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Area Forecast Discussion (AFD)

Issued by Seattle/Tacoma, WA (SEW)

FXUS66 KSEW 200336

Area Forecast Discussion
National Weather Service Seattle WA
836 PM PDT Mon Aug 19 2019

.SYNOPSIS...A warming trend will take place through Tuesday 
across the region as a ridge of high pressure builds in. This will
be brief as a trough of low pressure pushes in from the Pacific 
into central B.C. and provides cooler temperatures and rain 
across the region through Wednesday night. A weak high pressure 
will build back into the region for the late week with 
temperatures near normal. There may be a few showers around on 
Saturday, but otherwise it will be a mainly dry stretch of weather
to close out this week.


.SHORT TERM /TONIGHT THROUGH THURSDAY/...Mild conditions in place
tonight with just few to scattered cirrus clouds aloft.
Temperatures will be a few degrees cooler Tuesday morning with
lows mostly in the 50s. The current forecast is on track. 33

Previous discussion...A mid level trough of low pressure will 
slide northeast out of the Pacific from later tonight into 
Tuesday. Ahead of the system, increasing 500mb heights across 
Western Washington will set the stage for a warmup tomorrow. Highs
will be a good 5-10 degrees warmer than today, with many low 80s 
developing. Skies will be sunnier as well, but should steadily 
cloud up from west to east in advance of this approaching system. 
The system will bring rain to the coast by early Wednesday morning
and spread steadily east through the afternoon hours. Heaviest 
rain totals will be on the Olympic peninsula to the coast with 
lighter amounts farther east. Temperatures will remain mainly in 
the 60s on Wednesday with the clouds and showers. While this 
system has plenty of moisture, it looks to remain progressive as 
the trough axis crosses the area by late Wednesday afternoon and 
conditions become just showery overnight Wednesday with chances 
diminishing by daybreak Thursday.

Heights remain relatively high on Thursday and even begin to
increase again as a strong ridge over the eastern Pacific builds
south of the area. The ridge will not make too much northern
progression this week however as a trough well to the north will
hold it at bay and keep more of a flat ridge or zonal flow in
place. This will mean Thursday should be near normal with
decreasing cloudiness. 

.LONG TERM /FRIDAY THROUGH MONDAY/...Previous discussion...For 
the extended portion of the forecast, the zonal flow will continue
with a strong mid level ridge staying well south of the area. 
Some indication of possible showers on Saturday but the bulk of 
the system should remain north and so will include only small 
chances of rain on Saturday. Either way, temperatures will linger 
around normal through the weekend, in the upper 60s and lower 70s.
By early next week the focus will begin to shift on northward 
expansion of the strong ridge to our south. This may set the stage
for an eventual return to hot weather to close out August. 
Although this is a little past the current forecast period, it is 
worth noting that CPC highlights the area for a risk of excessive 
heat the last few days of the month. Will have to continue to 
monitor this possibility but there is a lot of time between now 
and then where the forecast may change. 


.AVIATION...Dry conditions tonight and Tuesday with a ridge moving
into the Pac NW. The flow aloft is southwesterly. Morning low
level stratus clouds will likely be limited to the coast.
Otherwise, expect VFR conditions. Rain may reach the coast late Tuesday
night or early Wednesday as a cold front approaches. 33

KSEA...VFR conditions tonight with N/NE wind around 5 kt. Winds 
becoming S to 10 kt by 18z Tues. 33


.MARINE...Light low level flow will continue tonight. Southerly
wind will increase Tuesday night as a cold front approaches the
region. Expect Small Craft Advisory winds over most waters. The
flow will turn westerly and onshore on Wednesday as the front
moves through western WA. High pressure will build over the 
coastal waters on Thursday. A weaker front will move by to the 
north later Friday. 33


.HYDROLOGY...The daily hydrology discussion has ended until the
start of the next rainy season; it will only be updated as needed
until then.