textproduct: Seattle/Tacoma

SYNOPSIS

An upper level trough will move into the region on Wednesday bringing cooler temperatures alongside showers and thunderstorms over the North Cascades Wednesday and Thursday. An upper level ridge begins to rebuild into the region late in the week, but onshore flow is expected to keep temperatures near or only slightly above normal for mid August.

SHORT TERM /TONIGHT THROUGH THURSDAY NIGHT/

Less in the way of morning cloud cover is resulting in somewhat warmer temperatures this afternoon with daytime highs expected to land right around seasonal normals for most spots. An upper level trough will dip southward out of British Columbia on Wednesday with clouds and modestly cooler temperatures. The main concern with this system will be in the fire weather realm with the threat of showers and thunderstorms in the North Cascades, and to a somewhat lesser extent in the Olympics, on Wednesday afternoon and evening. Broad upper troughing remains over the region on Thursday. Lowland areas will see mainly dry conditions with a slight bump in temperatures for interior areas. The presence of the trough will retain some degree of instability for the Cascades with a continued threat of convection again in primarily the afternoon and evening hours.

LONG TERM /FRIDAY THROUGH TUESDAY/

Upper ridging centered well to our east begins to exert a little more influence on the region on Friday. Despite a small recovery in heights, the low level flow remains firmly onshore for little appreciable change in temperatures from Thursday. There is still some uncertainty in the forecast for the weekend and beyond, but ensembles are converging on a somewhat clearer picture. A broad upper trough is expected to dig southward well offshore while strong upper ridging centered east of the Rockies builds westward somewhat. This leads to a modest warming trend toward the weekend and early next week, but onshore flow prevails through the period. As a result, the probabilities for Moderate HeatRisk have lowered for Friday through Monday for interior areas with a general 20% to 40% risk around Seattle metro. Corresponding high temperatures for the warmest portions of the interior are in the lower to mid 80s while coastal areas are held mostly in the 60s. At some point during the extended period, southerly flow aloft could import some monsoonal moisture northward near the Cascade crest and we'll need to watch for additional thunderstorm potential if this does indeed come to fruition.

27

AVIATION

Westerly flow aloft will become southwesterly on Wednesday as an upper level trough moves into the region. Flow aloft then likely to trend more variable Wednesday evening. Onshore flow continues in the low levels throughout the period. Low clouds will push back inland again tonight ahead of an approaching trough with LIFR-IFR ceilings near the coast and MVFR ceilings likely Wednesday morning for much of the interior. Currently, stratus has filled the coast and it's leading edge is near KSHN. Ceilings are expected to lift back to VFR for interior areas Wednesday afternoon. Instability associated with the trough will lead to scattered showers and isolated thunderstorms for the North Cascades and the Olympics Wednesday afternoon and evening.

KSEA...VFR is expected for the remainder of the evening. Low clouds are expected to return on Wednesday morning, but ceilings will more likely fall into MVFR category. Surface winds W/SW under 7 knots will veer N/NW again briefly this evening before backing S/SW again tonight.

27/41

MARINE

Broad surface high pressure will remain centered over the offshore waters with lower pressure east of the Cascades through much of the week ahead. This will maintain varying degrees of onshore flow over the next several days and bring diurnally driven small craft advisory strength westerlies to the central and east Strait of Juan de Fuca at times.

Seas across the coastal waters of 2-4 feet today are expected to increase to 5-7 feet near midweek before subsiding back to 3-5 feet heading into the weekend.

27

FIRE WEATHER

A dry, onshore flow pattern with seasonable temperatures and will continue this afternoon. Ensembles continue to very confidently show an upper level shortwave clipping the area Wednesday afternoon as recent models have brought the shortwave a bit more south over the CWA. This shortwave will provide instability to help develop thunderstorms Wednesday afternoon and evening for the northern and central Cascades. Latest guidance continues to show generally a 20-30% probability over the North and Central Cascades, with the higher probabilities along the border and on the east side. Warm and continued dry conditions remain on track through the second half of the week, although in latest guidance, temperatures have slightly gone down from previous forecasts. It is a little early to back away from expecting warmer and dry conditions into the start of next week...resulting in elevated fire weather concerns. Additionally, potential southerly flow over the weekend could continue low chances for thunderstorms over the Cascades. We'll continue to monitor the evolution of the pattern as it develops.

SEW WATCHES/WARNINGS/ADVISORIES

WA...None. PZ...Small Craft Advisory until 5 AM PDT Wednesday for Central U.S. Waters Strait Of Juan De Fuca-East Entrance U.S. Waters Strait Of Juan De Fuca.