Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 130427
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
827 PM PST Mon Feb 12 2018

.SYNOPSIS...High pressure will keep the area dry into Tuesday
with low temperatures near or below freezing. An upper trough
arriving from the northwest will bring showers and mountain snow
Tuesday night, then showers taper off late Wednesday. Rain will
develop again late Thursday and Friday. A stronger storm over the
weekend could bring wet and windy weather with low snow levels
Sunday.

&&

.SHORT TERM...
No changes to the previous thinking, which is as follows:
Skies will remain mainly clear tonight as the upper ridge holds
over the region one more night. The dry air mass and light winds
will bring more widespread freezing or below freezing temperatures
Tuesday morning. Low lying areas and areas inland from the water
will dip to the 20s. The ridge will weaken on Tuesday as a system
approaches from the northwest. Mid and high clouds will increase
through the day.

Models are in good agreement with the next system to bring rain into
Western Washington Tuesday evening. Snow levels will rise a little
ahead of the system but should remain below the lower passes. While
the system is progressive, models do give a pretty good shot of
precipitation Tuesday night, over a half inch in the higher
mountains. Colder air behind the front will provide higher
snow/liquid ratios and orographic effects. Snow amounts in the
mountains look to be advisory level, possibly 5 to 10 inches. Will
hold off on an advisory and defer to the night crew to allow a look
at the 00z models this evening.

Showers in the lowlands taper off late Wednesday afternoon with
mostly dry conditions by Wednesday night. Models are out of sync
with timing of the next weaker front. Some spotty light rain is
possible by later Thursday afternoon or night.

.LONG TERM FROM THE PREV DISCUSSION...
Long range models are advertising a strong low pressure system to
develop Friday and reach the area this weekend. Rain should
increase Friday ahead of the main low which looks to arrive from
the northwest on Saturday. The 00Z ECMWF shows rapid deepening of
the low as it tracks inland across northern Washington. This would
bring windy conditions to most the lowlands and mountains. Other
models do now show as much development with the low but would
still potentially bring breezy or locally windy weather. Models
also show good dynamics and decent moisture coming from the
northwest. Rain should transition to showers over the weekend. The
big question will be how much cold air makes it into the area
before showers end. It`s still a ways off but something to keep an
eye on as snow levels could reach 500 feet or lower by Sunday
evening. Mercer

&&

.AVIATION...
No changes to the previous thinking, which is as follows:
The air mass is dry and stable with VFR conditions. The flow
aloft is northerly. A cold front will spread mid and high clouds
inland Tuesday evening with rain reaching the coast by midnight.
33

KSEA...VFR. Winds will generally be light and variable.

&&

.MARINE...
The flow will be nearly flat tonight. A 1024 mb high off the
Washington coast will result in weak onshore flow on Tuesday.
Expect a weak cold front extending from a 1011 mb low to move SE
across the area during the late Tuesday night through early
Wednesday time frame. Onshore or westerly flow will prevail in the
wake of the front on Wednesday.

&&

.HYDROLOGY...
River flooding is unlikely the next seven days.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...None.

&&

$$

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