FOUS30 KWBC 020809 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 409 AM EDT Wed Sep 2 2026 Day 1 Valid 12Z Wed Sep 02 2026 - 12Z Thu Sep 03 2026 ...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF SOUTHEAST TEXAS... ...Eastern Texas and Southwest Louisiana... Raining/backbuilding with high rainfall rates from tropical cyclone Edouard are ongoing although there should be some weakening by late in the morning, but periodic backbuilding clusters will remain a possibility. Another uptick late Wednesday night into Thursday morning likely as the low level vorticity should remain fairly well defined. PWs of 2-2.25 inches will support continued heavy rain with hourly rates of 2-3 inches/hour. There is potential for a locally enhanced inflow band on its south side which could prolong and enhance the rain across areas that receive heavy rain. The Moderate Risk was expanded to include Freeston, Anderson and Henderson counties as well more of Polk county. The eastern boundary of the Slight Risk was adjusted eastward a bit to reflect the latest QPF guidance and WPC forecast. ...Northern Mid Atlantic & Southern New England... A stronger shortwave will interact with the slow-moving warm front over the Northeast to trigger scattered to widespread showers and thunderstorms across the region. There continues to be ample support of training of clusters of thunderstorms with 1-2"/hr rain rates. The guidance focuses this heaviest rainfall across portions of Upstate New York, and while soil moisture is generally below normal according to NASA SPoRT, this should support up to 3" of rain in some areas leading to scattered instances of flash flooding, with more isolated responses elsewhere. ...Florida... The tropical moisture plume remains entrenched across Florida, reflected by PW anomalies of +1.5 sigma stretching across most of the state. Both coasts could experience slow moving storms with 2+"/hr rain rates, as well as across the inland portions of the peninsula where storm mergers/collisions can occur. A Marginal Risk was maintained for this period. ...Arizona... Monsoonal moisture will return to southern Arizona to trigger diurnal convection. Locally excessive rainfall may be possible therefore a Marginal Risk is in effect. Campbell Day 2 Valid 12Z Thu Sep 03 2026 - 12Z Fri Sep 04 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF SOUTHERN ARIZONA AND EXTREME SOUTHWEST NEW MEXICO... ...Southwest... A Slight Risk remains in effect across southern Arizona and into New Mexico for this period. An uptick in moisture should result in increased convective coverage and a scattered flash flood risk. This region will continue to be monitored over the coming days. ...Texas, Gulf Coast and Florida... The remnant vort center of Edouard should be less defined by this time, but still present over east Texas with lingering tropical rain over sensitive soils. It could still help focus some organized convective clusters, and thus an isolated flash flood risk could linger into Thursday. Elsewhere onshore flow within the pooled high PW values and stalled boundary could trigger convection along the coastline and across the Florida Peninsula leading to isolated areas of flash flooding/ponding. A Marginal Risk spans from eastern Texas to Florida. ...Upper Midwest to Mid-Atlantic... While the details on where the heaviest rain will occur there continues to be a signal for one or more rounds of organized convection to track trough portions of this area through the day into Thursday night. Parts of this region have had recent rains to lower FFG and increase sensitivity to additional heavy rainfall. A Marginal Risk was maintained from Minnesota/Iowa to Del-Mar-VA and New Jersey. ...California into the Intermountain West... A Marginal Risk continues from northern California into Idaho. During this period a strong synoptic system will bring an axis of heavy rainfall to the region. Not much instability, but just enough showing up in model forecasts to suggest some potential for localized higher rates and an isolated flash flood potential. Campbell/Chenard Day 3 Valid 12Z Fri Sep 04 2026 - 12Z Sat Sep 05 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FRIDAY ACROSS PORTIONS OF ARIZONA... ...Northwest/Northern Rockies... Marginal Risk area is in effect across portions of the interior Northwest, particularly the area surrounding the northern Idaho Panhandle. This area will be much closer to the upper level trough, so forcing should be much better than in the Southwest. However, moisture and instability will be much lower, resulting in more likelihood of stratiform rainfall. Despite this, the rainfall should still have modest amounts of instability to work with in a generally dry, and therefore flash flood prone, area of the country. ...Southwest/Four Corners/Central-Southern Rockies... The signal for the return of heavy rain areas across the Southwest into the Four Corners and central-southern Rockies remains strong GEFS and ECENS PWAT anomalies increase back to 2-2.5 standard deviations above normal with the axis of best monsoonal moisture influx shifting slightly west. Still remains to be seen how much influence the eventual tropical system that ultimately pulls west of Baja will have in terms of the upper level moisture (potential uptick in the PW anomaly). A Slight Risk was maintained for Arizona while a broad Marginal covers southeast California the Four Corner. ...Upper Midwest/Ohio Valley/Central Appalachians... Expect more additional organized/widespread convection downwind of the upper ridge axis. Precipitable water values are high enough with sufficient instability that embedded cells could produce local downpours and isolated flooding. This will likely start over eastern portions of the Upper Midwest, where the upper jet streak to the northeast is expected to strengthen as the pivoting northern stream trough impinges on the broad-scale ridge to the south. Have essentially traced the Marginal Risk area southeast along the mean 850-300 mb thickness fields, where the upper level forcing along/ahead of the surface front will be most supportive of more organized activity. A Marginal Risk is in effect from Wisconsin to West Virginia. Campbell/Hurley Day 4 and Day 5 Valid 12Z Sat Sep 05 2026 - 12Z Mon Sep 07 2026 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL SATURDAY AND SUNDAY ACROSS PARTS OF THE SOUTHWEST... A deep upper low over the Pacific Northwest working in tandem with a strong ridge centered over the central U.S. will direct southwesterly flow into the southwest U.S. both Saturday and Sunday. Deterministic and ensemble guidance continue to advertise an impressive influx of moisture over the region, featuring PWs surging in excess of 150% of normal on Saturday and 200% of normal on Sunday. Models are somewhat split on whether or not that moisture can translate to significant QPF, and no doubt that likely has to do with the degree of influence from Tropical Storm Marie. With that being said, many of the models that keep Marie on a more northwesterly and eventually westerly track away from southern California still show a good deal of moisture, which is also likely monsoonal in nature, being pulled northward from the Pacific. Given the above, thought it would be prudent to have at least some cover for the potential for localized flash flooding from any heavy rain that may develop over the weekend. As a result, decided to largely keep the MRGL risk inherited from yesterday's Day 5 for the new Day 4/Saturday ERO with only minor changes. For the new Day 5/Sunday ERO, which will likely be the day with the more impressive moisture surge, decided to introduce a MRGL risk which encompasses portions of southern California, southeast Nevada, much of Utah, and western Arizona. Should the moisture signal diminish due to even less tropical interaction in the coming days, future updates may reduce or remove the MRGLs entirely. Miller 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 $$