Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Mar 13 2025 08:01:07 FOUS30 KWBC 130800 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 400 AM EDT Thu Mar 13 2025 Day 1 Valid 12Z Thu Mar 13 2025 - 12Z Fri Mar 14 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF SOUTHERN CALIFORNIA... A strong Pacific cold front associated with an amplified upper trough will continue to drop southeastward along the southern California coast this morning. Hi-res guidance indicates that moisture advection and modest instability ahead of the front will be sufficient to maintain rainfall rates of 0.5+ in/hr along the coast into the upslope areas of the Peninsular Ranges early in the period. Rates are expected to diminish as the front moves south=20 and east into the Colorado Basin and the Baja Peninsula by late=20 morning, but not before an additional 1-2 inches of rain falls=20 across portions of the highlighted area. Neighborhood probabilities indicate the greatest threat for heavier amounts is centered along=20 along the orographically favored regions of the Peninsular Ranges. A Slight Risk was maintained for areas at or below 3500 ft, where=20 precipitation is expected to fall as mostly or all rain. Pereira Day 2 Valid 12Z Fri Mar 14 2025 - 12Z Sat Mar 15 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE LOWER TO MID MISSISSIPPI VALLEY INTO PARTS OF THE OHIO AND=20 TENNESSEE VALLEYS... A strong mid-to-upper level shortwave crossing the Southwest on Day 1 is expected to move east of the Rockies, with a negatively-tilted low developing over the central Plains early in the period. Powerful forcing is forecast to interact with what is expected to be a relatively modest plume of moisture advecting north ahead of the system. Storms are forecast to develop initially over the=20 lower to mid Mississippi Valley Friday afternoon before moving=20 east into the Tennessee and Ohio valleys during the evening and=20 overnight. Cell motions will likely be quite progressive given the=20 very strong wind fields in place. Therefore, this convection will=20 likely pose more of a severe risk than a flash flood threat.=20 However, given the strong dynamics, along with sufficient=20 instability for deep convection, expect that rainfall rates will be briefly intense enough to produce localized amounts that exceed=20 current model forecasts (maybe locally over 2 inches).=20 Still, this appears to be a lower-end flood threat, and probably=20 confined to localized impacts over more sensitive urban or low=20 lying areas that are impacted by a quick burst of intense rainfall. Pereira Day 3 Valid 12Z Sat Mar 15 2025 - 12Z Sun Mar 16 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE TENNESSEE AND OHIO VALLEYS... Even as the previously noted low lifts to the north through the Great Lakes into Ontario, the upper pattern will remain amplified across the central U.S. as an upstream shortwave dives southeast into the southern Plains on Saturday. A surface wave developing along the leading system's trailing cold front will help focus deeper moisture tapped by strengthening southerly winds. While differences in the details remain, the general consensus of the 00Z guidance indicates that widespread accumulations of 2-4 inches centered over parts of the Tennessee and lower Ohio Valleys are=20 likely. A large Slight Risk was maintained across the region, reflecting the higher probabilities for 2 inches or more shown by by the GEFS/ECENS and where deeper moisture, interacting with strong forcing, along a relatively slow moving front, is expected=20 to raise the potential for heavy amounts late Saturday into early Sunday. This remains a higher-end Slight Risk for parts of the area, with an upgrade to a Moderate Risk possibly forthcoming, especially once the period moves within the hi-res window and details concerning rainfall rates and the placement of the heaviest amounts become more clear.=20 Pereira Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!7VWmhCEKxv-iBVHT1gKILbxdcogxrMMqJ0gYj4LeIgCV= 7qdHaX7ylntyv9qi0HghMZpwA4LaGIwa0CTLRV-wIoFEkiE$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!7VWmhCEKxv-iBVHT1gKILbxdcogxrMMqJ0gYj4LeIgCV= 7qdHaX7ylntyv9qi0HghMZpwA4LaGIwa0CTLRV-wHvsGE7k$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!7VWmhCEKxv-iBVHT1gKILbxdcogxrMMqJ0gYj4LeIgCV= 7qdHaX7ylntyv9qi0HghMZpwA4LaGIwa0CTLRV-wbfnKLDk$=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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