Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Dec 30 2022 08:24:13 FOUS30 KWBC 300824 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 323 AM EST Fri Dec 30 2022 Day 1 Valid 12Z 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. Changes from the inherited forecast were primarily to trim the southern portions of the risk areas...as the atmospheric river appears to be slower in beginning its move southward. Thus, rainfall amounts that could threaten flash flooding are less likely from Fresno south and over the Bay Area on Friday. ....Central Gulf Coast & Mississippi River Valley... In coordination with the MOB/Mobile, AL and LIX/New Orleans, LA forecast offices, trimmed the previous Slight to the south closer to the Gulf Coast. Guidance is nearly unanimous that thunderstorm activity capable of rainfall rates high enough to trigger flash flooding will be confined to the Gulf Coast, as little in the way of instability makes it very far inland. Further, the lines/cells of thunderstorms that are able to form are likely to be progressive moving towards the east, limiting the amount of rain any one location receives from any thunderstorm activity. The Marginal Risk area was maintained further north along the MS River into southern IN, but antecedent very dry conditions combined with thawing of the ground from the recent cold spell should allow the soils to effectively absorb much of the rainfall. As mentioned above, instability will be very lacking in this area, so cells or instances of heavier rainfall are unlikely in this area, further reducing the flooding threat. Wegman Day 2 The Day 2 outlook will be updated by 0830Z. Day 3 The Day 3 outlook will be updated by 0830Z. Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!74SCJwTp1VVMVMO_UYK2L_ILIrF_R0bLfkn9v8XUXIyC= woENWtIrZDpoe1O_tmzM54p1qImKdaRptsv8nBBWHUrRdjg$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!74SCJwTp1VVMVMO_UYK2L_ILIrF_R0bLfkn9v8XUXIyC= woENWtIrZDpoe1O_tmzM54p1qImKdaRptsv8nBBWciNigkM$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!74SCJwTp1VVMVMO_UYK2L_ILIrF_R0bLfkn9v8XUXIyC= woENWtIrZDpoe1O_tmzM54p1qImKdaRptsv8nBBW6-4VAnU$=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) .