Mesoscale Precipitation Discussion 0121 NWS Weather Prediction Center College Park MD 546 PM EDT Sun Mar 31 2024 Areas affected...western Transverse Ranges of southern California Concerning...Heavy rainfall...Flash flooding possible Valid 312143Z - 010330Z Summary...A few slow moving showers/thunderstorms will be capable of producing high rainfall rates of 0.5+ in/hr across western portions of the Transverse Ranges through ~03Z. Localized flash flooding will be possible with potentially higher impacts over areas which received heavy rain over the past 48 hours and/or sensitive burn scars. Discussion...GOES West visible imagery at 2130Z showed the southern and low level portion of a slow moving, vertically stacked low about 150 miles southwest of Los Angeles. This low was slowly moving east and in the process of opening up but weak steering flow was located beneath and northeast of the low center. Visible imagery and local radar imagery from KVBX showed numerous showers over the southern Coastal Ranges into the western Transverse Ranges where the 0-6 km AGL mean wind was from the north at 5-15 kt. Daytime heating has allowed for the development of weak MLCAPE, less than 500 J/kg per 21Z SPC mesoanalysis data, which was supporting ongoing shower activity across the region. An increased risk for flash flooding this afternoon/evening will come from the fact that steering flow is fairly similar in direction and magnitude to low level inflow from the north, which could promote some slow moving and/or training cores of heavy rain. The weak instability and modest moisture (PWATs 0.5 to 0.8 inches) should limit updraft strength but isolated thunder could still occur over the next few hours. The slow movement of heavy rain cores could allow for rainfall rates to exceed 0.5 inches in an hour or less. Due to heavy rain which has impacted Santa Barbara and Ventura counties over the past 2 days, with over 5 inches reported in portions of the Santa Ynez and San Rafael Mountains, sensitivity to flooding has increased in addition to already sensitive burn scar regions. Localized rainfall totals through 03Z of 1-2 inches will be possible, but with much of the rainfall falling over a relatively short period of time. A reduction of any lingering instability after sunset should reduce the threat for flash flooding but conditions will need to be monitored for a lingering threat beyond 03Z. Otto ATTN...WFO...LOX... ATTN...RFC...CNRFC...NWC... LAT...LON 35362017 34871943 34731904 34451880 34161879 33981905 34131948 34221991 34332048 34592073 35022086 35312060Download in GIS format: Shapefile | KML
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