FOUS30 KWBC 100059 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 859 PM EDT Fri Oct 9 2026 Day 1 Valid 01Z Sat Oct 10 2026 - 12Z Sat Oct 10 2026 ...THERE IS A HIGH RISK OF EXCESSIVE RAINFALL FROM THE FLORIDA BIG BEND INTO SOUTHERN GEORGIA... Weakening Hurricane Isaias is expected to make landfall on the Central Gulf Coast shortly not far from Eglin Air Force Base, bringing a threat of heavy rainfall and flash flooding to the region. Numerous instances of flash flooding are likely with this cyclone, some of which will be significant. Core convection should continue to diminish and rain rates north of the center should decrease with a lack of available instability. The threat of flash flooding near the track of the center for some distance inland lies from the FL Panhandle into southern Alabama where a Moderate Risk continues to exist. Gradients on the western edge across southwest AL and MS were tightened, based on recent mesoscale guidance and convective trends thus far. Further east across the FL Big Bend, a training Band Of Deluge is poised to be efficient from roughly 06z onward, whose incipient form appears to have two foci -- one east of Apalachicola which is moving eastward and another broken shower/thunderstorm band that lies from offshore northwest FL into the Big Bend. This band has much greater access to instability (2000+ J/kg of ML CAPE) and more than sufficient effective bulk shear (25-50 kts) to organize. Such bands have produced hourly amounts in the 3-6" range in other, previous tropical cyclones in and near the Gulf Coast, and there's little reason to believe this band will be any different. If the rain band trains in a specific location for several hours -- as hinted at by both the 18z HREF and 12z REFS -- double digit rainfall totals will be possible (10+ inches), potentially the event maximum storm total rainfall, as well as significant flash flooding. While there has been recent dryness in the region and the geology of the area favors quick percolation into the local aquifer, 3-6" in an hour would be extremely hazardous for any towns/transportation routes across the region. The signal in the 12z REFS/18z HREF along with conceptual models of such events in the past led to coordination with the Tallahassee/TAE Forecast Office to raise to a High Risk within this potentially dangerous rain band. Roth Day 2 Valid 12Z Sat Oct 10 2026 - 12Z Sun Oct 11 2026 ...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE SOUTHEAST UNITED STATES... ...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF SOUTHERN CALIFORNIA, NORTHWEST ARIZONA, SOUTHERN NEVADA AND SOUTHWEST UTAH... 21Z update... ...Southwest (Rachel)... A Moderate Risk area was raised for the Peninsular Range and adjacent area as tropically enhanced rainfall ahead of Rachel makes its way across southern California. Rainfall rates of 1-3 inches will be possible, particularly for the mountains of southern California. ...Southeast (Isaias)... A Moderate Risk of Excessive Rainfall was introduced for portions of the western Carolinas southward into northern Georgia given latest QPF trends. Guidance shows persistent south-southeasterly flow up against the higher terrain to help bolster the already tropically enhanced rainfall from Isaias as it. Neighborhood probabilities for reaching/exceeding 5 inches is over 70% for much of this part of the region. There is also a 60-70% likelihood that somewhere within that same area could reach/exceed localized 8 inches, so confidence is fairly high that there will be numerous areas with flash flooding from north- central Georgia to western North Carolina. The exact location will be dependent on the track of Isaias and its transition to being an extratropical feature. Campbell Previous Forecast Discussion... The threat of heavy rainfall and flash flooding from Isaias will continue over a large portion of the Southeast on Saturday and Saturday Night. The latest outlook is very consistent with the previous forecast, with mostly small changes to account for the latest guidance and updated forecasts from NHC. See www.hurricanes.gov for the latest forecast information on all hazards associated with Isaias. Although the Slight Risk area is large, there are several areas of note where the threat is somewhat higher (on the high end of the 15 to 40 percent probabilistic range associated with that risk category), and could warrant an upgrade to a Moderate Risk in the future if guidance becomes more consistent and/or the potential for locally significant rainfall and flash flooding increases. 1. Convective rain band east of the center of Isaias (Florida Big Bend into much of Georgia). There is high confidence in a convective rain band setting up about 200 miles east of the track of the center of Isaias. This will ramp up Friday Night and likely be ongoing on Saturday morning. Given the access to moderate instability and precipitable water values approaching 2.5 inches, the rain rates could be intense at times (2-3 inches per hour). The key uncertainties are how far north the strongest convection will be able to reach, and how stationary the band will be overall. The 00Z HRRR suggested some northern limit to the extent of the instability due to a wedge of cooler air associated with a surface ridge and northeasterly low-level flow from the Piedmont of the Carolinas down into northern Georgia. However, other models suggest the effective warm front will lift further north and bring locations near Atlanta under risk of greater rain rates. This will be monitored for the need for a risk upgrade. 2. Southern Appalachians (far northeast Georgia, far upstate South Carolina, western North Carolina). Model QPF has been increasing in general in this region; for example, the NBM QPF in the upslope areas of western North Carolina has increased from a general 1-3 inches to 3-5 inches in the past 12 hours of updated runs. A contributing factor is the event coming into view of the hi-res model runs, of which many are showing more terrain enhancement and a wetter forecast, but there are some overall adjustments to the ensemble envelope with some trends in the overall forecast of Isaias. Not all hi-res models are suggestive of the highest rain rates reaching this region -- the 00Z HRRR kept instability suppressed further south with a wedge of cooler air in place closer to the mountains. However, other hi-res models are beginning to suggest double digit totals are not out of the question depending on the mesoscale details (particularly strength and orientation of the low-mid level flow and available instability). It is noted here that a Moderate Risk upgrade was strongly considered, but after collaboration with WFO GSP and the uncertainties described here, it was held at a high-end Slight Risk for now. Continued model trends in a wetter direction and reduction in rainfall uncertainty would likely drive an upgrade later today. 3. Coastal Carolinas. Significant coastal convergence will favor occasional clusters or bands of low-topped convection capable of producing efficient rainfall along the coastline of the Carolinas. Although this area has been relatively dry of late, sustained heavy rain rates could overcome unfavorable antecedent conditions to produce a more concentrated area of heavy rainfall and flash flooding. Additionally, hi-res ensembles (HREF and REFS) do place the axis of heaviest rainfall offshore, although it would only take a small displacement error to put this over the coastline. Therefore, this area will also be monitored for a Moderate Risk, especially with potential complications from higher than average tides expected this weekend. ...Southwest (Moisture from Rachel)... Scattered convection may occur during the daytime on Saturday over the Southwest as precipitable water values increase in the tropical moisture plume downstream of Rachel over the eastern Pacific. By Saturday afternoon PWs over much of the region should be over the 99th percentile for early-mid October. However, the scattered nature of the convection and lack of a prominent focusing mechanism may keep the individual convective clusters or cells moving enough and isolated enough to prevent more widespread instances of flash flooding despite the arrival of the deep moisture. Later in the forecast period -- on Saturday night -- the threat may begin to ramp up in a more concentrated fashion from southeast California into southern Nevada and northwest Arizona as Rachel approaches the Pacific coast of Baja California. A Slight Risk was added in these areas, as the RRFS and REFS members show potential for a greater concentration of stronger convective bands and clusters in an area of increasing convergence. This will likely continue into the Day 3 period. Lamers Day 3 Valid 12Z Sun Oct 11 2026 - 12Z Mon Oct 12 2026 ...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL IN THE SOUTHWEST... 21Z update... Remnant energy of Rachel and tropically enhanced rainfall will be lifting northeast during this period spreading from southern California into the Great Basin and toward Central and Northern Rockies. The greatest amounts will likely be in proximity to the low track from southern California to northeast Utah/southwest Wyoming where a Moderate Risk was raised for this update. Campbell Previous discussion... Regardless of the exact timing of the post-tropical transition of Rachel as it approaches the Pacific coast and then moves into the southwest U.S., the ingredients will be there for heavy rainfall regardless. A broad Slight Risk was maintained over the region with deep tropical moisture in place and models showing increasing instability during the day on Sunday, particularly over the low desert regions of southeast California, southern and western Arizona, and southern Nevada. Additionally, increasing low-mid level convergence just ahead of the track of Rachel (or eventually the remnant circulation aloft), coinciding with the combination of instability and deep moisture, should lead to a focused area of convection. Some training of heavier rain rates may occur and lead to narrow corridors of more significant rainfall and flash flood potential. The area of greatest concern is from Riverside County in southeast California into northwest Arizona, southern Nevada (including the Las Vegas area), and then into the slot canyon regions of southwest Utah. There is still some uncertainty as to the exact track of Rachel, so that will play a key role in the placement of the heaviest rainfall. However, as uncertainty is reduced, it's possible a Moderate Risk upgrade could be required in the highest confidence areas. Lingering rainfall associated with Isaias will be will persist along the Mid-Atlantic region and may lead to isolated areas of flash flooding to sensitive areas. A Marginal Risk area was maintained. Lamers Day 4 and Day 5 Valid 12Z Mon Oct 12 2026 - 12Z Wed Oct 14 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF ARIZONA INTO UTAH MONDAY INTO EARLY WEDNESDAY... West... The combination of Rachel's prior movement into the region and an amplified upper level trough closing off aloft while moving through the Great Basin imports relatively high moisture (anomalous for mid- October) across portions of the Southwest & Great Basin. Enough moisture (0.5-0.75" with sub 5610 m 1000-500 hPa thickness) and low-level frontogenesis (650-850 hPa layer) should be present for heavy rain concerns. The degree of MU CAPE is expected to be in the 100-250 J/kg range, which on its own would support hourly rain amounts up to 1-1.25", which would be problematic in box/slot canyons, burn scars, arroyos/dry washes, and lead to at least some ponding in urban areas. The day 4/Monday-Monday night Slight Risk remains close to continuity, while a new day 5/Tuesday-Tuesday night Slight Risk has been introduced. Southern High Plains into the Central Plains... On Tuesday and Tuesday night, a confluent/convergent 850 hPa boundary moves through the region in the warm advection pattern east of the Western upper-level trough. Precipitable water values rise to 1.5-2" and MU CAPE of 250-750 J/kg are forecast for the region, which when combined with sufficient effective bulk shear would help support hourly rain amounts to 2.5". The main problem is the movement of the low-level focus, which should keep amounts from getting especially excessive. Still, there is a chance that as we move closer to the event, into the mesoscale guidance window, that a Slight Risk could become necessary, particularly near and just north of the Red River of the South Tuesday night into early Wednesday. Roth 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 $$