010
FXUS66 KPQR 011020
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
320 AM PDT Wed May 1 2024

.SYNOPSIS...Mostly dry conditions prevail today before another
system arrives this evening into Thursday morning. This will
bring another round of rain and mountain snow above 4000 ft.
Heaviest snowfall will be above 5500 ft. A more robust system
arrives Friday, with forecast precipitation around 0.75-1.5 inch
across northwest Oregon and southwest Washington. Snow levels
Friday will rise above 6500 ft. Rivers still have a less than 5%
chance of reaching action stage. Wet, unsettled weather likely
continues over the weekend through at least early next week.

&&

.SHORT TERM...Today through Thursday night...Radar imagery as of
3 AM PDT show a few lingering showers over the Cascades. Otherwise,
most places look dry with some low to mid-level cloud cover.
Expect to see some cloud breaks and clearing this morning, which
would support radiational cooling and potential frost development.
The current Frost Advisory for the North Oregon Coast Range,
North Oregon Coast Range Lowlands, Willapa Hills, Tualatin
Valley, and West Central Willamette Valley remains in effect
through 8 AM this morning. These places may also see some patchy
fog development as well (20% chance). The highest chances
(25-45%) for frost will be in the Coast Range and Willapa Hills.
Uncertainty remains for frost development in the Willamette
Valley since there could be stratus backbuilding against the
Cascades. In addition, dew points in the upper 30s don`t look
promising for frost.

Conditions will be mostly dry today before the next system
arrives this evening. We`ll have some relatively warmer high
temperatures today compared to what we`ve experienced the last
couple days. Afternoon highs are forecast in the low 50s along
the coast, upper 50s/low 60s for inland valleys, and 40s for
higher terrain.

The next low pressure system will arrive Wednesday evening to
Thursday morning, bringing another round of precipitation.
Expect lowland rain showers and Cascade snow showers, with snow
levels around 4000-4500 ft. The bulk of the snow will be above
the Cascade passes. 24 hr snow amounts ending 5 PM Thursday are
forecast around 2-5 inches above 4000 ft, except up to 10
inches above 5500 ft. Have decided not to issue a Winter
Weather Advisory for the Cascades considering that the heaviest
snowfall for this system will be above the passes and forecast
snow amounts look less than what we`ve seen with the previous
systems earlier this week. Current NBM guidance suggests a less
than 5% chance that the Cascade passes see 24 hr snowfall greater
than 5 inches ending 5 PM Thursday.

Thursday afternoon to late Thursday night, conditions will dry
up briefly before another system approaches the Pacific
Northwest on Friday.      -Alviz

&&

.LONG TERM...Friday through Tuesday...The long term forecast is
characterized by wet, unsettled weather as a series of systems
pushes through the Pacific Northwest. WPC cluster analyses are
in agreement of another trough pushing through the region on
Friday. This looks like a stronger system with abundant moisture
as GEFS and ECMWF ensemble members show IVT values peaking
between 250-500 kg/ms on Friday.

The latest NBM guidance suggests a 25-45% chance that 24 hr QPF
ending 11 AM Saturday exceeds 2 inches for the Coast Range and
Cascades. For inland valleys, this probability is less than 5%.
For now, our current 24 hr QPF forecast ending 11 AM Saturday
calls for 0.75-1 inch for inland valleys and 1-1.5 inch for the
coast, Coast Range, and Cascades. This will also be a relatively
warmer system, so snow levels during this time are forecast well
above the Cascade passes (around 6500-7000 ft).

Over the weekend (Sat-Sun), WPC clusters are in agreement that
this robust upper level trough will continue to progress
southeast toward Northern California and the Great Basin. As it
does, additional precipitation will continue through the
weekend. Chances for 48 hr QPF exceeding 1 inch ending 5 AM
Monday are around 40-60% for the Coast Range and Cascades, and
5-10% for inland valleys.

Currently, HEFS guidance shows all rivers across northwest
Oregon and southwest Washington having a less than 5% chance
of reaching action stage. However, we will still continue to
monitor the potential for hydro impacts to our rivers. Flood
control reservoirs are now full for summer water supply, so the
Willamette River can be more susceptible to flooding now. Those
with river interests should continue to monitor the forecast
over the next few days.

WPC cluster analyses are in agreement of additional troughing
over the Pacific Northwest Monday to Tuesday. There still
remains some uncertainty with how strong the troughing will be,
but the majority of ensemble members suggests we`ll maintain a
wet pattern through at least early next week. Don`t put your
rain jacket or rain boots away just yet!           -Alviz

&&

.AVIATION...Post frontal showers winding down and mainly confined
to the Cascades as of 09Z with VFR conditions across the area.
Satellite shows clouds decreasing over the coast and coast
range, while valley cigs remain around 5000-6000 ft. HREF
guidance shows less cloud cover than current satellite, but HREF
forecast clouds to decrease over the next several hours. This
could bring enough radiational cooling for fog or low stratus.
HREF shows 20-50% chance for low MVFR to IFR conditions in the
valley 12-17Z Wed. Next front arrives tonight with rain reaching
the coast by evening and to interior valley late in the evening
with MVFR to IFR reaching the coast 05-07Z Thu, and MVFR inland
09-11Z Thu.

Notes: The ASOS at KTTD is only available via the dial in option.
There is no estimated time of full repair and restoration for
this location.

PDX AND APPROACHES...Conditions are currently VFR, but two
possibilities exist that could deteriorate conditions this
morning. Chances are around 20% for dense fog to form 12Z-17z
Wed that could potentially reduce visibilities down to LIFR
thresholds. However there is a 60-80% chance for for VFR and 60%
chance for MVFR.  /mh

&&

.MARINE...Active weather pattern continues through the week as
Pacific front move through the waters about once a day, bringing
periods of Small Craft Advisory winds. The next front arrives
tonight and is expected to bring southerly winds gusts to 25 kt,
primarily over PZZ271 and 272. The next front for Friday looks
a little stronger with gusts 20 to 30 kt across most of the zones.
A weaker front follows on Saturday. Seas around 9 ft with a period
around 11 seconds early this morning. Seas will subside to around
4 to 6 ft later today through Friday, then expected inch up to
near 8 ft late in the weekend.
/mh

&&

.PQR WATCHES/WARNINGS/ADVISORIES...

OR...Frost Advisory until 8 AM PDT this morning for ORZ104-106-109-
     114.

WA...Frost Advisory until 8 AM PDT this morning for WAZ203.

PZ...Small Craft Advisory from 6 PM this evening to 5 AM PDT Thursday
     for PZZ271-272.
&&

$$

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