textproduct: Seattle/Tacoma

SYNOPSIS

Weak weather systems moving down the backside of an upper level ridge offshore will move through Western Washington the next few days. One will arrive this morning with another one later tonight into Tuesday. A third weak system will arrive Thursday. The ridge offshore will weaken Thursday night opening the door for a series of stronger systems to reach the area Friday into the first part of next week.

SHORT TERM /TODAY THROUGH WEDNESDAY/

Satellite imagery shows weak upper level ridge off the coast with a weak weather system moving down the backside of the ridge through British Columbia. High clouds over the area. Clear skies Sunday evening allowed fog to form over the Southwest Interior with visibility at Olympia and Chehalis a half mile or less at 3 am/11z. Temperatures were mostly in the 30s.

Weak system moving through Western Washington this morning. Doppler radar is picking up some echoes over the Northwest Interior but with cloud heights above 10000 feet very little if any of the precipitation is reaching the ground. This will continue to be the case as the system moves through the area. With temperatures near freezing in the Northwest Interior if any precipitation does reach the ground in the next few hours could see a few snow flakes/flurries in both Whatcom and Skagit county. For the remainder of the area just a chance of light rain. Fog in the Southwest Interior will dissipate later this morning. Cloud cover will keep high temperatures in the mid 40s.

Another system will follow the same track tonight moving into the area overnight into the early morning hours. This system is a little stronger than the one this morning and actually carves out a bit of a trough with it. Likely pops common across the area but rainfall amounts will be light, less than a tenth of an inch for most locations. Cloud cover will keep temperatures from dropping tonight into Tuesday morning with lows just a few degrees below the highs, upper 30s to mid 40s.

System moving out of the area from north to south later Tuesday morning into Tuesday afternoon. Dry afternoon from Seattle northward. Not much clearing behind the front keeping skies mostly cloudy in the afternoon. Best chances for a little clearing along the North Coast. Highs in the mid 40s to lower 50s.

Upper level ridge offshore nosing into the British Columbia coast Tuesday night and Wednesday with dry northerly flow aloft over Western Washington. Light flow in the lower levels combined with the ridge axis offshore will keep skies mostly cloudy. The cloud cover will limit fog formation with just patchy fog in the fog prone locations like the Southwest Interior Tuesday night and wednesday morning. Lows in the mid 30s to lower 40s. Cloud cover will also keep high temperatures down, mostly in the mid 40s.

November in Seattle was a warm one. The average monthly temperature of 48.6 degrees was 2.1 degrees above normal. This is the 7th warmest November in 81 years of records. The normal average temperature in Seattle for December is 42.0 degrees. In the top 10 warmest Novembers not including this year 8 out of the 10 Decembers had an average temperature below 42.0 degrees. Felton

LONG TERM /WEDNESDAY NIGHT THROUGH SUNDAY/

Extended models and ensembles in good agreement with another weak system moving down the backside of the ridge into Western Washington Thursday. Ridge offshore weakening Thursday night opening the door to a series of stronger systems. The first one arriving Friday with another later in the weekend. The system on Friday could tap into some sub tropical moisture. Air mass over the area not excessively warm, snow levels in the 5000 to 6000 foot range, keeping some of the precipitation in the mountains in the form of snow. Snow levels slowly lowering over the weekend and could be below the higher passes for the round of precipitation later in the weekend. High temperatures in the 40s Thursday will warm a little into the lower 50s for the remainder of the period. Felton

AVIATION

A ridge will move out of the region this morning as a weak shortwave trough and front sweep through the region Today into Tuesday. Flow aloft will remain northwesterly. Radar has showers moving south across the Canadian border this morning (this will primarily affect the northern terminals). Skies are a bit clearer to the south still ahead of the front. The Chehalis Valley remains the primary fog/freezing fog location this morning (between KOLM and KCLS). Satellite/cameras are also showing a few thin patchy fog layers forming (mainly near rivers and cool/sheltered areas) up to Puget Sound and some of the central terminals that are not yet covered with high clouds. This keeps a chance of brief patchy fog/mist possible for central and southern terminals through 18Z as ceilings continue to fill in from the north through the morning. Expect ceilings to gradually lower from VFR down to MVFR from north to south this evening overnight to Tuesday as showers move through. There is also a 40% that the ceilings may dip into IFR criteria Tuesday morning (from Kitsap/Puget Sound through the south interior and Cascades). Calm/light north winds this morning under 5 kt will flip to the south 4-8 kt this evening, returning to the northwest Tuesday morning.

KSEA...Odds remain very low for fog impacting the terminal this morning (although thin patchy fog has been observed to the northeast of the terminal, but it may stay close to the Green River and go through KBFI to the north). Weak front brings a 30% chance of showers to the terminal area this afternoon. Ceilings will gradually lower through the day, with MVFR expected after 03Z Tuesday. There is a 30% chance ceilings may drop below 1,000 ft Tuesday morning. Winds light out of the northeast this morning 3-6 kt will shift to south this afternoon and increase to 4-8 kt overnight, before turning back to the northwest 3-6 kt Tuesday morning.

HPR

MARINE

High pressure will move out of the region today as a weak system moves over the waters today through Tuesday with showers. Northwest winds behind the system Tuesday will increase in the coastal waters with winds of 15-25 kt (especially in the outer coastal waters). Seas will also increase from 4-6 ft Monday to 9-12 ft Tuesday through Wednesday. Benign conditions are expected behind this system, with light winds, and seas returning to 4-6 ft. A stronger system Friday through the weekend may bring small craft advisory level winds to the coastal waters, and a few of the north interior waters. There is also a medium chance at this time for gale winds in the Strait of Juan de Fuca. Seas will also increase to 9-13 ft with this system Friday through the weekend into early next week.

HPR

HYDROLOGY

System reaching the area around Friday looks to possibly have some sub tropical moisture entrained with it. The pattern is progressive and snow levels are not excessively high, 5000 to 6000 feet. River flooding is not expected over the next 7 days.

November rainfall for Seattle was 5.71 inches, 0.60 inches below normal. Only March and August have had above normal rainfall this year. The yearly rain total for Seattle through November 30th is 25.68 inches. The 8th driest January through November in 81 years of records and the driest since 2002 with 25.38 inches. There have only been 6 years since records started in 1945 where the yearly rainfall total in Seattle has been less than 30 inches. The last time was 2000 with 28.66 inches. Felton

SEW WATCHES/WARNINGS/ADVISORIES

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