Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 191727

Area Forecast Discussion
National Weather Service Seattle WA
900 AM PST Mon Feb 19 2018

.SYNOPSIS...Dry and cold northerly flow aloft will prevail over
the region through tonight. A weak disturbance will bring some
light precipitation to mainly the southwest part of the state on
Tuesday. Another system will follow late Wednesday into Thursday.
Temperatures will remain below normal.


.SHORT TERM...Snow showers have ended and residual clouds are
clearing out this morning. Dry northerly flow will maintain a dry
and cold air mass over the region through tonight. Surface gradients
will gradually weaken with breezing north winds easing by tonight.
If skies remain clear, radiational cooling should allow low
temperatures to dip down to the low to mid 20s, and teens in colder
valleys. The latest 12Z GFS Time-height does show some thin mid and
high cloud arriving tonight as a system approaches from the north
off the B.C. coast. Cloud cover could hinder cooling by a few
degrees tonight and some slight adjustments to forecast lows may be

The new 00Z GFS is still the farthest west with the low pressure
system driving south along 130 W on Tuesday. 500 mb heights indicate
a split in the north flow with the bulk of energy well offshore. The
portion of the split trough digging south through Wrn Wa has less
moisture to work with but models do show a couple tenths liquid QPF
from the coast to the southwest interior which could give localized
accumulations of an inch or two. Further north across Puget Sound,
not much more than flurries with no accumulation expected.

Models show little change in the cold and dry northerly flow pattern
through Wednesday. Another weak embedded system will brush the area
Wednesday but moisture will be very limited. Mostly flurries or
light snow showers possible with little or no accumulation. This
system looks to dig southward quickly with dry weather late
Wednesday and Thursday between systems. A chance of snow is
currently forecast for Thursday and will need to evaluate all the
new 12z model data before trimming or removing pops.  Mercer

Previous discussion...The Wednesday night upper level low will move
southwest across the area on Thursday. Will need to keep an eye on
this system because there is a chance that the precip amounts could
end up higher than forecast, especially if the low`s trajectory ends
up slightly farther west.

Beyond Thursday, it looks like the region will remain under
northerly flow aloft due to the ridge axis remaining more or less
near 150W. This will keep the door open for systems in western
Canada to drop in the CWA during this period. Will need to keep
an eye on the end of the week system as it could bring the
potential for significant snowfall to parts of the lowlands.
Otherwise, the main message for this period is below normal temps
and near or slightly below normal precipitation.


.AVIATION...Northerly flow aloft and at the surface will continue
today and tonight. The air mass is stable and dry except for some
lingering moisture in the far south. VFR conditions will prevail
through tonight. Northerly surface gradients will ease through the
day bringing gusty northerly winds around the region to an end.

KSEA...VFR with no ceiling. Northerly wind 10-12 kt will fall below
10 kt after 00Z and become light southerly late tonight. CHB


.MARINE...Gradients have been easing all night and winds are now
also easing. Winds will diminish today but will remain at small
craft advisory levels over the inland waters until late this
afternoon. On the coast, northeast winds will likely fall below
advisory criteria by noon or so.

A low center will move southeast well off the coast on Tuesday. For
now the forecast calls for the outer coastal water winds to be
southeast 15-25 kt while the inner coastal waters remain at 10-20
kt; will revisit this issue in the afternoon. A weaker system is
forecast to move southeast through Western Washington on Wednesday
night or so, but this is not forecast to bring any advisory level
winds. A stronger system will arrive around Friday. CHB


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


PZ...Small Craft Advisory until 4 PM PST this afternoon for Central
     U.S. Waters Strait Of Juan De Fuca-Coastal Waters From Cape
     Flattery To James Island 10 To 60 Nm-Coastal Waters From
     James Island To Point Grenville 10 To 60 Nm-Coastal Waters
     From Point Grenville To Cape Shoalwater 10 To 60 Nm-East
     Entrance U.S. Waters Strait Of Juan De Fuca-Northern Inland
     Waters Including The San Juan Islands-Puget Sound and Hood

     Small Craft Advisory until noon PST today for Coastal Waters
     From James Island To Point Grenville Out 10 Nm-Coastal
     Waters From Point Grenville To Cape Shoalwater Out 10 Nm.


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