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Area Forecast Discussion (AFD)

Issued by Seattle/Tacoma, WA (SEW)

FXUS66 KSEW 180332

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


The forecast has been largely on track this evening and has been
left as is. A fairly tranquil evening has played out across
western Washington as a mid/upper level ridge built into the
region. As mentioned in the afternoon discussion, this ridge will
pass to east fairly quickly tonight into Thursday, opening way for
the next frontal system to move in. Have already to begun to
notice the influx of higher level cloud cover this evening from
the west out ahead of the system. For most locations overnight, 
conditions will be mostly cloudy to overcast with showers 
spreading into the coastal areas very early Thursday morning 
before sunrise. Overall timing of precip in the grids looks okay 
for now, with moisture spreading east across the area thru the 
day. It is still worth keeping a close eye on QFP across the 
Olympic peninsula and especially the northern Cascades, where 
several rivers are still expected to rise and could approach minor
flood. Refer to hydrology section-tho no updates were needed this
evening. Elsewhere, generally a half an inch or so is probable 
thru Friday.

Previous discussion has been included below with modified aviation
and marine sections.



.PREV DISCUSSION... /issued 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...A fairly tranquil evening has played out across most
terminals this evening as mainly mid and high level clouds spread
east well ahead of the next frontal system. The exception has been
HQM, where conditions along the coast have generally been reduced
vsbys and intermittent MVFR conditions. These trends should
largely be the rule at least into the first part of the overnight
period where mid and high clouds generally become overcast for all
terminals. With the aforementioned frontal system, rain will be on
the increase from west to east overnight tonight and thru the day
on Thursday. Showers/Rain should begin to spread to the coast 
/HQM/ not long after midnight, with rain/showers approaching the 
SEA metro terminals by late afternoon/early evening Thursday. Rain
will likely bring VFR cigs down to at least MVFR where little 
recovery will likely take place thru the remainder of the TAF 
cycle. South wind will prevail for all generally 8-12 kts by 
Thursday afternoon with some gusts near 20kts possible, expect CLM
where winds will mostly stay NW near 5 kts.

KSEA...VFR ceilings will continue through Thursday. Rain will
spread near the terminal by late afternoon, likely bringing
ceilings down to MVFR. Southerly wind 10-15 kt with a few gusts
neat 20kts. 



.MARINE...The next frontal system will approach the area from the
west overnight tonight and thru the day Thursday. The marine
forecast has largely been on track this evening and no
modifications were necessary. The small craft advisory for Puget
Sound was allowed to expire at 8 PM, as observations in the area
over the the last few hours have not reached 21kts. At 8 PM, the
small craft advisory for the northern coastal waters will remain
in effect thru 00z Friday as winds should begin to pick up ahead
of the front shortly. No other marine headlines appear necessary
at this time, though it is still wise to continue to monitor the
potential for gales down the Strait Friday evening as westerly
winds pick up behind the front and gradients strengthen above 

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

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