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Area Forecast Discussion (AFD)

Issued by Seattle/Tacoma, WA (SEW)

FXUS66 KSEW 172255 AAA

Area Forecast Discussion
National Weather Service Seattle WA
352 PM PDT Wed Apr 17 2019


.SYNOPSIS...Mostly dry conditions are expected this evening as a
brief upper ridge moves across the area. A front will approach the
area from the west on Thursday and move across the area late
Thursday night or Friday morning. This front will give increasing
rain chances to the area Thursday, and showers on Friday. Rainfall
may be locally heavy on the Olympic Peninsula and in the interior 
north of Seattle. Saturday and Sunday are expected to be dry and 
seasonably mild. Another series of fronts will give a chance of 
showers to the area for the first half of next week. 


light showers on the Olympic Peninsula and over the coastal
waters. These will dissipate early as a weak temporary upper 
ridge moves east across the area this evening. The ridge will be
short-lived as a frontal system now out around 134-135W will move
slowly east toward the area. Rain ahead of this front will likely
hit the coastline by the early morning hours then will move inland
midday into the afternoon hours. Rainfall totals during the day on
Thursday are likely to be rather light. 

A period of moderate to locally heavy rainfall is expected,
mainly on the Olympic Peninsula and in the Cascades north of
Stevens Pass, Thursday night into early Friday. The rainfall will
occur with snow levels above 7000 feet and some locations in the 
north Cascades west of Mount Baker could see 3 inches or more of 
rain in an 18 hour period. Precipitation will become showery early 
Friday behind the front as snow levels drop back down to 3000-4000 
feet and the air mass will gradually dry Friday afternoon through 
Friday night. Rainfall will be heavy enough that spring runoff 
combined with the rainfall will cause rivers to rise. See the 
Hydrology section below for further information concerning rivers. 

Once showers end Friday night, the weekend looks dry and mild.
Weak flat ridging aloft and some onshore flow means that the air
mass will still have moderate amounts of low level moisture
resulting in partly to mostly cloudy skies. Temperatures will
warm, but will rain generally in the upper 50s to mid 60s at most

Forecast generally follow a blend of model solutions that are in
reasonably good agreement through Saturday. Albrecht

another dry day in a passing ridge moving across the area. The
ridge is flat enough that there will still be partly to mostly
cloudy conditions. Highs will generally be in the upper 50s to 
upper 60s across the region.

The ridge moves off to the east Sunday evening and a series of
frontal systems brush the area for the first half of next week.
Expect mostly cloudy conditions with a chance of showers Monday
through Wednesday with high temperatures at or just below normal
for late April. 

The extended forecast also follows a blend of model solutions
today that are all in reasonable agreement at this range. Albrecht


.AVIATION...An upper level ridge will move slowly east tonight and 
Thursday, with west southwest flow aloft. A frontal system will 
approach the area tonight, then spread light rain over the area on 
Thursday. At the surface, a ridge along the coast will weaken 
tonight and Thursday. The air mass is moist and stable.

Most ceilings are VFR with numerous sun breaks. A few sites are 
still MVFR under stratocumulus clouds. The cumulus will dissipate 
shortly after sunset, but high clouds and then mid clouds will 
increase tonight and Thursday. In general VFR should be the rule 
tonight and Thursday, except the coast where ceilings around 1k ft 
are likely.

KSEA...VFR ceilings will continue through Thursday. Southerly wind 
10-15 kt. CHB


.MARINE...A front will approach the coast tonight, then slowly move 
inland Thursday through Friday. Advisory strength southerly winds 
are expected for most of the coast beginning tonight. Advisory level 
westerly winds will develop on the coast and in the strait Friday 
behind the front, as onshore flow increases. Gale force west winds 
are a slight possibility in the strait Friday evening. 

Onshore flow will prevail Saturday and Sunday with high pressure
offshore and lower pressure inland. Another front will approach the 
area Monday. CHB


.HYDROLOGY...Rivers like the Nooksack and the Stillaguamish are 
forecast to rise with the Thursday-Friday system. While flooding 
appears unlikely at this time, some models are forecasting 
significantly higher amounts of rainfall in the North Cascades. 
Should the higher amounts occur, rivers could approach minor flood 
stage. Elsewhere, flooding is not expected in the next seven days. 


PZ...Small Craft Advisory from 8 PM this evening to 5 PM PDT Thursday 
     for Coastal Waters From Cape Flattery To James Island 10 To 
     60 Nm-Coastal Waters From Cape Flattery To James Island Out 
     10 Nm-Coastal Waters From James Island To Point Grenville 
     10 To 60 Nm-Coastal Waters From James Island To Point 
     Grenville Out 10 Nm.

     Small Craft Advisory until 8 PM PDT this evening for Puget Sound 
     and Hood Canal.