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Area Forecast Discussion (AFD)

Issued by Seattle/Tacoma, WA (SEW)

FXUS66 KSEW 231640

Area Forecast Discussion
National Weather Service Seattle WA
940 AM PDT Fri Mar 23 2018

.SYNOPSIS...A cold upper low will produce showers and scattered 
thunderstorms in the afternoon and evening with small hail possible.
Showers decrease late tonight, with lingering showers over the 
weekend along with sunbreaks. The next front brings light rain to 
Western Washington Monday, then decreasing Tuesday. High pressure 
will build over the Pacific Northwest the middle of next week with 
mostly dry and somewhat warmer weather.


.SHORT TERM...A sprawling upper low is currently centered west of
Vancouver Island near 50N 132W. There is very cold air aloft over
a large area around the low, with 500 mb temps of -37C to -40C. 
Such cold air aloft is leading to an unstable air mass, with 
Western Washington in the more unsettled east-southeast quadrant 
of the upper low.

This is a classic setup for spring convection, with showers
pivoting up from the south-southwest today. Models show the
coastal side of the Olympic Peninsula being most unstable this
afternoon, with CAPE values forecast to reach the 400-700 J/kg
range. Forecast already has thunder in the forecast this
afternoon, and that seems good. This will be the type of day when
a shower produces a single clap or two of lightning, seemingly out
of the blue and without lead-up. So if you see dark clouds 
approaching, even you don't hear thunder, take your lightning 
precautions anyway by most ideally going indoors. With such cold 
air aloft, could easily get small hail in convective showers, 
similar to yesterday.

As for this morning, have gotten several reports of snow falling
on hills above about 300 feet along the I-5 corridor. Hard to be 
excited about anything more than brief accumulations given air 
temperatures in the 36F-39F range, well above freezing. 

Showers decrease later tonight into Saturday. The main low and 
dynamics shift south with some lingering diurnal showers still 
possible over Western Washington. Highs will stay cool in the mid to 
upper 40s. Sunday will probably remain mostly dry across the area 
between systems. However, there are some inconsistencies in timing of 
the next front. The forecast is based off the ECMWF which shows 
light rain arriving as early as Sunday night. Haner

.LONG TERM...From Previous Discussion: High pressure offshore 
Monday will gradually build through at least mid week. Fronts 
arriving in northwest flow will bring periods of light rain Monday
and Tuesday. Eventually the ridge shifts eastward shifting fronts
further inland. Dry weather should develop later Tuesday through 
about Thursday. Expect some partial sunshine and temperatures a 
bit warmer in the 50s. Mercer


.AVIATION...A cold upper level low west of Vancouver Island will 
drop southeastward and onto the southwestern Oregon coast on 
Saturday. The air mass is moist and unstable with showers and some
thunderstorms rotating around the upper level low. Thunderstorms 
will be most likely along the Washington coast, but will be 
possible in the interior during the afternoon and early evening 
hours as the air mass inland destabilizes with daytime heating.

There are multiple cloud layers with showers rotating around the
upper low. Heavier showers will bring ceilings down to around
015-020 through this afternoon, but for the most part, ceilings
will be around layers at 3500 and 5000 feet. Small hail or 
graupel, along with brief changeover to snow or mixed rain and 
snow, is possible in heavier showers through early this afternoon.
Showers will slowly decrease from the north this evening as the 
offshore upper low drops southward offshore and toward the 
southern Oregon coast. Areas of MVFR conditions are likely again 
late tonight into Saturday morning as low level moisture lingers 
and we get nocturnal cooling. Albrecht

KSEA...Heavier showers are producing MVFR conditions with ceilings
down to 015 this morning. Expect ceilings to improve to the higher
end of MVFR or lower end of VFR this afternoon and evening as the
air mass destabilizes and we get some daytime heating. Snow levels
are low, so heavier showers may produce a brief mixture of rain
and snow or give small hail or graupel. Thunderstorms cannot be
ruled out through this evening, but the chances are low enough at
the terminal to keep the mention out. Will amend the terminal
forecast as needed if tstms are seen moving in that direction. 
Winds will be south 10 kt with gusts to 20 kt in the afternoon. 
Winds will ease to east to southeast at 5 kt or less this evening 
through the overnight hours. Albrecht 


.MARINE...A surface and upper level low is centered near 50N 135W.
This low will move slowly southward onto the southwest Oregon
coast on Saturday as it slowly weakens. Small craft advisory
conditions are expected to continue through this afternoon over
all waters with gale conditions until about noon over the eastern
entrance of the Strait. 

Winds will diminish as the upper low moves south of the area
tonight into Saturday and a weak ridge of high pressure builds
into the waters. 

A frontal system will move southeast through the waters Sunday
night. Small craft advisory conditions are possible over some of
the waters as this system moves across the area. Albrecht


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


PZ...Small Craft Advisory until 5 PM PDT this afternoon for Admiralty 
     Inlet-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-Northern 
     Inland Waters Including The San Juan Islands-Puget Sound 
     and Hood Canal-West Entrance U.S. Waters Strait Of Juan De 

     Small Craft Advisory for rough bar until 11 PM PDT this evening 
     for Grays Harbor Bar.

     Small Craft Advisory until 5 PM PDT this afternoon for East 
     Entrance U.S. Waters Strait Of Juan De Fuca.

     Gale Warning until noon PDT today for East Entrance U.S. Waters 
     Strait Of Juan De Fuca.