Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Oct 01 2025 00:30:09 FOUS30 KWBC 010029 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 829 PM EDT Tue Sep 30 2025 Day 1 Valid 01Z Wed Oct 01 2025 - 12Z Wed Oct 01 2025 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Bann Day 2 Valid 12Z Wed Oct 01 2025 - 12Z Thu Oct 02 2025 ....THERE'S A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE OLYMPIC MOUNTAINS AND THE EAST COAST OF FLORIDA... ....Olympics... An anomalous mid-level closed low will be slow to move off of the coast of the Pacific Northwest on Wednesday into Thursday. Moisture anomalies along the coast are forecast to remain neutral to weakly positive but onshore flow will keep the region unsettled and left-exit region upper level jet induced divergence may aid in rainfall intensities early on Wednesday. Low level flow will be oriented from the SSW into the Olympics with a high likelihood for showers (isolated thunder given weak instability near the coast) throughout the 24 hour period with occasional bursts of rain rates in excess of 0.5 in/hr. 12Z HREF probabilities of QPF over the 24 hour period ending Thursday at 12Z were 80-90 percent for 3+ inches, and 50-60 percent for 5+ inches. Given forecast rainfall of 1 to 3 inches ending Wednesday morning, and the additional rainfall on Wednesday into Thursday of 3 to 5 inches locally (with embedded high rates), localized runoff in and around the Olympics may result. ....East Coast of Florida... With the eastward departure of Imelda and Humberto from the western Atlantic, a large surface ridge will build across the eastern U.S., with low level easterly flow setting up perpendicular ot the east coast of the Florida Peninsula. 925-850 mb winds are forecast by the 12Z GFS to peak in the 20-30 kt range and offshore instability of at least 500-1000 J/kg is forecast by the latest model consensus to set up just offshore and along the coast. Upper level support for ascent will be aided by the right-entrance region of a 70-80 kt ahead of an upper level shortwave tracking east from the central Gulf Coast. While moisture parameters are forecast to be only near average for early October, PW values are still forecast to range from 1.5 to 2.0 inches at times with low level convergence helping to focus small but intense cores of heavy rain with hourly rainfall of 2 to 3 inches expected at times. The flash flood threat is expected to remain rather localized, but given the presence of the highly populated eastern Peninsula and potential for localized 4 to 5+ inch totals, a Marginal Risk was introduced for this update. Otto Day 3 Valid 12Z Thu Oct 02 2025 - 12Z Fri Oct 03 2025 ....THERE'S A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE GREAT BASIN AND EASTERN FLORIDA... ....Great Basin... The upper low from Wednesday is expected to dig into the West Coast on Thursday directing diffluent flow and left exit region jet divergence into the Great Basin. Weak instability (less than 250 J/kg MUCAPE from most models) along a developing cold front could generate repeating rounds of thunderstorms and short term training capable of producing some isolated flash flooding. While PWATs are forecast to be anomalous for early October (+2 to +3 standardized anomalies), moisture and instability values will be relatively modest in an absolute sense which may limit rain rates. However, local sensitivities, including burn scars, could be at risk of impactful rainfall rates. ....Eastern Florida... As an upper level trough over the Southeast slowly edges eastward on Thursday, a similar setup as seen on Wednesday will be in place across the east coast of Florida. This includes instability of at least 500-1000 J/kg, PWAT values of 1.5 to 2.0+ inches and low level onshore flow of 20-30 kt at times. Slow cell motions could allow for hourly rainfall of 2 to 3 inches in a couple of locations with a focus along coastal/high population density sections of eastern Florida. While hires model data is limited to the 12Z RRFS for the day 3 time frame, the RRFS shows a spot of 6-7 inches in northern portions of the outlook area, indicative of the potential for locally high rainfall totals. Otto Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!--vzlDqCKAME7aiKq4Jdwed_ujN-XNs14mrnuXUUpEc5= ZUmK_p9kuVQ-1lZS3_VG3iQ8bkt5sWiwLmAc8k1lXf45ZBo$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!--vzlDqCKAME7aiKq4Jdwed_ujN-XNs14mrnuXUUpEc5= ZUmK_p9kuVQ-1lZS3_VG3iQ8bkt5sWiwLmAc8k1lK0gAEaY$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!--vzlDqCKAME7aiKq4Jdwed_ujN-XNs14mrnuXUUpEc5= ZUmK_p9kuVQ-1lZS3_VG3iQ8bkt5sWiwLmAc8k1l9TZNecA$=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) .