Subj : MESO: Nws Weather Prediction Center College Park Md To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Dec 04 2023 18:01:34 AWUS01 KWNH 041801 FFGMPD ORZ000-WAZ000-050600- Mesoscale Precipitation Discussion 1188 NWS Weather Prediction Center College Park MD 100 PM EST Mon Dec 04 2023 Areas affected...Pacific Northwest Concerning...Heavy rainfall...Flash flooding possible Valid 041800Z - 050600Z SUMMARY...A well-defined surge of atmospheric river activity will arrive this afternoon across the Pacific Northwest which will strengthen this evening. Increasing rainfall rates and storm totals over time will increase the threat of areal runoff problems and urban flooding. DISCUSSION...The latest GOES-W Airmass RGB and WV suite shows a strong deep layer closed low focused over the northeast Pacific Ocean to the south of the Gulf of Alaska near 50N 145W. This energy is gradually advancing off to the east and will be driving a rather significant atmospheric river into the Pacific Northwest this afternoon and tonight. The nose of a 110+ kt upper-level jet is forecast to round the base of the broader upper trough over the next 6 to 12 hours which will shift the height falls closer into the coastal ranges, Olympic Peninsula, and the Cascades along with a well-defined cold front. Strong warm air advection and moisture transport ahead of the cold front and in the wake of a warm front lifting northeast across the Pacific Northwest today will foster increasing rainfall rates into especially the southwest facing slopes of the Olympic Peninsula initially, and then with time inland into western slopes of the Washington Cascades heading through the late afternoon and evening hours. Coastal ranges farther south down into southwest Washington and far northwest Oregon will see much of this concentration of heavier rainfall rates arriving tonight. Enhanced deep layer southwest flow out ahead of the upper trough is forecast to drive IVT values upwards of 750 to 1000 kg/m/s into the terrain, with these values peaking over the Olympic Peninsula in the 21Z to 03Z time frame, and the Cascades and the adjacent coastal ranges down to the south peaking more into the 00Z to 06Z time frame. The overall strength of the moisture transport may begin to subside just a tad by late this evening, but the overall axis of the atmospheric river will also be slowing its advance off to the east as the approaching cold front prepares to cross the Olympic Peninsula and adjacent coastal ranges. The 12Z HREF guidance supports rainfall rates increasing into the 0.50" to 0.75"/hour range in connection with the arrival of the stronger IVT values, with some peak rates, especially over the Olympic Peninsula likely getting into the 0.75" to 1"/hour range which aside from very strong orographic ascent will also be aided by some instability with MUCAPE values locally reaching over 250 J/kg. At least for the next 12 hour period, additional rainfall amounts of 3 to 6 inches are likely over the Olympic Peninsula with slightly lesser amounts over the northern Washington Cascades. Areas farther south over the Cascades and the coastal ranges will see somewhat lesser totals that may reach 2 to 3 inches. This next round of heavier rainfall rates/totals coupled with rising snow levels, snowmelt over the higher terrain, and wet antecedent conditions will increase the threat of areal runoff problems and urban flooding. Orrison ....Please see https://urldefense.com/v3/__http://www.wpc.ncep.noaa.gov__;!!= DZ3fjg!-2hMdPmSfYYSpNPI6zB0M2jpXZ_zskNMz9xABIhBgobRGU2toMSB2Fktk2mNOzkUrdVe= 00i52qHjkYDzPYKv2LthDzQ$ for graphic product... ATTN...WFO...OTX...PDT...PQR...SEW... ATTN...RFC...NWRFC...NWC... LAT...LON 48982142 48642104 48142099 47592106 47312123=20 47042147 46682139 46452145 46002156 45642202=20 45772247 45342320 44922336 44722381 45172420=20 46172427 47482459 48032485 48432445 48232390=20 48072350 47842306 47802268 47822244 48172242=20 48622250 48972199=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) .