Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA
Issued by NWS Seattle/Tacoma, WA
634 FXUS66 KSEW 171718 AFDSEW Area Forecast Discussion National Weather Service Seattle WA 1018 AM PDT Wed Jul 17 2024 .SYNOPSIS...An upper level disturbance will pass over Wednesday with a chance for scattered thunderstorms along the Cascades and Olympics. The mostly dry nature of these storms pose a critical fire weather risk. Surface high pressure will remain the predominant feature through the weekend. A longwave trough slowly begins to approach from offshore into early next week, potentially bringing cooler temperatures. && .UPDATE...No major changes to the forecast below. Continue to watch the potential for thunder this afternoon along the Cascades and Olympics (thunder seen to the south by Hood River). Please see discussion below for more details. HPR && .SHORT TERM /TODAY THROUGH FRIDAY/...A bit busier of a satellite picture this morning as mid-level clouds are making their way from south to north over western Washington. In the low levels, marine stratus is evident over the southern half of the Washington coast, slowly moving northeastward. Water vapor imagery shows the shortwave vorticity maximum centered just offshore of southern Oregon. As clouds continue to move into the region this morning, cooling will be inhibited and low temperatures will only bottom out in the mid 60s for most, mid to low 50s along the coast. Temperatures as of 3AM are in the upper 50s to mid 60s. The negatively-tilted shortwave trough will move northeastward over Oregon and Washington throughout the day today, bringing with it chances for thunderstorm development over the Cascades, with the best chances in the early afternoon to early evening. With the trough directly overhead by this evening, the atmosphere will contain sufficient instability as well as increasing mid-level moisture. A well-mixed and somewhat deep boundary layer (LCL heights reaching above 2000 ft) will limit how much rain reaches the surface, and this will also pose a threat of gusty downdraft winds. The greatest threat of these storms will be lightning, with implications for fire weather as well as outdoor recreation. There is a lesser but non-zero chance of thunderstorm development over the Olympics, especially after the morning inversion has burned off. However, the Olympics will be farther from the main source of forcing and overall model ensemble guidance is much more confident in the thunder potential over the Cascades. Wednesday will be another warm day through the interior, with highs in the mid to upper 80s, reaching the low 90s through the Cascade valleys and foothills. How warm temperatures get will depend on the duration of cloud cover Wednesday morning, as well as the degree of returning onshore flow Wednesday afternoon. The best probability for seeing a high temperature of 90 or greater will be across the south and eastern sides of the Seattle metro area and into the eastern King County lowlands (reaching 35-45%). Thursday and Friday look to be cooler with continued onshore flow behind the through, pushing the marine layer inland each morning. High temperatures Thursday and Friday look to be in the mid 70s to mid 80s, cooler along the coast. Lows will be in the mid 50s to near 60 in the Seattle metro. .LONG TERM /SATURDAY THROUGH TUESDAY/... The large upper level ridge centered over the Four Corners will amplify and retrograde slightly, bringing another warm day to the area with highs in the 80s to low 90s. Temperatures look to begin to cool off starting Sunday as a large upper level low begins to creep towards the area. Ensemble cluster analysis reveals differences in how quickly this trough will make in into the region. Expect a slow cooling trend, with highs potentially as cool as the upper 70s to near 80 into the beginning of next week. Lows may reach the upper 40s to low 50s. LH && .AVIATION...Southerly flow aloft with weak upper level disturbance moving through later this afternoon into the evening hours. Flow aloft becoming southwesterly behind the disturbance. Onshore flow in the lower levels. Air mass somewhat unstable over the higher terrain. Stratus along the central and southern coast this morning spreading inland into the Lower Chehalis Valley with IFR ceilings. Stratus staying west of the Puget Sound and retreating back to the coastline by 17z. Weak upper level disturbance this afternoon could trigger some thunderstorm activity over the Cascades and to a lesser extent the Olympics after 21z. Expecting the storms to stay over the higher terrain with light southerly flow aloft. Thunderstorm chances decreasing rapidly after 04z. Increasing low level onshore flow today with stratus moving inland after 06z with IFR ceilings getting into the Puget Sound after 09z Thursday. KSEA...Mid-level cloud deck this morning thinning out this afternoon. Stratus returning to the terminal around 09z Thursday with IFR ceilings. Light northwest wind becoming southerly 6 knots or less around 15z. Winds trying to come around to northwesterly this afternoon ending up in the 250-280 degree range 22z-04z. Winds returning to southerly 4 to 8 knots after 04z. Thunderstorms over the Cascades 21z-04z could be in the approach path for planes arriving from the east. Felton/McMillian && .MARINE...Onshore gradients increasing through tonight as high pressure builds over the waters. High pressure centered over the offshore waters Thursday through the weekend with varying degrees of onshore flow. Small craft advisory westerlies in the Central and Eastern Strait of Juan de Fuca tonight and again Thursday night. Small craft advisory westerlies also possible in the Strait Sunday night. Felton && .FIRE WEATHER...High resolution guidance continues to suggest the likelihood of abundant lightning moving from south the north this afternoon, particularly in the Cascades. Expect this threat to increase toward midday as this lifts north into the Washington Cascades. There`s a non-zero risk in the Olympics as well, and so the current Red Flag Warnings will be continued unchanged. After, we remain warm and dry across the region which maintains elevated concerns through the remainder of the week. && .CLIMATE...In 80 years of records at Seattle-Tacoma airport this was the warmest first half of July. The average temperature of 72.0 degrees was fractionally warmer than the previous warmest in 2015. The average high temperature of 85.3 degrees was two degrees warmer than the previous mark also set in 2015. The average low temperature of 58.7 degrees was 4th warmest ( 60.6 2015, 58.9 2019 and 58.8 2007 ). Felton && .SEW WATCHES/WARNINGS/ADVISORIES... WA...Red Flag Warning until 11 PM PDT this evening for East Portion of the Olympic Mountains-West Portion of the Olympic Mountains-West Slopes of the Central Cascades Generally above 1500 Feet-West Slopes of the North Cascades Generally above 1500 Feet. PZ...Small Craft Advisory from 6 PM this evening to 5 AM PDT Thursday for Central U.S. Waters Strait Of Juan De Fuca-East Entrance U.S. Waters Strait Of Juan De Fuca. && $$