Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 150459

Area Forecast Discussion
National Weather Service Seattle WA
959 PM PDT Wed Mar 14 2018

.SYNOPSIS...An upper low pressure over the northeast Pacific will drift south
along the coast through Friday and then into northern California.
The upper low will weaken and slide east through the weekend but a
broad trough of upper level low pressure will remain over the
area. Temperatures will be near or slightly below normal over the
next several days.


.SHORT TERM...Satellite imagery shows broad upper level low
offshore plenty of shower activity over the coastal waters and
just along the coast. Over the interior skies are clearing from
the south. Doppler radar has isolated showers over Whatcom and
San Juan county along with the showers along the coast and
offshore. Temperatures with the clearing skies have already
dropped into the 30s in some locations with most places in the
lower to mid 40s at 9 pm/04z.

Upper level low moving southeast tonight into Thursday with the
low center off the central Oregon coast by 00z Friday. Southerly
flow aloft over the area overnight will keep most of the shower
activity confined to the coast. Flow aloft turning southeasterly
on Thursday with the shower activity wrapping around the upper
level low moving inland south of the area. Temperatures aloft are
on the cool side so even with plenty of sunshine highs on Thursday
will be near normal, in the lower to mid 50s in most locations.

Upper level low continuing to move south Thursday night and
Friday leaving western Washington on the far north side of the
upper level low. The flow aloft will be light. Showers will
continue to be mostly south of the area. The air mass will be
slightly unstable in the afternoon so there is a chance of showers
mostly over the mountains. High temperatures will remain the the
lower to mid 50s. With the lack of cloud cover Thursday
night/Friday morning the cooler locations will drop below freezing
early Friday morning.

A much weaker upper level low developing over Vancouver Island on
Saturday. Flow aloft remaining light with low level onshore flow
at the surface. With a little more moisture to work with due to
the onshore flow and the air mass slightly unstable with the upper
level low in the vicinity expect a little more cloud cover on
Saturday and will have to keep a chance of showers in the
forecast. Highs will be a little cooler, a couple of degrees
either side of 50.

Current forecast has the trends covered. No update this evening.

.LONG TERM...Previous discussion follows. The upper low will
weaken and dissipate as it meanders well south of the area.
However the Pacific Northwest will remain under the influence of a
broad upper level trough which will help to keep at least some
showers around over the weekend, although coverage should not be
tremendous. Temperatures over the weekend will continue on the
same idea of near to slightly below normal. The quiet weather will
persist into Monday and early next week. However the mid level
heights will rise with some weak ridging building in and this
should allow temps to warm a few degrees during the Monday-
Wednesday time period but it will still just be a few degrees
above normal. So for now, no repeats of last weekends spring
preview - with a very typical mid March pattern settling in. By
very late in the forecast period there is indication of stronger
troughing off to the west and maybe increased rain chances again
by late next week. But this is quite far out and any confidence is
quite low. Johnson


.AVIATION...Southerly flow aloft becoming southeasterly on
Thursday. Low level flow becoming weakly offshore. Skies clearing
over the interior this evening and with the transition to weak
offshore flow overnight do not expect the cloud cover to fill back
in over the interior overnight into Thursday morning. Temperature
dew point spread are already down to 2 degrees at Olympia with
calm winds. Could see some restricted visibilities for a couple
hours in the morning. For the remainder of the area over the
interior just scattered clouds through Thursday. VFR ceilings
along the coast overnight at times with skies scattering out
there Thursday morning.

KSEA...Scattered clouds through Thursday. Light and variable winds
becoming northerly 4 to 8 knots late Thursday morning into the
afternoon and evening hours. Felton


.MARINE...Weak onshore flow will transition to offshore flow
overnight into Thursday morning. Southeasterly winds over the
coastal waters becoming more easterly during the day Thursday and
staying below 20 knots. For the interior variable winds overnight
becoming light offshore on Thursday. Low level flow turning back
onshore Friday but not strong enough for any advisories. The weak
onshore flow pattern will continue into Saturday. Felton


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



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