive rainfall threat spills over into Thursday across the Olympics as the upper low begins to dig down into the West Coast. 100-200 J/Kg MUCAPE and 0.75-1" PWATs should sustain the potential for excessive rainfall, particularly earlier on in the period (12-18z) before the trough pushes farther east into the interior and high pressure builds over the Olympic Peninsula. CMCE 1" exceedance probabilities are around 15-35% within the marginal risk area. ....Great Basin/Sierra... Strong diffluence associated with the digging upper trough will spread throughout the interior West on Thursday. An 80-100 kts RER jet with a 100 J/Kg MUCAPE should support convection with PWATs between 0.75-1", which will be 3-4 standard deviations above=20 average. This will provide enough moisture to produce high enough=20 rain rates to overtake flash flood guidance, which is below 1". ....Eastern Florida... Shortwave vortices within a mean upper trough will propagate south through the Florida Peninsula on Thursday. Sufficient instability (500-1000 J/Kg MUCAPE) and moisture (2"+ PWATs) associated with Imelda may produce excessive rainfall along the immediate coast early on in the day. The latest qpf trend is decreasing, partly because the heavy rainfall threat is split between two days (00z Wednesday--15z Thursday), but urban areas continue to be susceptible to runoff. Kebede Day 3 Valid 12Z Fri Oct 03 2025 - 12Z Sat Oct 04 2025 ....THERE'S A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF EASTERN FLORIDA... ....Eastern Florida... Continued troughing in the Southeast will promote more unsettled weather across Florida on Friday. Easterly flow will continue advecting rich tropical moisture (2-2.25" PWATs) from Imelda into the Peninsula with plenty of instability (500-1000 J/Kg MUCAPE) to generate scattered thunderstorms. 2" ensemble exceedance probabilities are between 5-15%, which supports the inherited marginal risk area. Again, the greatest threat for flash flooding is in urbanized areas. Kebede Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!6avUEPTz7w5KE2suAXqsVP1dVLAbmfdPmxgnZEtLOWFq= zArm3NgCYckK9c96cz_B9_Kd_Zo1KG9KS1hlKIMbFB9gD5g$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6avUEPTz7w5KE2suAXqsVP1dVLAbmfdPmxgnZEtLOWFq= zArm3NgCYckK9c96cz_B9_Kd_Zo1KG9KS1hlKIMby97pyUw$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6avUEPTz7w5KE2suAXqsVP1dVLAbmfdPmxgnZEtLOWFq= zArm3NgCYckK9c96cz_B9_Kd_Zo1KG9KS1hlKIMbgHk9OV4$=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 3.28-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .