Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA
Issued by NWS Seattle/Tacoma, WA
480 FXUS66 KSEW 230242 AFDSEW Area Forecast Discussion National Weather Service Seattle WA 742 PM PDT Sun Sep 22 2024 .SYNOPSIS...A flat ridge will give way to a trough over British Columbia tonight. The system will bring a chance of light rain showers overnight for the northern half of the coverage area. A eastward tilted ridge will build back high pressure into the region Monday into Tuesday, with flow becoming slightly offshore leading to warmer temperatures via a thermal trough along the coastline. A couple of frontal systems will move through Wednesday into Friday, with widespread precipitation expected across the entire coverage area. && .SHORT TERM /TONIGHT THROUGH THURSDAY/...The current forecast remains on track with no updates made this evening. A weak shortwave trough will continue to pass close by over British Columbia late today into the overnight. It is a pretty weak one, given it being sandwiched in between two different ridges, and it not being able to dig southward over Washington. High cloud coverage has already settled in much of the region this afternoon with the system approaching. A weak warm front will also pass to the north with this shortwave. This will keep precipitation chances primarily in the northern portion of the coverage area tonight (affecting the North Cascades, lowland areas north of Everett, Olympics and most of the coastline). Areas getting precipitation tonight will get scattered showers, with lowland areas receiving several hundredths of an inch of QPF, and North Cascades/Neah Bay areas potentially seeing locally heavier amounts over a quarter of an inch. A stronger upper level ridge will move into the region Monday into Tuesday, tilting northeastward as it moves inland over much of central Canada. At the same time, a thermal trough will build underneath the ridge along the coastline. This will cause the flow to turn slightly offshore, resulting in increasing high temperatures for the first part of the work week. The cloud coverage holding for part of Monday morning will most likely put a hinder on daytime temperatures (and will only see highs in the upper 60s to low/mid 70s once cloud coverage clears). Cloud coverage will be significantly less Tuesday morning, which will not put as much of a damper on afternoon temperatures with this pattern. Highs will top out in the mid/upper 70s to low 80s, with the warmest temperatures in the South Interior. HeatRisk remains predominately minor (given the short duration of the warmer temperatures). The pattern changes once again going into Wednesday, as the ridge rotates counter-clockwise giving way to a deepening trough over the Pacific. This will swing a significant cold front through the region on Wednesday (dropping high temperatures by around 10 degrees). Southwest flow out ahead of the trough will help tap in moisture for widespread precipitation during the day. Some of the models hint heavy precipitation rates in the far north Cascades, and the Olympics/coastline early Wednesday morning. Any convective banding/thunder that takes place within the precipitation bands may be able to produce locally heavy rainfall rates. The predictability in the amounts of rain remain tough to determine at this point (the 25th and 75th percentiles in the NBM spread about 0.75-1 inch from the mean and 50th percentiles over the coastline). Details on possible rainfall will be refined getting closer to the event. .LONG TERM /THURSDAY THROUGH SUNDAY/...The ensembles agree significantly on the cool/wet pattern continuing through the end of the week. Another shortwave trough will drive another frontal system through the region Thursday/Friday. Again, convective banding with thunder/locally heavy rainfall totals will be possible with this second round, but predictability in amounts remain uncertain (due to wide spreads in the ensembles in possible precipitation amounts). In addition, snow levels falling to around 6,000 to 7,000 feet Thursday/Friday may lead to some snow accumulations at higher mountain elevations in the Cascades. The wet weather will wrap up Friday, and next weekend looks drier with a mix of clouds and sun. High temps will stay cool in the low to mid 60s (significantly cooler in the 40s and 50s at higher elevations). HPR && .AVIATION...Westerly winds will become more northwesterly overnight into Monday as a weak frontal system moves into British Columbia and ridging builds offshore. Surface winds light and variable this evening for most terminals before shifting more southerly with speeds largely 4-8 kts early Monday morning and into the afternoon. VFR conditions in place over the majority of the area with LIFR conditions along the coast. Cigs will continue to lower tonight with widespread MVFR to IFR conditions expected by very early Monday morning. Overnight precip with aforementioned frontal system will be a concern for terminals north of Everett...including BLI and CLM. PAE will merit watching and while precip cannot be ruled out there, confidence is shaky. The mention of vicinity showers seems to cover this well and will opt to leave that classification in place for now. While cigs are expected to lift late Monday morning/early Monday afternoon...improvement will be limited, allowing cigs to get to high-end MVFR at the highest. KSEA...VFR cigs with high clouds. Mid level clouds will increase this evening into tonight. Cigs are expected to lower to MVFR tonight (around 09-10z) and continue through early Monday afternoon. There is also a 25% of IFR cigs Monday morning, mainly from 13-18z. Cigs will slowly lift and scatter later Monday afternoon. Winds light and variable this evening before S/SW winds tonight into Monday morning with speeds 4-8 kts. 18/JD && .MARINE...A weak frontal system will move into British Columbia tonight into early Monday. High pressure will then build back into the region Monday and Tuesday as a thermally induced trough expands northward along the Oregon coast. Weak offshore and northerly flow expected Monday night through Tuesday. A frontal system will then move into the coastal waters Wednesday, bringing a return of onshore flow and increased winds, especially along the coast and Strait of Juan de Fuca. A stronger front looks to move into the area Thursday afternoon into early Friday, perhaps bringing SCA winds for the Coastal Waters and localized SCA winds for interior waters. Seas of 4 to 6 feet will build to 7 feet for the outer Coastal Waters on Monday. Seas will then continue to build from 7 to 9 feet Tuesday afternoon through Wednesday. This period will need to be monitored for near 10 foot seas for the outer Coastal Waters. Seas build further Thursday afternoon into Friday, with GEFS probabilistic wave guidance suggesting a 50 to 70 percent chance of seas greater than 12 feet over the Coastal Waters. JD/18 && .SEW WATCHES/WARNINGS/ADVISORIES... WA...None. PZ...None. && $$