FOUS30 KWBC 271533 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1133 AM EDT Sun Sep 27 2026 Day 1 Valid 16Z Sun Sep 27 2026 - 12Z Mon Sep 28 2026 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PARTS OF THE NORTHEAST, PARTS OF THE HILL COUNTRY TO NORTH-CENTRAL TEXAS, AS WELL AS PARTS OF ARIZONA... ...16Z Update... ...Northeast... In coordination with OKX/Upton, NY and BOX/Taunton, MA forecast offices, the Slight Risk was dropped with this update. The Nor'easter continues across much of the Northeast this morning. The storm has begun weakening, with any heavier rainfall concentrated in rather narrow bands. The most significant is the one southeast of Boston, which is nearly stationary. Even within that band, rates are between 1/2 and 3/4 inch per hour. More bands are expected to impact the NYC metro, northern NJ, and southern New England late this afternoon through tonight, but they too are not expected to feature much heavy rainfall. Thus, the flash flooding threat is likely to be isolated at best, leading to the Marginal Risk downgrade. ...Texas... A surge of moisture, the leading edge of a multi-day low level jet which will fuel a multi-day heavy rainfall event in this area, will begin to advance northward from off the Gulf and into the Southern Plains today. Upper level ridging and a distinct lack of forcing will put a cap on any significant rains today. However, along frontal boundary/interface, a few isolated storms can't be ruled out, especially along the Balcones Escarpment, where topography oriented perpendicular to the southeasterly flow may allow for a stationary storm or two. Into north central Texas, it will be the warm frontal interface having to do all the work to cause any storms capable of flash flooding. Soils remain well drier than normal, so this is a very low-end Marginal, with only an isolated instance of flash flooding or 2 possible. The inherited Marginal was left largely unchanged, save for a few very minor tweaks in southern Oklahoma. ...Arizona... The beginning of a surge of moisture associated with the upper half of East Pacific Tropical Storm Odalys will move into the Southwest today. Ingredients for a multi-day heavy rain and flash flooding event across the Southwest, which will unfold over the coming days, will only begin to come together today. Thus, while widely scattered showers and storms are likely, resultant flash flooding should be isolated at best. No significant changes were made from inherited. Wegman ...Previous Discussion... ...New Jersey, Southern New York, and Southern to Central New England... A slow-moving nor'easter lingering along the coast near northern New Jersey and eastern Long Island will continue to funnel moisture across southern New England, southern New York, and New Jersey. Persistent northerly flow will keep instability low, limiting instantaneous rainfall rates; however, saturated soils from coastal inundation and prior heavy rains will maintain an elevated risk for flooding, especially in urban and poor drainage areas. A Slight Risk was maintained from eastern Massachusetts to the NYC Metro where additional rainfall amounts of 1-3 inches, with isolated heavier amounts, are expected. A Marginal Risk remains farther north into parts of Upstate New York and central New England, where orographic effects could help support some heavier totals. ...Southern Plains... The overnight CAMs guidance is showing some signal for localized heavy amounts developing along a ribbon of higher PWs extending from the Texas Hill Country into south-central Oklahoma. The HREF and REFS guidance indicate some potential for localized amounts of 2+ inches and some modest probabilities for amounts exceeding 3-hr FFGs, especially around the Texas Hill Country. Therefore, a Marginal Risk was introduced for this area. ...Southwest... Flow is expected to turn southwest ahead of a trough digging into California, which will begin to draw moisture from Hurricane Polo into the region. Overall, conditions are expected to remain relatively quiet, in what is expected to be last day before conditions greatly deteriorate across the the region. However, there is a low-end threat for isolated flooding, especially across portions of northern Arizona, where favorable mid-to-upper level dynamics may interact with the deepening moisture. Pereira Day 2 Valid 12Z Mon Sep 28 2026 - 12Z Tue Sep 29 2026 ...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE SOUTHWEST/FOUR CORNERS... ...Southwest/Four Corners... A predecessor rainfall event (PRE) is likely to impact much of the region beginning by midday and continue into the end of the period. Moisture preceding the arrival of the primary moisture plume associated with Hurricane Polo will continue to spread across the region as it is drawn northeastward ahead of a mid-to-upper level low digging south across California toward the lower Colorado Basin. As PWs increase to 2-3.5 std dev above normal across much of the region, strong upper level divergence combined with amplified southwesterly flow orthogonal to the terrain will support significant synoptic and orographic forcing. A limiting factor will be instability, hampered by thick cloud cover and shortening daylight. However, the strong forcing and anomalous moisture is likely to support very efficient warm-rain processes that will compensate for the limited CAPE. A broad Moderate Risk remains in place -- covering the complex terrain and burn scars of southern Utah, southwestern Colorado, as well as much of northern and central Arizona and New Mexico. One smaller adjustment this issuance was to extend the Moderate Risk a bit farther southeast into the extremely vulnerable areas of southeastern New Mexico, where soils remain nearly saturated from recent rounds of heavy rain. ...Dakotas/Northwestern Minnesota... Coupled upper jet forcing along with deepening moisture (PWs ~1.5 inches) on the nose of a southerly low level jet will promote training showers and storms across portions of the eastern Dakotas into northwestern Minnesota. A Slight Risk was maintained from northeastern North Dakota into northwestern Minnesota, where there remains a good model signal for widespread totals of 1-2 inches, with locally heavier totals. Pereira Day 3 Valid 12Z Tue Sep 29 2026 - 12Z Wed Sep 30 2026 ...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FROM THE SOUTHWEST INTO THE SOUTHERN HIGH PLAINS... ...Southwest to the Central Plains... Widespread heavy rain and potential numerous instances of flash flooding are expected to continue across portions of the Southwest while spreading into the southern High Plains this period. With some lingering differences in the details, the majority of the guidance shows the remnant mid-to-upper level energy associated with Tropical Cyclone Polo becoming incorporated within the upper trough moving across the Southwest. As the trough moves east, this will bring the better forcing and focus for heavier rains farther southeast into southeastern Arizona and New Mexico, including the extremely vulnerable southeastern parts of New Mexico. As the trough moves and east, a strong low level jet is forecast to develop from eastern Mexico to the central Great Plains, contributing an additional influx of deep moisture to parts of the southern and central Plains. The overnight guidance generally indicated the heaviest amounts of the period will focus over the Texas Panhandle-Northwest Texas region as this moisture interacts with increasing ascent afforded by strong mid-to-upper level forcing. This evolution of events continues to warrant a broad Moderate Risk that was maintained from southeastern Arizona to the Panhandle Region. A Slight Risk was also maintained farther to the northeast across parts of the central Plains into eastern Iowa, where moisture from the south pooling ahead of a surface-to-low level cold front and ahead of the remnant tropical system will support the growing potential for heavy rainfall. Pereira Day 4 and Day 5 Valid 12Z Wed Sep 30 2026 - 12Z Fri Oct 02 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PARTS OF THE SOUTHWEST, SOUTHERN AND CENTRAL PLAINS, AND PORTIONS OF THE MISSISSIPPI VALLEY WEDNESDAY... ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PARTS THE SOUTHERN CENTRAL PLAINS AND LOWER MISSISSIPPI VALLEY THURSDAY... Tropically enhanced heavy rainfall will spread from the Southwest, Southern/Central Plains and into the Mississippi Valley. The threat for excessive rainfall will remain elevated given areas of overlapping heavy rain footprints from prior days. Locations particularly sensitive will be dry washes, burn scars, slot canyons, and areas of higher terrain. 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 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 ECMWF EFI continues to signal a potentially significant rainfall event, particularly on Tuesday across eastern AZ and western-central NM. A broad Slight Risk for Arizona/Texas to Iowa/Illinois on Wednesday and from Texas to the Arkansas/Missouri state line on Thursday. 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 $$