Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 170954

Area Forecast Discussion
National Weather Service Seattle WA
300 AM PDT Sat Mar 17 2018

.SYNOPSIS...Upper level low pressure over the Pacific Northwest will
bring scattered light showers to Western Washington this weekend,
but for the most part the weather will be dry. An upper ridge will
move across the region Monday and Tuesday for dry weather. A frontal
system will bring a return to wetter weather Wednesday through the
end of the week.


.SHORT TERM...There are mostly just high clouds over the interior of
Western Washington early this morning. Western sections have a deck
of low stratus clouds, brought in by light onshore flow. Temps are
mostly in the 30s. Stratus over the interior will not be too long
lasting or extensive this morning. Today will be partly to mostly
sunny with highs in the low to mid 50s. An upper level low will
remain over the area through about Monday. The air mass is not
particularly moist, but the low has enough punch to generate diurnal
showers over the Cascades this afternoon and evening. These showers
could drift west over the lowlands.

Onshore surface flow will increase tonight, and stratus will be more
widespread on Sunday. The upper low will move closer to the interior
and the chance of showers will spread to all areas Sunday. Highs
will be in the low to mid 50s.

The upper low, now mostly a trough, will move south by Monday. There
could be a few residual showers over the mountains, but overall
Monday should be mostly dry. There could even be some clearing in
the afternoon. Highs will be in the mid 50s. Burke

.LONG TERM...After another mostly dry day Tuesday, a deep upper
trough off the coast will turn the upper flow southerly by
Wednesday. Models bring moisture from the south on Wednesday and
especially Thursday. The upper low is forecast to move inland on
Friday. The pattern is cloudy and moist, and by Friday it will be
cool as well. The snow level will fall to 1000 feet or so Thursday
night and Friday. After highs in the 50s all week, on Friday the
high temperatures may not get above the 40s. Burke


.AVIATION...Lingering upper level low pressure will keep flow aloft
from the east and fairly light today and into Sunday. Transition to
onshore surface flow in progress at the time of this writing and
once in place will continue throughout the day. The air mass remains
generally dry and stable. Cigs generally VFR area-wide although HQM
looks to have dipped into MVFR. That is expected to continue or even
drop further into IFR this early morning before recovering by mid to
late morning. OLM may also dip down with the possibility of
fog...but current obs are showing a BKN mid-level deck...and as such
this might be sufficient to nix any fog development. Will monitor
and update TAF as necessary. Proximity of the aforementioned upper
level low and moisture associated with it may cause scattered
showers to redevelop over the mountains this afternoon and early
this evening.  SMR

KSEA...VFR conditions to persist throughout the day for the most
part although some early morning stratus may be possible near the
terminal that may bring cigs down to MVFR. The potential for
scattered showers will steadily bring cigs down throughout the
day...but not expected to fall below VFR. While cannot rule out a
stray shower...POPs not significant enough to warrant inclusion in
TAFs. Southerly winds today with speeds 4-8 kt.  SMR


.MARINE...Onshore flow will increase today. Small craft advisory
strength westerlies are likely in the strait Saturday evening. Have
left it to the next shift to hoist the advisories. Onshore flow will
be somewhat lighter on Sunday, then will increase again on Monday. A
weather system will reach the area around Wednesday and turn the
winds more southerly. Thursday or Thursday night could be breezy
over all waters. CHB


.HYDROLOGY...River flooding is not expected in the next 7 days.




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