Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Jun 13 2023 20:24:46 FOUS30 KWBC 132024 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 422 PM EDT Tue Jun 13 2023 Day 1 Valid 16Z Tue Jun 13 2023 - 12Z Wed Jun 14 2023 ....THERE ARE SLIGHT RISKS OF EXCESSIVE RAINFALL FROM THE ARKLATEX INTO SOUTHWESTERN GEORGIA & THE CENTRAL HIGH PLAINS... 16Z Update... The forecast largely remains on track this morning with the Slight Risk stretching from the ArkLaTex to central Alabama remains most favorable for potential flash flooding today. Did introduce a Slight Risk in portions of the central High Plains where soils are overly saturated for recent days of heavy rainfall. Latest MRMS 48 hour precipitation analysis depicts as much as 2-8" worth of rain has fallen over east-central CO. AHPS rainfall totals over the past 14 days have ranged between 300-400% of normal as well. As the upper low ejects east this evening, thunderstorms emerging off the CO Rockies will make their way east into a more unstable environment where >1,000 J/kg of MUCAPE is present. PWs up to 0.75", which are values encroaching upon the 90th climatological percentile for mid-June, are also likely to be present. The combination of overly saturated soils with available moisture and instability this afternoon will favor strong thunderstorms capable of excessive rainfall rates in flash flood prone areas. In collaboration with PUB/GLD/BOU, have introduced a Slight Risk for this afternoon and into tonight. Minor changes to the Marginal Risk in the West with adjustment based off latest QPF trends and to include areas that have seen heavy rainfall in recent days. In the South, expanded the Marginal and Slight Risks farther west into TX where a strengthening LLJ intersecting the stationary front looks to reinvigorate thunderstorm activity tonight. Some portions of northeast TX have already seen strong-to-severe storms this morning, making them more vulnerable to potential flash flooding this evening. Farther east, decided to trim back the Slight Risk in southwest GA where the flash flood threat is still present, but is less likely compared to their neighbors to the west. Mullinax ---Previous Discussion--- ....ArkLaTex Through Southeast... Low level convergence north of a warm front over central TX in a moist and unstable environment should trigger rapid development of heavy thunderstorms around the Red River early this morning. This activity develops and shifts east in the 40kt westerly mean layer flow, likely spreading across the AR/LA border by midday and then central MS/AL into GA this afternoon. This will approach or cross areas that had heavy rain yesterday, particularly near the northern FL Panhandle borders with AL/GA. A shortwave impulse currently over the Desert Southwest reaches the southern Plains by this evening and tracks over the stationary frontal zone and into AR producing a second round of heavy thunderstorms over a similar area to this morning from northeast TX along the AR/LA border. Through this time, precipitable water values of 1.5-2" will be maintained along the frontal zone with ample instability (CAPE of 3000-4000 J/kg) will persist just south in the warm sector. There should be enough low-level inflow and effective bulk shear to organize convection with cell training in WNW-ESE motion and occasional mesocyclone formation leading to hourly rain totals up to 2.5". 00Z guidance and now the 06Z HRRR guidance maintains a consistent signal for local 3-5" totals among the two rounds from northeast TX into MS. The Slight Risk was expanded west to northeast Texas where the threat of two rounds is greatest. Farther southeast over AL into GA there is less confidence in the CAM consensus, but conceptually the presence of the front draped across this area along with scattered areas that received heavy rain yesterday warrants maintenance of the Slight Risk farther east. ....Great Basin/Northern Intermountain West to the Southern Plains... An upper trough currently off the BC coast will amplify and close into an upper low as it digs southeast to the northern Rockies tonight. This will enhance divergence aloft over the northern Great Basin into the northern Rockies. Pockets of 0.75-1.25" PWs combined with diurnal max of 1000+ J/kg of MLCAPE will allow for convection to develop in the afternoon and evening hours. Hourly rain totals to 2" are possible within such an air mass where cells merge and/or train. 00Z guidance QPF continues to indicate a localized 2" risk. Diurnal convection over the CO Rockies will spill onto the High Plains where above normal moisture persists in southerly low level flow. Maintained the Marginal Risk for from the Sierra Nevada through the northern Intermountain West and over the CO Rockies through southwestern KS into the OK/TX Panhandles. Jackson 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!7vUn2Pwd5_KHyGUTwq4Ocke92BUlT0DpJRSx-B8--ozP= l8SWc6wzG6wdyxM7GCqC3PhAEKrUS9kfSjS3qcFk59Liucc$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!7vUn2Pwd5_KHyGUTwq4Ocke92BUlT0DpJRSx-B8--ozP= l8SWc6wzG6wdyxM7GCqC3PhAEKrUS9kfSjS3qcFkUDYUdRs$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!7vUn2Pwd5_KHyGUTwq4Ocke92BUlT0DpJRSx-B8--ozP= l8SWc6wzG6wdyxM7GCqC3PhAEKrUS9kfSjS3qcFkAof0Cdg$=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) .