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Area Forecast Discussion (AFD)

Issued by Seattle/Tacoma, WA (SEW)

FXUS66 KSEW 210353

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

.UPDATE...The forecast is largely on track this evening with no 
major changes planned. Satellite data continues to show a ridge of
high pressure in the mid and upper levels of atmosphere 
progressing through the region this evening, while sfc analysis of
the lower levels shows high pressure offshore and lower pressure 
inland. Should be an overall tranquil evening for western 
Washington as mid and high clouds continue to stream in atop the 
ridge. These clouds should gradually lessen in coverage through 
the morning hours Saturday. However, overnight, given onshore flow
marine stratus will move inland and likely encompass most lowland
locations by daybreak. Some areas of fog could materialize as 
well. These clouds will gradually retreat back towards the coast 
thru the morning hours into the afternoon, allowing for some sun 
breaks. This will be short lived as the next frontal system will 
begin moving into the area tomorrow evening.

Have included the previous discussion below with an updated marine
and aviation section.



.PREV DISCUSSION... /issued 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...Around 830 pm PDT, most TAF sites were VFR with some
mid and high clouds streaming through from north to south. The
main exception to these conditions is HQM where marine stratus
deck appears to have settled in for the night. Expect this marine
layer to push inland thru the night with all terminals at least
lower end MVFR if not IFR by daybreak. Some fog likely to
accompany some sites, particularly OLM. Stratus should gradually
retreat back to the coast thru the afternoon with an improvement
to VFR expected.

The next frontal system will approach the area late Saturday,
bringing the next chance for rain and low ceilings.

KSEA...VFR ceilings this evening will give way to an MVFR-IFR
stratus deck overnight. These low ceilings will linger thru the
morning hours before conditions improve back to VFR. Winds
expected to remain on the south side under 10kts.



.MARINE...Onshore flow overnight will yield another round of
stratus clouds through Saturday morning with some areas of patchy
fog. The next frontal system is still expected to approach the
area late Saturday night and through Sunday morning, carrying with
it the next chance for rain. This forecast update follows closely
with the previous forecast with 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. 



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