Subj : MESO: Nws Weather Prediction Center College Park Md To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Feb 03 2025 17:58:07 AWUS01 KWNH 031757 FFGMPD CAZ000-040600- Mesoscale Precipitation Discussion 0017 NWS Weather Prediction Center College Park MD 1256 PM EST Mon Feb 03 2025 Areas affected...Northern California & San Francisco Bay Area... Concerning...Heavy rainfall Valid 031800Z - 040600Z SUMMARY...AR continues though orientation will become more NE-SW with time as weak height-falls and shortwave press slow moving cold front southwest toward the Redwood Coast of California and increasing rainfall potential with some embedded convective elements possible with hourly rates of .33-.5". DISCUSSION...18z surface analysis depicts an atypically well defined surface front across NW NV into the northern portions of the CA Sierra Nevada into the southern slopes of the Trinity mountain and toward a weak surface wave near Cape Mendocino.=20 Strong southerly veering to southwesterly flow through the northern Valley continues to intersect and ascend with moderate moisture into the upper 40s/lower 50s in Tds. Cold air into SW OR allows for a fairly steep isentropic ascent pattern to maintain light to moderate rainfall across the rim of mountains across the northern Valley with snow levels still above most but the highest peaks of the terrain. As such, prolonged .15-.25"/hr rates are likely to continue for a few more hours before the orientation of the AR/plume changes with the advance of the front; generally with a fulcrum/pivot point of the plume centered in N CA near Shasta/Siskyou county line. GOES-W IR and WV suite show a defined shortwave comma east-southeast of the 40N130W benchmark continuing to advance rapidly northeast under the influence of the rapidly exiting 150kt jet streak and associated right entrance region. As the ascent/shortwave-ridge pattern passes with this entrance region, influence of surface to 850mb northwesterly flow will allow for eastward progression of the well defined frontal zone. CIRA LPW sfc-850mb layer shows the stark moisture difference across the boundary with solid .5" over-topped by .25-.3" at 850-700mb and further solid core of subtropical moisture already at 700-500mb along/downstream of the shearing shortwave to further increase total moisture to over 1" nearing 1.25" intersecting the Redwood Coast southeast of Cape Mendocino. The shortwave is expected to further elongate/shear while reaching the NW CA coast by 00-02z period and the associated cold front intersection with the coast will drop southeastward. Forty-five to 60 degrees of sfc-850 25-35kt confluent flow will support IVT to increase slowly from 300-400 to 400-600 kg/m/s as the core of deepest moisture overlaps after 00-06z. Some mid-level CAA may support very weak vertical development given 50-150 J/kg possible; though driving mechanism is likely to be more low level convergence and orographic ascent across the Coastal Range. Typical .25-.33"/hr may occasionally increase to .5"/hr across S Mendocino county into Sonoma county after 00z. Scattered showers may extend into the Bay and Santa Cruz mountains late in the period (03-06z), though rates are not likely to be sizable for any flash flooding with exception of most susceptible urban locations. ....Northern Valley/Lower Slopes of Sierra Nevada... Southerly flow will veer to more southwesterly and increase orographic ascent toward perpendicular as the shortwave/DPVA crosses the region after 00z. Solid remaining moisture in the valley will ascent and present solid potential for .25-.5"/hr rates resulting in spotty totals of 2-2.5" by 06z. Given recent heavy rainfall and increased upper soil saturation increased run-off will occur resulting in areal expansion of increased riverine stream flows. All locales and rates are likely to be below critical values for any flashy style flooding and so will keep the tag on this discussion at Heavy Rainfall but will continue to monitor for any highly localized issues that may unfold.=20 Gallina ....Please see https://urldefense.com/v3/__http://www.wpc.ncep.noaa.gov__;!!= DZ3fjg!5NT9hdA1JDhaj4Y1NVc-AQ5Osz_eKHK8Hq3u18ZgRFhg6FFoR-LJP0Siff8Lgcpr-N_u= S4j1gv4W9wxECn3Yf4sNI44$ for graphic product... ATTN...WFO...EKA...MFR...MTR...STO... ATTN...RFC...CNRFC...NWC... LAT...LON 41412218 41392159 41082138 40842145 40682170=20 40402166 40062151 39822115 39602081 39192055=20 38772036 38372059 38532160 38292196 37802207=20 37182183 36892198 36952237 37972305 38442344=20 38962384 39722393 40052434 40502446 40972405=20 40732365 40942346 40832288 41112243=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 3.20-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .