fxus66 ksew 221602 

Area forecast discussion 
National Weather Service Seattle Washington 
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 European model (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 


Sew watches/warnings/advisories... 

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