textproduct: Seattle/Tacoma

SYNOPSIS

Weak upper level ridge with light onshore flow in the lower levels today. Upper level low approaching from the southwest Friday will move into the coastal waters Saturday and into Southern British Columbia early Sunday morning. System spinning out of low moving through Saturday with showers and possible thunderstorms. Upper level ridge rebuilding Sunday will move east Monday. Weak system moving through late Tuesday night as an upper level trough develops well off the coast.

SHORT TERM /TODAY THROUGH SATURDAY/

Satellite imagery shows stratus along the coast into the lower Chehalis Valley and about half way down the Strait of Juan de Fuca. Clear skies elsewhere. Temperatures at 3 am/10Z were in the 50s to mid 60s.

Slight change in the pattern today. A little bit weaker onshore flow has resulted in a shallower marine layer. Stratus way behind schedule versus the other mornings in the last week getting into Shelton about two hours later than the last few days. With the weaker onshore gradients stratus not getting east of Puget Sound this morning. A little more sunshine for the interior will push high temperatures up a couple of degrees with highs in the mid 70s to mid 80s. Stratus retreating off the coastline this afternoon warming highs along the coast into the mid 60s to mid 70s.

Changes in the pattern starting tonight. Upper level low well to the southwest beginning to move northeast. This in turn will build the upper level ridge out ahead of the low over the Pacific Northwest. Flow aloft going southerly tonight and remaining southerly Friday as the low continues to slowly move northeast. Light flow in the lower levels will keep the stratus west of Puget Sound again Friday morning but with the southerly flow aloft there will be plenty of middle and high level clouds. Lows tonight in the 50s. Cloud cover thick enough Friday to lower the highs a little, 70s and lower 80s inland and mid 60s to lower 70s coast. Upper level low continuing to move northeast Friday night and Saturday ending up just west of Grays Harbor by 00z Sunday. Upper level disturbance spinning out of the low moving through Western Washington Saturday morning into the afternoon. With some upper level diffluence air mass becoming somewhat unstable as this feature moves through producing showers and a chance of thunderstorms even for the lowlands. Breezy with and just behind the system Saturday. Lows Friday night in the mid 50s to lower 60s. Highs Saturday much cooler with the convective activity, in the 60s and lower 70s. Felton

LONG TERM /SATURDAY NIGHT THROUGH WEDNESDAY/

Upper level low moving into Southern British Columbia by 12z Sunday with shower activity just near the Canadian border early Sunday morning. Lows in the 50s.

Upper level ridge building over Western Washington Sunday moving east Monday. Onshore flow in the lower levels Sunday through Tuesday for an areas morning low clouds and afternoon sunshine forecast. High temperatures near or just slightly below normal.

Deep upper level low in the Gulf of Alaska Monday night through Wednesday. Southwesterly flow aloft over the area. Weak system kicking out of the low brushing the area Tuesday night for a slight chance of showers. Increasing onshore flow behind the system lowering high temperatures back down into the mid 60s to mid 70s for Wednesday. Felton

AVIATION

Light west southwesterly flow aloft becoming southerly tonight and remaining southerly Friday. Light onshore flow today with the low level flow going flat tonight and Friday.

LIFR ceilings west of Puget Sound this morning with local visibilities less than a mile in terrain above 300 feet. Ceilings lifting then dissipating 17z over the lowlands and by 20z along the coast as the marine layer retreats back off the coastline. Puget Sound eastward just some patchy valley fog until 16z.

Increasing middle and high level clouds tonight with increasing southerly flow aloft. Stratus staying west of Puget Sound again Friday morning.

KSEA...Mostly clear skies today with northerly winds 4 to 8 knots increasing to 8 to 12 knots between 23z and 04z. Felton

MARINE

Broad high pressure over the waters will weaken Friday. A weak surface low will move into the coastal waters Saturday with the associated system moving through the inland waters Saturday morning into early afternoon. The low will dissipate Saturday night. High pressure will rebuild Sunday and remain over the area through Monday.

Winds picking up with and behind the system Saturday but at this point looks like the winds will peak around 20 knots over the waters. Diurnal westerly pushes through the Strait of Juan de Fuca also producing up to 20 knot winds in the evening hours.

Seas remaining mostly in the 3 to 5 foot range for the next few days. Felton

FIRE WEATHER

Above normal temperatures the next couple of days but morning lows dropping into the 50s with good relative humidity recoveries keeping fire weather concerns at a minimum.

Upper level moving off the coast Saturday and into Southern British Columbia Saturday night/early Sunday morning. Fire weather concerns elevated Saturday as this feature will bring convection to the area late Friday night through Saturday morning. The highest chance for convection is over the Olympics and Cascades but even the lowlands will have a threat of thunderstorms. Rainfall amounts have increased for this system, and localized wetting rains are possible with any storms that develop alongside gusty and erratic outflow winds and abundant lightning. Cool day Saturday with the convective activity. Increasing onshore flow behind the low will keep temperatures cool and the air mass moist Sunday into Monday. Any lightning strike may not have long to smolder with minimum relative humidity values bottoming out in the 60-70% range Saturday.

Low level onshore flow continuing Monday into Tuesday bringing high relative humidity recoveries in the overnight hours reducing the fire weather concerns. Felton

SEW WATCHES/WARNINGS/ADVISORIES

WA...None. PZ...None.