FOUS30 KWBC 282020 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 420 PM EDT Mon Sep 28 2026 Day 1 Valid 16Z Mon Sep 28 2026 - 12Z Tue Sep 29 2026 ...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE SOUTHWEST... ...16Z Update... A few smaller changes were made to the risk areas with this update. The Moderate Risk was expanded southwest down the Colorado River to include the CA/AZ/NV tripoint region in coordination with VEF/Las Vegas, NV forecast office. Numerous pieces of guidance suggest a small cluster or line of slow-moving thunderstorms will develop there later this afternoon during peak heating, driven by clearing skies there now allowing maximum heating there by this afternoon, with plentiful moisture available. The Marginal and Slight risk areas were expanded northward for many of the same reasons along the NV/UT border. Showers developing there now could develop into nearly stationary lines by this afternoon, leading to a higher flooding threat. Fortunately, much of the rainfall ongoing currently across Arizona has been stratiform in nature, due to the lack of instability. Well above normal levels of moisture and the flash-flood-prone nature of the area due to wet soils still favor flooding occurring, though it's on the low end of the MDT risk category. The very flashy nature of southern UT, with plentiful slot canyons and burn scars, the San Juan Mountains of Colorado oriented perpendicular to the southwest flow, and much of NM due to saturated antecedent conditions still appear on track, with no changes made. No changes were made to the Slight Risk area across ND/MN, as that event is ongoing. Wegman ...Previous Discussion... ...Southwest... The predecessor rainfall event (PRE), from the combination of Odalys and the approach of Polo is still expected to produce a widespread 1-3 inches of rain across the Four Corners Region, where most soils are already at or near saturation from recent heavy rainfall. 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. The Moderate Risk forks a bit further south in New Mexico to include the Lordsburg, NM area, where numerous burn scars are, and to include the rest of the Sacramento Mountains in south- central Colorado. The flow will be uniformly be out of the southwest, south and west facing slopes along the Mogollon Rim, and the San Juans are likely to see the most rainfall. Rocky soils, burn scars, and slot canyons will all locally exacerbate any flooding from the widespread rainfall. ...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. The latest model guidance and WPC forecast increased the amounts across central North Dakota. A Slight Risk was maintained with some expansions further southwest toward southern North Dakota. A Marginal Risk extends from northern South Dakota to the North Dakota and far northwest Minnesota. Campbell/Pereira Day 2 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... ...2030Z Update... Not too many major changes were needed for Tuesday's forecast. The remnants of Hurricane Polo will make their way out of Mexico, near El Paso, then track generally northward, roughly following the NM/TX border. The heaviest rains of the event are likely to follow along the track of the center. However, a much broader rainfall shield will continue to envelop the Southwest. Thus, the large Moderate Risk from southeast Arizona through southeast CO and extreme southwest KS remains largely unchanged. A higher-end Moderate Risk remains from about the middle of New Mexico to the northwest corner of the TX Panhandle; Cimarron County, Oklahoma; and the southeast corner of Colorado, including the Sangre de Cristos. The combination of nearly saturated soils, low FFGs, instability associated with Polo's center, as well as moisture influx from the south will all work to highlight this region as the area most likely to experience locally catastrophic flooding from the 2-4 inches (locally higher) of rain expected through the period. Changes with this update generally included trimming the eastern end of all the risk areas due to both late dry slotting, and the track of the center of Polo remaining west. Easterly flow out ahead of Polo continues to paint a risk of locally catastrophic flooding into the east facing slopes of the Sangre de Cristos in far southern Colorado. Elsewhere, changes were relatively minor. Guidance generally shifted back south and east across the Plains from Nebraska through Iowa, with a lack of instability along that warm front still suggesting that most rain that occurs here will be stratiform, and therefore limit the rates enough to avoid numerous and/or widespread cases of flash flooding. Wegman ...Previous Discussion... ...Southwest to the Upper Mississippi Valley... 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. Model consensus depicts 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. A strong low level jet is forecast to develop from eastern Mexico to the central Great Plains as the trough shifts eastward, contributing an additional influx of deep moisture to parts of the southern and Central Plains. The heaviest amounts of the period will focus over the juncture of the Texas Panhandle, northeast New Mexico and southeast Colorado as this moisture interacts with increasing ascent afforded by strong mid- to-upper level forcing. A broad Moderate Risk was maintained from south-central Arizona to the Panhandle Region and north into western Kansas. A Slight Risk was also maintained farther to the northeast across parts of the Central Plains into southern Minnesota, 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. Campbell/Pereira Day 3 Valid 12Z Wed Sep 30 2026 - 12Z Thu Oct 01 2026 ...THERE ARE MODERATE RISKS OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE CENTRAL/SOUTHERN HIGH PLAINS AND SOUTHERN PLAINS... ...2030Z Update... The developing Plains low that by Wednesday will have effectively absorbed the remnants of Polo will strengthen over the High Plains. The low will take on a much more classic comma shape, with the warm front extending towards the Upper Great Lakes. Thus, the rainfall pattern by Wednesday will take on a backwards Greek letter tau shape. The Moderate Risks were generally little changed. Focusing on topographic effects, the Colorado/New Mexico Moderate risk area was expanded on its western side to highlight the upslope flow into the southern Front Ranges. The top of the tau extends from south- central Colorado through to southern Wisconsin. The heaviest rainfall along the northern end of the precipitation shield extends from southeastern Nebraska through much of central Iowa. Instability will be the limiting factor here as well, with most of it stuck further to the south. Thus, despite near saturated soils in this area, the rates should stay muted enough to prevent widespread flash flooding, instead favoring scattered instances consistent with a Slight. However, amounts are close to maybe needing a Moderate Risk in this area as well, so the area will need to be evaluated for a Moderate with future updates. The Moderate Risk in Texas/Oklahoma was nudged a bit to the west, with growing consensus that a line of convection will be slow- moving. Thus, the storms may form a bit west than previous forecasts had, generally from Wichita Falls southwest through San Angelo and the Hill Country. Interactions with the dry slot to the west will ultimately determine where the storms form and how much flash flooding results. Finally, speaking of the dry slot, growing consensus that it will track through eastern New Mexico and the Panhandles, (which makes sense given the track of Polo), means that fortunately many of these very hard hit areas from Tuesday will finally get to dry out under the dry slot. Thus, the no-risk area and Marginals were expanded northeast to include these areas as the risk of flooding rainfall is very low. Wegman ...Previous Discussion... Tropically enhanced heavy rainfall will continue to spread from the Southwest to the Southern/Central Plains and into the northern Mississippi Valley. Guidance continues to show a fairly persistent theta-e/PW axis across this corridor (PWs aoa 2.25"), with the confluence of anomalous mid- upper level moisture (subtropical connection, including Polo's remnant upper level moisture plume) as well as the uptick in low- level moisture transport from the western Gulf with the strengthening LLJ (850 mb flow +4 to +5 standard deviations above normal) ahead of the deepening/amplifying upper trough. Multiple days with overlapping heavy rain footprints will keep elevated threats for excessive rainfall and flash flooding. Locations particularly sensitive will be dry washes, burn scars, slot canyons, and areas of higher terrain. Some persistent model spread present, however hinting the location and (especially) timing of heaviest rain as Polo's remnant moisture plume becomes absorbed by the deepening SW US upper trough. Not withstanding the timing uncertainties, the ECMWF EFI continues to signal a potentially significant rainfall event, particularly on Tuesday across eastern Arizona and western- central New Mexico. A Moderate Risk was maintained over southeast Colorado, northeast New Mexico, extreme portions of the Texas panhandle and southwest Kansas. Another Moderate Risk was maintained yet expanded N-S across central Texas and eastern Oklahoma. A broad Slight Risk encompasses New Mexico/Texas to southern Minnesota/Wisconsin. Campbell/Hurley Day 4 and Day 5 Valid 12Z Thu Oct 01 2026 - 12Z Sat Oct 03 2026 ...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE SOUTHERN PLAINS ON DAY 4... A positively-tilted upper trough is forecast to gradually weaken as it drifts out into and then stalls over the central and southern Plains. However, additional heavy rains and potentially numerous instances of flash flooding are expected, especially on Thursday (Day 4) across portions of the southern Plains. A Moderate Risk was maintained from south-central Texas to southeastern Oklahoma, where larage-scale ascent afforded in part by favorable upper jet forcing will continue to interact with an axis of anomalous moisture (2-2.25+ inch PWs). Raising the threat for flash flooding will be the possible overlap of heavy rains spreading into this region the previous day. Also on Day 4, a broad surrounding Slight Risk extending north into parts of the central Plains and through the mid Mississippi Valley was also maintained. While areal average rainfall amounts are expected to be less in this region, moisture drawn north and focusing ahead of a cold front settling south across the region is likely to fuel the potential for widespread moderate to localized heavy rainfall, resulting in at least isolated runoff concerns. By Friday (Day 5), with the system continuing to weaken, so too is the potential for additional widespread heavy rains over the southern Plains. However, there is likely to remain some areas with an elevated chance for heavy rain and flash flooding concerns. A Slight Risk was placed over parts of the southern Plains -- from South Texas to southeastern Oklahoma -- where models show a lingering axis of deeper moisturure supported by increasing low level convergence. This moisture may interact with slow-moving mid-level energy remaining over the region to support organized storms. Once again, the possible overlap of heavy rainfall from previous day(s), may elevate the threat for flash flooding. Farther north and east, the previously noted front will continue to settle southeast through the Ohio Valley, with some potential for heavy rain along and ahead of the boundary. Guidance shows a broadening pool of deep moisture developing over the southeastern U.S. as southeasterly flow increases along the western edge of the subtropical ridge. Mid-level energy embedded within the flow may help produce areas of heavy rain, with some guidance indicating heavy amounts along the Gulf Coast from Mississippi to the Florida Panhandle this period. Pereira 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 $$