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Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA
Issued by NWS Seattle/Tacoma, WA
392 FXUS66 KSEW 161545 AFDSEW Area Forecast Discussion National Weather Service Seattle WA 845 AM PDT Tue Jul 16 2024 .SYNOPSIS...High pressure will continue to remain over the region for the majority of the week. Onshore flow will continue to allow the marine layer to move inland each night, with slight day-to- day variations. An upper-level shortwave impulse will bring a chance for elevated convection over the Cascades Wednesday. Upper level ridging will remain in place over the region through at least this weekend as a longwave trough slowly begins to approach from offshore into early next week. && .UPDATE...Little bit of a marine layer developed over Puget Sound (but remains relatively isolated and elevated over the water concerns). Couple other spots may have seen some patchy fog this morning, but given that several areas are already approaching 70 degrees approaching 9 am, this will all burn off very quickly. Today remains the warmest day of the week with highs in the upper 80s (with low 90s to the south). Still some uncertainty in temperature spread for Wednesday (depending on cloud cover from a trough and isolated thunderstorm coverage in the Cascades). Please see below for further details (as well as refreshed aviation, marine, and fire weather discussions). HPR && .SHORT TERM /TODAY THROUGH THURSDAY/...Latest satellite imagery shows the marine stratus layer slowly creeping eastward, south of the Olympic mountains into the lower Chehalis Valley and through the Strait of Juan de Fuca and over Whidbey Island. Patchy fog is being observed along the Pacific Coast and the Strait of Juan de Fuca. Stratus and patchy fog will continue to push inland this morning, but weaker pressure gradients compared to yesterday, it will not be able to make it as far inland and skies should remain mostly clear south of Everett, north of Skagit County, and north of Tacoma. Low temperatures this morning should reach the low to mid 50s. With the warm low-level air mass advecting northward into the region and with less marine stratus this morning, temperatures should warm quickly across the region, with high temperatures 8 to 12 degrees warmer compared to yesterday, reaching the mid 80s to near 90, with low to mid 90s possible through the Southwest Interior and Cascade valleys. Mid to upper level cloud cover will increase tonight as a shortwave trough approaches the area from the southwest. This will result in limited cooling overnight, with overnight lows in the low to mid 60s for most of the region. As a result, Tuesday`s HeatRisk values are Orange/Moderate for most of the region, with pockets of Major/Red for the Cascade Valleys and the Duwamish/Green River valley in the Seattle metro. Wednesday will start cloudy as the shortwave trough passes over the region, but skies will clear throughout the day. With the shortwave trough directly overhead, this will be the day with the best chance for elevated convection over the Cascades. There will be some moisture to work with with PWATs from around 0.75 to 1 in (around or slightly positive anomaly), however, it will be mostly contained in the upper levels with a deep and dry boundary layer, with little to no precipitation making it to the surface. The marine layer will help maintain a low-level inversion over the Olympics, though there may be a brief period in the late afternoon when the inversion dissipates in the higher elevation locations and the thermodynamic profile could be realized. Confidence and probability of occurrence is much lower than over the Cascades. The most likely scenario for temperatures is that cloud cover and substantial onshore flow should keep high temperatures slightly cooler than Tuesday, in the low to mid 80s, in the 70s along the coast. This corresponds to Moderate/Orange HeatRisk through the interior north of the Tacoma metro to the Canadian border. If skies clear early, there`s a chance that temperatures could reach the low to mid 90s through the interior and the Cascade valleys. Probabilities for seeing a high temperature of 90 or greater reach up to 30% across the south and eastern sides of the Seattle metro area and into the eastern King County lowlands. Thursday looks to be cooler with continued onshore flow behind the shortwave trough, pushing the marine layer into the interior Thursday morning. .LONG TERM /FRIDAY THROUGH MONDAY/...The pattern look to remain warm and dry with the ridge axis slowly moving eastward with highs through the weekend remaining in the mid 70s to upper 80s and lows in the low to mid 50s. Saturday looks to be the warmest day in the long term with the ridge momentarily amplifying as it retrogrades back towards the west coast. Deterministic and ensemble models are showing a large upper level trough creeping in from the northwest early next week, which could allow temperatures to drop back into the 70s and lows in the upper 40s to low 50s into the beginning of next week, though it depends if the trough will actually able to push the ridge is actually able to sweep the ridge away that quickly. LH && .AVIATION...Upper level ridge over Western Washington with southwesterly flow aloft. Flow aloft becoming southerly later this afternoon through tonight with upper level trough moving into Western Oregon. Onshore flow in the lower levels this morning becoming light northerly this afternoon and tonight. Increasing low level onshore flow Wednesday. Stratus along the coast, down the Strait of Juan de Fuca to near Whidbey Island and inland over the Lower Chehalis Valley is decreasing in coverage. The layer is shallow with tops near 1000 feet. With the stratus, LIFR cigs with local visibility 2 sm or less in fog until 16z. Ceilings lifting to IFR 16z-18z with the stratus clearing out 18z-20z. For the remainder of the area clear skies. Stratus reforming along the coast 03z-06z moving inland after 07z tonight but staying west of Puget Sound. As upper level trough moves into the northern portion of Western Oregon late tonight increasing middle level clouds over the area. There is a chance for a colorful sunrise Wednesday morning with the increasing alto cumulus clouds. KSEA...Clear skies until around 09z Wednesday. Increasing middle level clouds through the morning hours Wednesday. Northerly winds 4 to 8 knots switching to southerly 09z-12z Wednesday. Felton/McMillian && .MARINE...Light onshore gradients this morning becoming northerly this afternoon. Onshore gradients increasing later Wednesday morning into Wednesday afternoon and evening. High pressure over the waters Thursday through the weekend with varying degrees of onshore flow. Small craft advisory winds in the Central and Eastern Strait of Juan de Fuca Wednesday evening into early Thursday morning and possibly again Thursday evening. Felton && .FIRE WEATHER...High pressure will persist over the Pacific Northwest, maintaining warm and dry conditions. Low-level onshore flow across the region will keep marine air present, limiting minimum relative humidity values to 30-35%, with localized pockets as low as 25%. Good relative humidity recovery is expected each night. The primary period of concern is Wednesday as an upper-level disturbance moves across the region, providing ample instability for thunderstorm development over the Cascades. With an unstable atmosphere, as well as lightning risk with very low chances of wetting rains, a Fire Weather Watch remains in effect for zones 658 and 659. It is worth noting that with this pattern and very receptive fuels, even limited lightning could result in significant impacts. LH/Mazurkiewicz && .SEW WATCHES/WARNINGS/ADVISORIES... WA...Fire Weather Watch from this evening through Wednesday evening for West Slopes of the Central Cascades Generally above 1500 Feet-West Slopes of the North Cascades Generally above 1500 Feet. PZ...None. && $$