FOUS30 KWBC 270059 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 859 PM EDT Sat Sep 26 2026 Day 1 Valid 01Z Sun Sep 27 2026 - 12Z Sun Sep 27 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PARTS OF THE COASTAL NORTHEAST AND IOWA... 0100Z Update: Some adjustments were made to the Slight Risk areas across the Midwest and along the Northeast Coast. These updates were largely based on observational trends (radar, satellite, mesoanalysis), and also from most recent CAM guidance, including the 18Z HREF and REFS QPF exceedance probabilities. Because of this, the Slight Risk area shifted a little farther south and east across Iowa and northern Missouri. Across the Northeast Coast, rainfall rates and thus the flash flood threat will remain limited to areas right along the coast, especially in urban areas, where some (albeit weak) elevated instability will be present. This would explain the narrow corridor of both the Slight and Marginal Risk areas. Hurley Previous discussion below.. ...Coastal Northeast... A large Nor'easter continues to snail crawl off the coast of Delaware/southern Jersey this morning. The combination of the strength of the Nor'easter (986 mb as of this writing), its snail crawl movement, and a full moon have all worked to worsen coastal flooding concerns. While the most rain from the storm will also fall along the coast, coastal and tidal flooding are likely to be substantially more impactful than any rainfall flooding. Nonetheless, at high tide, runoff from the rainfall trying to drain into already full bays and inlets will worsen local flooding. The Slight Risk is largely unchanged, save for small expansions of the Slight to include urban northern New Jersey as well as urban central Connecticut. Otherwise, the inherited higher end Slight for eastern Mass, Rhode Island, and eastern Long Island was dropped with this update. All of the Northeast is on the cold/northwest side of the storm. Thus, instability needed to generate heavy rain is sorely lacking. This storm has and continues to be a prolonged light to moderate rain event, not a heavy one. Thus, while urban areas may still struggle to keep up with the combo of prolonged rain and tides (where applicable), the flood from rainfall threat is low with this storm relative to several other meteorological threats, such as coastal/tidal flooding and high winds. ...Iowa... The Slight Risk for central and southwestern Iowa was left unchanged with this update. Several of the high-resolution guidances suggest showers and storms will impact the area on the nose of a low level jet this evening and tonight. The area has wet soils from previous rainfall, which has lowered FFGs to the 1-1.5 in/hr range. Instability will be the primary limiting factor, with values struggling to exceed 500 J/kg. Moisture transport with the LLJ should be able to overcome this, given the lowered FFGs, so a low-end Slight still looks reasonable. Wegman Day 2 Valid 12Z Sun Sep 27 2026 - 12Z Mon Sep 28 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PARTS OF THE COASTAL NORTHEAST... ...2030Z Update... No significant changes were made to any of the risk areas. The Noreaster will continue spinning off the coast of Cape Cod, bringing soaking rains to eastern New England through NYC and the Jersey shore. With the storm's center east of New England, the northerly flow will continue to keep instability low, hampering any chances of heavy rain occurring anywhere. Thus, this remains a long-duration light to moderate rainfall event. Most areas outside of urban centers can generally handle a slow-falling rain. Thus, the Slight remains primarily for urban and poor-drainage areas. A Marginal Risk remains in North Dakota. It's a low-end Marginal, as any storms will occur over a relatively flat, very rural, and generally sandy portion of the Plains, all of which work against any significant chances for flooding. In the Southwest, the Marginal Risk remains largely in place, other than cutting out a portion of southeast California due to an eastward shift with the heaviest rain. It will very much be the calm before the storm, as the moisture plume with Polo/Odalys will be making its way north up the Baja California Peninsula. Wegman ...Previous Discussion... ...Northeast... The potent nor'easter will begin to weaken as it lingers in the vicinity of the northern Mid-Atlantic coast. How far inland, if at all, the storm drifts will help dictate how far beyond the coastal regions its associated heavy rains will reach. Heavy rainfall is expected to linger overhead, leading to an additional 1-2.5 inches of rainfall over much of the same area. These rains will overlap areas impacted during the Day 1 period thus maintaining an elevated threat for excessive rainfall and flash flooding concerns. A Slight Risk remains in effect from coastal New Jersey to eastern Massachuesetts with a Marginal from southern New Jersey to southern Maine. ...Southwest... Increasing southwesterly flow ahead of an amplifying trough digging south across California will begin to tap moisture and mid- to-upper level energy, supporting an increase in shower and thunderstorm activity and the potential for localized areas of flash flooding from Southern California into the Southwest. ...Northern Plains... A Marginal Risk area was raised for portions of the Northern Plains as a northern stream trough exits the Rockies and begins to tap into moisture over the plains. Rainfall rates exceeding 0.50 inch/hour may lead to isolated instances of flash flooding across the area. Campbell/Pereira Day 3 Valid 12Z Mon Sep 28 2026 - 12Z Tue Sep 29 2026 ...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR THE FOUR CORNERS REGION... ...2030Z Update... ...Southwest... A Predecessor Rainfall Event (PRE) will impact much of the 4 Corners Region on Monday. The rain will be preceding the arrival of the primary moisture plume associated with East Pacific Hurricane Polo. Some contribution from Hurricane Odalys can also be included, though it's likely Odalys will Fujiwhara around the stronger Polo, and therefore not fully merge with it. See the NHC forecast for more details. A large upper level trough and low will gradually drift southeastward through Monday. However, this trough will be the catalyst that draws the moisture plume northeastward into the Southwest. It will also provide the upper level cold air that will allow for convective activity to impact western portions of the risk area in Arizona, Nevada and southwest Utah. PWAT values will be 2-3 sigma above normal, or over an inch in some cases higher than climatology. This means there will be ample atmospheric moisture in place to be wrung out as heavy rain. Much of the Southwest remains in a very wet pattern. While wonderful for long-term drought improvement, the soils are now fully saturated over nearly the whole of New Mexico, spilling into portions of all the neighboring states. This means that the soils cannot take anymore rainfall. Thus, despite both today being dry and tomorrow only seeing negligible rainfall in this area, the soils will not be able to take much even by Monday before converting the rest to runoff. Much of the Southwest is a very flash-flood-prone area due to rocky soils, slot canyons, and a plethora of burn scars. Adding much heavier than normal amounts of rain to a landscape like this means major impacts are likely in the most prone areas. Instability will once again be a limiting factor as far as rainfall rates are concerned. Shortening daylight and extensive cloud cover will work against heavy rainfall. However, given very dynamic upper level support, well above normal levels of atmospheric moisture, nearly saturated soils to begin with, topographic effects and rocky soils, this should more than compensate for the above-noted limitations. Thus, in coordination with all the forecast offices from Phoenix and Vegas through southern Colorado and New Mexico, a large Moderate Risk area was introduced to cover the slot canyons and burn scars of southern Utah and southwest Colorado, and for the heavier rains across Arizona and New Mexico. The "main event" will be into Day 4/Wednesday. ...North Dakota/Northwestern Minnesota... A small Slight Risk was introduced with this update in coordination with FGF/Fargo-Grand Forks forecast office. The nose of the low level jet in that area will be uplifted by an approaching upper level shortwave, this will enhance lift and promote training thunderstorms across eastern North Dakota and northwestern Minnesota. The area has some history of flash flooding. While the area will struggle for instability, once again good forcing from aloft should work to compensate for that. ...New England... The Nor'easter impacting the area will slowly weaken and drift away from New England. Diminishing rainfall rates associated with the Nor'easter may still cause flooding issues into interior New England especially, where topography will enhance any ongoing flooding started in the prior days. A Marginal Risk remains in place, with a possibility for a small Slight that may be needed, especially across interior Massachusetts and far southern Vermont & New Hampshire, but that will largely depend on how impactful any flooding is from prior days, and any upgrade would likely be the continuation of an earlier Slight from Day 2/Sunday. Wegman ...Previous Discussion... ...Southwest, Southern Rockies, and adjacent High Plains... There will be a strong influx of tropical moisture from tropical cyclones Odalys and Polo directed into the Desert Southwest. This abundant moisture when coupled with the afternoon diurnal instability max will trigger scattered to widespread convection across the region. An interaction between the eventual closed upper low over California and what will be Polo as it begins moving northward towards Baja over the course of the weekend into early next week. The ensemble means for this time period continue to depict a broad expanse of heavier rainfall increasingly likely for the Southwest /Four Corners area due to the ascent pattern downstream of the upper low swinging into the region. PW anomalies during this period will be between 2-3.5 standard deviations above seasonal normal for late September. A broad Slight Risk for excessive rainfall and flash flooding remains in effect while a higher-end Slight Risk was raised for east-central Arizona and western New Mexico., ...Northern and Central Plains... Progressive northern stream trough coupled with prevalent moisture advection from the Southwest will lead to a period of convective concerns and isolated flash flood potential. There is a broad Marginal Risk in effect from northern Kansas to North Dakota/northwest Minnesota. Remnant energy from what is currently tropical cyclone Odalys will get drawn northeastward out of the Pacific prior to the period reaching the Northern and Central Plains Monday into early Tuesday leading to quite a larger coverage of convection over the above area. Areal coverage of convection will increase Tuesday as there will be a stronger PW surge with the remnant energy from Odalys. Mean QPF is currently between 1-2.5 inches with locally higher at 5-day leads, a testament to the threat in question as there will likely be more significant rainfall potential with this setup. ...New England... Lingering rainfall expected as the coastal low begins to drift away from the region. The cumulative effect of rainfall over the long period into the urban corridor of eastern Massachusetts will cause hydrologic concerns, including some flooding in poor drainage locations. A Marginal Risk area is in effect from eastern Connecticut up to coast to southern Maine. Campbell/Santorelli Day 4 and Day 5 Valid 12Z Tue Sep 29 2026 - 12Z Thu Oct 01 2026 ...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL OVER PARTS OF THE SOUTHWEST AND SOUTHERN HIGH PLAINS TUESDAY AND TUESDAY NIGHT... 2030Z Update: Per collaboration with the WFOs, the biggest change was adding the Moderate Risk area across eastern AZ, much of NM, and into the western portions of the Southern PLains. While the guidance is slowly converging on a solution, there is still spread with the location and (especially) timing of heaviest rain as Polo's remnant moisture plume becomes absorbed by the deepening SW US upper trough. Timing issues are most notable by Day 5 (12Z Wed-12Z Thu), with most of the 12Z operational global model runs and both AIGFS and EC AIFS slower (farther west with the heaviest QPF) than the NBM and ensemble mean QPFs. Not withstanding the timing uncertainties, the latest ECMWF EFI continues to signal a potentially significant rainfall event, particularly on Tuesday across eastern AZ and western-central NM. Hurley Previous discussion below... The eventual absorption of tropical cyclone Polo's remnants into an amplifying Southwest U.S. upper trough have led to a slower and wetter trend across the Southwest-Four Corners and into the High Plains by midweek. The latest suite of guidance persists with the spreading of tropically enhanced heavy rainfall from the Southwest into the Plains overlapping some areas with heavy rainfall in prior days increasing the threat for flash flooding particularly over sensitive washes, burn scars, slot canyons, and areas of higher terrain. A broad Slight Risk for Tuesday spans from Arizona to Texas and northeast to Iowa/Illinois and from Texas to Iowa/Illinois on Wednesday. This part of the country will continue to be monitored for QPF/track of remnant energies trends as they may impact future upgrades of the Risk levels. Campbell/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 $$