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Area Forecast Discussion (AFD)

Issued by Seattle/Tacoma, WA (SEW)

FXUS66 KSEW 242151

Area Forecast Discussion
National Weather Service Seattle WA
300 PM PDT Wed Apr 24 2019

.SYNOPSIS...A weak upper ridge will keep the weather dry Thursday. 
An upper trough will bring a chance of showers and cooler weather to 
the region by the weekend. Some warming and drying is possible by 
the middle of next week.


.SHORT TERM /TONIGHT THROUGH SATURDAY/...Clouds have dissipated all 
day and now Western Washington is mostly sunny. Highs today will be 
in the mid 50s to lower 60s. 

A flat upper ridge will keep dry weather Thursday, but high and mid 
level clouds are likely to increase in the afternoon. Highs will be 
in the low to mid 60s. A weak upper trough will cross the area 
Thursday night and Friday. While it looks mostly dry, a stray shower 
cannot be ruled out in the mountains. Low level onshore flow will 
increase on Friday with westerly gales possible in the strait. Highs 
will be a few degrees lower.

An upper low will develop over southern British Columbia Friday 
night, and swing through Washington on Saturday. This will bring 
more widespread chances for showers, and pops will rise to likely at 
times in the Cascades. Temperatures on Saturday will be noticeably 
lower, with highs in the 50s. The snow level will be around 3000 
feet. A few inches of snow could fall at Stevens Pass, and even 
Snoqualmie Pass could get an inch or so. Burke

.LONG TERM /SUNDAY THROUGH WEDNESDAY/...A longwave upper trough will 
remain in the region all of next week. Most days will be partly 
sunny with a chance or a slight chance of showers. Temperatures will 
be within a few degrees of normal Sunday and Monday, with perhaps a 
few degrees of warming Tuesday and Wednesday. Burke


.AVIATION...A weak ridge of high pressure aloft over the offshore
waters will give moderate westerly flow aloft through Thursday.
High pressure extending from the central interior of British
Columbia into the offshore waters will give low level NW to N flow
at lower levels tonight through Thursday. The air mass is drying 
and stable. Some mid and high level moisture will spread into the 
area from the west Thursday afternoon. 

Visible satellite imagery and observations show flat cumulus
clouds decreasing this afternoon. Expect only a few cirrus
overnight. Models have slowed with the approach of mid and high
level moisture into the area on Thursday, now showing its arrival
during the afternoon hours. Albrecht

KSEA...Good VFR conditions through Thursday afternoon with only a
few cumulus clouds early this evening and increasing cirrus and
altocumulus clouds Thursday afternoon. Winds will generally be 
northerly 7 to 11 kt. Albrecht


.MARINE...High pressure extending from the interior of central 
British Columbia southwestward into the offshore waters will give 
northwest to north flow over the waters tonight through Thursday. 
this afternoon and tonight. West to northwest onshore flow will 
increase Thursday afternoon as pressures fall east of the 
Cascades. Small craft advisory conditions are expected Thursday 
afternoon and evening over the coastal waters and through the 
eastern two thirds of the Strait of Juan de Fuca. Mesoscale models
appear to be overforecasting the strength of northwesterly winds 
through the Strait of Georgia getting into the northern inland 
waters north of the San Juan Islands. Winds from the San Juans
northward generally follow the NAM12 that performs better in these
situations there. 

Expect onshore flow to significantly increase later Friday as a
trough of low pressure aloft moves by to the north of the waters.
Gale force winds are indicated for the eastern two thirds of the
strait Friday afternoon and night with small craft advisory
conditions across the remainder of the waters. 

Onshore flow will diminish Saturday then possibly become offshore
late in the weekend as a cold front moves into the area from the
north. Albrecht


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


PZ...Gale Watch from Friday afternoon through late Friday night for 
     Central U.S. Waters Strait Of Juan De Fuca-East Entrance 
     U.S. Waters Strait Of Juan De Fuca.