Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 210354

Area Forecast Discussion
National Weather Service Seattle WA
854 PM PDT Thu Apr 20 2017

A ridge of high pressure in the upper levels of the atmosphere
will bring warm and dry weather to Western Washington on Friday,
with the ridge lingering through the first half of Saturday. A
large upper level low pressure off the west coast will push cooler
temperatures, clouds and showers back into the area by Sunday.
This will be followed by a return to cool and wet weather for most
of next week.


Scattered shower activity across the area this afternoon and
evening. One thunderstorm did briefly develop across Western
Skagit County but was short lived. A developing convergence zone
in west central Snohomish County continues to produce some
lingering, slow moving downpours with considerable amounts of
small hail. The trend will continue to be for gradual weakening as
daytime heating fades and warming in the mid levels should help to
stabilize the atmosphere. Friday will be the best day for quite
some time and looks to be the warmest day thus far of the year for
most areas. A well pronounced mid level ridge of high pressure
will crest atop the region by Friday afternoon with heights to
near 570dm. Not only will this produce widespread sun, it will
send temperatures above normal as well. Expecting highs to top out
in the lower and mid 60s for most of the area, and mid to upper
60s south of Seattle. Would not be surprised to see a 70 degree
reading show up somewhere over southwest Washington. Temperatures
will remain on the warm side Friday night with mostly 40s and
lower 50s expected. The ridge axis will be rather progressive and
by daybreak Saturday will already be shifted to a position across
Idaho. Heights will fall quickly in the area with the large upper
low over the Pacific advancing behind the departing ridge. Will
spin clouds and showers this way on Saturday with the first half
of the day remaining dry east of the Sound and damp on the coast.
Temps will still remain on the warm side of normal Saturday
however, so it shouldn`t be too bad of a day before rain increases
in the afternoon. Sunday starts dry but also sees increasing
showers as a shortwave trough crosses the area and temperatures
dip a couple more degrees.

A cooler than normal and unsettled pattern with bouts of showers
will continue through much of the coming week. A more significant
system per GFS on Monday but the focus of the heaviest rain
appears south of the forecast area. Continued westerly flow with
heights below 550dm for several days. Normal high for this time of
year is in the low 60s. Looks like the area will be on the cooler
than normal side of that next week. Not expecting any particularly
high impact weather events next week, but will expect a prolonged
period of cool and wet weather with snow levels in the 3500-4000
foot range for several days.


.AVIATION...Scattered showers will end late this evening. The moist
unstable air mass will dry and stabilize into Friday as the upper
trough is replaced with an upper ridge. NW flow aloft. There are
likely to be a few areas of morning low clouds and patchy fog,
otherwise Friday will be sunny. Surface high pressure builds tonight
then weakens and shifts inland on Friday.

KSEA...Scattered showers have mostly avoided Seattle this evening
and they will end by midnight as the air stabilizes. Friday will be
mostly sunny after patchy morning low clouds.


.MARINE...High pressure will move over the area this evening and
winds will become light and variable tonight. The flow will become
easterly Friday and Friday night as the high pressure moves east of
the Cascades and low pressure moves to the offshore waters. A front
will move across the area on Saturday with southerly winds and the
weather pattern will be active next week.





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