Subj : MESO: Nws Weather Prediction Center College Park Md To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Mar 09 2023 18:12:53 AWUS01 KWNH 091812 FFGMPD CAZ000-ORZ000-100600- Mesoscale Precipitation Discussion 0092 NWS Weather Prediction Center College Park MD 111 PM EST Thu Mar 09 2023 Areas affected...Much of Northern and Central California Concerning...Heavy rainfall...Flash flooding possible Valid 091800Z - 100600Z Summary...An atmospheric river will come ashore the northern/central California coast today, with localized rainfall rates increasing to as high as 0.5-1.0"/hr (with localized totals over 6-9 hours reaching 2-5 inches). While snowpack will likely help to minimize the extent flooding impacts in the short term, isolated instances of rapid onset flooding will be possible. Discussion...An atmospheric river (AR) is nearing initial landfall along the northern/central California coast this morning, with radar imagery indicating light to moderate rainfall beginning to gradually overspread the area. An upper-level low is evident via GOES-West imagery, about 1000 miles north of Hawaii, directing copious amounts of low-level moisture northeastward from the sub-tropics into the mid-latitudes. Blended total precipitable water (TPW) imagery depicts this defined AR well, indicating values of 1.5-2.0 inches streaming as far northward as 30deg N latitude (arcing northeast to about halfway between Hawaii and California). The AR becomes less defined/consolidated north of 30deg N, with broadening TPW values generally between 1.0-1.5 inches (which is still about 150-200% of normal between 35-40deg N latitude, per the TPW imagery). This level of tropospheric moisture is also at or above the max moving average of the associated OAK sounding climatology. Another upper-level low digging southward off the coast of WA/OR should also help to focus the moisture and lift into CA, as the associated polar jet phases with the subtropical jet. This is expected to lead to the development of a 140+ kt jet streak over north-central CA, with the right-entrance region likely focusing additional lift into the area by late this afternoon and evening. At the lower levels (~850 mb), the flow is expected to accelerate to 50-70 kts, allowing for rain rates locally to reach 0.5-1.0" (primarily forced by terrain, whereas more typical peak rates will be closer to a quarter to half inch per hour). The bulk of the precipitation is expected to occur after 21z, with forecast localized totals through 06z of 2-5 inches (primarily forced by terrain along the northern/central CA coastline, but also to some extent along the upslope portions of the Sierra). Peak 3-hourly localized totals during this time frame are expected to range from 1-2 inches. Precipitation of this magnitude is already approaching the associated flash flood guidance thresholds, but a major complicating factor for realizing flooding impacts is preexisting snowpack across the region (even well into the lower elevations). On the one hand, snowpack will act like a sponge to absorb much of the rainfall and limit rapid runoff, but on the other hand the rapidly warming conditions could lead to increased runoff over a longer period of time (and is more likely to occur at lower elevations where the snowpack is more shallow). In the nearer term (for purposes of this MPD), the snowpack is expected to primarily limit the extent of any flash flooding impacts, but localized/isolated instances of rapid runoff are still considered possible where the higher localized totals are realized. Churchill ....Please see https://urldefense.com/v3/__http://www.wpc.ncep.noaa.gov__;!!= DZ3fjg!8CIkQGjGXKwwkK-BSWobynkvVGpXsfIPqGV3JJdFVuh0zuAqmZz3WMI00hpy2VJLennJ= 0D1Yo4KQvqzGT5htKyWGLmw$ for graphic product... ATTN...WFO...EKA...HNX...LOX...MFR...MTR...REV...STO... ATTN...RFC...CNRFC...NWRFC...NWC... LAT...LON 42772364 42262318 42282277 42062248 41732237=20 41302165 40922138 40272098 39622028 38962012=20 37971956 37601927 37081908 36571985 35782031=20 35302070 35382127 36052194 37232268 37972335=20 38712389 39682422 40612454 41392441 42512457=20 =20=20=20=20=20=20=20=20=20=20=20=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) .