Subj : HVYRAIN: Excessive Rainfa To : All From : Mike Powell Date : Wed Jun 14 2023 17:21:00 FOUS30 KWBC 142004 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 403 PM EDT Wed Jun 14 2023 Day 1 Valid 16Z Wed Jun 14 2023 - 12Z Thu Jun 15 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE SOUTHEAST... ....Lower Mississippi Valley through the Southeast... Morning convection over AL/GA continues eastward with renewed activity along and south of a frontal boundary draped across the Southeast. Morning soundings indicate a moist environment (tall, skinny CAPE saturated through the column) with precipitable water values around 1.5-1.75 inches which is near to just above climo values. Organized convection is likely to continue and expand this afternoon as multiple lower-amplitude shortwaves progress eastward on the north side of the instability pool characterized by mid 70s dew points. CAM guidance has come into better agreement on a focus for higher QPF over southeastern AL into southwestern GA this afternoon/evening and have targeted this area for a rather small Moderate Risk area. There is a lot to be determined as convective areas work through and modify the atmosphere, but with this morning's activity getting soils moistened there is enough support for the upgrade. The westerly mean layer flow running parallel to the stationary fronts will support training and backbuilding convection over some areas with the potential for additional rounds tonight, before lessening in coverage overnight. ....Intermountain West and Northern Rockies onto the Northern Plains... An upper low pivoting over the Northern Rockies sends a cold front to the central portions of the Intermountain West and the renewed upper trough extending over southern CA send a surge of Pacific moisture up through this front. An axis of QPF with a risk for local 1"+ is forecast from the Sierra Nevada across the Great Basin to the CO Rockies in diurnally enhanced convection which continues to warrant a Marginal Risk. The cold front crosses the northern Rockies with lee-side convective activity in PW exceeding 1.25" by this evening (2 sigma above normal) over the already saturated central portions of Montana warrants maintenance of the Marginal Risk for much of Montana into the western Dakotas. Fracasso/Jackson Day 2 Valid 12Z Thu Jun 15 2023 - 12Z Fri Jun 16 2023 ....THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF THE SOUTHEAST AND WYOMING... Lower Mississippi Valley through the Southeast... Further impulse activity coming across the southern tier of the CONUS in a westerly subtropical jet will continue to enhance lift along a persistent frontal zone that is anticipated to be farther south toward the Gulf coast/onto the FL Peninsula on Thursday compared to today. Continued pooling of 1.5 to 2" PWs along this frontal boundary along with ample instability and bulk shear allows for further passage of organized activity over this saturated zone from central OK ESE through southern GA/northern FL. With increased confidence in a footprint of 2-3" additional rainfall in an area with increasingly sensitive soils, maintained the Slight Risk over this region. Intermountain West onto the Great Plains... Added a Slight Risk area to central/southeastern Wyoming into far northern CO and western NE, coincident with lower FFG values ~1"/hr due to recent rainfall. The upper low over the northern Rockies today lifts northeast to the Canadian Prairies tonight while the associated troughing digs over the Intermountain West, connecting a trough axis from the northern Rockies SSW through southern CA. This will allow for a similarly placed, but greater magnitude QPF, compared to today over the Great Basin and into CO/WY Rockies which warrants another Marginal Risk. EFI QPF values (ECMWF EPS) are noted around 0.7-0.8, and 12Z HREF neighborhood probabilities of 1"/hr rates/amounts are in the 10-35% range over central to southeastern WY atop 7-day rainfall percent of normal values >400%, forming the rough outline to the Slight Risk area. Farther east on the northern and central Plains, Gulf moisture drawn up ahead of the slowing cold front allows for increased PW around 1.25" (1.5 sigma above normal) and diurnal development of locally heavy thunderstorms. Slower southwesterly mean flow should allow some cell mergers, continuing to warrant a Marginal Risk along 100W. Southern Sierra Nevada... Weak instability combined with PW of 0.75 to 1" (2 to 3 sigma above normal) will support locally heavy rainfall rates in diurnal convection. Some instances of flash flooding are possible, particularly in areas where soils are overly saturated from multiple recent days of rain and in rugged terrain warranting a Marginal Risk. Fracasso/Jackson Day 3 Valid 12Z Fri Jun 16 2023 - 12Z Sat Jun 17 2023 ....THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL OVER EASTERN COLORADO TO WESTERN KANSAS... Intermountain West onto the Great Plains... The upper trough axis shifts east which takes the QPF focus area farther east compared to Thursday - generally over the CO Rockies, central High Plains then over a fair latitudinal range from a slow moving frontal system with a lee-side low over eastern Neb. 1.25" to 1.5" PW over the Plains is generally 1.5 sigma above normal and the slow motion of the frontal zones warrants a Marginal Risk extending to the Corn Belt. Within this area, introduced a Slight Risk area where there is a good, but not universal, signal for some >1+" amounts and rates >0.75"/hr, roughly over the Eastern Plains of CO into western KS. Here, FFG values are currently depressed around 1"/hr due to recent rainfall (7-day QPE anomalies 400-600%), and banked on some decent ensemble agreement to form the Slight Risk outline. This also aligned with EFI values around 0.6-0.7 and the CSU Machine-Learning first guess which also showed a Slight Risk. Mid-South through eastern Gulf Coast... The stalled frontal boundary looks to linger at least into Friday from AR to the FL/GA line. Global guidance is generally in agreement for 1 to 2" QPF through this zone which would be over mostly saturated soils from multiple prior days of heavy rain. The Marginal Risk was maintained with notable potential for a Slight Risk once confidence increases in placement in light of observed rainfall and lower FFG values. Fracasso/Jackson $$ --- SBBSecho 3.14-Linux * Origin: Ilink: CCO - capitolcityonline.net (454:1/105) .