Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Sep 02 2025 23:57:39 FOUS30 KWBC 022357 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 757 PM EDT Tue Sep 2 2025 Day 1 Valid 01Z Wed Sep 03 2025 - 12Z Wed Sep 03 2025 ....THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF SOUTHERN ARIZONA... ....Southwest... An anomalous surge in monsoonal moisture into the Sierra Nevada continues due to the position of the upper ridge axis to the east=20 and a weak upper low along the central California coast to the=20 west. PW anomalies of 2 to 3 sigma are forecast from the lower=20 Colorado River up trough the Sierra Nevada and even into southern=20 Oregon. Impressive CAPE has led to periods of heavy rainfall so far today, with some of the better activity in and near the persisting Slight Risk in southern AZ. While most of the region is expecting a general downturn in activity, there is some chance of renewed activity across CA closer to the upper low overnight, so other than the dropping of the previous Slight Risk across SoCal, left the Marginal Risk more or less intact. Activity winding down across TN led to the dropping of that Marginal Risk, and so far, activity across the Northern Plains/southern MN has been progressive enough to generally preclude flash flooding/excessive rainfall. Roth Day 2 Valid 12Z Wed Sep 03 2025 - 12Z Thu Sep 04 2025 ....THERE IS A SLIGHT RISK FOR PORTIONS OF SOUTHERN CALIFORNIA AND SOUTHER ARIZONA... 2030Z Excessive Rainfall Discussion Update... Main change was to remove the Marginal Risk area over the Great Lakes region. Model and deterministic QPF had come down to values under half an inch...and in many cases the areal average amounts were closer to one-quarter inch. Model-derived instability does maximize above 500 J per kg during the afternoon...enough instability to support locally heavy rainfall. However...the cells should be progressive enough to limit the risk of excessive rainfall to less than a 5 percent probability. Farther south, the concern about excessive rainfall due to the threat of repeating convection remains and the Marginal Risk area remained in place. Concern for excessive rainfall across the Southwest US remains as described below. Previous Excessive Rainfall Discussion... ....Southwest... Anomalous monsoonal moisture will continue to affect northward from the lower Colorado Valley through the Sierra Nevada and much of Nevada. Diurnally- driven activity can be expected again over the Sierra Nevada which may be locally excessive, as well as much of Nevada through western Arizona and southwest Utah. A broad Marginal Risk cover much of Arizona,southeast Utah, southern Nevada, and eastern/southern California. The presence of strong CAPE embedded with the deep column of moisture, especially across southern Arizona and across the Transverse and Peninsular Ranges of southern California will be conducive for heavy rainfall and scattered areas of flash flooding. These locals will be vulnerable to the high rainfall rates, especially in the flashy prone locations. A Slight Risk was raised in coordination with the local forecast offices. ....Central U.S... Elevated moisture ahead of the strong cold front sweeping down the Plains will trigger heavy rainfall again. Enough focus is present to warrant maintenance of the Marginal Risk around the central Great Lakes though timing is in question. While much of the activity should be progressive, there is a repeating threat on the southern end of activity along the southern Kansas/Missouri border- a Marginal Risk was carried on for this period. ....Florida... Enhanced rainfall expected to develop within the very moist airmass along the stalled frontal boundary. Individuals cells will focus heavy rainfall over the southern peninsula and the Keys thus keeping an elevated threat for flash flooding. Campbell Day 3 Valid 12Z Thu Sep 04 2025 - 12Z Fri Sep 05 2025 ....THERE IS A MARGINAL RISK AREA FOR PORTIONS OF FLORIDA, SOUTHWEST/GREAT BASIN, AND THE NORTHEAST... 2030Z Excessive Rainfall Discussion Update... Shrunk the Marginal risk area in the eastern US...keeping portions of New York and Pennsylvania in the Marginal area where there is some overlap between instability and moisture. The shear profile has become less favorable for cell training compared with 24 hour ago...which should work against the risk of excessive rainfall. Only a few changes needed elsewhere. Bann Previous Excessive Rainfall Discussion... ....Florida... The environment will remain nearly the same as the Day 2 period, thus convection will persist. Heavy rain will be possible therefore the Marginal Risk was maintained southern Florida and expanded southward to cover the Keys. ....Northeast... The cold front passing through the Lower Great Lakes and into the Northeast will be fairly progressive, however there are signals for higher QPF to focus across central and northern portions of New York. Parts of the region have been drier with some areas with notable drought conditions, which will limit the potential for impacts from heavy rainfall. The highest amounts are expected for the more elevated locations which will increase the threat for flooding concerns. A Marginal Risk area was maintained from the previous Day 4 but with portions of western and eastern new York trimmed out given the D0/D1 drought conditions. ....Southwest, California and southern Great Basin... The monsoonal moisture will continue to advect into the southwestern U.S. underneath the mean upper ridge, promoting scattered showers and thunderstorms. Moisture anomalies are expected to be the greatest for portions of California and Nevada. A broad Marginal Risk encompasses eastern California, most of Nevada, southeast Utah, Arizona and west/central New Mexico. Campbell Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!7OHwFFM0UeoUJPmMZyyF86_PrDOR8MFOzUnq1n830L9o= tIJhu3GiSlwz8yKvN9kXZFXqLyeaK2wnavYJzX6a8EU0ogg$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!7OHwFFM0UeoUJPmMZyyF86_PrDOR8MFOzUnq1n830L9o= tIJhu3GiSlwz8yKvN9kXZFXqLyeaK2wnavYJzX6aGWCHwNs$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!7OHwFFM0UeoUJPmMZyyF86_PrDOR8MFOzUnq1n830L9o= tIJhu3GiSlwz8yKvN9kXZFXqLyeaK2wnavYJzX6aa57OlwM$=20 =3D =3D =3D To unsubscribe from WX-STORM and you already have a login, go to https://lists.illinois.edu and use the "Unsubscribe" link. 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