Subj : MESO: Nws Weather Prediction Center College Park Md To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Dec 17 2024 22:04:14 AWUS01 KWNH 172204 FFGMPD ORZ000-WAZ000-181000- Mesoscale Precipitation Discussion 1177 NWS Weather Prediction Center College Park MD 503 PM EST Tue Dec 17 2024 Areas affected...western WA into northwestern OR Concerning...Heavy rainfall Valid 172202Z - 181000Z SUMMARY...Areas of heavy rainfall will continue to affect western WA into northwestern OR through tonight. While high rainfall rates will not not be constant over the next 6-12 hours, rates will occasionally surpass 0.5 in/hr and locally approach 1 in/hr into the favored terrain leading to an additional 2-5 inches of rain through Wednesday morning. DISCUSSION...GOES West water vapor imagery at 21Z showed a vorticity max embedded within a broader mid-upper level trough axis near 41N 141W, tracking toward the east and preceded by a 130-150 kt upper level jet. A cold front was analyzed at the surface out ahead of the upper trough with a surface wave near 42N 135W. An atmospheric river was noted with peak PWAT values of 1.2 to 1.3 inches on CIRA Advected TPW imagery and RAP analysis data ahead of the cold front to the west of WA/OR. PWAT values along the coast, near the mouth of the Columbia River, were just below 1.2 inches via GPS data, but when combined with southwesterly 850 mb winds of 40-50 kt, IVT values were estimated to be near 700 kg/m/s along the southern WA/northern OR coast. Downstream rainfall rates into the northern OR Coastal Ranges have already been observed in the 0.5 to near 1.0 in/hr range from earlier today, but radar/satellite imagery indicated the heaviest rain was shifting north into western WA. Water vapor imagery does not suggest the base of the upper trough is amplifying any more to the south and may even be beginning to lift slightly north. A general E to ENE motion to the upper trough is forecast by RAP guidance through the overnight which will continue to translate the greatest magnitude of IVT values northward across western WA, with the peak axis focusing primarily into Vancouver Island just after 00Z, perhaps clipping far northwestern Clallam County in WA. Peak rainfall rates along the WA Coastal Ranges into the Olympics within the northward shifting IVT axis should reach into the 0.5 to 0.8 in/hr range, perhaps isolated values near 1.0 in/hr. A temporary lull in heavy rain is expected for much of the Pacific Northwest just after 00Z followed by another surge as the upper trough and surface cold front approach overnight. From roughly 06Z onward, IVT values are forecast to surge to near 1000 kg/m/s along the WA/OR coast via recent RAP guidance, but only last for an hour or two at that magnitude. The overnight surge is likely to produce rainfall rates in the 0.5 to 0.8 inch range with localized values near or in excess of 1.0 in/hr. Additional rainfall totals of 2-4 inches are likely to occur through 09Z for the WA Coastal Ranges into northwestern OR, along with the upslope regions of the WA Cascades. Isolated totals in excess of 5 inches cannot be ruled out, especially in any favored southwest facing slopes of the Olympics where the duration of higher rainfall rates is expected to be the longest. Gauges showed 24 hour totals of 1-2 inches for the Coastal Ranges and Cascades so far, with localized maxima near 3 inches. Earlier collaboration with the National Water Center and NWM output suggests flooding should be isolated at best, partially due to dry antecedent conditions, but 48 hour rainfall totals of 5-7 inches through Wednesday morning will likely support notable rises on area creeks/streams. Otto ....Please see https://urldefense.com/v3/__http://www.wpc.ncep.noaa.gov__;!!= DZ3fjg!-E06sBTiKv_1XE9fjB-Sa5EcS8WulePp0i71Bzc_2R7jrYf7wCg9heGD1yHABLvMe-a7= 6QL8Ggj5rYHY1DZKV4_0uSQ$ for graphic product... ATTN...WFO...OTX...PDT...PQR...SEW... ATTN...RFC...NWRFC...NWC... LAT...LON 49102238 49072164 48862117 48662101 48372094=20 48102088 47782096 47562096 47322103 47122107=20 46952103 46782107 46562129 46332135 46172137=20 45982159 45782159 45592169 45472197 45512216=20 45702243 45852246 46072267 46182270 46422248=20 46632258 46742255 46872228 46962223 47042238=20 47012275 46822305 46672311 46502308 46332320=20 46272340 46112344 45962345 45672333 45552337=20 45382341 45282347 45172354 44982347 44792356=20 44722384 44982433 45492455 46732462 48242509=20 48442453 48282397 48302321 48622290 48872289=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 3.20-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .