Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Jan 04 2023 00:57:37 FOUS30 KWBC 040057 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 756 PM EST Tue Jan 03 2023 Day 1 Valid 01Z Wed Jan 04 2023 - 12Z Wed Jan 04 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF SOUTHERN ALABAMA AND SOUTHWEST GA... ....Central Gulf Coast to the Lower Ohio Valley and Southern Appalachians... Maintained the Moderate Risk for excessive rainfall over portions of the Lower Mississippi where MRMS showed 1 to 3 inches of rainfall earlier today in portions of Alabama and where there is the potential for additional convection as a cold front approaches from the west. Given the amount of instability which lingers in the wake of this round of convection which precludes removal of the Marginal or Slight Risk areas over parts of Louisiana and Mississippi due to re-development of convection later tonight.=20 Short term radar trends also suggested an expansion of the Slight Risk farther into Georgia was needed...an idea supported by the 00Z soundings which showed an air mass with precipitable water values in excess of 1.5 inches positioned upstream and being advected ahead of the convection by west southwesterly low level flow of 35 kts to 45 kts and forced ascent of a coupled upper level jet aloft. About the only area where confidence was sufficiently high to make significant change to the on-going outlook was to remove the Marginal Risk area extending into the central Appalachians where enough dry air has moved in to shut down showers overnight in the most hydrologically sensitive areas. Bann Day 2 Valid 12Z Wed Jan 04 2023 - 12Z Thu Jan 05 2023 ....THERE IS MODERATE RISK FOR EXCESSIVE RAINFALL ALONG NORTHERN AND CENTRAL CALIFORNIA COAST... ....2030 update... No changes were made to the moderate risk area. Minor changes were made to the slight and marginal risk areas to encompass some canyons and mountain passes along the Sierra/southern Cascades. The latest guidance is in reasonable agreement with the overnight suite regarding qpf, rain rates, the low level jet and PWATs. Elsewhere, a marginal risk of excessive rainfall was introduced over portions of the central Appalachians and the Carolina piedmont. ....Previous Discussion... ....California... A deepening upper low is forecast to move across the eastern Pacific toward the Northern California and Oregon coasts on Wednesday. This will direct a long fetch of deep moisture into California, fueling widespread heavy precipitation, including heavy rain, along the coast and the interior valleys.=20 Precipitation is expected to begin with the arrival of a well-defined warm front on Wednesday, which by itself may produce locally heavy amounts across the region. However, the heaviest amounts are expected to unfold later in the day and continuing into the overnight as the trailing cold front and coinciding plume of copious moisture moves onshore. Strong onshore flow along with favorable upper jet forcing will help contribute to heavy precipitation. Widespread accumulations of 2 to 4 inches are expected across northern and central California, with localized heavier totals in the favored terrain. This includes much of the northern and central California coast from the Redwood Coast south through Monterey Bay, where rainfall totals are most likely.=20 Given the wet antecedent soil conditions produced by recent heavy rains and the likelihood for additional heavy amounts, a Moderate Risk was introduced this period, indicating that numerous flash floods and runoff concerns are likely across this area. Kebede/Pereira Day 3 Valid 12Z Thu Jan 05 2023 - 12Z Fri Jan 06 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF CALIFORNIA... ....2030z update... Few minor changes were made to the marginal and slight risk areas along the Sierra and portions of northern California to incorporate some of the gaps and foothills where heavy rainfall may cause flash flooding. QPF has increased over much of northern California since the overnight issuance, therefore an upgrade to a slight risk may be considered if the trend continues. ....Previous discussion... ....California... An atmospheric river event will continue into Thursday, with the best onshore flow and moisture advection translating south across California. A well-defined shortwave moving through the base of the broader scale trough will push the axis of deeper moisture south, focusing the threat for heavy rainfall farther south from the central to southern California coasts and interior valleys.=20 Diminishing moisture and the more transient nature of the system will likely to begin limit the potential for widespread heavy amounts this period. However, the threat for localized heavy amounts should continue, which given the wet antecedent soil conditions may produce localized flash flooding and runoff concerns. Kebede/Pereira Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!6jg3u0LCRBXXE0KB6xVygbcdf54Op-m9hL4uotNNJHdE= HdSszGeWwBC09j6aG-piDVGk3bDMiK68F7pDZzWPjSot-VQ$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6jg3u0LCRBXXE0KB6xVygbcdf54Op-m9hL4uotNNJHdE= HdSszGeWwBC09j6aG-piDVGk3bDMiK68F7pDZzWPfGxI1C8$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6jg3u0LCRBXXE0KB6xVygbcdf54Op-m9hL4uotNNJHdE= HdSszGeWwBC09j6aG-piDVGk3bDMiK68F7pDZzWPyeklMqg$=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) .