Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Nov 29 2022 16:10:18 FOUS30 KWBC 291600 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1100 AM EST Tue Nov 29 2022 Day 1 Valid 16Z Tue Nov 29 2022 - 12Z Wed Nov 30 2022 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF SOUTHERN MISSISSIPPI AND ALABAMA... ....16z Update... Little change to the previous outlook, with the main notable adjustment being an expansion of the eastern flank of the Slight Risk a bit southeastward into more of central GA (per the faster 12z HREF guidance). The far northern portion of the Slight Risk area (across portions of TN) was also trimmed back a bit (per the updated guidance). The overall expectation for this event has not changed much, though better exceedance probabilities have consolidated farther north into north-central MS, northern AL, and far northwestern GA (where the 12z HREF suite indicates a corridor of 50-80% neighborhood exceedance probabilities for greater than 3"). Much of this rainfall will be spread out over a 12-hr period, and while the exceedance probabilities certainly support a higher-end Slight Risk area it is difficult to pinpoint an embedded Moderate Risk at this time (especially given the expected supercell storm mode and overall fairly dry antecedent conditions). Farther south across southern MS/AL, probabilities are less impressive overall.. but the wet antecedent conditions and the somewhat prolonged nature of the event supports a low-end Moderate Risk area. Subsequent updates to the Day 1 ERO may be required later today (including a possible targeted Moderate Risk upgrade for north-central MS/northern AL) as the event gets underway. Churchill ....Previous Discussion... A strong shortwave trough ejecting out into the Plains today will lead to deepening cyclogenesis over the Upper Midwest with an associated cold front progressing across portions of the lower Mississippi Valley. This surface low and its associated cold front is expected to track quickly across the Plains and into the Midwest. Deep-layered moisture will expeditiously advect northward within the swift southerly low-level flow ahead of this front all the while steep mid-level lapse rates spread out across the region and the airmass becomes more buoyant. The environment will quickly become favorable for organized convection ahead of and along the approaching front. A severe weather outbreak likely develop over portions of the Lower Mississippi Valley, Gulf states and into the Tennessee Valley. While strong winds and hail will likely be the primary threats with any of these storms, especially over the Mississippi/Tennessee Valleys there is a strong potential for brief periods of very intense rainfall particularly across southern portions of Mississippi and Alabama. Some of the latest hi-res guidance show as much as 3 inches/hour streaking across the central Gulf area as one, possibly two, squall lines push through. Even though the bulk of the storms may occur overnight, they will still be strong enough to cause local flash flooding, especially given the moist soils from recent rains over far southern Mississippi and Alabama but forward speed will be a mitigating factor. That said, there may be some stalling of that squall line over far southern Mississippi and Alabama, but a bit inland from the coast. If that happens the flash flooding threat locally could be substantially higher. Areal averages of 2 to 4 inches are forecast for the south-central U.S. but isolated maxes of 5 to 8+ inches will certainly be possible. Amounts in that quantity would soon exceed local FFG guidance and likely lead to scattered to locally widespread flash flooding. A Moderate Risk was raised across southern Mississippi and Alabama. The ongoing Slight and Marginal Risk areas were adjusted to reflect the latest trends and WPC QPF. Campbell/Wegman Day 2 Valid 12Z Wed Nov 30 2022 - 12Z Thu Dec 01 2022 ....THERE ARE MARGINAL RISKS FOR EXCESSIVE RAINFALL ACROSS PORTIONS OF THE SOUTHEAST AND ALONG THE COAST OF OREGON AND NORTHERN CALIFORNIA... ....Southeast... At the start of this period there will likely be a squall line that has maintained itself from the overnight hours and will be trekking across central Alabama and Georgia. These storms should be fairly fast moving by this point, using up the last of the instability from Tuesday. Recent rains including the previous day's amount have left parts of the Southeast with nearly saturated soils. Additional rains during this period will keep an elevated threat for excessive rainfall and flash flooding. The forcing and instability diminish through the day, so the Marginal Risk is largely for remaining thunderstorms Wednesday morning. ....West Coast... A fast approaching low pressure system will continue to usher in 1 to 3 inches, with isolated higher amounts probable, along the west coast of Oregon and northern California. The highest amounts are expected to focus over southwest Oregon and northwest California as the low begins to sink south by late Wednesday afternoon/evening. Due to the speed of the front and cold air punching in from the north, snow levels will plummet over Oregon. This will reduce the degree of possible flooding concerns. However, the primary area of rainfall will stay ahead of any falling snow levels. Thus, all rain is expected both at the coast and the adjacent mountains. Campbell Day 3 Valid 12Z Thu Dec 01 2022 - 12Z Fri Dec 02 2022 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL ACROSS ALONG COASTAL CALIFORNIA AND SOUTHWEST OREGON... The low will continue to sink south along the West Coast and move inland. The surge of moisture inland will increase during this period, especially for central and southern parts of coastal California. The PW values will be closing in on 1 inch, which is +1.5 to +2 sigma. The nearly perpendicular flow to the terrain will reduced in a fair amount of orographically enhanced rainfall. Recent rains will have already raised soil saturation and increase sensitivity prior to this period. A Marginal Risk area is in effect from southwest Oregon to southern California. Campbell Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!9CDYUp9XYtHtx7t2arQkjVGS-uuy5fdkSeX3i4_zrctZ= FX16BAgdMJjv_iV2VAVKe_03MDne6olX87s-hI9lQhptStQ$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!9CDYUp9XYtHtx7t2arQkjVGS-uuy5fdkSeX3i4_zrctZ= FX16BAgdMJjv_iV2VAVKe_03MDne6olX87s-hI9lw9Yj508$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!9CDYUp9XYtHtx7t2arQkjVGS-uuy5fdkSeX3i4_zrctZ= FX16BAgdMJjv_iV2VAVKe_03MDne6olX87s-hI9lwcniwQk$=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) .