Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Jun 14 2023 00:52:22 FOUS30 KWBC 140052 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 851 PM EDT Tue Jun 13 2023 Day 1 Valid 01Z Wed Jun 14 2023 - 12Z Wed Jun 14 2023 ....THERE ARE SLIGHT RISKS OF EXCESSIVE RAINFALL FROM THE ARKLATEX INTO SOUTHWESTERN GEORGIA & THE CENTRAL HIGH PLAINS... 01Z Update... The previous outlook largely remains in tact and much of the previous forecast reasoning remains valid. The two changes made this cycle were to remove the Slight Risk in Colorado and to expand coverage of the Marginal Risk area in Texas where convection began to flare up during the late afternoon hours.=20 Thinking was that the activity in Colorado will be waning during the evening but felt it was too early to entirely remove the Marginal Risk area given the lingering dynamics and instability with some overlap of areas where 2 to 8 inches of rain has fallen recently. The western portion of the outlook...the Maringal Risk extending into Colorado/Nevada/Idaho...still looks good into the late evening or early morning hours. In a similar line of reasoning...maintained the Slight Risk across portions Texas due to additional showers and thunderstorms moving across a swath where 4 to 7 inches of rain has fallen in parts of Texaw within the past 24 hours. Maintained the Slight risk area farther east in Mississippi and Alabama due to the signal from the HRRR about additional convection overnight.=20 Bann Day 2 Valid 12Z Wed Jun 14 2023 - 12Z Thu Jun 15 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE SOUTHEAST... 20Z Update... Focusing on the Southeast, the Slight Risk areas was expanded farther south and west deeper into the South to account for newer guidance coming in wetter in these areas, but also because central MS and AL will have seen soils moistening throughout Tuesday. The soils will grow increasingly more saturated as a result, while the atmosphere becomes primed to produce rounds of strong-to-severe storms. NAEFS between 12Z Wed - 06Z Thurs showed IVT values peaking around 750 kg/m/s over southern GA and southern SC while there is also large footprint of 97.5 climatological percentiles for IVT values throughout the south. Westerly mean 850-300mb winds will run parallel to the stationary front draped across the region and could support training and backbuilding convection over areas that are dealing with ongoing rounds of thunderstorms. There was some contemplation to hoist a Moderate Risk this forecast period, but opted to hold off to allow today's convection to conclude first to see where the most sensitive soils take shape. There were also lingering disagreements in CAMs in where the axis of heaviest precipitation would occur. Should confidence and probabilities increase further in the potential impact of flash flooding, a Moderate Risk could be needed in future forecast cycles. The remainder of the forecast rationale remains on track below. Mullinax ---Previous Forecast--- ....Lower Mississippi Valley through the Southeast... Ongoing activity from Tuesday night should be across MS/AL/GA as an impulse shifts east along the stalled frontal boundary over the Southeast. Ample Gulf moisture (PWs of 1.5 to 2") continues to pool along the frontal zone which will continue to meander north and south ahead and behind this first and subsequent impulses given the instability pool along and north of the Gulf Coast. Expect redevelopment Wednesday night over potentially the same areas from the AR/LA border and east. Therefore, the Marginal Risk was shifted west over much of southern AR while the inherited Slight Risk matches up well with the 00Z consensus. There remains a risk for this repeating heavy activity to focus over similar areas in MS/AL/GA to the FL Panhandle as today which may warrant an eventual Moderate Risk upgrade. ....Intermountain West and Northern Rockies onto the Northern Plains... An upper trough pivoting over the Northern Rockies sends a cold front to the central portions of the Intermountain West and a renewed upper trough extending over southern CA allows an elevated surge of Pacific moisture up through this front. An axis of QPF from the Sierra Nevada across the Great Basin to the CO Rockies is forecast with a risk for local 1"+ precip in diurnally enhanced convection warrants a Marginal Risk. The cold front crosses the northern Rockies with lee-side convective activity over the already saturated central portions of Montana warrants maintenance of the Marginal Risk into the western Dakotas. Jackson Day 3 Valid 12Z Thu Jun 15 2023 - 12Z Fri Jun 16 2023 ....THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FOR SOUTHERN GEORGIA... ....Southern Georgia... The stalled frontal boundary responsible for repeated rounds of strong-to-severe storms will stick around another day in the Southeast. The IVT remains at or above the 97.5 climatological percentile and a similar setup with mean 850-300mb winds running parallel to the front. Latest guidance is currently keying in on southern Georgia where they will also have received heavy rainfall two days prior. With growing consensus in a footprint of up to 2-3" of rainfall in an area with increasingly sensitive soils, decided to introduce a Slight Risk to the region. It is possible the Slight Risk may need to expand should rainfall forecasts increase in subsequent forecast cycles. A Marginal Risk was introduced to the southern Sierra Nevada. Some weak instability combined with PWATs closing in on 0.75" (90-97.5 climatological percentile) will support locally heavy rainfall rates. Some instances of flash flooding are possible, particularly in areas where soils are overly saturated and in rugged terrain. Mullinax ---Previous Discussion--- Lower Mississippi Valley through the Southeast... Further impulse activity coming off the upper trough over southern California tracks across the southern tier of the CONUS in a westerly subtropical jet, enhancing lift along a persistent stationary front over the Deep South Thursday. 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 the eastern OK/TX border east through southern GA/northern FL. However, some uncertainty on placement and timing of these waves allows for a Marginal Risk at this time. However, should activity focus over areas that have been hit multiple times over the past few days, upgraded risks may be warranted. Intermountain West onto the Great Plains... The upper low over the northern Rockies on Wednesday lifts northeast to the Canadian Prairies Wednesday night, leaving trough over the northern Rockies SSW to the upper trough over southern CA. This will allow for a axis of QPF over the Great Basin to CO/WY Rockies somewhat similar to the one the day before warranting another Marginal Risk. 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" and diurnal development of locally heavy thunderstorms. Slower southwesterly mean flow should allow some cell mergers and warrants a Marginal Risk. Jackson Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!5x3EHq7BAgXPxbQE54YjbOUUQi_ZrUtZwmIfijPuD7sW= _AYp5IXE18K3JgKiFeOk_imQVfLhJ-iYzdFlg1iccF5sPuo$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!5x3EHq7BAgXPxbQE54YjbOUUQi_ZrUtZwmIfijPuD7sW= _AYp5IXE18K3JgKiFeOk_imQVfLhJ-iYzdFlg1icK5jm7mk$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!5x3EHq7BAgXPxbQE54YjbOUUQi_ZrUtZwmIfijPuD7sW= _AYp5IXE18K3JgKiFeOk_imQVfLhJ-iYzdFlg1icUyEzq4A$=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) .