Subj : MESO: Nws Weather Prediction Center College Park Md To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Sep 25 2023 16:41:59 AWUS01 KWNH 251641 FFGMPD CAZ000-ORZ000-260200- Mesoscale Precipitation Discussion 1101 NWS Weather Prediction Center College Park MD 1241 PM EDT Mon Sep 25 2023 Areas affected...Coastal northern CA and southern OR Concerning...Heavy rainfall Valid 251640Z - 260200Z Summary...Transient rainfall rates exceeding 0.5"/hr are likely to spread onshore the coasts of northern CA and southern OR this afternoon within a strong atmospheric river. Training of these rain rates could result in locally more than 3 inches of rainfall. Discussion...The ALPW imagery this afternoon depicts a narrow channel of impressive and deep PW shifting towards the Pacific coast ahead of a cold front. This channel of moisture is occurring as a strong atmospheric river (AR), characterized by IVT exceeding 750 kg/m/s according to CW3E, within which PWs are measured by GPS to be 1.2 to 1.5 inches. Concurrent with this AR, a plume of modest instability analyzed by the SPC RAP is reaching 250-500 J/kg, helping to produce rain rates measured by IMERG that have recently eclipsed 0.4"/hr directly within the greatest overlap of IVT/instability. During the next several hours, the surface cold front analyzed by WPC should push onshore and then continue to drop southeast into the Great Basin by this evening. Immediately preceding this front, the strongest IVT should pivot onshore and angle SSW to NNE to favorably intersect the North Coast of CA. Although instability is progged to remain modest, forecast to peak at only 250 J/kg over land, impressive ascent through low-level convergence ahead of the front, some frictional convergence, and increasing upper diffluence within the RRQ of a potent jet streak should be sufficient to produce heavy rain rates. The HREF neighborhood rain rate probabilities reach 20% for 1"/hr later this aftn, highest near Cape Mendocino, coincident with HRRR 15-min rainfall that peaks around 0.25", and this will occur despite a slow weakening noted in the IVT probability forecasts. Although the highest rain rates should be confined to the immediate coast, impressive 850-700mb moisture flux reaching +3 standard deviations on the SREF indicates that upslope flow into terrain, especially into the Klamath, Shasta/Siskiyou, and southern Cascades Ranges, could also result in isolated 0.5"+/hr rain rates. Despite fast general cell motions noted by 850-300mb winds of 60 kts, training of rainfall will likely still result in heavy rain accumulations reaching 1-2", with some locations potentially receiving more than 3" as reflected by HREF and RRFS probabilities reaching 30% and 20%, respectively. Much of this area has been dry, and outside of the scattered 0.5"+/hr rain rates, even where training occurs this should generally be beneficial rain. However, in axes of the heaviest rainfall, or where training of the most intense rain rates occur within vulnerable terrain features or atop recent sensitive burn scars, some runoff impacts are possible. Weiss ....Please see https://urldefense.com/v3/__http://www.wpc.ncep.noaa.gov__;!!= DZ3fjg!8DMlmRxecg3hRGblRkAhz0vTqhICspHtgRtr4FFQMWKlKHSMg2yH1N362h0NSY7vRi8J= hGwDdkUifH61YLa75x6idZE$ for graphic product... ATTN...WFO...EKA...MFR... ATTN...RFC...CNRFC...NWRFC...NWC... LAT...LON 43012281 42792225 42222232 41522256 40782292=20 39852321 38922349 38852367 39332411 39682439=20 40072460 40562452 41302448 41832446 42172453=20 42722447 42992351=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) .