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Area Forecast Discussion (AFD)

Issued by Seattle/Tacoma, WA (SEW)

FXUS66 KSEW 202227

Area Forecast Discussion
National Weather Service Seattle WA
327 PM PDT Fri Sep 20 2019

.SYNOPSIS...High pressure will give mostly dry but cloudy
conditions through Saturday. A cold front will spread rain to the
area on Sunday. Wet and showery weather will continue through
early next week with temperatures trending below normal. 


.SHORT TERM /TONIGHT THROUGH MONDAY/...An upper level ridge is
nudging into the Pac NW with northerly flow aloft over western 
WA. A weak disturbance in the flow is generating isolated showers
in the area but accumulations are light. 

The upper level ridge will bring mostly dry weather on Saturday.
The ridge will be dirty, though, with increasing clouds through
the day. The next incoming cold front, now slower in the models,
will spread rain inland late Saturday night into Sunday. We should
see widespread wetting rains with this front, similar to the
system this past Tuesday. Thunderstorms will be possible but the 
best dynamics and instability will be mostly south over Oregon. 
Onshore flow will increase Sunday afternoon/evening with a 
possible Puget Sound Convergence Zone developing in the central 

We're in westerly/zonal flow on Monday with a jet over the region,
with ongoing low level onshore flow. A system passing through
southern B.C. may clip western WA for a chance of showers. 
Temperatures through the period will remain cooler than normal. 33

.LONG TERM /TUESDAY THROUGH FRIDAY/...Onshore flow will continue
on Tuesday for cloudy and cool conditions. Models are still
generating showers over the area, mostly along the coast and in
the mountains. We may see a brief break in the weather on
Wednesday before more wet and unsettled weather on Thursday. The 
air mass will remain cool through the end of the week with a deep 
upper level trough over the region. Temperatures are trending
around 10 degrees below climo with highs near 60 and lows dipping
into the 40s. 33


.AVIATION...Upper level ridge centered offshore with northerly
flow becoming northwesterly as the ridge shifts inland later
tonight. Ridge shifting east late Saturday with the flow aloft
becoming southwesterly. Light flow continuing in the lower 

Ceilings mostly between 4500 to 6000 feet tonight with the cloud
layer scattering out in a few places after 04z. Stratus filling 
back in overnight with ceilings lowering back down to below 1000 
feet 09z-13z Saturday morning. Conditions improving late morning
with just high clouds over the area by early afternoon. 

KSEA...Ceilings aoa 4500 feet with the cloud deck scattering out
04z-08z. Ceilings lowering back down to under 1000 feet around 
10Z overnight with ceilings remaining below 1000 feet until late
morning. Lower deck scattering out midday Saturday with just high
clouds in the afternoon. Variable wind less than 5 knots becoming 
southwest wind 4 to 8 knots Saturday morning. Felton


.MARINE...Light onshore flow will continue through Saturday. A 
frontal system will move through the area Sunday. This system will
produce small craft advisory level southerly winds over the 
coastal waters ahead and with the front and small craft 
westerlies behind the front in portions of the Strait of Juan de 
Fuca. Another frontal system will move through the area around 
Monday night with surface high pressure building behind the front
for Tuesday and Wednesday. Felton


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