Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Jan 05 2023 00:45:56 FOUS30 KWBC 050045 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 745 PM EST Wed Jan 04 2023 Day 1 Valid 01Z Thu Jan 05 2023 - 12Z Thu Jan 05 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF CALIFORNIA... ....Previous discussion... ....California... An atmospheric river event has begun across portions of California...with models showing an increasing in rainfall rates overnight. While some impacts have already been seen, the concern for flooding is increasing as heavy rainfall becomes more widespread. Deep, westerly to southwesterly flow to the south and ahead of a powerful low over the eastern North Pacific will continue to direct a long fetch of copious moisture into the state and a portion of the adjacent Oregon coast...which will lead to widespread rainfall totals of 2-4 inches across much of Northern California, with locally amounts exceeding 6 inches in some of the favored terrain. In addition to strong onshore flow and moisture flux, favorable upper jet forcing is expected to enhance the potential for widespread heavy amounts across Northern California. By late tonight/early this coming morning, a well-defined shortwave moving through the base of the broader scale trough which will help to orient the better onshore flow and deep moisture farther south, bringing the threat for heavy rain into the Southern Coast and northern Transverse ranges.=20 Given the already saturated grounds produced by the previous weekend's storm, these amounts are expected to produce widespread rapid runoff concerns...along the coast, interior valleys, the Sierra Nevada foothills as well as in the immediate vicinity of several burn scars. ....Southeast... Removed the Marginal Risk area. The forcing along a trailing cold front has weakened and convection over the land has begun to dissipate. Bann Day 2 Valid 12Z Thu Jan 05 2023 - 12Z Fri Jan 06 2023 ....THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS OF CALIFORNIA... ....20:30z update... Minor changes were made to the marginal or slight risk areas from the overnight issuance. Slight risk along the southern Sierra foothills was expanded southward to account for additional burn scars. ....Previous discussion... ....California... Axis of deeper onshore flow and moisture advection will continue to translate, south bringing the threat for heavy precipitation through the remainder of the Transverse Ranges and into the Peninsular Ranges of Southern California, with some additional heavy amounts expected along the Sierra foothills as well. A Slight Risk was maintained for both areas.=20 A Slight Risk was also placed along parts of the Northern California coast, where some of the heaviest amounts in the Day 1 period are expected. While widespread additional heavy amounts are not expected, some enhanced forcing and moisture advection just ahead of the low as it lifts north along the Oregon coast may support some localized heavier amounts, especially in the terrain on Thursday. And given the anticipation of very heavy amounts across this region, there are concerns that even additional light to moderate amounts may accentuate any ongoing runoff concerns across the region. Kebede/Pereira Day 3 Valid 12Z Fri Jan 06 2023 - 12Z Sat Jan 07 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF NORTHERN CALIFORNIA... ....20:30z update... Minor changes were made to the marginal risk as the latest model guidance is in reasonable agreement with what is described in the previous discussion. ....Previous discussion... ....Northern California... Following the expectation of widespread heavy precipitation covering much of California Wednesday and Thursday, a brief break is expected with drier conditions forecast for much of the state early Friday. However, a weakening shortwave and modest plume of moisture are expected to swing across Northern California late Friday into early Saturday. This will likely produce some light to moderate accumulations across the region, which given the anticipated saturated soil conditions may produce some additional runoff concerns. Kebede/Pereira Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!7UhlgSoqs7cddr2-5saK0tjh2YmkEQqf6_Rssb7qggno= sDVgcw1XuWYYY3VZvJxJaJA8jU6KwNoycCdyPX2-6kch0F8$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!7UhlgSoqs7cddr2-5saK0tjh2YmkEQqf6_Rssb7qggno= sDVgcw1XuWYYY3VZvJxJaJA8jU6KwNoycCdyPX2-wpYUICA$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!7UhlgSoqs7cddr2-5saK0tjh2YmkEQqf6_Rssb7qggno= sDVgcw1XuWYYY3VZvJxJaJA8jU6KwNoycCdyPX2-qh_EkUk$=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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