Area Forecast Discussion
Issued by NWS Portland, OR

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FXUS66 KPQR 261025
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
325 AM PDT Sun Mar 26 2017

.SYNOPSIS...The next in a series of Pacific frontal systems is
approaching the Pac NW coast early this morning. This system will
spread rain across much of the forecast area by sunrise, with snow
developing in the Cascades. The cold front will push onshore this
afternoon, with rain tapering to showers late this afternoon and
evening. Onshore flow behind the front will cause showers to continue
through Monday, with snow affecting the Cascade passes. Some drying
is likely Monday night and Tue, especially Salem southward. However
the next potent frontal system will spread more rain across the
forecast area Tuesday night and Wednesday, along with some potential
for gusty winds. Higher pressure and northerly flow behind this
system may lead to drier weather toward the end of the week.

&&


.SHORT TERM...(Today through Tuesday)...Very active and progressive
pattern continues across the North Pacific today, with the latest in
the series of fronts approaching the Pac NW coast early this morning.
KLGX radar is starting to show echoes filling in offshore; a couple
hours faster than most guidance had suggested. As a result, we will
be starting the Winter Weather Advisory for snow a couple hours
earlier, at 7 AM this morning. Expect rain to develop along the coast
by 5 AM, then spread quickly across much of the forecast area by
sunrise. Similar to our last couple of systems, this front is
occluding while moving onshore. This will allow cooler air to linger
over the Cascades as precipitation develops, especially Mount
Jefferson northward. As a result, snow levels are likely to start off
below pass level this morning. The Oregon Cascades may briefly get
into the warm sector for a few hours this afternoon; the combination
of that and stronger late March solar heating (despite being behind
clouds) may be enough to allow the passes to be wet or slushy this
afternoon. The higher ski resort elevations will see primarily snow
from this system, and with overall QPF possibly over an inch in the
Cascades through Monday, the higher Cascades could see up to a foot
of snow by the time snow showers taper off Mon night.

For the lower elevations, this will be another modest rainfall with
around 0.50" for the inland valleys, 0.50-1.00" for the coast, and
1-2 inches for the Coast Range. This will continue to push our March
rainfall total closer to record levels...for example, PDX March
rainfall is now 8th all-time. The 6.34" recorded thus far is only
1.55" shy of the all-time March record of 7.89" set in 2012.

The occluding front will push east of the Cascades late this
afternoon and evening, with cooler air aloft pushing in behind the
front with the parent upper trough. It appears the core of coldest
air aloft will move through well before peak heating Monday, so the
thunder/convection threat is not quite what it was last Friday.
Nonetheless, with the 00z GFS showing 500 mb temps as low as -30 deg
C over the coastal waters late tonight and early Mon, we decided it
would be prudent to mention a slight chance of thunder along the
coast and over the coastal waters. 00z GFS 1000-700 mb lifted indices
remain below zero deg C through Monday, so showers will likely be
numerous across the forecast area Monday. However, a strengthening
cap just below 500 mb will probably keep convection shallow enough to
mitigate the threat of thunder.

Showers decrease Monday night as the onshore flow decreases,
subsidence associated with high pressure increases, and warm
advection aloft stabilizes the atmosphere. Based on latest guidance,
flat upper level ridging will probably succeed in building across
Oregon, shoving the jet stream a little further north into Washington
as the next system moves toward the coast Tuesday. The result will
likely be a mostly dry and mild day Salem southward, with more in the
way of cloudiness for the PDX metro and a continued chance of rain
across much of SW Washington Tuesday.  Weagle

.LONG TERM...(Tuesday night through Saturday)...Upper-level pattern
becomes more amplified later next week, behind a trough that will
bring more rain Tuesday night into Wednesday, transitioning to
showers on Thursday. This means a ridge builds up over the Pacific
Northwest Friday for a dry and mild day. Models are in remarkably
good agreement with this ridge for how far out it is in the forecast
period, increasing confidence. Models however have the ridge
flattening with an approaching broad trough, with significant
differences in timing between the ECMWF (Saturday) and the GFS
(Monday). Either scenario will bring rain back to the area with the
flattening of this ridge. Snow levels generally hover around 4000 to
5000 feet, except for Wednesday and Friday when they go up to 6-8000
feet. Temperatures generally remain within a few degrees of late
March normals through the extended period, with Wednesday being the
coolest day as the upper trough swings through. -McCoy/Weagle

&&


.AVIATION...A strong frontal system to move into SW Washington
and NW Oregon today. Light rain will increase this morning as a
warm front approaches, but conditions should generally remain VFR
through around 15Z. Rain will increase at the coast during the
late morning, with MVFR conditions likely to develop between
15Z-18Z. Rain will increase over the interior between 18Z-21Z.
Confidence is not as high that the interior TAF sites will see
conditions drop below VFR, but think at least some sites will
see MVFR conditions for a few hrs in the afternoon. Rain will
taper to showers later in the afternoon and in the evening behind
the cold frontal passage. Conditions should improve to mainly
VFR as mixing increases in the post-frontal air mass.

KPDX AND APPROACHES...Expect VFR conditions to persist through at
least 18Z. Light rain will spread in this morning, then become
heavier in the afternoon. Think there is a decent chance at a few
hrs of MVFR conditions in the afternoon. Rain will taper to
showers during the late afternoon and evening. Conditions to
improve to generally VFR. Pyle

&&

.MARINE...A strong frontal system will pass through the coastal
waters today, bringing a period of solid gale force southerly winds.
The strongest winds will be over the outer waters, where gale
gusts should begin by around sunrise. Winds will have an offshore
component, so the effects will be limited somewhat over the
nearshore waters. However, still think there will be some gusts
to 35 kt for a brief period right ahead of the cold front, which
will move onshore during the early to mid afternoon time frame.
Seas will see a sharp increase this later this morning and this
afternoon in response to the gusty winds. Expect them to top out
in the 13 to 15 ft range. Winds will subside and veer to the
west later this afternoon and this evening. Seas will also drop
fairly quickly tonight, likely falling below 10 ft by around
midnight.

There will be a lull in the weather later tonight and Mon as
weak high pres builds over the waters. However, a longer period
westerly swell will arrive Mon night, pushing seas back into the
teens. Then another low pres system will affect the Pac NW Tue
and Wed. This will bring a period of winds gusting 25 to 30 kt,
with potential for some low-end gales Tue night into Wed morning.
Higher pres is then expected for Thu and Fri, bringing a period
of more benign conditions to the waters. Pyle

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...Winter Weather Advisory from 7 AM this morning to 4 PM PDT
     Monday for Cascades in Lane County-Northern Oregon
     Cascades.

WA...Winter Weather Advisory from 7 AM this morning to 4 PM PDT
     Monday for South Washington Cascades.

PZ...Gale Warning from 11 AM this morning to 4 PM PDT this afternoon
     for Coastal Waters from Cape Shoalwater WA to Florence OR
     out 10 nm.

     Gale Warning from 6 AM this morning to 3 PM PDT this afternoon
     for Waters from Cape Shoalwater WA to Florence OR from 10
     to 60 nm.

     Small Craft Advisory for Rough Columbia River Bar from 1 PM
     this afternoon to 7 PM PDT this evening.

&&

$$

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This discussion is for Northwest Oregon and Southwest Washington
from the Cascade crest to 60 nautical miles offshore. The area is
commonly referred to as the forecast area.



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