Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Jun 15 2023 00:54:12 FOUS30 KWBC 150054 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 853 PM EDT Wed Jun 14 2023 Day 1 Valid 01Z Thu Jun 15 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... Convection redeveloped during the afternoon along and south of a frontal boundary draped across the Southeast. Special soundings from 18Z across the region showed a moist environment with tall, skinny CAPE through the column and precipitable water values at or above 1.75 inches which is somewhat higher than climo values. Multiple lower-amplitude shortwaves propagating eastward on the north side of the instability pool characterized by mid 70s dew points and 850 mb equivalent potential temperatures in the range of 345 K to 355K. Convection should continue into the overnight hours pretty much along the same corridor...although outflow from the convection the afternoon has pushed an outflow boundary into the southern portion of Alabama and Mississippi. As a result...expanded the Slight Risk southward to cover more of the Florida peninsula and to nudge the Moderate Risk a bit farther south as well. The 18Z suite of HREF probabilities have the highest risk of 1- and 2-inch per hour rates slightly outside the Moderate Risk area. With the RAP still showing convective redevelopment later this evening and in the overnight hours that tracks over a similar portion of Mississippi and Alabama later so no change the coverage there. ....Intermountain West and Northern Rockies onto the Northern Plains... An upper low pivoting over the Northern Rockies sent a cold front across the western portion of the Intermountain West during the day allowing the removal of a part of Montana from the Marginal risk area...but an upper trough extending parts of CA send a surge of Pacific moisture up through this front will keep the the threat of isolated flash flooding a concern into the late night. 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 warrants maintaining the Marginal Risk. The cold front crossing the northern Rockies with lee-side convective activity in an air mass with precipitable water values at or above 1.25" by this evening (2 sigma above normal) over the already saturated central portions of Montana warrants maintenance of the Marginal Risk for the eastern half of Montana into the western Dakotas and southward into Colorado and Utah. Bann 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 Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!8P67uZB-6XY88d2kfj1dER9MIg56z_TQKAL6ZgilF-Vp= JZ4TrOBQhYUiG60hhqnlcpuoKXFwNIwlTMQPsYq39SjKETs$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!8P67uZB-6XY88d2kfj1dER9MIg56z_TQKAL6ZgilF-Vp= JZ4TrOBQhYUiG60hhqnlcpuoKXFwNIwlTMQPsYq35RTURv0$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!8P67uZB-6XY88d2kfj1dER9MIg56z_TQKAL6ZgilF-Vp= JZ4TrOBQhYUiG60hhqnlcpuoKXFwNIwlTMQPsYq35JDPNDU$=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. Otherwise email Chris Novy at cnovy@cox.net and ask to be removed from WX-STORM. --- SBBSecho 3.14-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .