Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Sep 13 2024 15:59:46 FOUS30 KWBC 131559 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1159 AM EDT Fri Sep 13 2024 Day 1 Valid 16Z Fri Sep 13 2024 - 12Z Sat Sep 14 2024 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF CENTRAL AND NORTHERN ALABAMA... 16Z update... The latest global and CAM guidance continue to focus the heaviest QPF across north/north-central Alabama as the remnant circulation of Francine persist over the region. While these rain amounts will likely be beneficial to many locations that have been in a prolonged dry stretch, the rainfall intensity and duration over sensitive terrain areas may quickly lead to localized flooding problems. The ongoing Moderate Risk had a small expansion to the southeast for this update as supported by the latest guidance. Elsewhere across the region the level of threat still stands as already identified with the Marginal and Slight Risks in place. Campbell ....Deep South... Remnant circulation from Francine will be pivoting back south and then southeast in retrograde thanks to a strong block from a stout mid- level ridge positioned over the Great Lakes through Ontario/Quebec. This will allow for the a migration of elevated moisture centered under the disturbance to pivot back across MS/AL where drier air has punched through under prevailing southwest flow aloft, mainly above 500mb. This is well- documented within the 00z sounding analysis out of KJAN where moisture is pooled between the surface to 500mb with a large wedge of drier air focused above. The sounding from KBMX was beginning to show signs of the dry tongue and can be noted within the radar presentation as deep convective development is struggling to initiate this evening. Tomorrow afternoon and beyond is when the pattern shifts to a more favorable convective regime across far Eastern MS into much of AL, especially the northern 2/3's of the state thanks to the stacked synoptic circulation sliding back overhead. Guidance is keen on the deeper moisture return as indicated within the 700-500mb RH field leading to an uptick in regional PWATs back above 1 deviation above normal with ample mid- level energy accompanying. The latest 00z HREF remains very aggressive within its depiction for a broad axis of heavy rainfall in-of Northern and Central AL during the late- afternoon and evening time frame in conjunction with the increased upper forcing approaching from the west along the tightened theta-E gradient located across much of the state. Surface ridge to the northeast will also begin wedging south into Northern GA with an axis of convergence developing upstream between the approaching circulation from now Post-Tropical Francine. This will place a good portion of AL into the crosshairs for a better convective training episode as the deep layer flow aligns perfectly from the southeast. HREF neighborhood probability for >5" is robust between the area of KHUN down to KBHM indicating a 50-70% prob with an EAS signal of 50-80% for at least 2", a signal correlated highly for a higher end risk. Hourly rate probs are also aggressive in the means with a corridor of elevated probs for between 2-3"/hr within the HREF, along with some intra-hour rate potential exceeding 4" given the efficient rainfall processes and expected instability presence within the eastern fringe of the general circulation. All these prospects weighted on the decision to maintain the inherited MDT risk with some minor adjustments to correlate with the blended mean QPF off the latest HREF, as well as the EAS prob field coverage of higher probs for at least 2" of rainfall. The changes made were not extensive in the grand scheme, so the overall forecast was close to general continuity. ....Coastal Carolinas... A weak inverted trough located off the Southeast coast will allow for a targeted axis of heavy rainfall across the Eastern Carolinas, mainly north of KCHS with a bullseye being forecast along the coast of Southeast NC. The prevailing easterlies off the Atlantic within the integrated water vapor field (IWV) from the GFS signal the axis very well with a nose of elevated PWATs being advected right into the coastal areas of NC during the first half of the period. 00z CAMs also signal the threat for a period of heavy rainfall with some varying degrees of the latitudinal push of where the heaviest precip would occur. The greatest probabilities for >3" are centered from Wilmington up through Morehead City into Cape Hatteras. Considering the area FFG's, the main flooding risks will be relegated to more urbanized areas where run off potential is highest. The signal was good enough for a continued MRGL risk with some expansion further inland, mainly west of Raleigh where some convergence from the damming high to the north and the prevailing easterlies may intersect allowing for a small axis of enhanced precip away from the coastal plain. Not anticipating a large areal coverage of flash flooding in any case for the region, but a few pockets are plausible leading to a low to middle ground MRGL for the forecast. ....Florida Panhandle... Deep tropical flow pulling north from the Gulf will allow for an axis of heavy convection to push north out of the Gulf into the Big Bend of FL. Areal FFG indices for 1/3/6 hr rates are pretty substantial, so the threat is low-end within the MRGL risk category. Still, the signal for amounts >5" is relatively high right along the coast leading to the MRGL continuation from previous forecast. Kleebauer Day 2 Valid 12Z Sat Sep 14 2024 - 12Z Sun Sep 15 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE SOUTHEASTERN U.S... ....Southeast... The remnant mid and upper circulation from Francine will get stuck under the blocking pattern positioned across the northern and eastern U.S with little propagation of any kind during the period. This will allow for a continuation of broad, cyclonic flow across the Southeast with a steady convergence pattern positioned over parts of the Deep South. A steady southeast flow pattern will preside over much of AL and Western GA with a sharp convergence signal focused over the central portion of the outlined area thanks to the persistent anti-cyclonic "wedging" occurring around the southern flank of the ridge, banking against the cyclonic regime centered around Francine's remnants. Ensemble QPF distribution is a bit further south compared to the previous forecast allowing for an adjustment in that direction for the SLGT risk prospects. Totals are generally favored to be within 1-3" overall, but some pockets of higher totals are depicted within the 00z deterministic suite leading to some locally impactful flash flood prospects, especially when coupled with the previous period(s) of rainfall lowering the area FFG's across AL/GA. The SLGT risk is on the higher-end of the spectrum with a higher risk potential relying on the contingency of how the D1 period plays out across the region. There's a non-zero chance of an upgrade, but the slowly degrading axis of instability may thwart the potential all together. Regardless, the SLGT risk is sufficient just given the variables at hand. ....Arizona... A surge of deep tropical moisture from now Tropical Storm Ileana will make progress into the southern portion of AZ by the middle to end of the forecast period. With sufficient diurnal heating Saturday afternoon, environment across Southern and Southeast AZ will destabilize allowing for isolated to scattered thunderstorms to initiate in-of the terrain situated within Pima/Santa Cruz counties to possibly as far east as Cochise county in proxy to the Huachuca mountains. MUCAPE forecast is relatively favorable for convective development with the ensemble SBCAPE forecast between 750-1250 J/kg off the latest HREF mean. The combination of increasing moisture anomalies and environmental buoyancy adds favor to an isolated flash flood risk, especially within the terrain and adjacent locales. A MRGL risk was maintained from previous forecast, but trimmed on the northern edge given the expected timing of the moisture push to be delayed for a greater convective threat within Pinal/Maricopa counties. This area will have more favorability later on D3. Kleebauer Day 3 Valid 12Z Sun Sep 15 2024 - 12Z Mon Sep 16 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE MID AND LOWER MISSISSIPPI VALLEYS INTO THE SOUTHEAST, AS WELL AS THE DESERT SOUTHWEST... ....Southeast... The remnant circulation and moisture from Francine will stick around for one more period to round out the weekend with scattered thunderstorm activity prevailing across much of the Southeast up through the Mid-Mississippi Valley. Amounts currently are lower given the reduced ascent pattern as the upper energy starts becoming more diffuse in its representation, stunting the potential for greater convective impact compared to previous periods. Local totals >2" are still plausible in the setup and the fact there will be overlap in some of the harder hit locations from the D1-2 time frames, there could end up being a targeted SLGT risk just based off the ensemble QPF distribution. For now, a broad MRGL will suffice given the expected synoptic pattern and convective environment forecast. ....Southwest... Elevated moisture returns over the Desert Southwest will aid in more widespread thunderstorm development during peak diurnal instability leading to scattered flash flood concerns across much of AZ, Western NM, into the Four Corners. The primary areas of interest will be the favored complex terrain locations, urbanized zones, slot canyons, and remnant burn scars littered across the region. A targeted SLGT is possible in future updates, especially across the terrain in AZ. Kleebauer Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!40P41T32eAltOXiDX3rGfjc9EVhfm5KU9-___wEbaO98= VCQNOVBugSZq0-FcpykZQDS9Y_LrXVjOdwLhJ9-9X-kmSPY$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!40P41T32eAltOXiDX3rGfjc9EVhfm5KU9-___wEbaO98= VCQNOVBugSZq0-FcpykZQDS9Y_LrXVjOdwLhJ9-9tnaIA2w$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!40P41T32eAltOXiDX3rGfjc9EVhfm5KU9-___wEbaO98= VCQNOVBugSZq0-FcpykZQDS9Y_LrXVjOdwLhJ9-9nB21ew0$=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 3.20-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .