194 FOUS30 KWBC 071950 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 350 PM EDT Wed Oct 7 2026 Day 1 Valid 16Z Wed Oct 07 2026 - 12Z Thu Oct 08 2026 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR THE LOWER TEXAS COAST AND MUCH OF THE FLORIDA PENINSULA... 16Z Update... ...Lower Texas Coast... A small Marginal Risk was added along the southern portion of the Lower Texas Coast where persistent northeasterly flow is expected to support streaming showers and thunderstorms that may produce isolated heavy accumulations and runoff concerns. The Marginal Risk highlights the area where the HREF shows high probabilities for amounts exceeding 3 inches through the remainder of the period. Also within this area, the HREF shows some 20-40 percent probabilities for amounts exceeding 3-hr FFGs later this morning into the early afternoon. Pereira Previous discussion... ...Florida... A very slow moving, but strong front will remain draped across the Florida Peninsula today, specifically over the northern Florida Peninsula. A tropical air mass to the south of the front, over the Peninsula, characterized by PWATs above 2.25 inches will be uplifted against a much cooler and drier air mass to the north. This clash of air masses will promote a continued rinse- and- repeat pattern, with widespread light to moderate rainfall continuing along I-4, and generally south of I-10. As on previous days, localized convection will remain possible. Where convection forms, the abundance of moisture will support locally heavy rainfall. The sluggish movement of any convection will prolong any associated heavy rainfall. Thus, a Marginal Risk remains in place for the possibility that any convection that develops will impact an urban or specifically flood-prone area. ...Four Corners Region... Widely scattered showers and a few thunderstorms are possible across the Four Corners Region this afternoon. Instability values are expected to peak on either side of 500 J/kg, which could allow for any storms that form to produce locally heavy rainfall. Convective coverage appears to remain isolated, so the subsequent flash flooding potential appears to be sub-Marginal, but the area will need to be monitored. Wegman Day 2 Valid 12Z Thu Oct 08 2026 - 12Z Fri Oct 09 2026 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR MUCH OF FLORIDA... 20Z Update... No significant changes were made to the previous forecast. Pereira Previous discussion... A stationary front draped across north Florida to start the day on Thursday will begin to retreat northwestward into the Southeast through Thursday night. Increasing southeasterly flow ahead of the circulation of T.S. Isaias will overcome any reinforcement of the drier air to the north, allowing the front to begin moving. Tropical air to the south of the front running into it will allow for the continuation of light to moderate rain across much of the Florida Peninsula. As on previous days, and with increasing moisture advection, localized convection and still generally slow storm movements will make for an isolated flash flooding threat. Late Thursday night, the outermost rain bands associated with T.S. Isaias will begin to sweep northwestward across the Apalachicola area, which could lead to an isolated flash flooding threat. Wegman Day 3 Valid 12Z Fri Oct 09 2026 - 12Z Sat Oct 10 2026 ...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR FAR SOUTHERN ALABAMA AND THE WESTERN FLORIDA PANHANDLE... 20Z Update... The daytime update continued some of the overnight adjustments -- most notably trimming away some of the western extent of previous outlook areas and creating a tighter gradient. This is a reflection of a slight eastern shift in the preferred track guidance and the model indication of drier air wrapping around the west side of the system as it moves inland. The Moderate Risk was maintained along the Gulf Coast and expanded a little farther north and east to cover a larger portion of the Florida Panhandle and southern Alabama. Following landfall, this area is expected to remain a focus of heavy rainfall that will be supported by a long axis of onshore flow remain centered over the area into early Saturday. Pereira Previous discussion... The circulation of T.S. Isaias, likely as a hurricane, will approach the Gulf Coast, make landfall, and move inland across the Southeast Friday and Friday night. See the latest forecast from the National Hurricane Center at www.hurricanes.gov on the track forecast. Rainfall-wise, increasing southwesterly shear over the storm and an accelerated forward motion will make the rainfall shield along and east of the track become increasingly lopsided, leaving a sharp gradient of rainfall on the west side. With continued eastward nudges to the track, implying a faster forward motion, the inherited Day 3/Friday forecast was significantly changed with this update. The Slight Risk was expanded well to the north, now including Huntsville, AL and Atlanta. Meanwhile, New Orleans, and all of Louisiana for that matter, look to dodge this cannon ball. Thus, the Louisiana Gulf Coast was downgraded to a Marginal, and will likely to continue to be trimmed with future updates. A Moderate Risk was introduced with this update for far southern Alabama, including Mobile, as well as the western Florida Panhandle. It remains probable with the aforementioned eastward shifts in the track, that the Moderate may be too far west, and the Mobile area may be able to be dropped from the Moderate with future updates. Conversely, the Moderate will likely be expanded north with future updates to include much of central Alabama and possibly western Georgia. The Moderate was drawn to focus on the Gulf Coast for now as this area has been included in all of the guidance for getting hit with very heavy rainfall associated with the core eyewall of the storm. There remains a contingent of guidance that keeps the storm further west and with a slower forward motion. While this appears to be increasingly unlikely, to account for this possibility, the Moderate was limited in its northward extent in case the storm takes its time making it to the coast. A faster forward speed may necessitate a much higher flooding threat with future updates for portions of the highly flood-prone southern Appalachians. As the center of circulation tracks due north into the Tennessee and Ohio Valleys, an upslope southeasterly flow into the western Carolinas and north Georgia could pose a significant flash flooding threat for that region, though for now that threat is greatest in the Day 4/Saturday time frame. Wegman Day 4 and Day 5 Valid 12Z Sat Oct 10 2026 - 12Z Mon Oct 12 2026 ...THERE ARE SLIGHT RISKS OF EXCESSIVE RAINFALL ACROSS PARTS OF THE TENNESSEE VALLEY AND SOUTHEAST SATURDAY, AND THEN ACROSS PARTS OF THE SOUTHWEST ON SUNDAY... 20z Update: Only minor changes needed to the risk areas associated with Isaias. The Slight risk was combined across portions of MS/AL/TN to account for some eastward trend in the storm track and uncertainty on the track with this inland extent. A higher end Slight risk remains for portions of the southern Appalachians. It does look like some instability gets into these areas, which combined with upslope enhancement, should lead to an area of 3-5" and scattered flash flooding. Some of this could be locally significant, but the forward speed of the system will hopefully keep more widespread higher end impacts at bay. At the moment NBM probabilities of exceeding 5" are pretty low, supporting holding at higher end Slight. Can not rule out an eventual MDT risk upgrade, especially if rainfall amounts trend more towards ~6-8", which while a low chance at the moment, is still a possibility given the degree of moisture transport into the terrain...so we will continue to monitor. A slow moving band of convection may extend south across GA and into the FL Panhandle on saturday. This could lead to locally significant flood impacts, but too much uncertainty on location and magnitude to justify anything higher than a Slight risk at this time. Out West, we added a Marginal risk to AZ on day 4 (Saturday). Isolated to scattered convection appears likely as higher moisture begins to advect into the area. What remains of Rachel and the abundant tropical moisture associated with it, will impact more of the Southwest on Sunday. New models remain in line with the overnight forecast, so little to no change was needed to the inherited Slight risk area on day 5 (Sunday). Chenard ...Previous Discussion... Day 4 (12Z Sat-12Z Sun) -- There was a significant departure from yesterday's Day 5 ERO, largely due to the trend in the guidance (led by the AI ECMWF and GFS Guidance), which align with the official NHC forecast track of T.S. Isaias. There are three main areas of concern (thus 3 separate Slight Risk areas) -- one across northern MS-western TN and far NW AL, associated along and just west of track of Isaias's circulation which by that time will be going through extratropical transition with sharpening mid-level deformation on the western side. The other areas include the south and east facing slopes of the southern Appalachians with the strong upslope south-southeast flow (850 mb wind anomalies some 5+ standard deviations above normal) along with the high-PW environment east of Isaias's remnant low (PWs ~2.5 standard deviations above normal per the GEFS). Over this area, the ERO risk a bit more concerning with much of the guidance indicating amounts between 2.5-5.0+ inches on day 4, which is why we included a 25% area or "high-end Slight" in the ERO. This area will need to be monitored for a potential upgrade to Moderate Risk in later outlooks. Lastly for Day 4, included another separate Slight Risk along the Carolina coast, given the high PW environment (PWs up to 2.25-2.5") along with an uptick in deep-layer instability with strengthening trajectories advecting off the Gulf Stream. Day 5 (12Z Sun-12Z Mon) -- The Eastern U.S. will not the only part of the country that will be affected by a remnant tropical cyclone. Much of the global guidance, led by both the GFS, ECMWF and their AI versions, show the remnants of Rachel having an impact across parts of the Southwest into the Lower Great Basin. The latest GEFS/ECENS/CMC ensemble output show the highest QPF exceedance probabilities across southeast CA-southern NV into southwest UT and western AZ, where PW anomalies peak between 4-5 standard deviations above normal while 850 mb wind and 850-700 mb moisture flux standardized anomalies reach +5. Most certainly sufficient multi-model support for at least a Slight Risk (which is supported by the GEFS first-guess ERO), however for now have refrained from including anything higher given (a) the event still over 4 days out and (b) the degree of instability (and thus rainfall rates) is too much of a unknown at this point. Hurley 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 $$