Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA
Issued by NWS Seattle/Tacoma, WA
064 FXUS66 KSEW 231644 AFDSEW Area Forecast Discussion National Weather Service Seattle WA 944 AM PDT Mon Sep 23 2024 .UPDATE...Low clouds and areas of mist/drizzle this morning will gradually give way to clearing skies through the day. Minor tweaks were made the the forecast this morning to account for the latest trends and short range guidance. Will be taking a closer look at the Wednesday system today. -Wolcott- && .SYNOPSIS...Upper level ridge building over Western Washington today pushing weak system near the Canadian border further north this morning. Ridge continuing to build tonight before shifting east Tuesday. Thermally induced surface trough moving north along the Oregon coast today reaching the Washington coast late tonight with the low level flow going briefly offshore. The thermally induced trough will move inland Tuesday. With the ridge well to the east door open for front to arrive Wednesday with another frontal system Thursday into Friday. Drying trend this weekend. && .SHORT TERM /TODAY THROUGH WEDNESDAY/...Upper level ridge building over Western Washington today. This will push the weak front near the Canadian border north ending the chance of showers for the North Coast, Strait of Juan de Fuca and the Northwest Interior this morning. By 00z Tuesday 500 mb heights in the mid 580 dms. Surface gradients northwesterly this afternoon which will put a little damper on the highs. Increasing sunshine will get highs to near normal, mid 60s to mid 70s. Thermally induced surface trough moving up the Oregon coast today will reach the Washington coast late tonight wit with the low level flow turning briefly offshore. The low level offshore flow will keep temperatures elevated overnight with lows only in the mid 50s to lower 60s. Some places could set record high minimum temperatures ( the Seattle record is 60 set in 2011 ). Upper level ridge shifting east Tuesday with thermally induced trough moving inland. Tuesday the warmest day of the week and could be the warmest day until next spring. Highs in the 70s with the Southwest Interior reaching the lower 80s. Warmest location likely to be the Cascades foothills where mid 80s are possible. Upper level ridge continuing to move east Tuesday night with the thermally induced surface trough now east of the Cascades. Frontal system approaching overnight spreading clouds over the area making for another mild night with lows in the mid and upper 50s. Like Tuesday morning record high minimums are possible. Rain out ahead of the front reaching the North Coast after midnight with a chance of rain mainly west of Puget Sound and over the Northwest Interior. Front moving through Western Washington Wednesday with rain across the area. Highs in the lower 60s. Front east of the Cascades by early Wednesday evening. Stability indexes not that impressive with lifted indexes above 0, lapse rates 5 to 6C and CAPE for the most part less than 200 J/KG. A little extra lift from the Cascades and a convergence zone over King and Snohomish county could add enough lift to produce a thunderstorm Wednesday evening. .LONG TERM /THURSDAY THROUGH SUNDAY/...Extended models continue to show good run to run consistency. Next system right on the heels of Wednesday`s front with a warm front creating a chance of rain Thursday ( looks like it will mostly go by to the north ). Trailing cold front arriving Friday morning with rain out ahead of the front Thursday night. Post frontal showers Friday afternoon into Friday night. Weekend looks dry at this point with the models indicating some weak trofiness over the area but little in the way of moisture to create any shower activity. Temperatures will be on the cool side with highs only in the lower to mid 60s. Felton && .AVIATION...Widespread MVFR to IFR cigs over western Washington this morning. Drizzle has largely ended, but areas of mist linger throughout Puget Sound. Surface winds this morning are southerly, but are expected to transition to a northwesterly this afternoon into the evening. Speeds should generally be 5 kt or less. Cigs are expected to lift to 3000 ft or better generally after 21z-22z. For most terminals, the improvement in conditions should last until Tuesday morning, but for areas along the coast, there will not be much improvement at all and conditions will stay IFR throughout the TAF period. Another round of low cigs and visbys likely after 10Z Tuesday for all terminals. KSEA...MVFR cigs expected at the terminal throughout much of the rest of the day. Improvement will be slow, with ensemble guidance suggesting a 40 to 50 percent chance of cigs above 3000 ft by 21z-22z. VFR conditions likely through Tuesday morning around 11z when another round of lower cigs and visbys arrives. Surface flow will be southerly through 00Z, with veering to northwesterly thereafter. Speeds will be 5 kt or less and the switch should occur largely after more appreciable ceiling improvement occurs. Kristell && .MARINE... weak frontal system will continue to move into British Columbia this morning. High pressure will then build back into the coastal waters today into Tuesday as a thermally induced trough expands northward along the Oregon coast. This will promote northerly and weakly offshore flow tonight through Tuesday. A frontal system will then move into the coastal waters on Wednesday, allowing for flow to transition to southerly and back to onshore. Westerly winds will increase and approach small craft criteria in the wake of the front along the central and eastern Strait of Juan de Fuca Wednesday night into Thursday. A stronger front then looks to move into the region Thursday afternoon into Friday and will bring more widespread small craft winds to the western Washington waters. Seas 4-7 ft will persist across the coastal waters today, before building towards 7-9 ft Tuesday afternoon into Wednesday. Seas may even briefly build to 10 ft across portions of the outer coastal waters Tuesday night into early Wednesday. Seas will then build towards 10-13 ft Thursday afternoon into Friday, with latest GEFS probabilistic wave guidance indicating roughly a 50 to 70 percent chance of seas building to greater than 12 ft over portions of the coastal waters. 14/Kristell && .SEW WATCHES/WARNINGS/ADVISORIES... WA...None. PZ...None. && $$