Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA
FXUS66 KSEW 231617
Area Forecast Discussion
National Weather Service Seattle WA
917 AM PDT Sun Apr 23 2017
.SYNOPSIS...An upper level disturbance over the coastal waters
will push inland this afternoon and tonight giving an increase in
shower activity to the area. An upper level low will give mostly
cloudy conditions to the area on Monday and scattered showers.
A front will move across the area later Tuesday and Tuesday night
for another round of showers. Weak disturbances will give
continued showery weather to the region Wednesday through the end
of the week. Temperatures will generally be near or slightly
below normal for late April.
.SHORT TERM...Radars show rain on the coast and through the
Chehalis Gap gradually spreading northeastward into the interior
of Western Washington this morning. This precipitation is
associated with a vertically stacked low near 47N 129W. The
precipitation will spread in to about the Seattle area aroun 1030
AM then all the way to the Canadian Border by early afternoon.
Precipitation will quickly tend to become more showery in nature
during the afternoon hours, with numerous showers continuing this
evening then tapering off somewhat from the north overnight.
Clouds and precipitation will hold high temperatures in the 50s to
around 60 today.
The next system upstream is centered near 45N 140W and is moving
rapidly eastward on a rather strong westerly jet. The forecast
models now all center the landfall of this feature as a 1000-1004
mb low on the northwestern Oregon coast late tonight into early
Monday morning. The movement of this low to our south is being
forced by its interaction with the upper low now over the
Washington offshore waters, and the upper feature gradually shifts
eastward into Western Washington Monday afternoon then east of the
Cascades Monday night. Cool air aloft associated with this upper
level feature will keep the threat of showers in the area Monday
through Monday night, but with the more unstable air mass, expect
some sunny intervals Monday afternoon from the north Coast and
Seattle or so northward. The warm late April sun will allow
temperatures over the northern half of the forecast area on Monday
to approach 60 in the afternoon, while temperatures over the
southern half of the area are held in the 50s with the more
numerious showers there.
After a bit of a lull in shower activity late Monday night into
Tuesday morning, a negatively tilted warm occlusion will spread
precipitation onto the coast midday Tuesday then inland later in
the day. Showery weather will persist behind the front Tuesday
Minor updates were made to the forecast this morning to refine
cloud cover forecasts for Monday, POPS, and precipitation amounts.
Otherwise the forecasts are consistent with the previous package.
.LONG TERM...From the previous long term discussion: Upper level
ridging progged to push into the area Thursday night might be the
first promising hint of having more than 24 hours of dry
conditions. While models keep the initial onset of this ridge a
touch on the dirty side...which may allow for a shower here or
there...the bulk of the period from Thursday night at least into
Saturday morning looks mostly dry. Even better is the fact that
current progs also nudging next upper level low on a more northern
track...pushing it into northern portions of B.C. If that ends up
being the case...dry conditions could persist for most of the CWA
(except the northern-most portions) into Sunday morning. Sunday
afternoon and evening may see some showers associated with this
system drift down into remainder of the CWA...as apparently the
bullet can only be dodged for so long. Good news from that is
ridging builds in again overnight Sunday and may just persist into
the first half of next week. SMR
.AVIATION...An upper level trough will swing NE across W WA
through 06Z bringing light rain to the area. Rain will spread
across Puget Sound through 19Z and across the N interior by 21Z.
CIGS over the interior will gradually lower to BKN-OVC025-050 by
00Z as the rain moistens the lower air mass.
The trailing upper level low will approach the coast tonight then
move inland 18Z-00Z on Monday. This will keep the air mass moist
with scattered showers and MVFR or low end VFR conditions.
KSEA...Light rain will spread over the terminal by 19Z as a weak
upper level trough moves across the area today. CIGS will lower to
BKN-OVC025-050. Surface winds will remain southerly 5-12 KT. Kam
.MARINE...A weak surface trough will move NE across W WA today.
Pressure gradients across the area will remain weak, allowing
winds to remain below small craft advisory levels. The associated
upper level trough will bring areas of light rain.
An offshore 1003 MB surface low will approach the region from the
W tonight then slow and weaken as it nears the N Oregon coast
Monday morning and then moves inland. A weak surface ridge
following the low will increase the onshore flow Monday evening
with SCA W winds expected in the central and E Strait of Juan de
An occluded front will approach the coast Tuesday before moving
inland Tuesday evening. Stronger onshore flow is possible behind
the front with some potential for westerly gales in the strait
later Wednesday. Kam
You can see an illustrated version of this discussion at