097
FXUS66 KPQR 232305
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
405 PM PDT Tue Apr 23 2024

.SYNOPSIS...High pressure aloft is leading to a mostly sunny and warm
day today across much of the Pac NW, though sea breezes are keeping
the coast cooler with some patches of low stratus. High pressure will
weaken and shift eastward through Wednesday; meanwhile a weak Pacific
frontal system will approach the Pac NW coast. Drizzle or light rain
will be possible along the coast by Wed morning, but inland areas
will likely see rain hold off until Wed night or Thu morning as the
next, better organized frontal system approaches the coast. This
frontal system will mark the beginning of a prolonged period of cool
and unsettled weather, with snow possible down to the Cascade passes
as soon as Thu night/Fri morning.


&&

.SHORT TERM...Now through Friday...Dry and unseasonably warm weather
persists across much of SW Washington and NW Oregon today, with the
only real exception being along portions of the immediate coast where
sea breezes have kept temps in the mid 50s to lower 60s (as is often
the case in the summer). Steepening lapse rates have led to
significant thunderstorm development to our south, but this activity
has remained south of the OR/CA border. A drier air mass has kept
this type of convection out of our CWA - and as of 3 PM no cumulus
development has occurred yet in the Lane County Cascades or
elsewhere.

Upwelling from NW breezes along the coast are allowing a shallow
marine inversion to develop, with some low clouds already beginning
to develop along the coast as of this afternoon. Suspect these low
clouds will become more widespread overnight as 500 mb heights
gradually lower and the marine layer becomes deeper. By Wednesday
morning, we could start to see some areas of drizzle along the coast,
and the deepening marine layer may also lead to some marine stratus
pushing inland Wednesday morning. Suspect this cloud layer will be
shallow and broken enough to clear out for the afternoon, allowing
afternoon temps to approach 70 degrees for some inland valleys later
Wed afternoon. That said, 18z runs of the NAM/GFS have trended colder
aloft, with 850 mb temps generally in the +1 to +3 deg C range. Even
when mixed down dry adiabatically to the surface, the Willamette
Valley would struggle to get above the 60s with 850 mb temps that
cool.

Clouds lower/thicken in earnest Wednesday night as a fairly well
organized (but weak) low pressure system approaches the Pac NW coast,
along with its attendant warm front. Rain becomes likely throughout
the forecast area late Wed night/early Thu morning. With total
precipitable water values in the 0.8-1.0" range, precipitation will
probably be modest despite decent forcing. NBM deterministic QPF
suggests the lowlands will receive 0.50-0.75" of rain by Friday
morning, with QPF in the mountains ranging from 0.75-1.25" south of
Highway 20 and 1-2" to the north. So, after about two weeks of
relatively dry weather, this system will be a beneficial rain maker
for the valleys. After this warm front moves through, the associated
upper trough and low pressure will settle in near the Pac NW, pushing
a weak cold front through and allowing for some cold advection later
Thursday into Friday. NBM probabilistic guidance shows a 50/50 chance
of snow levels lowering to the Cascade passes by 5 AM Friday, so any
snow for the passes themselves will probably be wet with a very low
snow-to-liquid ratio. The higher ski resort elevations above 5000 ft
should see a few inches of snow, but advisory-level accumulations of
6 inches or more will likely be relegated to elevations above 6000
ft. For the lowlands, the upper trough will maintain slow moving rain
showers, with about a 10-20% chance of enough instability to support
a thunderstorm or two. Temperatures will be considerably cooler
Friday afternoon, with highs struggling to climb above the 50s
anywhere in our CWA.  Weagle

.LONG TERM...Friday through next Tuesday...Longer range forecast
models strongly suggest that once the upper trough arrives Thursday,
cool and unsettled weather will prevail through the weekend and well
into next week. The majority of 12z ECMWF ensemble members bring in
another upper low/trough with a reinforcing shot of cool air,
maintaining the possibility of late season snow for the Cascade
passes and above into early next week. For the lowlands, just expect
cool and showery weather more reminiscent of early April rather than
the end of April. Depending on the amount of clearing each night, the
air mass will be cold enough to maintain the threat of morning frost
as early as Saturday and likely well into next week. Therefore it is
probably still a bit early to plant sensitive vegetation, regardless
of how warm we have been today.  Weagle

&&

.AVIATION...A front is beginning to move in, but high pressure
persists enough to keep all inland terminals VFR throughout the TAF
period. High clouds are moving in throughout today, with ceilings
beginning to lower (though remaining VFR) at all terminals
late Tuesday night. After 03z this evening, the front approaches
close enough for low marine clouds to begin setting in at coastal
terminals, with a 70% chance of MVFR cigs developing first at KONP,
then KAST. Chance of IFR cigs is about 10-20%, but MVFR looks to be
the dominant possibility.

Notes: The ASOS at KTTD is only available via the dial in option.
The AWOS at KMMV is still out of commission. As it is not
maintained locally by the National Weather Service, there is no
estimated time of repair.

PDX AND APPROACHES...VFR conditions continue throughout, with fairly
light winds and high clouds. In the latter half of Tuesday night, an
approaching front is expected to begin lowering cigs, but not enough
to drop to MVFR thresholds. /JLiu

&&

.MARINE...Observations at buoy 46050 at 2:30 PM PDT are showing
northerly winds 15 to 20 kts, with gusts to 25 kt. Winds are
currently low-end Small Craft Advisory level, though winds will
slightly turn NNW over this afternoon. Gusts are in the process of
dying down, with the Small Craft Advisory expiring at 8pm PDT;
winds calm considerably for the rest of the night. Swells are
currently around 4-6 ft at 9 seconds, and will remain around that
level until Friday.

The next front arrives Thursday, with around a 80% chance of SW
gusts meeting Small Craft Advisory criteria. Seas will rise
slowly on from Thursday onwards, with possibility for double digit
seas by the end of the weekend. /JLiu

&&

.PQR WATCHES/WARNINGS/ADVISORIES...

OR...None.
WA...None.
PZ...Small Craft Advisory from 11 AM this morning to 8 PM PDT this
     evening for PZZ210-251.

     Small Craft Advisory until 8 PM PDT this evening for PZZ252-253-
     271>273.
&&

$$

www.weather.gov/portland

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