Subj : MESO: Nws Weather Prediction Center College Park Md To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Dec 05 2023 15:33:48 AWUS01 KWNH 051533 FFGMPD ORZ000-WAZ000-060300- Mesoscale Precipitation Discussion 1189 NWS Weather Prediction Center College Park MD 1025 AM EST Tue Dec 05 2023 Areas affected...Northwest Oregon...Southwest Washington & Washington Cascades... Concerning...Heavy rainfall Valid 051530Z - 060300Z SUMMARY...Prolonged moderate Atmospheric River (AR) will continue to narrow but remained focused along Central OR Coast and Downstream into S WA/N OR Cascades. Additional 3-6" are possible highly dependent on favorable orographic ascent, adding to ongoing flooding conditions. DISCUSSION...GOES-W WV suite and AMVs denote a bit of shortwave ridging starting to develop across S OR/N CA as the core of the 150kt 3H jet continues to lift out of the Pacific Northwest across Vancouver Island; this is resulting in broadening mid-level baroclinic leaf just off the NW CA, OR coast line which supports some cyclogenesis but also reduces eastward propagation of the low to mid-level steering flow and moisture flux within the core of the AR. Deep layer moisture remains mildly anomalous but has narrowed within the channel with total PWats of 1.2-1.4" confined to about 200 miles in width oriented from SW to NE with the core focused on NW Oregon from N Lincoln/Tillamook county and extending across the Columbia/N Willamette Valley to the Southern Washington Cascades toward Pierce/Lewis county. Due to weak cyclogenesis and exiting upper-level forcing, the gentle isentropic ascent pattern continues to flatten and winds continue to reduce from 50-55kts at 850mb into the upper 30s by after 00z tonight while 700mb winds of 55-60kts drop into the lower 50kt range at similar times. So a slow reduction of IVT from 700 kg/m/s toward 500-550 kg/m/s is expected. Wind direction seems to be static and while not ideally orthogonal to the coastal range (and eventually Cascades), southwest facing terrain will be continually exposed. Wind directions are expected to back toward more southerly by 06z further reducing ideal orographic enhancement. As such, rainfall rates will be highly dependent on orographic ascent/intersection and so will be highly variable spatially if fairly consistent at any given exposure to the wind. As such only the highest peaks will see close to .5" over the next few hours, mainly near the coastal range if at all, but rates near.25-.33"/hr are highly probable for the next 12 to 15hrs across the area and into the southern Washington Cascades and perhaps the northern most Oregon Cascades, while valleys behind the terrain may see less than .1"/hr. As such, additional 3-6" are probable through 06z tonight adding to the 2-5" that have already fallen (highest generally north) . NASA SPoRT LIS soil saturation ratios are over ..85 to nearing 1 through 40cm, so much of the rain will be runoff and those levels of saturation may see debris/mud slides especially in areas of recent burns or removed ground cover. Freezing levels are expected to drop a bit throughout the morning/afternoon from northwest to southeast, but should remain above all but the highest peaks, so there will continue to be rain on snow, while models have been over-estimating snow melt over the last day or so, hourly rates of .1 to .2"/hr are possible adding to those rainfall rates and helping to swell already flooded rivers and streams; however, rapid inundation/flash flooding is considered unlikely moving forward. Gallina ....Please see https://urldefense.com/v3/__http://www.wpc.ncep.noaa.gov__;!!= DZ3fjg!-GzhFmSoKwhaMAuhCrRWd4KcZZWld7Gc-_RPxNEX9BjKpCH8nykuLkk4ht8PEyrUZART= JpzHNgkP3H-T32FSL_ZAIzc$ for graphic product... ATTN...WFO...MFR...PDT...PQR...SEW... ATTN...RFC...NWRFC...NWC... LAT...LON 48262194 48082140 47692128 46482130 45142190=20 44402296 43562385 43432449 43822455 45082428=20 46332403 46832368 46812319 46892257 47182222=20 47892216=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) .