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Area Forecast Discussion (AFD)

Issued by Seattle/Tacoma, WA (SEW)

FXUS66 KSEW 192310

Area Forecast Discussion
National Weather Service Seattle WA
330 PM PST Mon Nov 19 2018

.SYNOPSIS...High pressure will provide one more dry day across the 
region Tuesday. Patchy fog or freezing fog will develop again later 
tonight and Tuesday morning, with filtered sunshine in the afternoon 
as high clouds increase. A series of low pressure systems will bring 
wet weather Wednesday through Friday. After a break between systems 
on Saturday, A weak front could bring some light rain Sunday.


.SHORT TERM...IR satellite imagery shows the frontal system expected 
to affect the area Wednesday currently out near 135w. The front 
already shows signs of splitting, with cloud top brighting on the 
southern extent where a secondary low is developing. This will tend 
to slow the system a bit, with the ridge holding over the region 
through Tuesday. Expect patchy fog or freezing fog again later 
tonight and Tuesday morning. Otherwise sunshine will become filtered 
sun by afternoon as high clouds increase over the area. 

Models generally agree that light rain reach the coast early 
Wednesday morning, then spread into the rest of Western Washington 
by midday. This first front will be the weakest as it encounters the 
ridge pushing eastward. QPF amounts look rather light, perhaps a 
third of an inch over the mountains and about half that over the 
lowlands. Snow levels may start just above pass levels but will fall 
later Wednesday and Wednesday night down to at least 4000 feet. This 
could allow a couple inches of snow to fall at some passes such as 
Stevens Pass, and White Pass. Ski areas may pick up several inches 
but likely sub-advisory level. 

The next system arrives for Thanksgiving with steady rain picking up 
during the afternoon. This front will have significantly more 
moisture to work with and better lift, both dynamic and orographic. 
snow levels will fall, probably down to all mountain passes by 
afternoon. Models still show a split flow pattern with the heaviest 
precipitation again headed into CA and Oregon. There is a strong 
orographic push with onshore flow behind the front, and this would 
be the likely be the period of heaviest snowfall. An advisory could 
be necessary, but with the split flow and progressive push through 
the region, snow amounts in the passes come up just short of 
advisory level. It will become breezy with this system as well, but 
gradients do not support strong or damaging winds. However, 
visibility in the passes could be reduced with snowy pass conditions 
at times. 

.LONG TERM...Snow levels will lower to around 2500 feet Thursday 
night into Friday. Additional snow is likely, and accumulations in 
the passes will probably cause travel difficulty through passes. 
Those planning travel through mountain passes Wednesday through this 
weekend should pay close attention to current forecasts and check on 
road conditions by calling 5-1-1 before departing. 

Snow will taper to snow showers in the mountains Friday afternoon 
and night. Cool and showery conditions will affect the lowlands. The 
Puget Sound convergence zone could become active as well. 1000-850mb 
thickness values are not as low as might be expected with the -31C 
cold pool overhead at 500mb. Low elevation snow does not appear to 
be a concern at this time. 

A break in precipitation is expected for Saturday. The GFS and ECMWF 
both show this brief dry period. Both models also bring a weak front 
into the area Saturday night and Sunday with a round of light rain 
and mountain snow. Rain amounts should not be more than a couple 
tenths at best given the progressive flow pattern. 

The progressive flow pattern continues into early next week. This 
makes timing individual fronts difficult. Wet weather is shown by 
the ECMWF returning by late Monday and Monday night, while the GFS 
holds rain off until Tuesday. Either way, expect unsettled weather 
and timing can be refined as we get closer to the day 6-7 period. 


.AVIATION...High pressure over the region will start to shift east 
of Tuesday as a trough approaches. Light southerly flow aloft will 
become moderate by Tuesday. Stable and dry conditions, except moist 
near the surface with patchy fog later tonight and Tuesday morning. 
Fog or freezing fog is most likely to develop over parts of the 
southwest interior and south Puget Sound. Fog will lift by afternoon 
as high clouds increase. Deeper moisture and rain will reach the 
coast late Tuesday night or early Wednesday morning.

KSEA...Northwest wind 5 to 10 kt becoming Northeast 2-4 kt after 
04z. VFR conditions through Tuesday, except a chance of IFR fog most 
likely between 10-16z. Any fog should lift by noon as high clouds 
increase.  dtm


.MARINE... High pressure will remain in control with mainly easterly 
flow through Tuesday. A series of low pressure systems will move 
across the area from late Tuesday night through the end of the week. 
Stronger onshore flow is likely with periods of small craft advisory 
conditions from time to time. It's possible there could be a period 
of gales over the outer coastal waters Thursday into Friday. CEO


.HYDROLOGY...A pattern change will get underway late Tuesday with a 
series of low pressure systems moving across the area Wednesday into 
the end of the week. Snow levels will be lowering with these 
systems, down to around 3000 feet by Friday. At this time, rainfall 
amounts do not look high enough to cause any river flooding but 
river levels will be on the rise. Still some uncertainty with the 
model precipitation forecasts so will continue to monitor. CEO