000
FXUS66 KSEW 310334
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
834 PM PDT Mon Mar 30 2020

.UPDATE...The current forecast remains on track and no changes are
planned for this evening. Scattered showers are expected to
continue through the overnight hours. However, with the loss of
daytime heating, the threat for isolated thunder will become
confined to the outer coastal areas where the latest meso-analysis
indicates 100-200 J/KG of CAPE at this hour. This will not be
enough to support strong enough updrafts that yield small hail
production that was observed earlier today across parts of
Western Washington. Snow showers will continue in the Olympics and
Cascades through the overnight hours.

&&

.AVIATION...SCT -SHRA will continue across western WA through the
period. While CIGS will generally lower to low end VFR/high end MVFR
levels (between 2 and 3K feet), and showers that move over a
terminal will yield brief and localized reductions in CIGs and
vsbys. Winds will remain out of the south overnight at speeds
generally less than 10 kts, increasing to 10 to 20 kts Tuesday
morning with periodic gusts out of the south-southwest. Like today,
gusty, variable outflow winds will be possible preceding any showers
approaching a terminal on Tuesday.

KSEA...High end MVFR to low end VFR CIGS will be observed overnight,
with periodic -SHRA moving over the terminal. Winds will remain
southerly at speeds generally AOB 10 kts. Brief reductions in CIGs
and vsby will be possible if any SHRA moves directly over the
terminal. Expect SCT SHRA to continue through Tuesday. Winds will
remain predominately out of the south-southwest at speeds around
10 kts. Briefly higher gusts will be possible, particularly ahead
of any showers that approach the terminal during the daytime
hours. CIGs will gradually scatter to VFR levels during the
daytime hours with brief reductions under any SHRA.

Pullin

&&

.MARINE...Breezy westerly to southwesterly winds continue across
most of the waters and southerly winds through Puget Sound.
Advisory strength winds will continue through the next several
hours, with strong onshore flow persisting in the offshore waters
and into the entrance of the Strait of Juan de Fuca. Winds ease a
bit into early Tuesday, but a weak low moving down the coast
Tuesday will bring stronger offshore flow to the north of the low
later Tuesday and into Wednesday. Expect that high end advisory
strength winds will develop across portions of the northern inland
waters, with some potential for gales across the Strait of
Georgia and near the San Juan Islands. Winds ease further toward
midweek and through much of the rest of the week in a weaker
onshore flow pattern with relaxed surface pressure gradient. The
next organized system may approach closer to the weekend. Over the
coastal waters, combined seas remain above 10 feet today and are
steep with a large wind wave/fresh swell component. Seas will
remain elevated into midweek as a longer period west swell enters
the waters, but continue to diminish through the rest of the week
with several small swells. A larger west swell arrives late in the
week.

Cullen

&&


.PREV DISCUSSION... /issued 154 PM PDT Mon Mar 30 2020/

SYNOPSIS...Unsettled weather will continue through the week with
daily chances for showers. A few thunderstorms are possible
through Wednesday.

SHORT TERM /TONIGHT THROUGH THURSDAY/...As expected, radar
activity has picked up over the past several hours with plenty of
convective cells popping up. The strongest returns have produced
small hail with several reports of accumulations. No lightning
strikes as of 1 pm but enough instability around with moderate
mid-level lapse rates to continue the mention of thunderstorms in
the forecast. Expect this activity to continue into this evening.

Tuesday looks similar to today as upper level low continues to
drop south toward the area. Expect additional showers and enough
forcing for isolated thunderstorms. Snow showers will continue in
the mountains with additional light accumulations for the Cascade
passes. Upper level low finally skirts into SW portions of the
state on Wednesday with another shortwave moving across the area
from the north. This will bring another round of showers. Have
added thunder mention to the forecast, mainly from Seattle
southward with best instability across southern portion of the
state. Wednesday night will be pretty chill with morning lows in
the low to mid 30s. GEFS data highlighting the potential for more
widespread freezing temperatures than current forecast suggests
but is on cool side of the envelope. Have nudged temperatures down
slightly but will continue to watch trends.

As upper level trough departs, models attempt to bring in a period
of more zonal flow for Thursday but another weak shortwave will
likely swing through during the afternoon/evening hours. Shower
coverage likely won`t be as widespread as earlier in the week with
the potential for a few sunbreaks in the morning. We`ll see.

CEO

LONG TERM /FRIDAY THROUGH MONDAY/...Active pattern continues in
the extended across the Pacific Northwest. Deterministic models
continue to have trouble resolving the details and timing of each
additional system for the end of the week into early next week but
ensembles in good agreement with anomalous troughing across the
area. This will bring periods of additional rainfall and mountain
snow as well as below normal temperatures.

CEO

HYDROLOGY...Several rounds of lowland rain and mountain snow
expected through the week. Amounts are not forecast to be heavy
enough to cause any river flooding concerns into the weekend.

CEO

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Winter Weather Advisory until 11 AM PDT Tuesday for Cascades of
     Pierce and Lewis Counties-Cascades of Snohomish and King
     Counties.

     Winter Storm Warning until 11 AM PDT Tuesday for Cascades of
     Whatcom and Skagit Counties.

     Winter Weather Advisory until 5 AM PDT Tuesday for Olympics.

PZ...Small Craft Advisory until 2 PM PDT Tuesday 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 3 AM PDT Tuesday for Admiralty Inlet-
     Northern Inland Waters Including The San Juan Islands-Puget
     Sound and Hood Canal.

     Small Craft Advisory until 1 AM PDT Tuesday for Central U.S.
     Waters Strait Of Juan De Fuca.

&&

$$

www.weather.gov/seattle

NWS SEW Office Area Forecast Discussion

Discussion Script by Ken True at Saratoga Weather - WFO Script by SE Lincoln Weather