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Area Forecast Discussion (AFD)

Issued by Seattle/Tacoma, WA (SEW)

FXUS66 KSEW 230411

Area Forecast Discussion
National Weather Service Seattle WA
911 PM PDT Sun Apr 22 2018

.SYNOPSIS...An offshore upper ridge will build into the Pacific 
Northwest on Monday and remain there through Thursday, bringing a
stretch of sunny days to Western Washington. Low level offshore 
flow will result in the warmest weather since last September, 
with highs in the 70s over much of the area. An upper trough will
bring a return to more seasonal weather Friday and the weekend.


.SHORT TERM...Clear skies and a strong warming trend are forecast
through Thursday. Record high temperatures are possible for some
places on Wed and Thu. 

A fairly strong upper level ridge axis will move eastward across 
Western Washington on Monday, with 500 mb heights under the ridge 
axis near 574 decameters. The ridge will then linger over the
Pacific Northwest until about Thursday. Meanwhile at the surface,
a thermal pressure trough will develop along the coast and remain
in place until Thursday morning, barring minor diurnal
fluctuations. 850 mb temps will rise from around +7C on Monday to
around +14C on Wednesday. As we have been advertising for several
days now, this will result in sunny warm days with highs well 
into the 70s away from cooler shorelines over northwest 
Washington. Haner

.LONG TERM...From Previous Discussion: The models are a little 
inconsistent -- and hence there is some uncertainty -- with 
regard to the transition from the warm weather to a more typical 
spring pattern. The models agree that the upper ridge will shift 
further east late in the week, and an offshore closed upper low 
will begin moving east toward northern California; that will 
induce a switch to onshore flow across Western Washington and 
bring a chance of showers to the area in the high-based 
instability with southerly flow aloft. The difference in the 
models is primarily the timing of this transition, with the GFS 
showing a switch to onshore flow beginning Thursday afternoon, and
the ECMWF delaying that until Thursday night. For now we will 
stick with the current forecast, which reflects the slower timing 
of the ECMWF. The models all agree that the right forecast for 
Friday through Sunday is onshore flow and a chance of showers as 
the low moves inland, followed most likely by another trough from 
the west during the weekend. McDonnal


.AVIATION...A fairly strong upper ridge axis will move eastward
across Western Washington on Monday. Light to moderate southerly
flow aloft through Monday will become light southerly on Monday
night, and drying low-level offshore flow will strengthen. The air
mass will be dry and stable, with few or no clouds.

KSEA...Clear skies and a northerly surface wind component will
continue for the next 30+ hours.  Haner


.MARINE...Thermally induced low pressure will set up along the 
Oregon and Washington coasts tonight and persist through about
Wednesday night. This pattern leads to northerly offshore flow. 
Will probably see small craft advisory easterly winds at the west 
entrance to the Strait of Juan de Fuca by daybreak Tuesday. The 
thermal low pressure will move inland late Thursday with onshore 
flow and cooler marine air pushing back into the area. Haner