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Area Forecast Discussion (AFD)

Issued by Seattle/Tacoma, WA (SEW)

FXUS66 KSEW 200435

Area Forecast Discussion
National Weather Service Seattle WA
930 PM PDT Wed Sep 19 2018

.SYNOPSIS...Showers and a Puget Sound convergence zone will 
dissipate Thursday morning. A stronger Pacific system will bring a 
warm front to Western Washington late Thursday. The associated 
surface low and trailing front will bring some rain and breezy winds 
to the area Friday and Saturday. Showers will decrease Sunday. Dry 
offshore flow develops Monday and persists through mid week.


.SHORT TERM...Onshore flow has strengthened with enough flow through 
the Strait and south Sound to produce a broad convergence area of 
showers. Radar and obs indicate spotty light rain with mostly trace 
amounts so far. The WRF4km 12z run shows this area of showers but 
not developing until later tonight. Convergence zone showers should 
dissipate rapidly Thursday morning as gradients ease. 

A warm front will brush the area late Thursday and Thursday night. 
Rain will reach the coast, Olympics, north interior, and 
north/central Cascades. Much lighter amounts will occur south over 
Puget Sound with most the deep moisture tracking into the north. 
Showers will likely ease behind the front but breezy south winds 
will develop. Winds will continue to be breezy Friday into Saturday 
as a 995-1000 mb low moves into Vancouver Island. It could be 
locally windy, especially across the north interior. Over an inch of 
rain is expected by Saturday north of around Everett. Much less rain 
to the south where shadowing will also hinder rainfall in the 
central Sound. Mercer

.LONG TERM...Previous discussion...Onshore flow with clouds and 
showers will gradually decrease Saturday into Sunday. Showers will 
gradually become confined to the mountains and to a possible Puget 
Sound Convergence Zone, especially later Saturday into early Sunday.
Temperatures will generally range from the mid 40s to mid 50s at
night, then into the 60s during the day.

Models show very good consistency and continuity next week as a
strong upper level ridge builds offshore and low level high 
pressure builds over British Columbia. The developing pattern 
results in the development of moderate to strong low-level 
offshore flow beginning on Monday and continuing through the 
upcoming week. Expect mostly clear skies next week with 
temperatures ranging from the 40s to lower 50s at night to the 70s
during the day. A model blend that favors the warmer GFS solution
was used to construct the temperature forecast. Albrecht


.AVIATION...Onshore flow is producing a broad area of convergence 
enhanced showers across parts of greater Puget Sound this evening. 
The WRF4km 12z run kept this going off and on through tonight and 
dissipating Thursday morning. Onshore flow and added moisture from 
rainfall will increase the chance for low MVFR stratus including 
KSEA/KBFI by 12z. A warm front will spread rain to the coast 
Thursday afternoon and barely clip the KSEA/KBFI terminals southward 
by the evening. dtm

KSEA...The light precipitation this evening around Puget Sound 
including the SEA/BFI terminals will dissipate later tonight and 
Thursday morning. This will bring additional moisture down closer to 
the surface with a higher chance of low MVFR cigs by 11-12z. The 
cigs were raised to 2k but given the current trends will likely 
lower them again with the 06z issuance. Cigs will be variable but 
MVFR until rising to VFR by late Thursday morning. The air mass will 
remain moist and stable on Thursday as a warm front clips the 
region late. dtm


.MARINE...Onshore flow has strengthened with over +1 mb through the 
Strait and Small Craft westerlies in the central/east portion until 
5 AM PDT. A lull in winds will occur Thursday morning. A stronger 
Pacific frontal system will arrive Thursday night and Friday with 
increasing southerly flow over the waters. The flow will turn 
onshore Friday night and Saturday as this system moves inland. 
Offshore flow will develop early next week as a thermal trough forms 
along the coast. dtm


PZ...Small Craft Advisory until 5 AM PDT Thursday 
     for Central U.S. Waters Strait Of Juan De Fuca-East 
     Entrance U.S. Waters Strait Of Juan De Fuca-Northern Inland 
     Waters Including The San Juan Islands.