Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 221552

Area Forecast Discussion
National Weather Service Seattle WA
852 AM PDT Thu Mar 22 2018

.SYNOPSIS...A cold front will move through Western Washington this
morning followed by a trough axis this afternoon. A deep upper
level low will keep the air mass cool and unstable through
Friday with isolated thunderstorms possible. The upper low will
weaken as it moves inland on Saturday. A ridge will build over the
Pacific Northwest early next week for warmer and drier weather.


.SHORT TERM...Rain is spreading across the region this morning as
a slow moving cold front shifts inland. Expect more showers
through this afternoon with increasing onshore flow and a passing
upper level trough axis. The mountains will see a few inches of
snow with snow levels lowering to around 1500 feet tonight.

A broad upper level low will remain over the region on Friday for
more cool and showery weather. We are still under moist,
diffluent flow and there`s a slight chance of thunderstorms across
much of Western WA. By this point in time, 500 mb temps will drop
to around -37 C, setting up steep lapse rates and a Lifted Index
near 0. Small hail is possible too.

The deep upper level low will finally weaken and shift inland as
an open trough on Saturday. This system is diving farther south
in the models with the bulk of moisture over Oregon. Across
Western WA, snow levels will remain low during the morning, only a
few hundred feet, but showers in the lowlands are light and
spotty. Significant snowfall accumulations are not expected.
Temperatures will continue to trend below normal with highs mainly
in the mid to upper 40s. 33

.LONG TERM...Previous discussion...Extended models in good
agreement Sunday with the low moving inland and a weak system
embedded in the northwesterly flow aloft over western Washington
moving into the area. Will keep the chance pops going into Sunday.
Some slight differences for Monday and Tuesday with the GFS a
little stronger with the building upper level ridge offshore which
will help push the next system in the northwest flow aloft north
and east of the area versus the weaker ridge ECMWF solution. Both
the GFS and ECMWF have the upper level ridge over the area on
Wednesday with 500 mb heights in the lower to mid 570 dms. Current
forecast leans toward the GFS solution with a dry forecast
beginning Monday night. Nothing in the 00z run to change this idea
with the early morning package. Felton


.AVIATION...Moderate southerly flow aloft continues over western
Washington with a moist air mass in place. A cold front making its
way across the western half of the CWA and will continue to move
eastward through the day ushering in a colder, more unstable air
mass. Rain will turn over to showers with the passage of the front
by early this afternoon. Cigs generally MVFR area-wide with some
isolated exceptions dipping down into IFR. Once the front passes and
the transition to showers occurs may see some locations rebound into
low-end VFR this afternoon and evening. Increasing low level onshore
flow behind the front will bring some gusty south to southwest
surface winds to the area from mid morning into the afternoon before
tapering off some in the evening. 27/SMR

KSEA...MVFR cigs to persist with rain this morning ahead of a cold
front...then low-end VFR possibly dipping down into MVFR in showers
from midday into the evening hours. Fairly strong post-frontal
onshore flow will lead to some gusty S-SW winds of 20-25 knots at
times from 18Z through 00Z. 27/SMR


.MARINE...A low pressure system and cold front will affect
Western Washington today. Stronger southerly winds are expected as
the front moves ashore. This low pressure area will linger offshore
Friday and then weaken and move ashore over northern California over
the weekend. High pressure will build into the area next week, but
weather systems will still brush the area.


.HYDROLOGY...River flooding is not expected in the next 7 days.


PZ...Small Craft Advisory until 5 AM PDT Friday for Coastal Waters
     From Cape Flattery To James Island 10 To 60 Nm-Coastal
     Waters From Cape Flattery To James Island Out 10 Nm-Coastal
     Waters From James Island To Point Grenville 10 To 60 Nm-
     Coastal Waters From James Island To Point Grenville Out 10
     Nm-Coastal Waters From Point Grenville To Cape Shoalwater
     10 To 60 Nm-Coastal Waters From Point Grenville To Cape
     Shoalwater Out 10 Nm-West Entrance U.S. Waters Strait Of
     Juan De Fuca.

     Small Craft Advisory until 6 PM PDT this evening for Admiralty
     Inlet-Central U.S. Waters Strait Of Juan De Fuca-East
     Entrance U.S. Waters Strait Of Juan De Fuca-Northern Inland
     Waters Including The San Juan Islands-Puget Sound and Hood


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