Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Nov 30 2022 00:55:56 FOUS30 KWBC 300055 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 754 PM EST Tue Nov 29 2022 Day 1 Valid 01Z Wed Nov 30 2022 - 12Z Wed Nov 30 2022 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF EASTERN MISSISSIPPI...NORTH-CENTRAL ALABAMA...AND NORTHWEST GEORGIA... ....01Z Update... A rather widespread severe weather outbreak is ongoing right now across portions of the lower MS Valley and the Mid-South with several southwest to northeast oriented clusters of supercell thunderstorms seen impacting areas of northern LA and far southeast AR eastward through a large area of central/northern MS and into northern AL. The activity continues to develop and evolve as a result of confluent and moist low-level southwest flow advancing up across the region from the Gulf of Mexico out ahead of a deep layer trough crossing the Plains and Midwest. The moisture transport is being aided by a low-level jet of 40 to 50+ kts, and this is also fostering the northeast advance of a warm front across the Mid-South. A much more unstable airmass is also advancing downstream from the lower MS Valley with MLCAPE values of 1500 to 2000+ j/kg over much of LA and central/southern MS seen gradually shifting eastward. The combination of strong thermodynamics and enhanced shear/wind profiles within a broadly divergent flow pattern aloft around the southeast flank of the aforementioned trough should maintain a significant threat of organized convection going well into the night across many areas of the South with an increasing threat of excessive rainfall as areas of repeating supercell convection transition gradually to more linear bands of convection capable of training over the same area. Rainfall rates with the ongoing supercell convection continue to be as high as 1 to 2 inches/hour with some spotty 2.5 inch/hour rates noted with the cluster of supercells advancing east across parts of northern AL. The concern overnight is well illustrated by several consecutive runs of the HRRR in conjunction with radar and satellite trends which are all pointing to the idea of convection increasingly becoming oriented with the deeper layer west-southwest flow such that there will be potentially multiple training QLCS type structures impacting areas of eastern MS, central to northern AL, and potentially to some extent later in the night across parts of northwest GA. Multiple runs of the HRRR suggest a swath of as much as 4 to 6 inches of rain across north-central AL in particular, with isolated heavier amounts. Given the low-level jet and convergent moisture feed as seen in the SFC/850mb layer via the CIRA-ALPW data, the rainfall rates are likely to remain high, and the repeating/training convective cell setup will favor high-end rainfall potential at least locally. Areas of flash flooding are likely, and locally significant impacts will be possible for the more urbanized corridors. The biggest change for the evening Day 1 ERO update will be to move the Moderate Risk area from southeast MS and southwest AL where organized convection by comparison has been struggling to gain focus, and place it farther north across portions of eastern MS, north-central AL, and northwest GA. This best aligns with the HRRR guidance and 18Z HREF probabilities, with some adjustments to account for the latest radar trends. Please consult WPC's latest Mesoscale Precipitation Discussions (MPDs) for more details on the evolving excessive rainfall threat overnight. Elsewhere, the Marginal and Slight Risk areas have been tweaked to account for these trends, but are otherwise largely kept intact and especially with respect to the TN Valley and Southeast which will see heavier rains arriving overnight and into early Wednesday. Orrison 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 CALIFORNIA... ....2030z Update... For portions of the Southeast: The Marginal Risk area was adjusted based on the 12z HREF suite, which resulted in a broadening of the southern half of the risk area to encompass more of southern GA and the FL Panhandle. Conversely, the northern extent of the Marginal Risk area was trimmed back significantly (to exclude the Carolinas) as instability looks to be insufficient that far north for the necessary rainfall rates. Overall the Marginal Risk is in good shape categorically, as rates look to diminish significantly relative to the Day 1 outlook. For western OR and northern CA: Small adjustments were also made to the Marginal Risk area across portions of the Northwest, mainly to bring the risk area a bit farther south into coastal northern CA (per the 12z HREF being a bit faster with the southern progression of showers). There is good agreement among the CAMs to indicate localized totals of 1-3+ inches, and rates may occasionally approach (or even exceed) 0.5-0.8"/hr. Churchill ....Previous Discussion... ....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 MUCH OF COASTAL CALIFORNIA... ....2030z Update... Marginal Risk area is in good shape for this update and was only adjusted slightly based on the new guidance and resulting changes in WPC QPF. The only significant change was to include a portion of the Los Angeles Metro and adjacent portions of the San Gabriel Mountains with localized rainfall totals of 1-2+ inches possible (with a small section of coastal southwest Oregon removed due to the faster progression of the system). Churchill ....Previous Discussion... 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!4ccgN-nEkeS6e2Jr7FYhuUgNVSCRLk7vNrPFSvwq_M-h= aR3zMtQ8YADxR3SjbXeQNtZhlV5BAxRqk_8U2bGFnu50UIQ$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!4ccgN-nEkeS6e2Jr7FYhuUgNVSCRLk7vNrPFSvwq_M-h= aR3zMtQ8YADxR3SjbXeQNtZhlV5BAxRqk_8U2bGF4kUNEv4$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!4ccgN-nEkeS6e2Jr7FYhuUgNVSCRLk7vNrPFSvwq_M-h= aR3zMtQ8YADxR3SjbXeQNtZhlV5BAxRqk_8U2bGFh6ZSxUA$=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. 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