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Area Forecast Discussion (AFD)

Issued by Seattle/Tacoma, WA (SEW)

FXUS66 KSEW 260255

Area Forecast Discussion
National Weather Service Seattle WA
755 PM PDT Sun Aug 25 2019

.SYNOPSIS...A strong ridge of high pressure will begin to build
into the area tonight thru mid-week. Low level onshore flow will 
also transition to offshore flow late Monday into Tuesday. This 
combination will bring unseasonably hot temperatures to Western 
Washington with temperatures peaking on Wednesday. A weak system will 
bring a brief rest on Thursday before another ridge builds over the 
area at the end of the week.


.SHORT TERM /TONIGHT THROUGH WEDNESDAY/...Current satellite imagery 
and a look out the window agree that skies are generally clear over 
the area. There are still a couple of patches out there...but the 
trend is for these bits to thin out. The exception to this may be 
along the coast where weak onshore flow may bring a return of 
marine stratus.

Monday will kick off a week full of unseasonably warm conditions as 
upper level ridging will continue to strengthen over W WA but the 
furnace does not really kick on until gradients turn the low level 
flow offshore later in the day. With flow being in transition 
through the increase in temps will be mainly from the upper 
level ridge and clear skies...thus only being a few degrees warmer 
than what was observed today with afternoon highs still generally in 
the 70s with the SW interior hitting around 80. Tuesday will see 
this combo of offshore flow and strong upper level ridging have a 
larger impact with afternoon highs in the lower to mid 80s along the 
coast and the mid 80s to around 90 for the interior. While this will 
be the peak for temps along the coast...Wednesday will bring the 
hottest temperatures to the interior...with locations from the 
Seattle Metro area south seeing temps in the upper 80s to around 90 
and the SW interior getting into the lower 90s.

This kind of heat will result in a moderate heat risk over much of W 
WA for both Tuesday and Wednesday...meaning that those more 
sensitive to heat will be at risk unless simple measures are taken. 
While this heat will not meet advisory thresholds...those finding 
themselves or their pets outdoors for any prolonged period of time 
Tuesday and Wednesday are encouraged to take regular breaks and 
hydrate often and if traveling or shopping...remember to never 
leave children or pets unattended in a hot vehicle.  SMR

.LONG TERM /THURSDAY THROUGH SUNDAY/...From Previous Discussion...On 
Thursday, the ridge will begin to dampen across the region as a 
shortwave trough approaches the area from the SW. Guidance is having 
a tough time determining the amount of moisture associated with this 
weak system as its projected path takes it across southern 
Washington into eastern Washington. For now will keep slight chance 
of showers in for the far SE portion of the CWA. Enough afternoon 
destabilization should yield enough CAPE for a chance for a 
thunderstorm, mainly along and east of the crest, which could pose 
fire concerns. Despite weak troughing with the shortwave, guidance 
still keeps temps in the low-mid 80s, so while it may not be quite 
as hot as Wednesday, it may still remain rather warm. Will have to 
watch trends over the coming days: more cloud cover and showers 
could keep things cooler, whereas a weaker system or a more 
southerly track could keep those conditions to our south.

By Friday, the ridge noticeably reestablishes itself across the 
area, primarily in response to a deepening closed low across the 
northern Pacific. Yesterday, guidance wanted to bring this low 
towards Washington rather quickly with some wetting rain, 
essentially ending our brief period of heat. Last few model runs 
now slow this system down considerably, with a more noticeable 
amplification of the ridge. It remains possible for temps to warm 
back up Friday and Saturday now. For now not going as high on 
temperatures as the middle of the week, but have nudged temps a 
few degrees. 

Models then diverge later in the weekend regarding closed Pacific
low. Unfortunately, ensembles don't really help clear anything up.
Deterministic EC brings front into the area with showers while 
GFS keeps ridge over area. Given solutions in previous runs have 
shown potential for showers, re-introduced just a slight chance, 
tho it remains possible the closed low could get "cutoff" from 
faster westerly flow to the north, which if is the case, western 
Washington gets to hang onto the ridge. Right now, leaning 
slightly towards the more progressive solution, which would favor 
the EC a tad.



.AVIATION...Northwesterly flow aloft as an upper level trough 
departs the Pacific Northwest. Onshore flow at the surface. 

VFR conditions in place over W WA this evening and are expected to 
remain in place for most of the area overnight and into Monday. The 
exception to this may be along the coast where marine stratus may 
cause for MVFR conditions during the overnight hours and early 
Monday morning before conditions clear by Monday afternoon.

KSEA...VFR conditions tonight and Monday. Northerly winds at 4-8 
knots. SMR


.MARINE...Onshore flow continues into Monday although gradients will 
not be as tight as previous days. As such...winds in the strait are 
just shy of SCA criteria. Will continue to monitor...but even the 
usually cantankerous Race Rocks is adhering to the no 
headlines in place for the evening forecast package. Surface flow 
will shift to offshore late Monday and into Tuesday and is expected 
to persist through much of the week.  SMR


.FIRE WEATHER...Very warm temperatures and low relative 
humidities are expected Tuesday and Wednesday with poor relative 
humidity recovery especially Tuesday night into Wednesday morning. 
Wednesday will be The warmest and driest day. Thursday will likely 
see cooler temperatures and improved relative humidities.