Area Forecast Discussion
Issued by NWS Portland, OR

Home | Current Version | Previous Version | Text Only | Print | Product List | Glossary On
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28
000
FXUS66 KPQR 251037
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
336 AM PDT Sat Mar 25 2017

.SYNOPSIS...An upper level trough along the coast and its associated
onshore flow will keep showers going overnight into Saturday, then
decrease later Saturday and Saturday evening as the upper trough
moves east and the onshore flow decreases. The next front will bring
another good dose of rain and some breezes to the area Sunday, with
snow in the Cascades...mainly Mt Hood northward. Its associated upper
trough will bring showers to the area Sunday night and Monday with
snow levels near or a little below the Cascade passes. The next
system will bring more rain to the area Tuesday and Wednesday of next
week.

&&

.SHORT TERM...(Today through Monday)...Radar imagery shows scattered
showers persisting this morning across SW Washington and NW Oregon,
but with much less coverage than was the case Fri afternoon. Friday
afternoon`s convection produced an EF0 tornado, which was caught on a
home security camera and relayed to the public via the media. The
storm had its origins around the west hills of Portland, crossed the
Columbia River, moved through the downtown Vancouver area, through
Minnehaha, close to Vancouver Mall, and through the Covington and
Orchards areas of northeast Vancouver. This storm produced some
damage in the Covington and Orchards areas around 315 pm that was
consistent with an EF0 tornado. An LSR and SPS were issued earlier
about this event.

Today should be considerably less active, as warm advection aloft
will increasingly cap any convection throughout the day. 00z NAM/GFS
soundings show a cap strengthening near 10 kft as the day progresses.
Lapse rates below the cap will remain decent, so there should still
be plenty of showers...just not the deeper/stronger convection we saw
Friday. Given the steep lapse rates below 700 mb and plenty of
low-level moisture, suspect stratocumulus clouds and showers will
fill in this morning with the start of daytime heating. However,
subsidence associated with a transitory shortwave ridge should
eventually bring an end to the showers late this afternoon and
evening.

Satellite imagery continues to show a progressive pattern across the
North Pacific, with our next front already crossing 145W. By the time
showers with our current upper trough taper off this evening, high
clouds will likely be increasing in advance of our next frontal
system. Rain will likely begin along the coast by daybreak, then
spread inland rapidly due to the strong Pacific jet. This system does
have an atmospheric river stretching back to the West Pacific
subtropics, with precipitable water values in excess of 1.50 inch as
far east as 150W. However this moisture plume will become
increasingly stretched and strained as the front approaches the Pac
NW, so rainfall totals will probably be fairly modest. Nonetheless,
it will be a significant rain with 0.33-0.50 inch totals likely to be
common in the inland valleys, up to 1 inch along the coast, and 1-2
inches in the higher terrain by the time precip tapers off throughout
the day Monday.

Similar to last Thu night/Fri, easterly flow ahead of the frontal
system will likely lock cool air in place over the Cascades Mt Hood
northward as precipitation develops Sunday. This will cause the front
to occlude for the most part as it moves inland, with little time
spent in the warm sector north of Mt Hood before cold advection
pushes snow levels below the Cascade passes Sun night/Mon. This
system may warrant a Winter Weather Advisory for snow in the S WA and
possibly N OR Cascades, as 6-12 inches are certainly possible above
the snow level. Wet snow will likely impact the passes, especially at
night. Given uncertainty in snow levels, will hold off on issuing the
advisory for now and defer to future shifts.

Cool onshore flow will persist into Monday as the upper trough moves
across the Pac NW. Unlike yesterday, it appears the coolest air aloft
will move through at night Sun night, with temps warming aloft Mon.
This should limit the strength of convection Monday, with just
perfunctory showers expected at this point. Showers should decrease
Mon night with the loss of daytime heating and increasing subsidence
as high pressure builds into the region. Temps likely remain a little
below late March normals for the next few days due to our troughy
upper level pattern.  Weagle

.LONG TERM...No Changes. Previous discussion follows...
(Monday night through Friday)...12z models remain in fairly good
agreement with regard to the long wave pattern of the upcoming work
week. They show the upper trough from this weekend departing east
becoming positioned over the Northern Rockies by early Monday
evening. A flat upper ridge crosses the region Tuesday through early
Wednesday before a flat upper trough takes its place Wednesday night.
The trough deepens as it crosses the PacNW Thursday which allows for
a little higher amplitude ridge to build for Friday. Unfortunately
(depending on your preferred weather), this low amplitude upper
pattern does not lend toward any day long dry period for the upcoming
work week. As of today`s model runs, best chance may end up being
Friday as the ridge builds, however the GFS is showing a short-wave
riding over the top with rain affecting the northern tier.

The overall cloudy/rainy/snowy sequence will keep temperatures a
little cooler than normal during the day and warmer at night. Snow
levels will stay around 4-5000 feet except for Wednesday and Friday
where they rise to 5-7000 feet. /JBonk

&&

.AVIATION...A showery pattern to continue today, but showers will
be diminishing during the afternoon. Expect mainly dry conditions
tonight into Sun morning. Conditions should be mainly VFR
through mid- morning, then think cigs of 2500 to 3500 ft will
form in the late morning as stratocumulus develop with daytime
heating. Any MVFR conditions that do form during the morning will
improve to VFR in the early afternoon and remain VFR through
tonight.

KPDX AND APPROACHES...Occasional showers expected today,
especially during the morning hours. Expect mainly VFR
conditions. But cigs of 2500 to 3500 ft likely form during the
mid-to-late morning, so a few hrs of MVFR possible. Then expect
VFR this afternoon and tonight. Pyle

&&

.MARINE...Weak high pres building in today will keep winds fairly
benign over the waters through tonight. Seas remain around 10 ft
this morning, so will allow the small craft advisory for seas to
continue through 5 AM. Seas should drop below 10 ft by
sunrise, and will linger around 8 to 9 ft through the rest of
today. Low pres will move toward the B.C. coast Sun, bringing a
strong front through our coastal waters. The fcst models continue
to show a period of solid gale force winds Sun morning and early
afternoon. The main impacts look to be over the outer waters,
but decided to extend the gale watch to include the nearshore
zones as well. Seas will also likely build back into the mid
teens Sun, and will be dominated by southerly wind waves and
fresh swell. There will be a lull in the winds Sun night and
Mon, but another front is expected to impact the waters on Tue
into Wed. There is some model disagreement with the track and
timing on this system, but it looks likely that it will result in
some small craft advisory winds. A longer period westerly swell
is modeled to arrive Mon night into Tue, which would push seas
back above 10 ft. The additional wind wave energy from the
frontal system could bring seas back into the mid teens. Pyle

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...None.
WA...None.
PZ...Gale Watch from Sunday morning through Sunday afternoon for
     Coastal Waters from Cape Shoalwater WA to Florence OR out
     60 nm.

     Small Craft Advisory for hazardous seas until 5 AM PDT early
     this morning for Coastal Waters from Cape Shoalwater WA to
     Florence OR out 60 nm.

     Small Craft Advisory for Rough Columbia River Bar until 6 AM
     PDT early this morning.

     Small Craft Advisory for Rough Columbia River Bar from 1 PM
     this afternoon to 6 PM PDT this evening.

&&

$$

Interact with us via social media:
www.facebook.com/NWSPortland
www.twitter.com/NWSPortland

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.



USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.