Area Forecast Discussion
Issued by NWS Portland, OR
FXUS66 KPQR 241703
Area Forecast Discussion
National Weather Service Portland OR
1002 AM PDT Fri Mar 24 2017
.SYNOPSIS...A slow moving cold front will push east of the Cascades
today, with rain turning to showers behind the front. A few
thunderstorms are 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...Radar and surface observations
reveal that the cold front is currently moving across the Cascade
Foothills this morning, with the front expected to push east of the
Cascades later today. As the front moves east expect conditions in
the Cascades to become showery. As such, will go ahead and let the
Winter Weather Advisory for the South Washington Cascades expire at
10 AM. Elsewhere, radar shows some scattered showers over the coastal
waters and along the coast this morning.
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...No Changes. Previous discussion follows...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 normal. Weagle
.AVIATION...A slow moving cold front will continue to move east
across the Cascades today. Flight conditions had improved to mainly
VFR category this morning, although there is still a chance for
temporary MVFR conditions at TAF sites, primarily through 20z, with
the best chances along the coast. Air mass turns more showery this
afternoon after 19z, but still primarily VFR. Cannot totally rule
out brief MVFR conditions in heavier showers or possibly even a
thunderstorm through 03z this evening, but this is less than a 20
percent chance at any given site.
KPDX AND APPROACHES...Mainly VFR conditions today. There is a chance
of seeing MVFR conditions through 18z this morning in rain, then a
less than 20 percent chance of seeing MVFR conditions in a heavy
shower or thunderstorm from 21z through 03z this evening.
.MARINE...Post frontal winds from the sw today expected to remain
gusty into the 20 to 25 kt range today, so will continue with a
small craft for winds through the afternoon. Southerly short period
swell on the order of 10 to 11 ft this morning will make for steep
seas. The southerly swell is expected to subside this afternoon and
evening. A westerly swell with a little longer period will increase
this afternoon, meaning overall combined seas will be slow to
subside below 10 ft, probably not until tonight.
A short break between systems will bring a period of quieter weather
through Sat. Another frontal system arriving Sun will bring
increasing winds and seas to the coastal waters late Sat night and
Sun. Gale force gusts appear likely at this time, mainly during the
afternoon Sun, with seas likely to build above 10 ft again. The next
frontal system in a continued active pattern is expected to arrive
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 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.