FOUS30 KWBC 082023 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 423 PM EDT Thu Oct 8 2026 Day 1 Valid 16Z Thu Oct 08 2026 - 12Z Fri Oct 09 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS CENTRAL FLORIDA... Given the increasing signal in both the HREF and REFS for localized very heavy amounts, parts of central Florida were upgraded to a Slight Risk with the 16Z Update. Thunderstorms developing along a stalled boundary in central Florida -- extending from the Tampa Bay area to the Space Coast -- will be capable of producing rainfall rates in excess of 3 in/hr. A very moist airmass (~2.25-2.50 inches), along with favorable upper- level jet dynamics and cloud layer shear will combine to support organized, highly-efficient, rain-producing storms. Given the antecedent moist soils and the potential for training storms, localized instances of flash flooding appear more likely, especially in urbanized and poor drainage areas. The Slight Risk reflects an axis where both the HREF and REFS show high probabilities for 3 inches or more, with both indicating that heavier amounts over 5 inches around the Tampa Bay area are likely. Refer to WPC MPD #1460 for additional information regarding the near-term threat for heavy rain and flash flooding in this area. Pereira Day 2 Valid 12Z Fri Oct 09 2026 - 12Z Sat Oct 10 2026 ...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR SOUTHERN ALABAMA AND THE WESTERN FLORIDA PANHANDLE... 20Z Update... As with the expected track for Isaias, the ERO remains largely unchanged with this update. The Moderate Risk was expanded a little farther north and east, extending from the anticipated landfall point near the western Florida-Alabama border over to the Florida Big Bend and far southwestern Georgia where guidance continues to indicate that a lingering inflow band may produce a prolonged period of training storms and very heavy amounts. REFS neighborhood probabilities for amounts exceeding 5 inches are greater than 70 percent within much of the Moderate Risk area. The only other adjustment of note was to continue to trim some of the western extend of the outlook as guidance continues to show drier air on the western extent supporting a tight accumulation gradient. Pereira Previous discussion... Hurricane Isaias will make its way out of the central Gulf, make landfall near the Alabama/Florida border, then continue north across Alabama on Friday. See www.hurricanes.gov for the latest track forecast. As regards rainfall, increasing southwesterly shear over Isaias as it makes landfall will push most of the associated rainfall east and northeast of the center. Thus, much of the higher risk area are along and east of the track. Frictional convergence on the southerly flow east of the center will locally enhance rainfall rates from Mobile east along the coast and into the Florida Panhandle. 3-7 inches of rain, with localized totals up to 10 inches are forecast. Considering the soils are already wetter than normal over much of this area from recent rainfall, this amount of heavy rain... much of it occurring in a 12-18 hour period will cause numerous flash floods, some of which will be significant. Urban areas such as Mobile and Pensacola are at greatest risk. Also at risk are areas around any concave shaped bays (which most are), that can funnel any storm surge up the rivers and block any drainage of rainfall. As the track of Isaias is little changed, the rainfall totals are also little changed. The risk areas over this region are also little changed. Isaias will be moving at a steady clip as it moves into the Southeast, so for most areas, any heavy rains will be relatively short in duration. This fast movement will be the saving grace preventing much more widespread and significant flooding issues. As the center moves inland Friday night, the heaviest rains will pivot northwest of center with the greatest forcing into the drier air mass to the north, as well as in lingering feeder bands that will promote localized training storms. The Slight Risk area continues to highlight much of the rest of Alabama, eastern Mississippi, and western Georgia. Frictional convergence will increase in intensity along the Atlantic coast of Georgia and South Carolina in the predawn hours Saturday morning. This phenomenon will be just beginning, so the Marginal Risk that extends to the Atlantic coast looks good. Wegman Day 3 Valid 12Z Sat Oct 10 2026 - 12Z Sun Oct 11 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR MUCH OF THE SOUTHEAST... 20Z Update... Similar to Day 2, the overall messaging remains much the same, with a few areas of higher concerns within a broad Slight Risk area that extends from the Florida Big Bend region to the Tennessee Valley, southern Appalachians, and the Southeast Coast. Following the 12Z model trends, adjustments included shifting the axis of the Slight Risk a little farther east across the Florida Panhandle and the northern Peninsula into the interior Southeast. Training storms along a persistent inflow band may continue to elevate the threat for flash flooding from the eastern Panhandle- Big Bend region northward across Georgia, into an area of orographically-enhanced amounts covering portions of the southern Appalachians. Currently considering this corridor a 'higher-end Slight,' with potential upgrades forthcoming if forecast confidence in the positioning of the inflow band increases. Also, remaining a 'higher-end Slight' is much of the coastal Carolinas, where strong inflow and onshore convergence, potentially enhanced by a weak wave is expected to raise the threat for heavy amounts and flash flooding. With the potential for the heaviest amounts to remain confined to the immediate coast or just offshore, opted not to upgrade to a Moderate with this update but will continue to monitor. Pereira Previous discussion... ...Southeast... The remnant circulation of Isaias over the Southeast will rapidly complete extratropical transition on Saturday. With the storm exhibiting much more of a classic comma or T shape, the heaviest rains will elongate more east to west. To the west, the bulk of the circulation will track across central Tennessee, with heavier rains expanding northwest to the Mississippi River near Memphis. The feeder bands from the Gulf will be nearly stationary from the Florida Panhandle near Apalachicola north over much of Georgia. Finally, the bulk of Isaias' moisture will track northeast up into the Carolinas. Within the large Slight Risk area, there are several higher risk areas worth noting. In the Florida Panhandle, feeder bands impacting many of the same areas that the core or at least the eastern portion of the core hit on Friday will continue through much of Saturday and possibly into Saturday night. While rainfall amounts will be significantly lower on Saturday as compared to Friday, especially on a larger scale basis, any storms will be capable of localized rates to 2.5 inches/hour. Rates that high will most likely be in any convection that is closer to the Gulf, where much of the instability will be. In the southern Appalachians, there will be a 12-18 hour period when the center of Isaias over Tennessee will support screaming winds at several thousand feet up, or for the Appalachians, around mountain top level. Winds will be in the 40-50 kt range. This will favor significant upslope along the mountains. Thus, much higher rainfall amounts are expected into the western Carolinas. The short time these conditions will be favorable, despite a complete absence of instability, will save this area from much more significant flash flooding. For this reason, no Moderate Risk upgrade is included with this update, though that area is in a higher-end Slight. Finally, along the Carolina coast, significant frictional convergence will favor much heavier rainfall amounts from about Hilton Head, SC north to around Morehead City, NC. Alone, this may favor a lower-end Slight, with widespread 2-4 inches of rain expected, and localized higher amounts to 6 inches. However, as Saturday is the New Moon, the king tides, especially Saturday morning, will be particularly high. While the high tide cycle will be slightly muted Saturday evening, the heavy convection will be in full force at that time. Thus, expect a combination of tidal and rainfall/fresh-water flooding to impact coastal communities. For this reason, a higher-end Slight has been included with this update. ...Southwest... On the other side of the country from Isaias, a deep positively tilted trough over the West Coast will begin to pick up deep tropical moisture from the Pacific and advect it across much of the Southwest on Saturday. Later on, the remnant circulation and associated moisture of Rachel will move in, though that's far more of a concern on Day 4/Sunday. A predecessor rainfall event of sorts will begin on Saturday. Strong southwesterly flow will uplift the moisture being transported into the lower Colorado River Valley, as well as further north across much of Utah. Any east-west oriented ranges will be particularly exposed to the incoming moisture, where upslope will locally increase rainfall amounts. Instability will be a limiting factor, due mostly to extensive cloud cover and the weakening October sun. Nonetheless, since many of the areas of the Southwest are simply poorly equipped to handle a surge of tropical moisture, a large Marginal was expanded across Utah to as far north as southeastern Idaho. Further north into the Yellowstone region, enough cold air will be in place to favor snow, which cuts the flooding risk significantly. Wegman Day 4 and Day 5 Valid 12Z Sun Oct 11 2026 - 12Z Tue Oct 13 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM SOUTHEAST CALIFORNIA INTO SOUTHERN NEVADA, WESTERN ARIZONA AND SOUTHERN UTAH... 2030Z Update...Given latest guidance from NHC on the track of Rachel, the previously issued Slight Risk encompassed the track and the heavier rainfall amounts shown by the envelope of model QPF. There was a northward nudge to the northern boundary of the Marginal based on the latest WPC QPF. Bann Previous discussion... Day 4 (12Z Sun - 12Z Mon)... The closed low off the southern CA coast will help push Rachel or its extra-tropical remnants northeastward into southern CA/western AZ and the southern Great Basin. While confidence is not high on the models handling of whatever surface center still remains, there is higher confidence that anomalous PW values will push northeastward day 4 across Southern CA into the Southwest and Great Basin. A fairly broad slight risk for day 4 was maintained across southeast CA/western AZ into southern NV and southern UT where model consensus is for widespread heavy rainfall potential. The previous slight risk was expanded farther into southern UT to include the sensitive slot canyon regions. Across the coastal Mid-Atlantic...the extra-tropical remnants of Isaias will continue to weaken day 4. While heavy rain is possible day 4, believe it will be over several hour period, precluding any significant flash flood threat. A marginal risk was maintained primarily for the urban corridor from NYC to DC....decreasing the western extent of it. The marginal risk was extended farther south into eastern NC where there is potential for convection along and ahead of the associated eastward moving cold front. Day 5 (12Z Mon - 12Z Tue) The mid to upper level trof along the West coast expected to amplify further Monday into Tuesday. This will help to draw Simon northward into western Mexico with anomalous PW values 3 to 5+ standard deviations above the mean moving into the Southwest and Southern High Plains. A marginal risk is depicted day 5 where there will be modest instability in the anomalous PW axis. Additional moderate precip totals also likely day 5 from Southern CA, northeastward into the Great Basin, but with more stable conditions expected, rainfall rates will likely be lower and no risk area depicted at this time. Oravec Day 1 threat area: https://www.wpc.ncep.noaa.gov/qpf/94epoints.txt Day 2 threat area: https://www.wpc.ncep.noaa.gov/qpf/98epoints.txt Day 3 threat area: https://www.wpc.ncep.noaa.gov/qpf/99epoints.txt $$