851 FOUS30 KWBC 092021 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 421 PM EDT Fri Oct 9 2026 Day 1 Valid 16Z Fri Oct 09 2026 - 12Z Sat Oct 10 2026 ...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL IN SOUTHERN ALABAMA, THE FLORIDA PANHANDLE, AND SOUTHWEST GEORGIA... 16Z update... The latest hi-res guidance is suggesting an increase in QPF amounts inland across Florida, Alabama and eastern Mississippi as Isaias advances onshore, and into Georgia with the convective feeder band that is expected to the east of the low center. As of now that feeder band looks to be in the vicinity of the Big Bend region. Rain rates with the immediate landfall may be as high as 3-4 inches/hour before reducing to 1-3 inches/hour during the overnight period as it tracks further north across the Gulf states/Coastal Plains. The Moderate Risk was expanded into central Alabama and across southwest Georgia. The high-end Slight Risk was expanded into northwest Alabama and west-central Georgia. Additional convection is expected along the coastline from northeast Florida to South Carolina as tropical moisture advecting northward interacts with the frontal boundary draped to the north. Cells will be capable of yielding 1-2+ inches/hour rain rates along this urban corridor. A Slight Risk was added to cover this elevated threat for localized excessive rainfall and flash flooding. Campbell Previous discussion... Hurricane Isaias is expected to make landfall on the Central Gulf Coast later today, bringing a threat of heavy rainfall and flash flooding to the region. Numerous instances of flash flooding are likely, some of which will be significant. Overall, the ERO for this forecast period is very consistent with the previous forecast. See www.hurricanes.gov for the latest forecast information on all hazards associated with Isaias. The northern fringes of the rain shield associated with Isaias are already beginning to approach the Gulf Coast, but the threat of heavier rain rates will ramp up closer to the time of landfall and thereafter, as the region of strongest instability in the southeast quadrant of the system will also begin to reach the coast. Thus, the threat of flash flooding should begin to increase markedly in the latter half of the forecast period, or from late Friday afternoon into Friday night. The primary areas of concern are related to (1) higher rain rates in the inner core and any eyewall convection that can be sustained as Isaias pushes onshore, and (2) a convective rain band to the east along the instability gradient. As Isaias advances inland, the inner core convection should diminish and rain rates near the center should decrease with a lack of available instability. Therefore, the threat of flash flooding near the track of the center will be highest closest to the coast, particularly in the Moderate Risk area of southern Alabama and the western Florida Panhandle. Heavy rain and flash flooding may extend along the track of the center further north and northwest into eastern Mississippi and central Alabama, but this will be highly dependent on the track and overall organization of the system as it reaches the coast. Shear will be increasing on Isaias; if it can resist that shear and maintain a well-organized inner core, the flash flood threat associated with the center may extend further inland. For now the risk level in these areas further inland has been held at a Slight Risk given the uncertainties. Further east, a training band of convection is expected. This band will have much greater access to instability, and the combination of that with extremely deep tropical moisture will promote very efficient rainfall and very high rain rates. If the rain band trains in a specific location for several hours, that could easily produce double digit rainfall totals (10+ inches), and potentially the event maximum storm total rainfall, as well as significant flash flooding. This would be about 200 miles east of the landfall location, so people in the threat area are encouraged not to concentrate too much on the landfall location to understand the rainfall threat from Isaias. The main changes to the ERO in these areas were to nudge the risk contours slightly to the east to account for guidance that suggests the rain band could set up even east of Tallahassee at times, and increase the area of the Moderate Risk into southwest Georgia as it is likely the rain band will affect areas at least 100 miles inland (possibly more). Lamers 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, sso 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 $$