Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 192134

Area Forecast Discussion
National Weather Service Seattle WA
234 PM PDT Mon Mar 19 2018

.SYNOPSIS...Isolated showers will mainly affect the mountains
tonight and Tuesday as the flow flips to onshore. Moist southwest
flow will develop on Wednesday. Rain will increase Wednesday night
as a cold front moves in. A deep upper level low will keep the
air mass cool and unstable Thursday through Saturday. A ridge will
bring drier weather on Sunday.


.SHORT TERM...A weak upper level trough will shift inland over
Oregon tonight. Across Western Washington, mainly quiet conditions
with just isolated showers popping up over the mountains. Showers
will diminish late this evening. Winds are light and may see
patchy fog overnight. Lows will drop into the 30s.

The pattern will remain rather benign on Tuesday with weak
onshore flow. Models show some moisture over B.C. which may clip
the area. Isolated showers are expected again, mainly in the
Olympics and north Cascades.

Wet weather will return Wednesday and Thursday as a deep upper
level low forms over the NE Pacific, setting up moist S/SW flow
over WA. A cold front will shift inland Wednesday night, the focus
for steadier precip. A trough axis will swing through on Thursday
keeping the air mass cool and unsettled. Model consensus looks
good so far. 33

.LONG TERM...The air mass will remain cool and unstable Friday and
Saturday as the deep upper level low crosses the region. Snow
levels are low, only a few hundred feet, with a chance of
rain/snow showers possible on the higher hills. Significant
accumulations are not expected since daytime temps will be in the
mid to upper 40s. Lows will be mainly in the mid 30s. A weak
upper level ridge will bring drier weather on Sunday. Temperatures
will be a few degrees warmer although still trend below normal.
Rain will return on Monday as the next Pacific frontal system
arrives. 33


.AVIATION...A ridge of high pressure centered over the area will
move slowly southward through Tuesday. North flow aloft will
become northwesterly on Tuesday. Light winds will prevail at lower
levels. The air mass is somewhat moist and is stable, but will be
weakly unstable in the mountains during the late afternoon and
evening hours due to daytime heating.

Conditions are mainly VFR across the area with some patches of
ceilings 2500-3000 feet in the south interior and near the
mountains. Expect VFR conditions to continue through most of
tonight with CU at 3500-5000 ft dissipating after sunset. Some
clouds at or above 080 are expected at times overnight. Some fog
or low stratus can be expected toward morning in favored river
valleys and near bodies of water. Any fog or stratus that does for
will dissipate quickly around 18Z. Albrecht

KSEA...VFR expected through tonight. Stratus forming near the
sound or around the terminal cannot be ruled out after 12Z with
the light pressure gradients and radiational cooling overnight.
Any stratus should be short-lived. Light NW winds this evening
becoming NE 4-7 kt after 03Z then variable after 10Z. Albrecht


.MARINE...A ridge of high pressure will remain over the waters
through Tuesday night giving light winds and weak
afternoon/evening sea breezes.

A frontal system will move slowly east through the coastal waters
on Wednesday and inland later Wednesday night or early Thursday.
Forecast models have trended weaker with this front and with the
post frontal onshore flow that develops behind it. Hence,
confidence in winds staying below gale force has increased. Still,
expect some small craft advisory conditions to develop around the
Strait of Juan de Fuca and portions of the coastal and inland
waters later Thursday or Thursday night.

Weaker frontal systems will affect the waters Friday through the
weekend. Albrecht


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



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