Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Mar 14 2023 00:44:16 FOUS30 KWBC 140044 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 842 PM EDT Mon Mar 13 2023 Day 1 Valid 01Z Tue Mar 14 2023 - 12Z Tue Mar 14 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF NORTHERN CALIFORNIA... 01Z Update... Radar imagery shows locally heavy rains continuing to advance north into southern New England ahead of the developing nor'easter offshore of the East Coast. The 18Z HREF and recent HRRR guidance favor some 1 to 2 inch rainfall amounts overnight into areas of Connecticut and Rhode Island as stronger onshore flow/forcing along an inverted surface trough sets up across the region. Moisture transport will increase with time, and the 18Z HREF does suggest the potential for some occasional 0.50"+/hour rainfall rates in the 06Z to 12Z time frame. While flash flooding in itself is not likely, the heavy rainfall may result in some areal runoff concerns and potential for localized urban flooding. As a result, the Marginal Risk area has been expanded a bit off to the northeast to include most of Connecticut and Rhode Island. Across southwest Oregon and northwest California, a Pacific cold front has crossed the region and allowed the back edge of heavier precipitation to push off to south and east. Still expecting heavy rainfall overnight though across much of northern California as the cold front slows down and gradually stalls out ahead of deepening low pressure offshore. The latest HRRR guidance supports locally an additional 2 to 3 inches of rain with isolated heavier amounts over the coastal ranges and adjacent foothills of the northern Sierra Nevada. Based on the latest radar and model trends, the Moderate and Slight Risk areas were removed from southwest Oregon and trimmed just a bit for northwest California where the front has passed through. Orrison Previous discussion... ....Southwestern Oregon and Northern California... Organized, heavier precipitation is expected to develop across the region later today as a well-defined shortwave moves south of a weakening low centered east of British Columbia. Southwesterly flow along the associated cold front will focus deeper moisture across southwestern Oregon and Northern California. This moisture along with favorable upper forcing is likely to support periods of moderate to locally heavy precipitation that may to continue into the overnight before the onshore flow is disrupted briefly by another shortwave approaching Northern California. While not expected to be an extreme event, significant amounts are likely over the next 24-hrs, with the HREF showing high probabilities for accumulations of 3 inches or more within the area highlighted by a Moderate Risk. This includes the coastal ranges, the southern Cascades, and the northern Sierra Foothills. For most areas, snow levels will be around 5500-6500 ft early on before dipping lower across southwestern Oregon into far northwestern California as the front moves into the region and stalls. This additional rain, combined with snowmelt, and wet soils are likely to contribute to short-term runoff concerns during this period. ....Coastal New Jersey, New York, and Connecticut... A powerful coastal storm is forecast to move along the Northeast Coast this evening and through the overnight, with heavy rain expected along the coastal region. This rain may accentuate flooding concerns along the coast, promoted by what are forecast to be strong easterly winds on the north side of the system. Pereira Day 2 Valid 12Z Tue Mar 14 2023 - 12Z Wed Mar 15 2023 ....THERE IS A HIGH RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF SOUTHERN CALIFORNIA AND THE SOUTHERN SIERRAS... ....California... 21Z update... The environment will be very favorable for prolonged, terrain-enhanced rainfall along the central and southern coastline and the Sierra foothills. The global guidance and a majority of the hi-res guidance hammer this part of the state with 3 to 6+ inches of rain, some occurring over areas still dealing with the high stream flows of the previous, not so distant AR event. Higher dew points are expected to infiltrate the region which will help prime the snowpack for melting and possibly expedited by the heavy rain, thus increasing the threat for rapid runoff and the areal coverage of flooding. Campbell Another strong storm is expected to impact a large portion of the state this period, including heavy to excessive rains from the central to the southern coast, and along the Sierra Foothills.=20 Models show a shortwave trough off of the Northern California Coast Tuesday morning. This system is expected to amplify as it approaches the coast and interacts with a trough to the north.=20 This will tap a long-fetch of deep moisture, with strong southwesterly winds ahead of the trough spreading moisture inland across the state on Tuesday. PWs are forecast to surge to 4-5 standard deviations above normal on Tuesday along an axis of 40+kt low level inflow centered across Central into Southern California. This anomalous moisture, coupled with strong forcing is likely to produce widespread heavy amounts, especially for the upslope regions in the coastal ranges and the Sierra. Within the Moderate Risk area, rainfall accumulations of 3-6 inches, with locally heavier amounts are expected. In addition to channeling copious moisture, the deep southwesterly winds will push snow levels higher across the region on Tuesday, promoting snowmelt and the threat for runoff concerns. Pereira Day 3 Valid 12Z Wed Mar 15 2023 - 12Z Thu Mar 16 2023 ....THERE IS MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF SOUTHERN CALIFORNIA AND ARIZONA... ....Southern California and Arizona... 21Z update... Minor tweaks were made to both risk areas to reflect the latest trends and WPC QPF, otherwise remain in good order. Campbell Atmospheric river event unfolding Tuesday may continue into early Wednesday, with heavy rain persisting across the Southern California coastal ranges before the upper trough moves further inland and the axis of deeper moisture advection pushes farther south.=20 Southwesterly flow and a coupled jet farther east may promote localized heavy amounts and an isolated concern for flooding in the upslope regions of Arizona on Wednesday. Pereira Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!_Gxr1_ifJsixH1oJNJjOYSIt_m0cwwx84YY-sEDJEqaq= p6LIB8dQSQbuKvvSsfNhdFu-7vpATDBFcuhTYr2d5Uc_JQ4$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_Gxr1_ifJsixH1oJNJjOYSIt_m0cwwx84YY-sEDJEqaq= p6LIB8dQSQbuKvvSsfNhdFu-7vpATDBFcuhTYr2dLiQsJ7Y$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_Gxr1_ifJsixH1oJNJjOYSIt_m0cwwx84YY-sEDJEqaq= p6LIB8dQSQbuKvvSsfNhdFu-7vpATDBFcuhTYr2dP9I-KpM$=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) .