Subj : MESO: Nws Weather Prediction Center College Park Md To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Dec 29 2024 04:26:57 AWUS01 KWNH 290426 FFGMPD CAZ000-ORZ000-291625- Mesoscale Precipitation Discussion 1204 NWS Weather Prediction Center College Park MD 1125 PM EST Sat Dec 28 2024 Areas affected...Southwest OR...Northern CA Concerning...Heavy rainfall...Flash flooding possible Valid 290425Z - 291625Z SUMMARY...Increasing rainfall rates associated with strengthening atmospheric river activity will be impacting southwest OR and northern CA overnight and through early Sunday morning. The wet antecedent conditions and additional rains will promote concerns for urban and small stream flooding, including a low-end flash flood threat for any sensitive burn scar locations. DISCUSSION...The latest surface analysis shows a warm front across northern CA and with an offshore extension of this to a developing surface low near 43N 130W. A cold front then extends well southwest away from the low center. The low center will be deepening overnight as it lifts northeastward toward the Pacific Northwest overnight in conjunction with an amplifying shortwave trough. The attendant warm front will slowly lift north with time, but this coupled with deep layer southwest flow should still help to focus a well-defined atmospheric river into the coastal ranges of southwest OR and northwest CA. GOES-W GeoColor satellite imagery shows the southwest extension of moisture offshore of the West Coast, and the latest CMORPH2 and NESDIS Blended Rain Rate data shows a pool of heavier rainfall rates kust offshore in association with the advance of stronger low to mid-level IVT magnitudes. The IVT values should increase to 750 to 1000 kg/ms/s in the 06Z to 12Z time frame across portions of Coos, Curry, Del Norte and Humboldt Counties going from north to south across southwest OR and northwest CA as the core of the strong low to mid-level flow arrives ahead of the offshore cold front and the south side of the aforementioned low center. Enhanced warm air advection, moisture transport and orographic ascent should favor rainfall rates increase to as much as 0.5" to 1.0"/hour across these coastal ranges. Rainfall rates may also approach and briefly exceed a 0.50"/hour for some of the adjacent terrain and counties extending inland into southern parts of the OR Cascades and also down into the Shasta/Sisikiyou ranges of northern CA including also eventually parts of the northern Sierra Nevada early Sunday morning. Some of the heaviest overall rates should tend to be associated with the final arrival and passage of a cold front, and there may be some convective elements associated with the front that will enhance the rainfall rates. After the cold front passes through, the rates will then quickly slacken, but additional rainfall totals over the next 6 to 12 hours are forecast to reach as high as 3 to 6+ inches for the favored coastal ranges, and with as much as 1 to 3 inches elsewhere. Some of these heavier rains may also reach down the coast into the Bay Area Sunday morning. Given the wet antecedent conditions and high streamflows, these additional rains are likely to increase the concern for areas of urban and small stream flooding, and there may be at least a localized flash flood threat should some of these heavier rainfall rates overlap any of the more sensitive burn scar locations. Orrison ....Please see https://urldefense.com/v3/__http://www.wpc.ncep.noaa.gov__;!!= DZ3fjg!8dOWZUGZOi6H9JYNkcqb2e7NJkXCb7ced17aC56rmajOFfoUdP0hiVS36up0JBynBsbt= KlEz6OyACTuHTemwR1GXNhc$ for graphic product... ATTN...WFO...EKA...MFR...MTR...PDT...PQR...REV...STO... ATTN...RFC...CNRFC...NWRFC...NWC... LAT...LON 44712205 44262132 43742131 43252177 42792210=20 42532272 42132301 41792271 41522238 41502185=20 41262169 41012151 40382131 39562038 39022023=20 38822057 39092111 39682161 40372216 40452237=20 40322278 39782256 39342255 38702206 38012172=20 37282169 36972215 37352271 38142330 39372423=20 40592459 41602465 42482464 43212459 43962399=20 =20=20=20=20=20=20=20=20=20=20=20=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 3.20-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .