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Area Forecast Discussion (AFD)

Issued by Seattle/Tacoma, WA (SEW)

FXUS66 KSEW 161602

Area Forecast Discussion
National Weather Service Seattle WA
902 AM PDT Tue Jul 16 2019

.UPDATE...A broad, open wave trough encompasses much of the west
coast of the US this morning, per water vapor imagery. Yesterday's
shortwave trough across western Washington can now be seen 
lifting into southern Alberta, with a weak area of vorticity 
passing through the area this morning. Most of the moisture/lift
associated with this wave are to the east of the CWA border.
Nonetheless, residual low level cloud cover is hanging around the
area this morning, although northern counties are seeing some 
clearing. A few showers were spotted offshore with dry conditions 
across the area. 

As weak wave/trough axis continue east today, heights will rise
slightly and western Washington will catch a break between
systems. This should lead to an overall quiet day with clouds
breaking across portions of the area by the afternoon. A few
showers will remain possible across the higher terrain but
coverage should remain more isolated than yesterday.

A strong cyclone will dig south across Gulf of Alaska today and
tonight, pushing a seasonably strong front towards the area with a
much better chance for widespread showers tomorrow. Overall have 
left bulk of ongoing forecast as is this morning, except for a few
adjustments to lower POPs thru 18z this morning and decrease 
cloud cover for the afternoon. Previous discussion follows below 
with a fresh marine and aviation section.



.PREV DISCUSSION... /issued 329 AM PDT Tue Jul 16 2019/ 

.SYNOPSIS...A weak upper level trough will linger over the area
today. A frontal system will drop down from the northwest on 
Wednesday. Another upper level trough will follow the front for 
Thursday into Friday. Dry weather over the weekend into first part
next week with an upper level ridge to the east and an upper 
level trough offshore. 


.SHORT TERM /TODAY THROUGH THURSDAY/...Satellite imagery shows a
few breaks in the cloud cover over the Northwest Interior but for
the most part skies are cloudy to mostly cloudy over the area at 3
am/10z. With the clearing in the north Friday Harbor has dropped
down into the lower 50s at 3am while under cloudy skies
temperatures remain in the lower to mid 60s in the Seattle metro

Not much going on today over Western Washington. Plenty of low
level moisture still in place with some weak troughiness aloft at
least into the afternoon hours. This will keep a slight chance of
showers in the forecast. With a little bit of sunshine in the
afternoon temperatures will warm up into the mid 60s to mid 70s.
Dew points in the mid 50s to lower 60s will keep the muggy
feeling going throughout the day.  

Clouds thickening up over the area tonight ahead of the next 
system moving down the British Columbia coast. Rain out ahead of 
the warm front could reach as far south as Bellingham and Forks 
by 12z Wednesday. Lows with the cloud cover a couple of degrees 
either side of 60. 

Warm front moving through Western Washington Wednesday morning
with the trailing cold front along the coast by 00z Thursday.
Rain shadow developing over the Central Puget Sound with westerly
flow aloft over the area. The moist air mass will eventually
overcome the rain shadow so will have a chance of rain for the
metro area. Highs on Wednesday only in the 60s.

Cold front moving through early Wednesday evening with an upper
level trough settling in to the north later Wednesday night into
Thursday. Low level onshore flow will keep the lower layers of the
air mass moist with showers remaining in the forecast. Lows 
Wednesday night in the mid 50s to lower 60s. Highs on Thursday 
only a couple of degrees warmer than Wednesday, in the 60s and 
lower 70s.

.LONG TERM /FRIDAY THROUGH MONDAY/...Models in good agreement
with the upper level trough hanging around on Friday keeping a
chance of showers in the forecast with highs in the mid 60s to mid
70s. Weak upper level ridge moving through the area on Saturday
will mark the beginning of a dry spell with a slight warming 
trend. Highs mostly in the 70s. Upper level trough well offshore 
with an upper level ridge to the east will keep the dry weather 
forecast intact Sunday and Monday. Southwesterly flow aloft will 
bring some warmer air to the area but the low level flow remains 
onshore keeping highs in the 70s and lower 80s. Felton


.AVIATION...A weak upper trough will move inland today. A front will 
reach the area Wednesday. The air will remain moist and there is a 
hodge podge of conditions this morning with areas of low stratus and 
in most areas another layer of clouds above that. There area a few 
holes in the overcast in the south, plus a large clear area north of 
Everett--with mostly clear skies for Whatcom and Skagit counties 
this morning. For the afternoon and evening, the low clouds near the 
water will burn off, most areas ought to have a broken layer around 
4 or 5kft and the clear areas this morning will probably fill in 
with cumulus. 

KSEA...Cloudy skies will improve into the afternoon and then fill 
back in overnight. A front will bring rain to the area Wednesday.


.MARINE...A front will reach Western Washington on Wednesday and 
south winds will pick up with that front. There will be westerlies 
in the Strait of Juan de Fuca this evening, and then again Wednesday 
night behind the front. Onshore flow through Friday will then weaken 
over the weekend--winds over the weekend should be more of the 
typical summer diurnal northerlies.


.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...Small Craft Advisory from 6 PM this evening to 2 AM PDT 
     Wednesday for Central U.S. Waters Strait Of Juan De Fuca-
     East Entrance U.S. Waters Strait Of Juan De Fuca.