Subj : MESO: Nws Weather Prediction Center College Park Md To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Mar 21 2023 05:42:55 AWUS01 KWNH 210542 FFGMPD CAZ000-211700- Mesoscale Precipitation Discussion 0114 NWS Weather Prediction Center College Park MD 142 AM EDT Tue Mar 21 2023 Areas affected...coastal central to southern CA Concerning...Heavy rainfall...Flash flooding possible Valid 210540Z - 211700Z Summary...A rapid increase in heavy rain is expected for the Coastal Ranges of central CA into portions of southern CA with the greatest probabilities for hourly rain totals over 0.5 inches over southern CA. Rainfall totals through 17Z are likely to range between 1 and 2 inches with peak localized totals of 2 to 3 inches possible. With the arrival of higher rainfall rates will come the potential for flash flooding. Discussion...GOES West imagery through 05Z showed a very dynamic pattern off of the West Coast with a baroclinic wave out ahead of a large closed low tracking east from near 39N 139W. Flow was divergent and diffluent out ahead of a GOES West DMV sampled upper level jet near 150 kt on the south side of the closed low. Infrared imagery showed an area of colder cloud tops approaching the south-central CA coastline at the leading edge of a lead vorticity max located on the southeastern side of the closed low. At the surface, a triple point low was estimated to be near 33.4N 126.8W, with a warm front extending ESE, just south of a region of colder cloud tops on IR imagery. Regional radar reflectivity showed echoes approaching the southern CA coast, but there is still some dry air to work through in the low-mid levels per the 00Z VBG sounding. Short term guidance from the RAP supports the rapid deepening of the triple point low as it moves into an increasingly diffluent/divergent regime of upper level flow, with the surface low becoming the main low as it tracks toward Monterey and San Francisco Bay through 18Z. Warm advection rainfall of moderate to locally heavy intensity will begin to overspread southern CA and the Transverse Ranges through 12Z while light to moderate rain moves into the central CA coast. As the developing storm system's occluded/cold front approaches the coast early this morning, a surge in low level moisture transport will impact areas of southern CA with precipitable water values peaking between 1.0 and 1.2 inches along the coast along with S to SW 850 mb winds of 35 to 50 kt. It is with this surge, after 12Z, that the greatest probabilities of hourly rainfall totals between 0.5 and 0.8 inches are expected, focusing across south to southwest facing terrain of Ventura, Orange, San Diego, southwestern San Bernardino and western Riverside counties. Rainfall totals through 17Z are expected to peak in the 1-2 inch range but with localized totals of 2-3 inches for southern CA. For locations farther north, from near Monterey Bay to Point Conception, stronger dynamics with 850 mb winds peaking between 60-70 kt (per recent RAP forecasts), along with strong diffluence aloft will combine with weak instability and low level winds nearly orthogonal to the Santa Lucia Range to support locally heavy rain but the character should be more showery in nature after the initial surge tied to the frontal passage. These increasing rainfall intensities through early morning will be accompanied by an expanding threat for flash flooding and debris flows due to wet antecedent conditions from previous events across the state of CA. Otto ....Please see https://urldefense.com/v3/__http://www.wpc.ncep.noaa.gov__;!!= DZ3fjg!78qwxOo3oWvTbnLIU8Fj68kcoZuV4LmG6MahvWPrXcXlRT_A-CvluwT2UVcxEEZ1sMYi= SiUPEGKZPPObVrmH08ZDyYc$ for graphic product... ATTN...WFO...HNX...LOX...MTR...SGX...STO... ATTN...RFC...CNRFC...NWC... LAT...LON 37652163 37412126 37042111 36772083 36532061=20 36342044 36082037 35962023 35722011 35491998=20 35281973 35021952 34811943 34621940 34561936=20 34571925 34581908 34621896 34701886 34741883=20 34761878 34741865 34691850 34631833 34541820=20 34411815 34301805 34281792 34311775 34261768=20 34251762 34271756 34321756 34401764 34431763=20 34421751 34381737 34271712 34081695 33901681=20 33711671 33451664 33181648 33021642 32781640=20 32531639 32511651 32401732 32571784 33171854=20 33541943 33882036 34412121 35032164 35932219=20 36922259 37482250 37632219=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) .