000 FXUS66 KSEW 032224 AFDSEW Area Forecast Discussion National Weather Service Seattle WA 324 PM PDT Sat Jun 3 2023 .SYNOPSIS...Upper level troughing will continue through the weekend, with low level flow transitioning to northerly flow on Sunday. Fire weather conditions will become more favorable Sunday into early next week as breezy winds combine with drying fuels and low relative humidity values. A ridge will slowly build into the area by mid-week bringing a warming trend with continued dry conditions. Afternoon and evening shower and isolated thunderstorm potential will also increase through the latter half of the week, with a return to a Pacific trough influence by next weekend. && .SHORT TERM /TONIGHT THROUGH TUESDAY/...The mid and upper levels of the atmosphere this afternoon are characterized by an area of closed low pressure along the BC Coast, while downstream from this feature exists a fairly strong ridge of high pressure over the northern tier of the US and into central and eastern Canada. Near the surface, a weak frontal boundary was noted near the WA Coast, while high pressure dominates much of the US West Coast and troughing continues east of the Cascades across E WA and E OR. Abundant sunshine this afternoon across most of W WA this afternoon, especially east of the Olympics. Closer to the Coast, more cloud cover is present, given the lifting of residual marine stratus and cloud cover associated with the weak frontal boundary. Through the evening and into the overnight period, weather conditions will be generally benign with just some passing thin cirrus across the area and continued lower level cloud cover near the coast. A shortwave trough will round the periphery of the aforementioned broader closed near the BC Coast. This system will pass by hardly noticed, although it may be able to squeeze out a shower or two near or just north of the US/Canada border. Onshore flow will strengthen behind it, bringing mostly marine- related impacts to the area and some stratus to the coast. Given mostly clear skies, expect low temps to drop into the upper 40s for most tonight (low 40s down near the Chehalis Valley and south of Olympia. Weak, loosely organized troughing will continue in the mid and upper levels on Sunday, with very weak shortwave activity passing through. Onshore flow will quickly transition to a more northerly flow, becoming breezy by the afternoon, especially nearest water and around the Puget Sound area. These conditions combined with persistent dry conditions will elevate fire weather concerns, particularly south of Puget Sound towards the OR border where the breezy N/NE winds will coincide with relatively dry fuels and min RH values below 30%. See the fire section below for more details, but have issued an SPS to account for the elevated fire threat in this area. Elsewhere, fire starts will still remain possible. Another swift-moving shortwave trough will skirt the area Sunday night and Monday, bringing brief height falls to the northern counties. This feature will also pass largely unnoticed with dry conditions continuing and lows dropping back to similar values as the previous night. Weak upper level troughing to continue on Monday with low level winds remaining breezy out of the north. Elevated fire weather concerns remain, especially south of Puget Sound. Very little change then expected synoptically in the upper levels for Tuesday, however, a warming trend is expected as thermal troughing builds northward, making for high temps back into the 70s (80s for areas south of Puget Sound). Elevated fire concerns to continue. Kovacik .LONG TERM /WEDNESDAY THROUGH SATURDAY/...A ridge will begin to build westward into the region on Wednesday, supported largely by ensemble guidance, with center remaining over the central US. To the west/southwest of this feature will exist a closed area of low pressure, with southerly flow on its eastern periphery extending into OR. Wednesday will be the warmest day of the week as subsidence increases from ridging aloft. Moisture advection from the south could help produce a few afternoon and evening showers and thunderstorms across the southern Cascades, but current forecast confidence in this is low as the deepest moisture looks to remain displaced just to the south. Winds will be light before transitioning back to onshore by the evening. Fire weather conditions will likely still remain elevated given warm and dry conditions. Upper level ridge influence will continue into Thursday, making for continued warm and dry conditions. Daytime heating combined with orographic support may help produce showers and isolated thunderstorms over the Cascades, aided in part by moisture advection from the weakening closed low to the south. Stronger onshore flow in the low levels should knock temperatures down a good 5 degrees or so from Wednesday. More of the same expected for Friday, except with perhaps a slightly more favorable environment for showers across the Cascades as well as slightly cooler temperatures area-wide as low level onshore flow continues and upper level trough influence over the Pacific increases and nudges the ridge slightly eastward. Upper level trough influence may increase a bit more into Saturday, as hinted in the ensemble solutions. This may again help aid in some precip over the higher terrain as well as increase cloud cover along the coast. Kovacik && .AVIATION...Southwest flow aloft continues as upper trough west of Haida Gwaii gradually shifts eastward. The air mass is dry and stable. Increasingly onshore today will likely bring some stratus a little further inland tonight for areas of IFR or low MVFR west of Puget Sound Sunday morning. Any stratus will quickly dissipate by late Sunday morning for VFR regionwide in the afternoon. KSEA...VFR. Surface winds W/NW 5 to 10 knots this afternoon turning northerly tonight...then increasing to 10-15 knots Sunday afternoon. 27 && .MARINE...Surface ridging parked over the coastal and offshore waters combined with lower pressure east of the Cascades will continue to produce onshore flow through the weekend. The onshore gradient will increase today into Sunday in response to a weak system moving onshore over British Columbia. This will produce westerly gales over the central/east strait and small advisory level winds over adjacent waters. In addition, northwesterlies will increase over the coastal waters as surface ridging strengthens offshore and thermally induced low pressure expands over northern CA and OR. The thermal trough to our south will expand northward early next week turning winds more northerly across area waters. 27 && .FIRE WEATHER...An area of low pressure along the BC Coast this afternoon will move slowly eastward through tonight, while at the surface onshore flow will strengthen. By Sunday, low level winds will then begin to turn northerly and will then become breezy by the afternoon. These winds combined with an overall already dry atmosphere column will cause min RH values to drop to around 25% across Puget Sound and lowland and foothill locations to the south towards to OR/WA border. South of Puget Sound, ERC values continue to creep towards 90th percentile values, raising the threat for elevated fire weather concerns. An SPSSEW product has been issued to highlight the potential for quick fire starts and spreads, particularly across this area Sunday afternoon and evening. Elsewhere, fire starts will still be possible. No improvements to conditions are expected into Monday, with fire starts possible area-wide, but with highest concern continuing to be south of Puget Sound to the OR border. Will continue to monitor conditions for possible need for a watch, but given some of the fuel moisture profiles will hold off for now. Overall, expecting breezy N/NE winds to continue with afternoon RH values dropping to near 20%. Conditions will likely remain similar for Tuesday and potentially on Wednesday, given continued warm and dry conditions. Kovacik && .HYDROLOGY...The daily hydrology discussion has ended until the start of the next rainy season; it will only be updated as needed. && .SEW WATCHES/WARNINGS/ADVISORIES... WA...None. PZ...Small Craft Advisory until 2 AM PDT Sunday for West Entrance U.S. Waters Strait Of Juan De Fuca. Gale Warning until 7 AM PDT Sunday for Central U.S. Waters Strait Of Juan De Fuca-East Entrance U.S. Waters Strait Of Juan De Fuca. Small Craft Advisory until 7 AM PDT Sunday for Admiralty Inlet- Northern Inland Waters Including The San Juan Islands. Small Craft Advisory from 11 AM Sunday to 8 PM PDT Monday for Coastal Waters From Cape Flattery To James Island 10 To 60 Nm-Coastal Waters From Cape Flattery To James Island Out 10 Nm-Coastal Waters From James Island To Point Grenville 10 To 60 Nm-Coastal Waters From James Island To Point Grenville Out 10 Nm-Coastal Waters From Point Grenville To Cape Shoalwater 10 To 60 Nm-Coastal Waters From Point Grenville To Cape Shoalwater Out 10 Nm. && $$
NWS SEW Office Area Forecast Discussion
Discussion Script by Ken True at Saratoga Weather - WFO Script by SE Lincoln Weather