textproduct: Seattle/Tacoma

SYNOPSIS

An upper level trough over Western Washington today will shift eastward tonight. A weak upper ridge will produce a brief drying trend on Sunday before a series of systems produce cool and unsettled conditions for the first half of the coming week. A pattern shift toward dry and considerably warmer conditions is expected late next week as strong high pressure aloft builds into the region.

SHORT TERM /TONIGHT THROUGH MONDAY NIGHT/

Daytime heating coupled with an upper trough axis over Western Washington will produce scattered showers, isolated thunderstorms and unseasonably cool temperatures into this evening. The higher terrain of the Cascades and Olympics above 4500 feet will see some light snowfall, but it is not expected to produce any travel impacts across the passes. The trough will shift eastward tonight and a weak shortwave ridge will slide onshore on Sunday allowing the area to briefly dry out though temperatures will remain below normal. The next in a series of troughs will spread additional clouds and showers into the region Sunday night into Monday. It's eastward progression will, however, be hindered as a weak surface low developing off the Oregon coast splits the system apart and takes the focus of the bulk of the precipitation into Oregon.

LONG TERM /TUESDAY THROUGH SATURDAY/

The cool and unsettled pattern continues into the middle of the coming week as another upper trough digging southward along the British Columbia coast keeps showers and cool temperatures in the forecast.

The narrative takes a decided turn from that point forward. Ensembles are virtually unanimous that we'll see a substantive change in the synoptic pattern heading into late next week and the days that follow. A building upper ridge will produce a warming trend that will result in temperatures climbing back to just above normal levels swiftly on Thursday. The climb upward continues as we round out the week and likely beyond. High temperatures by the beginning of next weekend could run 10 to 15 degrees above seasonal averages as anomalously strong upper ridging takes up residence just to our west.

Longer term guidance strongly suggests that the upward trend in temperatures will persist beyond the 7 day period. Probabilistic HeatRisk suggests a 90-100% chance of Minor HeatRisk across much of the lowlands by Friday and Saturday and a 40-70% probability of Moderate HeatRisk on Saturday for the interior lowlands of south Puget Sound and the Southwest Interior. Though it's early yet, current probabilistic data has a considerable portion of the CWA likely reaching Moderate HeatRisk as we enter the June 14th/15th period with potential daytime temperatures in the mid 80s to mid 90s for interior areas from Seattle southward. Stay tuned.

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AVIATION

An upper trough axis over the interior this afternoon will shift east of the Cascades tonight. Westerly flow aloft will become north to northwesterly tonight. A combination of daytime heating and cool temperatures aloft will destabilize the air mass this afternoon for scattered showers and isolated thunderstorms. Low level convergence will enhance this across central Puget Sound. Shower coverage will diminish this evening. MVFR ceilings can be expected around heavier showers and/or isolated thunderstorms. Remnant low level moisture and light surface winds will lead to greater coverage of MVFR ceilings across the region on Sunday morning. This will gradually lift to VFR Sunday afternoon.

KSEA...Tempo MVFR ceilings expected around some of the heavier showers today. Confidence in thunderstorm activity in the vicinity of the terminal remains fairly high. Gusty and erratic winds can be expected around any storms that do develop. More predominant MVFR ceilings are expected Sunday AM as remnant low level moisture remains in the area. These will lift to VFR Sunday afternoon. Surface winds south to southwesterly 9 to 13 knots into the afternoon. The convergence zone is expected to shift into the vicinity of the terminal for few hours this evening for a brief to switch to NW-NE winds. Winds will switch back to prevailing southerlies after 06Z.

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MARINE

A weak trough over the coastal waters will shift inland tonight as weak high pressure builds just offshore. Another trough and associated frontal system will approach the outer coastal waters Sunday evening then slow and dissipate in response to a weak surface low moving toward the Oregon coast. Onshore flow will increase Tuesday and Wednesday as broad high pressure gradually expands into the coastal and offshore waters. Onshore flow weakens late next week as thermally induced low pressure begins to expand northward across Western Oregon.

Coastal seas are expected to remain below 10 feet through the period.

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FIRE WEATHER

Although lightning will be possible today and rainfall amounts may not quite make it up to the 0.25 in threshold separating wet and dry storms, Min RH values exceeding 60 pct should limit any fire weather concerns today. As the generally troughy pattern remains in place over W WA in the first half of the upcoming week, again, some limiting moisture is expected to remain in place.

As upper level ridging builds into the area starting Thursday, conditions, including fuels, will need to be monitored as temperatures are expected to climb to above normal values entering into the week 2 time period.

SEW WATCHES/WARNINGS/ADVISORIES

WA...None. PZ...Small Craft Advisory from 5 PM this afternoon to 5 AM PDT Sunday for Central U.S. Waters Strait Of Juan De Fuca- East Entrance U.S. Waters Strait Of Juan De Fuca.