textproduct: Seattle/Tacoma

SYNOPSIS

Cool and showery conditions will linger across Western Washington today. A brief dry pattern follows Wednesday into early Thursday, with near-normal temperatures and morning fog. The next disturbance arrives by early Friday, bringing a return of rain and mountain snow, as well as cooler temperatures, this weekend through early next week.

SHORT TERM /TODAY THROUGH THURSDAY/

A few lingering showers remain evident on radar early this morning, mostly along the coast and over the mountains. Snow levels finally dipping to around 3000 ft or so, so this could bring an additional light accumulation to the Cascade passes (especially Snoqualmie northward) but most likely accumulations are little if any with the decreasing coverage. Given the abundant residual low-level moisture and clearing skies and light wind, fog is continuing to form in the South Sound south down the I-5 corridor through Lewis County. This will be dense is spots, and those on the road this morning should be prepared for reduced or rapidly fluctuating visibility. A dense fog advisory has been issued through 9 AM for these locations, and may need to be extended if fog is more widespread toward daybreak. Showers are expected to end this morning with chances less than 5 percent by mid morning.

The dry trend with clearing skies continued Wednesday with morning fog and low clouds likely a bit more extensive in coverage as weak high pressure builds overhead. Overnight lows drop down to near 30 with the clear skies and light winds, and a few degrees warmer closer to the city centers. Otherwise, morning fog/low clouds giving way to afternoon sunshine and temperatures around 50 should yield rather pleasant conditions Wednesday. Little change into early Thursday, but the weak high pressure moves on and the next disturbance approaches the coastline by Thursday afternoon spreading another round of cloudy conditions and precipitation into Western Washington.

LONG TERM /THURSDAY NIGHT THROUGH MONDAY/

Ensemble guidance remains in good agreement with the next system moving into the region Thursday, though some variability in just how strong this frontal system is. The common thread among the majority of ensemble guidance at this point is the message of a return of mountain snow, lowland rain as the pattern remains unsettled. Fairly strong agreement that the upper trough will remain positioned over the region through the weekend, maintaining additional disturbances moving through. While a single time period doesn't appear particularly heavy for snow in the mountains, the persistent rounds of showers will maintain winter conditions over the Cascade passes even if amounts remain short of advisory thresholds for any particular storm.

The ensemble mean - and in fact the vast majority of members - take the upper trough down into the northern portion of California by Sunday, opening the door for colder conditions into the region. As such, there is the potential for any showers during this time (especially during the cooler overnight time periods Sunday and Monday mornings) to bring some snow mixed in with rain down to the lower elevations. However, given the overall precipitation chances and amounts (along with temperatures near the surface), it remains very unlikely (less than 5%) for any accumulations in the lowlands through at least the start of next week.

AVIATION

Showers continue to persist across the Olympic Peninsula early this morning, but should continue to taper off by 18Z. Elsewhere, a mid-level cloud deck persist across much of the area, with some areas seeing MVFR ceilings, particularly across the northern half of the region. In the southwest interior, terminals are seeing areas of localized fog. These conditions are expected to persist this morning as clouds lift and scatter to the north and fog scatters in the south. VFR conditions will prevail through much of the rest of the day today. There is a strong signal for the development of low clouds and fog across much of the Puget Sound area, potentially as early as 06Z, but in particular from 09-12Z Wednesday morning. Fog will slowly lift and scatter late Wednesday morning into the early afternoon through the south Sound.

Southerly winds 5-10 kt early this morning will ease and become light by around 18Z Tue will flip around to northerly and increase back to around 5-10 kt by 00Z Wed. North winds continue overnight and into Wednesday morning.

KSEA...VFR conditions early this morning with a broken mid-layer deck. This should help limit fog potential this morning, but fog in the vicinity of the terminal cannot be ruled out for a brief period (10-15% prob. btwn 13-17Z). VFR conditions continue through much of the remainder of the period. Higher probabilities of low clouds developing after 09-12Z Wed (up to 35%). South winds 5-8 kt will decrease this morning, flip to N after 18Z Tue, and increase to 8-12 kt this evening and continue overnight.

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MARINE

Broad high pressure will begin to build across the NE Pacific today, allowing for conditions to dry out and winds to remain light over the area waters through Thursday. A relatively weak front will move across the area waters late Thursday into Friday, with additional systems possible through this weekend and into early next week. Periods of small craft winds may be possible during this weekend but confidence in currently low at this point.

Seas 8 to 9 ft this morning will continue to ease to 5 to 6 ft by Wednesday. Seas look to rise Thursday into Thursday night, potentially as high as around 15-17 ft as long period swell arrives to the area. Additional systems look to keep seas in the 10-13 ft range Friday into early next week.

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HYDROLOGY

River flooding is not expected during the next seven days.

SEW WATCHES/WARNINGS/ADVISORIES

WA...Dense Fog Advisory until 9 AM PST this morning for Lowlands of Lewis and Southern Thurston Counties-Middle Chehalis River Valley-Olympia and Southern Puget Sound.

PZ...None.