Area Forecast Discussion
Issued by NWS Portland, OR
FXUS66 KPQR 241052
Area Forecast Discussion
National Weather Service Portland OR
352 AM PDT Fri Mar 24 2017
.SYNOPSIS...Low pressure off the British Columbia coast is pushing a
cold front slowly onshore into the Pac NW this morning. Steady light
to moderate rain will turn to showers behind the cold front. A few
thunderstorms will be possible through this evening as cool air aloft
destabilizes the atmosphere. High pressure will cause showers to
taper off late Saturday, but the next frontal system will likely
bring more rain Sunday followed by showers Monday. Occasionally wet
weather is expected to linger through the middle of next week.
.SHORT TERM...Today through Sunday...A steady light to moderate rain
has been falling much of the night across the forecast area, with
totals thus far in the 0.33 to 0.50 inch range for the lowlands and
0.50 to 1.00 inch for the higher terrain. The rain is associated with
a cold front which has been slowed considerably by a wave of low
pressure which developed along the front near the OR/CA border. The
front is just now moving onshore into the Pac NW...Buoy 46050 just
experienced a wind shift and a drop-off in winds which indicates the
surface front moved through there around 3 AM.
Accumulating snow has generally been reserved for the higher Cascade
elevations...well above the passes. However, cold air trapped along
the east slopes of the Washington Cascades is allowing lower snow
levels for eastern Skamania County. Buck Creek RAWS has dropped to 32
deg F over the last few hours, indicating a likely changeover to snow
as low as 2500 ft elevation across eastern Skamania County and
possibly as far west as Indian Heaven Wilderness. Surprise Lakes
SNOTEL was up to 5 inches of snow by 2 AM. Given the combination of
lowering snow levels and plenty of precip upstream, decided to issue
a Winter Weather Advisory for snow in the S WA Cascades. An
additional 3-6 inches of snow are possible on top of the 3-6" that
have already fallen in that area. The snow advisory only runs through
10 AM this morning, as precipitation is expected to become more
showery behind the cold front later this morning through this
afternoon. For other areas, expect snow levels to lower to 4000-4500
feet behind the cold front this afternoon into tonight, but by then
precip will taper to showers with only modest orographics.
Conditions will need to be closely monitored today for thunder
potential. Some dry air will come in behind this precip and may be
sufficient enough to scatter out clouds and get some sun. If this
does occur, at least some surface based instability will develop. Any
instability will be sufficient for thunder given the falling heights
and colder air aloft. In fact, given freezing levels are expected to
be around 4000 feet, it won`t take much of an updraft to form small
hail. If surface instability strengthens enough for updrafts to
extend beyond 10 kft (would guess around 400 to 500 J/kg), would
expect a few hail showers which may even lead to minor accumulation.
This all depends on the amount of clearing we get behind the cold
front this morning into this afternoon, but the likelihood of
clearing is high enough to warrant a mention of the hail potential.
These storms will be pretty shallow, with tops generally 15-20 kft.
The effective shear will be significant enough, (25-30 kts)
to organize any stronger updrafts for hail formation, but these
shallow storms will lack the depth for any significant wind creation
either from evaporative cooling or water loading in the updraft.
It appears the previous shift made a good call in expanding the area
of thunder slightly eastward and adding the mention of small hail to
areas north of Salem; thus we made no changes to the thunder/hail
area with the morning forecast. Was tempted to boost the mention to
chance (vice slight chance) after seeing 750 J/kg of CAPE in the 00z
GFS sounding for PDX this afternoon; however we would like to see
some sunbreaks first before highlighting thunder chances further.
Enough terrain-guided shear may occur for there to be a couple
stronger cells this afternoon, along with the usual cold core funnel
potential in this type of pattern due to brief rotating updrafts if
we get a decent amount of sunbreaks. As usual, it is highly unlikely
that any funnels would touch the ground as a tornado.
Models seem to be trending a little more stubborn with the cold air
aloft tonight and Saturday...likely keeping numerous showers around.
Any lingering showers into Sat evening will likely collapse quickly
with the loss of daytime heating. Weak high pressure will probably
allow Sat night to be mostly dry, though high clouds will be on the
increase again ahead of our next frontal system due to move into the
Pac NW Sunday. This system looks very similar to the one over our
region this morning, with the main baroclinic zone and axis of
deepest moisture appearing destined for the OR/CA border. Rainfall
totals will likely be similar Sunday, with around a half inch for the
lowlands and around 1 inch for the higher terrain. Snow levels will
likely be just above the passes for most of this event, lowering to
3500-4000` as precip tapers to showers behind the cold front Sun
.LONG TERM...Sunday night through Thursday...Similar to the evolution
of our frontal system last night into today, steady rain Sunday will
likely taper to showers Sunday night and Monday as a cold front moves
eastward across the Pac NW. Cold air aloft behind the front may again
be able to support a few thunderstorms Monday, but confidence was not
quite high enough in this yet to warrant a mention. Models have been
more aggressive in holding the remnant baroclinic zone over the Pac
NW, keeping a significant chance of rain in the forecast through
Tuesday. 00z model suite continues to show fairly good agreement in
having a bit stronger of a moisture tap with the next organized
system Tue night/Wed. This will be followed by another cool upper
trough, which would keep showers and lower snow levels through
Thursday. Most models want to evolve this upper trough into a cutoff
low somewhere over the southwestern U.S. toward the end of the week.
Depending on how far west this occurs, it could set up a drier and
warmer pattern for the end of the week. Current forecast generally
trends drier late next week with temps rising to slightly above
.AVIATION...A slow moving front will bring a mix of MVFR and VFR
conditions early this morning before the atmosphere destabilizes
behind the front and conditions turn more showery and predominantly
VFR at all taf sites for the remainder of today. Nonetheless,
cannot rule out a temporary MVFR condition at any point today
near or under heavier showers, but these should be short lived
enough and hard enough to predict that will leave the mention out
of the taf for now. There is a slight chance of a thunderstorm
impacting any given terminal today with the primary window
falling between 21z Friday and 03z Saturday. Will also leave any
thunder/cb mention out of the tafs given their low predictability
this far out in this type of pattern.
KPDX AND APPROACHES...A slow moving front will bring a mix of
MVFR and VFR conditions early this morning before the
atmosphere destabilizes behind the front and conditions turn more
showery and predominantly VFR for the remainder of today.
Nonetheless, cannot rule out a temporary MVFR condition at any
point today near or under heavier showers, but these should be
short lived enough and hard enough to predict that will leave the
mention out of the taf for now. There is a slight chance of a
thunderstorm impacting the terminal today with the primary
window falling between 21z Friday and 03z Saturday. Will also
leave any thunder/cb mention out of the taf given their low
predictability this far out in this type of pattern. /Neuman
.MARINE...A strong cold front is currently pushing eastward over
the waters early this morning. Current Gale Force wind gusts of
35 to 40 kt should end shortly so will convert Gale Warnings to
Small Craft Advisories for winds and hazardous seas. Weak high
pressure will slowly push into the waters tonight and Saturday,
which will bring a gradual decline in winds. This will in turn
eventually allow seas to subside a few more feet. However, any
break in the weather and seas will be relatively short lived this
weekend as another front...similar to tonights although a bit
weaker...will push eastward into the waters and likely bring
another round of Gale Force winds Sunday. Seas should climb back
into the teens as a result. A weaker frontal storm system
appears on tap for Tuesday, but the weather looks to remain
relatively active for much of next week.
It should be noted there will be several sizable swells coming
from different directions with various dominant periods so these
were for the most part added manually to the coastal waters
forecast over the course of the next week to show the sea state
will be a bit more chaotic than a single primary swell might
WA...Winter Weather Advisory until 10 AM PDT this morning for South
PZ...Small Craft Advisory for winds until 5 PM PDT this afternoon
for Coastal Waters from Cape Shoalwater WA to Florence OR
out 60 nm.
Small Craft Advisory for hazardous seas until 9 PM PDT this
evening for Coastal Waters from Cape Shoalwater WA to
Florence OR out 60 nm.
Small Craft Advisory for Rough Columbia River Bar until 8 AM
PDT this morning.
Small Craft Advisory for Rough Columbia River Bar from 1 PM
this afternoon to 6 PM PDT this evening.
Small Craft Advisory for Rough Columbia River Bar from 2 AM to
6 AM PDT Saturday.
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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.