AWUS01 KWNH 260415 FFGMPD CAZ000-261615- Mesoscale Precipitation Discussion 1167 NWS Weather Prediction Center College Park MD 1114 PM EST Mon Nov 25 2024 Areas affected...Portions of Central and Southern CA Concerning...Heavy rainfall...Flash flooding possible Valid 260415Z - 261615Z SUMMARY...Moderately strong atmospheric river to continue overnight and through Tuesday morning across portions of the central and southern CA coastal ranges, San Joaquin Valley, and southern Sierra Nevada. Some areal flooding concerns and a low-end burn scar flash flood threat will exist. DISCUSSION...The GOES-W Airmass RGB satellite imagery in conjunction with CIRA-ALPW data shows a rather well-defined and moderately strong atmospheric river advancing inland across coastal areas of central and southern CA, the San Joaquin Valley and into the higher terrain of the southern Sierra Nevada. So far, the heaviest rainfall rates have been in the foothills of Fresno and Tulare Counties where some occasional rainfall rates of 0.50"+/hour have already been observed. The moisture transport over the coastal ranges and into the interior foothills is relatively strong with IVT magnitudes reaching as high as 500 to 750 kg/m/s, and this is associated with 850/700 mb moisture flux anomalies of 3 to 4 standard deviations above normal. There is a rather substantial subtropical moisture connection to this atmospheric river with PWs that are running anywhere from 2 to 4 standard deviations above normal, and CIRA-ALPW showing the deeper layer moisture axis extending well offshore of the West Coast all the way down to near 30N 140W where there is close proximity of a deeper layer trough. Embedded within this moist southern stream flow are multiple shortwave impulses interacting with a frontal zone and there has been at least some weak surface wave activity traversing this front, including one wave that has advanced inland to the northeast of Monterey. Some additional increase in IVT values are forecast overnight, and especially in the 06Z to 12Z time frame as additional stronger energy from offshore arrives, and this should promote some additional localized increase in rainfall rates. The coastal ranges of Monterey and San Luis Obispo Counties may see rainfall rates increase into the 0.25" to 0.50"/hour range on occasion. However, with the spillover of stronger moisture transport into the foothills of the southern Sierra Nevada, and with favorably orthogonal orientation of the flow into the terrain, the rainfall rates here below snow level should occasionally reach into the 0.50" to 0.75"/hour range with possibly a couple instances of rates higher than this which is supported by the 00Z HREF guidance. Additional rainfall amounts going through Tuesday morning may reach 1 to 3 inches for the coastal ranges, and as high as 3 to 6 inches for the foothills of the southern Sierra Nevada. Given the persistence overnight and into early Tuesday morning of heavier rainfall rates and the corresponding storm totals, there may be some areal flooding concerns along with at least a low-end threat for some burn scar flash flooding. Orrison ...Please see www.wpc.ncep.noaa.gov for graphic product... ATTN...WFO...HNX...LOX...MTR...STO... ATTN...RFC...CNRFC...NWC... LAT...LON 37841961 37471911 36941866 36351843 35881835 35581837 35451868 35721934 35511987 35201997 34761977 34532004 34582066 35352097 35722133 36332183 36852153 37232057 37722011