textproduct: Seattle/Tacoma

SYNOPSIS

High pressure aloft will briefly weaken over Western Washington today as an upper trough moves across British Columbia. Upper level ridging will rebuild into the region Sunday through Tuesday for a significant warming trend. The ridge will begin to weaken by the middle of the coming week for cooler conditions. A frontal system could bring the regions first chance of rain in more than two weeks by the end of next week.

SHORT TERM /TODAY THROUGH MONDAY/

No major forecast updates have been made this morning. Widespread marine stratus clouds will linger through around mid morning before breaking out for a clear afternoon. See below for the previous discussion and an updated aviation section.

A weak system passing well north of the area this morning has induced a decent onshore push with cooler marine air pouring inland. We'll see areas of morning low clouds getting at least part way inland and most interior locations will shave 7 to 10 degrees off their high temperatures over yesterdays readings. The cooling trend will be short-lived.

Upper level ridging will begin to rebuild into the area on Sunday. Our brief flirtation with low level onshore flow will revert to northerly Sunday afternoon then turn very weakly offshore Sunday night into early Monday as thermally induced low pressure at the surface expands northward into Western Washington once again. This will lead to a pretty good spike in high temperatures for Monday with much of the Puget Sound lowlands seeing highs in the 80s to around 90 in the Southwest Interior. Coastal high temperatures on Monday are far less certain. A number of models suggest the thermal trough will be oriented just inland from the coast. This makes the forecast from the 50th percentile NBM (near 80 at Hoquiam) used in the forecast a bit suspect. In any case, Moderate HeatRisk remains the central story for the interior lowlands on Monday.

LONG TERM /MONDAY NIGHT THROUGH FRIDAY/

An upper level ridge remains over the area into Tuesday, but the low level flow will be weakly onshore. This will cool the coast, but the NBM seems to discount this reality. The thermal trough remains in place across the interior lowlands where most locations away from the water will be well into the 80s and lower 90s with widespread Moderate HeatRisk.

Upper level ridging is expected to begin to weaken as we move beyond the middle of next week. This will lead to a downward trend in temperatures. Ensembles generally agree that anomalous upper troughing will take up residence over the region by the end of next week. Though they disagree somewhat on the overall strength of the trough, the bigger story is the introduction of Western Washington's first chance at measurable precipitation in more than two weeks.

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AVIATION

MVFR cigs for all terminals this morning except for KBLI and KCLM. Breezy southwesterly winds are expected through the morning, easing up somewhat in the afternoon to below 7 kts. The stratus is likely to linger until around 18Z when a more appreciable breakup of the clouds begins. Thereafter, expect a VFR afternoon and evening. Guidance is suggestive of another round of marine stratus impacting western Washington tomorrow morning (30% likelihood of MVFR for inland terminals, 50% likelihood for the immediate coast).

KSEA...MVFR at the terminal presently with cigs hovering around 019-020. A breakout to VFR cigs is still favored for 18Z, continuing to clear through the afternoon. Southwest winds 8-10 kts will continue until around 21Z, decreasing to 7 kts or less thereafter. Opted to put a return to lower clouds as a SCT025 in for 12Z tomorrow morning as guidance suggests another round of marine stratus to make it into the Sound.

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MARINE

High pressure continues to weaken today as a low pressure system moves through British Columbia. Once the low pressure system pushes east of the Cascades, high pressure will quickly rebuild over area waters late this evening and will strengthen through midweek. A frontal system will approach the waters on Thursday, leading to high pressure to weaken.

A Small Craft Advisory continues this morning for elevated seas and winds. Seas and winds will gradually subside this afternoon and remain at 4-7 ft through Monday. Gusty northwesterly winds through the Strait of Georgia this morning may lead to occasional 21 kt or greater gusts over the northern interior waters. These gusts are expected from the San Juan Islands north to Point Roberts.

Moreover, with high pressure strengthening over the waters early next week, increased northerly flow over the coastal waters will bring another chance for small craft winds and seas Monday through midweek. Latest probabilities range between 45-70% for winds 21 kt or greater. Diurnal pushes through the Strait of Juan de Fuca will continue into late next week. Latest guidance suggests that the best chances for for small craft winds will be Wednesday and Thursday. On Wednesday there is a 65-85% chance for small craft winds and a slight chance (25-35%) of gales. The push on Thursday will be slightly weaker, but there is moderate to high confidence (55-70%) for small craft winds, in addition to a slight chance (15-25%) for gales.

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FIRE WEATHER

Hot and dry weather returns in earnest for the first half of next week, with minimum relative humidity values dropping into the 20 percent range or less through Tuesday in the south interior and Cascade valleys. A thermal trough developing slightly inland from the immediate coast on Monday and Tuesday lends itself to a conditionally unstable atmosphere early in the week. Area fuels are getting much closer to critical levels as additional waves of hot and dry weather continues. Grasses are curing expeditiously and have already contributed to a number of roadside brush fires in recent days. Please continue to exercise caution with ignition sources.

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SEW WATCHES/WARNINGS/ADVISORIES

WA...None. PZ...Small Craft Advisory until 11 AM PDT this morning for Coastal Waters From Cape Flattery To James Island Out 10 Nm- Coastal Waters From James Island To Point Grenville Out 10 Nm-Coastal Waters From Point Grenville To Cape Shoalwater Out 10 Nm.

Small Craft Advisory until 11 PM PDT this evening for Coastal Waters From Cape Flattery To James Island 10 To 60 Nm- Coastal Waters From James Island To Point Grenville 10 To 60 Nm-Coastal Waters From Point Grenville To Cape Shoalwater 10 To 60 Nm.