textproduct: Seattle/Tacoma

SYNOPSIS

Next system to reach the area will approach the coast this afternoon then weaken and move inland tonight. Fast moving upper level trough following the system Friday with another upper level trough right over Western Washington by Saturday afternoon. Both Friday and Saturday will be unseasonably cool. Trough kicks out to the south Saturday night. Upper level ridge offshore building into Southern British Columbia Sunday. Weak system trying to move over the top of the ridge Monday. The ridge rebuilds Tuesday and Wednesday for warmer weather.

SHORT TERM /TODAY THROUGH SATURDAY/

If you were wondering if it was unusual to have a 24 degree drop in high temperatures from day to day in May ( 82 degrees Tuesday, 58 degrees Wednesday ) in Seattle the answer is yes. In 82 years of records there have only been two times where the day to day drop in the high temperature in May was greater than 24 degrees. There were 26 degree drops on May 13th to 14th, 2016 ( 84 to 58 degrees ) and on May 21st to 22nd, 1963 ( 93 to 67 degrees ). In addition to yesterday there have been two other times with a 24 degree drop in one day, May 11th to 12th, 1971 ( 87 to 63 degrees and May 13th to 14th, 1959 ( 80 to 56 degrees ).

Satellite imagery shows mostly cloudy skies over Western Washington this morning. Strong upper level winds over the area with wave clouds over the Cascades into Eastern Washington. Doppler radar shows the convergence zone hanging tough over Snohomish county with isolated showers elsewhere. Temperatures at 3 am/10z were a couple of degrees either side of 50.

Next system to reach the area currently back near 133W will approach the coast but still be offshore late this afternoon. Convergence zone dissipating this morning with continuing isolated showers. Precipitation out ahead of the front spreading over the area this afternoon. Westerly flow aloft will create a small rain shadow over the Central Puget Sound. Highs in the mid 50s to lower 60s.

Front falling apart as it moves inland tonight so will make the precipitation type showers. Showers coming to an end along the coast in the evening with decreasing showers over the interior overnight. A few breaks in the cloud cover will allow temperatures to drop into the 40s.

Fast moving upper level trough moving into Western Washington Friday keeping showers in the forecast. Slight chance, 20 percent or less, of afternoon thunderstorms with the trough becoming negatively tilted as it reaches the area. Very cool day with highs in the lower to mid 50s. Not record setting low maximums but close.

Upper level trough moving out of the area Friday night only to be replaced by an even colder trough Saturday. Snow levels down to around 3500 feet in the mountains by Saturday. Could see a couple inches of snow from Stevens Pass northward as well as at White Pass and Mount Rainier. Air mass somewhat unstable Saturday afternoon with the trough axis right overhead giving Western Washington a chance of thunderstorms. Another very cool day with highs just above record low maximums in the lower to mid 50s. Felton

LONG TERM /SATURDAY NIGHT THROUGH WEDNESDAY/

Upper level trough kicks out to the south Saturday night. Upper level ridge offshore building into Southern British Columbia Sunday. Weak system coming over the top of the ridge Monday with slight chance pops North Cascades. Ridge rebuilding Tuesday and Wednesday.

Warming trend through the period with highs in the lower to mid 60s Sunday and Monday warming to the 60s Tuesday and the 60s to lower 70s Wednesday.

Chilly morning Sunday with decreasing clouds overnight. Lows in the mid 30s to mid 40s.

AVIATION

Zonal flow aloft through this afternoon as a shortwave ridge moves through W WA before shifting northwesterly tonight. At the surface, southerly winds generally 5-10 kts with gusts having generally eased. This break will be short lived, as gusts are expected to redevelop later in the morning and remain in place for the majority of the day. While they should ease tonight, some model data suggests that they may hang in there beyond 06Z tonight. Even as gusts drop off overnight, wind speeds are still expected to remain elevated running 8-12 kts with direction only shifting slightly to south to southwesterly.

Cigs over the area somewhat mixed. Majority of terminals reporting VFR, although there is a band running from CLM through to PAE where a PSCZ has developed. Showers continue to linger there and as such have given rise to MVFR to IFR conditions. This feature has held on longer than anticipated and as such the forecast for PAE will likely need to reflect these current conditions extending into the mid to late morning time frame come the 12Z update. By noon today, widespread VFR conditions are expected to return for much of the day. It is worth noting that as another round of showers starts to move in this evening, this may bring cigs back down into MVFR, although models in some disagreement as to how long this might linger before recovery to VFR conditions once again during the overnight period. Will continue to evaluate.

KSEA...Some stubborn low clouds at or around 2000 ft causing the terminal to bounce between MVFR to VFR at least through the early morning hours...although generally trend in cigs suggest VFR conditions by mid-morning. As stated above, southerly winds have lost their gusts for now, but will see them redevelop by 16Z this morning and remain in place for the remainder of the daylight hours and into early evening. While the gusts fall off by 06Z, wind speeds will remain in an 8-12 kt range. Direction to remain southerly to southwesterly for the TAF period.

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MARINE

Onshore flow will continue, which will allow for daily pushes through the Strait of Juan De Fuca at least into the weekend. Speeds expected to largely remain below any headline criteria, although an occasional gust to 21 or 22 kts is possible. However, this is not considered to be a widespread threat in either area or time and as such will hold of on any short term headlines. A more compelling case for headlines remains on Saturday, where speeds appear to be more conclusively in Advisory thresholds. A system will pass through Sunday, allowing for the possibility for increased winds over the coastal waters.

Coastal seas 4 to 6 feet for much of today before rising to 7 to 9 feet tonight and remaining there into the weekend.

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HYDROLOGY

The daily hydrology discussion has ended until the start of the next rainy season; it will only be updated during this time as needed.

SEW WATCHES/WARNINGS/ADVISORIES

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