Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 221602

Area Forecast Discussion
National Weather Service Seattle WA
902 AM PDT Sun Apr 22 2018

.SYNOPSIS...A weak upper trough over Washington will move east
today, and clouds over Western Washington will give way to mostly
sunny skies. An upper ridge will build over the area tonight and
remain in place through Thursday for a stretch of sunny days and
the warmest weather since last September. A weak system will bring
a return to clouds and cooler weather Friday and Saturday.


.SHORT TERM...A weak upper trough is over the Pacific Northwest
this morning, with its trough axis along the Cascades at this
time. The cloud cover directly associated with the trough has
moved east of the crest for the most part, but there are areas of
lower clouds over the forecast area -- one over the greater
Seattle area and the other over the northwest interior and its
waters. Elsewhere skies are mainly sunny, with 9 am temperatures
in the 40s to lower 50s.

The trough will move across Eastern Washington this afternoon,
with dry northwest flow aloft over Western Washington ahead of the
upper ridge building offshore. The cloud cover mentioned above
should give way to sunny skies across the entire forecast area
within the next few hours. Highs today will be a couple degrees
above normal, in the upper 50s to mid 60s.

The upper ridge will continue building as it shifts inland tonight
through Tuesday, and the low level flow will turn offshore as a
thermally induced trough extends northward along the Pacific
Northwest coast. The 500 mb height will rise to around 5730
meters; the 850 mb temperature will rise to around +7C on Monday
and +11C on Tuesday. As we have been advertising for several days
now, this will set the stage for sunny warm days with highs well
into the 70s over much of the area. McDonnal

.LONG TERM...Here is the long term section from the early morning
forecast discussion -- Extended models in good agreement early on in the
extended with the upper level ridge remaining in place Wednesday
into Thursday with the low level offshore flow beginning to
weaken on Thursday. Model 850 mb temperatures peaking out in the
plus 14 to 16C range. Temperatures aloft this warm combined with
low level offshore flow and the April sun having plenty of juice
brings up the possibility of lower 80s in the warmer locations
Wednesday and Thursday. Temperatures on the Coast will cool a
little on Thursday from the 70s on Wednesday with weakening of the
offshore flow. Temperatures over the interior will mostly be in
the upper 60s to lower 80s both Wednesday and Thursday. Model
differences start to appear on Friday with the ECMWF a little
slower kicking the shortwave out of the upper level low off of
northern California which would keep the offshore flow pattern
intact for another day. The faster GFS solution has the low level
flow turning onshore later Thursday with the approach of the
shortwave. Current forecast more in line with the GFS and given
the rarity of 4 70 plus days in a row in Seattle in April ( see
climate section below ) will stay with the cloudier cooler slight
chance of showers forecast for Friday and Saturday. Felton


.AVIATION...There are several areas of morning clouds that should
break up and dissipate for the afternoon. An upper ridge will bring
dry and stable conditions once the morning clouds burn off. The
flow aloft is northwesterly.

KSEA...A patch of morning stratus is over the metro area--mainly
east of I-5 over King county. It should burn off by midday.


.MARINE...Northerly flow will prevail today. Then tonight,
a thermally induced trough of low pressure will set up along the
Oregon and Washington coasts and persist into Wednesday morning.
This will lead to light northerly offshore flow. The trough will
move inland on Thursday, with a southerly surge along the coast
and the start of a marine push into the inland waters.


.CLIMATE...It is a rarity to get 4 days or more in a row of 70
degree plus weather in Seattle in April. In the 74 years of
records at Sea-Tac it has only happened 4 times ( 4 days in row 3
times and 5 days once ). The last time it happened was the record
crushing 5 day streak from April 17-21, 2016 that included 4 days
in a row in the 80`s. The other 4 day streaks occurred on April
24-27, 1995, April 26-29, 1987 and April 27-30, 1976. Even getting
a streak of 3 days in a row of 70 degree plus weather in April in
Seattle is rare with only 6 streaks of 3 days in a row since
records started in 1945. Felton



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