Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Jan 04 2023 20:08:51 FOUS30 KWBC 042008 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 308 PM EST Wed Jan 04 2023 Day 1 Valid 2004Z Wed Jan 04 2023 - 12Z Thu Jan 05 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF CALIFORNIA... ....20Z Update... Modified the northern/eastern periphery of the Marginal Risk in northern California to better align with concerns about flooding from several hydrologically sensitive burn-scars. Also realigned the Marginal Risk area in the Southeast to remove areas behind the progressive line of convection. Bann ....Previous discussion... ....California... Another atmospheric river event will begin to impact much of California beginning later today and continuing into Thursday, bringing a variety of weather hazards, including heavy rain along the coast, interior valleys, and the Sierra foothills. Deep, westerly to southwesterly flow to the south and ahead of a powerful low will direct a long fetch of copious moisture into the state as it moves toward the Northern California and Oregon coasts today. Precipitation is expected to develop during the day with the arrival of a well-defined warm front and then intensify late in the day into the overnight hours as the trailing cold front and accompanying plume of deep moisture moves onshore. Widespread totals of 2-4 inches are expected 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 early Wednesday, a well-defined shortwave moving through the base of the broader scale trough 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. A Moderate Risk was maintained and expanded along the coast, with an encompassing Slight Risk extending into and covering much of the Sacramento Valley into the Sierra foothills. ....Southeast... Ongoing convection across the Southeast is forecast to persist into the early part of the period, with strong southwesterly flow maintaining an axis of deep moisture ahead of an advancing cold front. While widespread heavy amounts are not expected, instability will likely remain sufficient for heavy rainfall rates. This along with some potential for southwest to northeast training may produce localized heavy amounts and isolated runoff concerns. Kebede/Pereira 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... ....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. 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... ....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. Pereira Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!94ZRONoAOE-LHIGdcnajcoA9PbR3ENPbxIe_nwnwMCi2= iS9AulHDtgWjRkPgB0klnB6zh5GS7NpKAtnBs5yqJ2HFCi4$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!94ZRONoAOE-LHIGdcnajcoA9PbR3ENPbxIe_nwnwMCi2= iS9AulHDtgWjRkPgB0klnB6zh5GS7NpKAtnBs5yqOy5K4cA$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!94ZRONoAOE-LHIGdcnajcoA9PbR3ENPbxIe_nwnwMCi2= iS9AulHDtgWjRkPgB0klnB6zh5GS7NpKAtnBs5yqRIhtB90$=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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