National Weather Service Raw Text Product

838 
FXUS66 KPQR 201804
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
1004 AM PST Tue Feb 20 2024

.UPDATED AVIATION DISCUSSION..

.SYNOPSIS...
Shower activity continues across northwest Oregon
and southwest Washington today and tomorrow, with a slight
chance of thunderstorms this afternoon. Thursday to Saturday,
expect mostly dry conditions as upper level high pressure builds.
Sunday to Monday, a colder system will move in, returning Cascade
snow and the potential for low-elevation snow.

&&

.DISCUSSION...Today to Monday night...A 1000 mb low pressure 
system is centered around 42.68N 128.42W with a weak occluded 
front extending across southwest Oregon. As this weakening low 
progresses northward, expect showers to continue today across 
northwest Oregon and southwest Washington. The latest CAM 
guidance suggests that the heaviest showers will be mainly 
offshore and along our coast through late morning. 24 hour QPF 
ending at 4 AM Wednesday is generally around 0.50-0.75 in for 
the coast and Coast Range (around 1-1.25 in for the highest 
peaks). Expect around 0.10-0.25 in for the interior valleys and 
0.25-0.50 in for the Cascades. Come late afternoon and evening, 
expect a 15-20% chance of thunderstorms at the coast, Willamette
Valley, and Cascades. HREF shows some weak upper level forcing,
and some model soundings such as the RAP, HRRR, and NAM3km show
CAPE values up to 100-150 J/kg. Cloud cover could inhibit 
surface-based convection, but higher terrain above the clouds 
could be more susceptible to thunderstorm development.

Tomorrow (Wednesday), showers continue as the low pressure
system exits the region but will begin to diminish in the
evening. 24 QPF ending at 4 AM Thursday is generally around
0.10-0.25 in for the coast and interior valleys, and around
0.50-0.75 in for the Coast Range and Cascades. Snow levels today
through Wednesday will remain between 4000-5000 ft, so there
could be a couple new inches of non-impactful snow at pass 
level. Thursday to Saturday, upper level high pressure builds 
over the region, so expect predominately dry conditions during 
this time frame and maybe even a bit of sunshine by the end of 
the week (yay!).

Sunday to Monday, GEFS/EPS/GEPS ensemble guidance are in 
agreement of a colder system moving into the Pacific Northwest. 
This will lead to relatively colder temperatures and falling 
snow levels. Long range ensembles are showing 850 mb temps 
dropping to around -5C to -7C across northwest Oregon and 
southwest Washington (GEFS currently running the coldest). As a
result, snow levels could drop to around 1000 ft or lower. 
Fortunately, ensembles are suggesting that we will be maintaining
onshore flow, so this does not appear to be a scenario where 
arctic air filters in from the east. Currently, around 60-80% of
long range ensemble members show at least 0.01 in of snow or 
greater in the Willamette Valley on Monday (80% or greater for
the Columbia River Gorge and Hood River Valley). The latest NBM
shows a 20-30% chance for 0.10 in of snow or greater in the 
Willamette Valley. At the very least, the Cascades would receive
abundant new snow from this system. Forecast confidence is 
still low for low-elevation snowfall. Be sure to monitor the
latest forecast throughout the week for updates on this 
upcoming system. 							-Alviz

&&

.AVIATION...
An upper-level low currently off the southern Oregon coast will
continue to track northwards through the period with showers at
times, primarily the rest of this morning towards midday 
followed by a second distinct increase in showers later this 
afternoon into the evening(23-03z inland TAF sites). There 
remains a 10-20% chance of isolated thunderstorms in the 
central/southern Willamette Valley and along the central Oregon 
coast accompanying the latter wave of activity previously noted.
While conditions generally remain VFR this afternoon temporary 
periods of MVFR are possible accompanying stronger showers. 
However, guidance suggest CIGS to trend towards MVFR overnight 
into Wednesday morning at most sites. 

As far as winds are concerned, east winds will generally dominate
for locations north of KUAO into SW Washington as well as along 
the coast. For locations KUAO and south, expect more variable 
winds, slowly switching to a south wind throughout today. While 
LLWS is not necessarily expected throughout the entire area, there
will be some directional shear for locations that observe surface
winds from the east with speeds at 2000-3000ft around 30 kt from 
the S-SSW.

*NOTE: The observation out of KONP is generally limited to wind
 and pressure at this time. Periodically cloud cover is reported
 but is unreliable. 

PDX AND APPROACHES...VFR conditions continue this afternoon and
evening with light easterly breezes. Instability will maintain 
intermittent showers throughout the day, although chances for this
activity increases significantly between 00-02z - brief periods of
MVFR CIGS/VIS possible this evening with stronger showers. Weak
LLWS persists with ~20-25kt winds out of the SSW at 3000ft 
while lighter surface winds maintain a large easterly 
component. /Schuldt

&&

.MARINE...
The broad area of low pressure remains nearly stationary
around 43N/123 W and will continue to track north, northeast
through Wednesday towards the Puget Sound. There remains a 10-30%
chance for isolated thunderstorms across the coastal waters
Tuesday afternoon and into the evening. 

As the low progresses northward, expect winds to turn southerly 
with ensemble guidance indicating a high chance (>60%) of gusts 
up to 35-40 kt in the central and south central waters. Therefore,
the Gale Warning remains from Cape Falcon to Florence through 
this evening. The low is tracking slightly slower than previous 
model runs; as of 2 AM buoy 46050 is currently only gusting 21 kt,
however Port Orford is at 36 kts. Confidence remains high that 
these winds will make their way north across the southern and 
central waters by morning. For now, satellite and radar show cells
that are capable of producing isolated convective gusts up to 
Gale criteria across the coastal waters. 

As winds ramp up expect steep and choppy seas building to around 
12-15 ft at 10-11 seconds. North of Cape Falcon winds will 
increase but it appears they will be sub-gale force speeds. As 
the low weakens and finally shifts inland will see winds and seas 
ease. Maintaining sub-Advisory conditions into the weekend. 

A deepening trough out of the Gulf of Alaska will swing towards
the Pacific NW early next week. Sustained northwesterly flow in
the postfrontal air mass will drive seas into the 20s off the
coast of British Columbia. These seas will dissipate across the
coastal waters as a mid to upper teens wave group. -BMuhlestein

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...None.
WA...None.
PZ...Small Craft Advisory until 10 PM PST this evening for Columbia 
     River Bar.

     Hazardous Seas Warning until 10 PM PST this evening for Coastal 
     waters from Cape Shoalwater WA to Cape Falcon OR out 60 NM.

     Gale Warning until 10 PM PST this evening for Coastal waters 
     from Cape Foulweather OR to Florence OR out 10 NM.Coastal 
     waters from Cape Foulweather OR to Florence OR from 10 to 
     60 NM.
&&

$$

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