Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Dec 30 2022 20:26:52 FOUS30 KWBC 302026 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 326 PM EST Fri Dec 30 2022 Day 1 Valid 16Z Fri Dec 30 2022 - 12Z Sat Dec 31 2022 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF NORTHERN CALIFORNIA AND THE CENTRAL GULF COAST... ....California... An atmospheric river driving eastward along a 170 kt upper level jet is forcing light to moderate rain across portions of northern CA this morning. For the most part, this forcing will remain stationary, at least for the first half of the day. After that, a strengthening shortwave trough digging into the jet will both begin to weaken and shift it south overnight tonight. Since for most of the day the plume of moisture will be stationary, expect prodigious rainfall into the coastal ranges and the Sierras of northern CA. Rainfall totals of 4-7 inches are expected by Saturday morning in both mountain ranges, particularly between Eureka and the Bay Area and from Fresno north along the Sierras. Snow levels are not expected to move much today, hovering around 9,000 feet, meaning most areas will see rain except the highest peaks of the Sierras, where uplift may help locally lower snow levels. PWATs between 1.0 and 1.25 inches remain in the forecast with anomalies around +3 standard deviations above normal. As usual, the valley locations in between the aforementioned ranges will get significantly less rainfall due to downsloping off the coastal ranges. Finally, the most susceptible areas continue to be those areas high in soil moisture, burn scars, canyons, and urban areas. -Wegman No changes were made to the inherited outlook for the 16Z ERO. Bann ....Central Gulf Coast & Mississippi River Valley... The previously issued Slight Risk area from eastern Louisiana extending across the southern portion of Mississippi and Alabama into the western Florida panhandle looks to be in good shape as convection spreads from west to east. Radar has been showing the core of heaviest rainfall rates/amounts being increasingly confined to areas near the coast while 1-hr rainfall rates have been decreasing to just over 0.5 inches per hour farther north in Mississippi. In addition, the area of heaviest rainfall is expected to remain progressive through the remainder of the morning...which should mitigate some of the risk for excessive rainfall. Farther north...removed the Marginal Risk areas along and near the Mississippi and Ohio Rivers where the morning runs of the CAMs had rainfall amounts generally under 3/4 of an inch.=20 Considering the lack of instability that far north and the on-going heavier convective to the south...rainfall rates should remain sufficiently low to not challenge flash flood guidance. Bann Day 2 Valid 12Z Sat Dec 31 2022 - 12Z Sun Jan 01 2023 ....THERE ARE SLIGHT RISKS OF EXCESSIVE RAINFALL FOR THE COASTAL RANGES OF CENTRAL CALIFORNIA, THE TRANSVERSE RANGES OF SOUTHERN CALIFORNIA, AND THE WESTERN SLOPES OF THE SIERRA NEVADA... ....2030Z Excessive Rainfall Update... An on-going atmospheric river will continue to make its way south and east on Saturday into the early hours of New Years Day. Model guidance has been consistent with that feature and associated precipitation forecast. No changes were needed. ....California... The atmospheric river impacting much of northern California on Friday continues into New Year's Eve Saturday from the Bay Area east into the Sierra Nevadas. Unlike on Friday, by Saturday much of the heaviest rainfall totals are expected in the Sierra Nevada, and less-so into the coastal mountains. This is likely because the digging trough Saturday morning will place the Sierras and interior CA and far western NV more in the left exit region of the jet as it becomes much more amplified with time. Expect another 2 to 4 inches of rain across the northern Sierras Saturday morning before the rain associated with the AR finally begins shifting southward. Behind the atmospheric river, expect snow levels to rapidly fall to between 4,000 and 5,000 feet into New Year's night. This will change over any lingering upslope precipitation over to snow as it ends. The coastal ranges will see rainfall associated with the locally enhanced A.R. being helped by the digging trough being somewhat cancelled out by the increasingly fast southward movement of the moisture plume. The amplified nature of the moisture will direct the rainfall more south to north once it reaches SoCal on New Year's night. Therefore the Transverse Ranges will be favored by a short period of heavy rain during which 2-3 inches of rain are expected. The inherited Slight Risk along the coast was trimmed on the northern end as little lingering rain is expected on the northern CA coast, and also trimmed on the southern end to exclude the Peninsular Ranges. With the atmospheric river more directed south to north, the Peninsular Ranges will not effectively upslope the moisture...unlike the Transverse Ranges. The interior Slight Risk for the western slopes of the Sierras was left unchanged with this update.=20 Wegman Day 3 Valid 12Z Sun Jan 01 2023 - 12Z Mon Jan 02 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF CENTRAL AND SOUTHERN ARIZONA... The digging trough from New Years Night will press eastward into Arizona on New Year's Day Sunday. Expect rainfall to begin early New Year's morning and continue through the day across the Marginal Risk area. Generally expect rainfall totals of 1-2 inches, but with locally higher amounts in the mountains east of Phoenix. Meanwhile further north along the Mogollon Rim and points north, expect snow levels to be low enough that any rain that occurs initially will quickly change over to snow. PWATs of 0.5 to 0.75 inches, or +3 standard deviations above normal, will start out the day in the Marginal Risk area. Thus, moisture amounts will not be a problem on Sunday. Snow levels are expected to begin the day around 8,000 feet...but drop to around 5,000 feet by sunset on Sunday. This should allow many lower elevation areas to see all rain. Wegman The 12Z guidance was larely consistent with the previous runs and on-going outlook needed few...if any...changes. Bann Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!7j2q3A58IarORg1e88HFNcNiZXRFPnehCugJKWcttPSz= EKwkEvEp5Wkxl0Knv_qNU6woc_K_Bqf69IuvBWzzrs8-He8$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!7j2q3A58IarORg1e88HFNcNiZXRFPnehCugJKWcttPSz= EKwkEvEp5Wkxl0Knv_qNU6woc_K_Bqf69IuvBWzz5cCBPdU$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!7j2q3A58IarORg1e88HFNcNiZXRFPnehCugJKWcttPSz= EKwkEvEp5Wkxl0Knv_qNU6woc_K_Bqf69IuvBWzzQEXGaZM$=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) .