FOUS30 KWBC 261553 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1153 AM EDT Sat Sep 26 2026 Day 1 Valid 16Z Sat Sep 26 2026 - 12Z Sun Sep 27 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PARTS OF THE COASTAL NORTHEAST AND IOWA... ...16Z Update... ...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 ...Previous Discussion... ...Northern Mid-Atlantic Coast into Southern New England... The latest guidance is favoring a deeper nor'easter to approach the Northeast and then stall while spreading heavy rain generally along the I-95 Corridor from Philadelphia to Boston. Prevailing southeast to easterly flow will support a substantial influx of moisture into parts of southern New England, southern New York, and New Jersey. Increasing hydrologic concerns are expected. In addition to an increasing threat for flooding in urbanized and poor drainage areas, this heavy rain may also exacerbate ongoing coastal flooding. The Slight Risk highlights the areas where the general model consensus shows widespread amounts of 1-3 inches during the period. A higher-end Slight Risk encompasses much of Long Island, Rhode Island and eastern Massachuesetts. ...Southern Plains to the Upper Mississippi Valley... A Slight Risk was maintained for portions of eastern Nebraska, central/southern Iowa, and far northern Missouri where a corridor of cell-training is possible along a NW-SE oriented stationary front. Hi-res guidance is suggesting 24-hour accumulations may reach or exceed 2 inches, which could support scattered flash flooding as this rainfall overlaps with heavy rainfall from the previous day. The Marginal Risk was also expanded further south where CAMS signal the potential for locally heavy rainfall near sensitive soil, although coverage of storms is expected to be lower than previous days. Rainfall developing late in the Day 1 period, may continue into late Saturday morning across parts of Iowa and southern Minnesota. Expect a diminishing threat for heavy to excessive rain by late morning, followed a potential return later in the day centered farther to the southwest. While confidence is limited, there remains some signal in the guidance for storms developing back across across parts of the central Plains along a strengthening low level jet and axis of increasing instability and moisture. ...West Texas... Weak mid-level energy interacting with an uptick in low level moisture may produce some additional localized heavy amounts, impacting the more sensitive areas of West Texas around the Davis Mountains and Stockton Plateau. A Marginal Risk was maintained. Campbell/Pereira/Asherman 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 NORTHEAST... ...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 SLIGHT RISK OF EXCESSIVE RAINFALL FOR THE FOUR CORNERS REGION... ...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 SLIGHT RISK OF EXCESSIVE RAINFALL OVER MUCH OF THE SOUTHWEST, FOUR CORNERS REGION AND INTO THE SOUTHERN AND CENTRAL PLAINS... 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 $$