635
FXUS66 KPQR 051813 AAA
AFDPQR

Area Forecast Discussion...UPDATED
National Weather Service Portland OR
1113 AM PDT Sun May 5 2024

Updated short term and aviation discussions.

.SYNOPSIS...
Broad low pressure continues to dominate the weather for
much of the US West Coast, though precipitation has been dwindling
west of the Cascades. Light to moderate snow continues in the
Cascades above 4000 feet, largely due to increasing upslope flow as
relatively higher pressure builds onshore from the Pacific. Onshore
flow will briefly weaken, decreasing the snow in the Cascades this
evening. However the next front will bring a resurgence of snow in
the Cascades later tonight into Monday morning, with a brief shot of
rain in the lowlands followed by scattered showers Monday afternoon.
Post-frontal showers will continue into Tuesday, then a significant
warming and drying trend is expected later in the week as high
pressure builds over the region.

&&

.SHORT TERM...Now through Tuesday...The front continues to shift
inland with the main portion of precipitation falling over the
Cascades north of the Willamette Natl Forest. Precipitation
amounts have generally been around 0.15-0.25 inch over the last
6 hours, but there has been some fluctuation in regards to the
terrain. Currently, freezing levels are around 4500 ft based on
pilot reports which is consistent with the forecast. Will see
minimal shift in these temperatures and thus snow levels will
see very little change. One thing that has been a bit of a
surprise are the winds at higher elevations. Along Mt Hood (7000
ft), winds have been exceeding gusts of 55 mph. While this has
been fairly localized and at some of our highest elevations,
still were higher than forecast.

While there is high confidence in the evolution of the larger
scale features, confidence is lower in the smaller scale,
subtler details. For example, while precipitation has generally
been decreasing over western WA/OR, there has been an uptick in
rain over this morning across much of Clark, Multnomah, and
Clackamas Counties. This enhancement is largely due to
deformation banding, which is notoriously difficult to forecast.
Most of the latest hi-res model runs do not even have this
existing deformation band in their analyses or hour 1-3
forecasts. For elevations above 3500-4000 ft, this enhanced
precip is manifesting as heavier snowfall rates in the Cascades
between Santiam Pass and Mount Hood, and should last another few
hours before diminishing. Meanwhile, KTTD-KDLS gradients have
increased to +4.8 mb, suggesting increased orographic
enhancement that will persist throughout the day today. So,
while the lift from deformation decreases this morning, it will
be replaced by the increasing orographic lift, which should keep
light to moderate snow going in the Cascades through this
afternoon. This orographic lift should briefly decrease this
evening as an occluding cold front approaches the coast, so we
will maintain the existing Winter Wx Advisories for the Oregon
Cascades until 6 PM this evening for another 3-7 inches of snow.
For the lowlands, today looks to be a damp, dreary day with
periods of rain and temperatures once again struggling to climb
above 50 degrees.

The above-mentioned occluding cold front will move onshore tonight,
pushed along by an unseasonably strong jet stream that is 120-140 kt
at its core. This jet stream will also push the existing broad upper
low eastward into the Rockies, yielding a more typical evolution of
precipitation across the forecast area beginning with the occluding
cold front late tonight/Monday morning. Expect a band of enhanced
rain and Cascades snow to sweep across the CWA with the cold front
late tonight/early Monday morning, followed by post-frontal showers
through Tuesday. Lapse rates steepen behind the front Monday
afternoon and may be sufficient for a thunderstorm or two. With low
freezing levels for the season, any thunderstorms would be capable of
producing small hail. Warming temperatures aloft should lead to
shallower convection Tuesday, decreasing the chance of thunder and
hail. By then, high pressure will be strengthening over the NE
Pacific and building into the Pac NW, bringing an end to our period
of unseasonably cool and damp weather.-Muessle/Weagle

.LONG TERM...Wednesday through Saturday...Longer range ensemble
guidance depicts a much more straightforward forecast beginning
midweek than in the short term. All clusters in the 00z WPC cluster
analysis depict substantial positive 500 mb height anomalies across
the Pac NW by Friday, with at least some degree of offshore flow.
This will bring dry and much warmer weather to the region by the end
of the week. NBM deterministic guidance continues to show highs in
the 60s Wednesday, 70s Thursday, and 80s Friday and Saturday. In
fact, NBM probabilistic guidance shows a 50/50 chance of the PDX
metro area reaching 90 degrees Saturday, with a 10-30% chance of
doing so elsewhere across the interior lowlands. If it seems early
for us to be reaching 90 degrees in the interior lowlands, it is
early but not unheard of to reach 90 deg F this time of year. In
fact, PDX did so on 5 out of 6 days between May 12-17 last year.
Spring can be a time of large temperature swings and it appears the
upcoming week will be no exception for the Pac NW. -Weagle


&&

.AVIATION...Our broad upper-level low has shifted further
eastward this morning with moist westerly flow now presiding
across the region. Conditions are currently a variable mix between
IFR to VFR although most sites sit at MVFR CIGS/VIS. Expect this
variability to continue through the day with drops in CIGS
primarily associated with shower activity moving through the
region. High resolution guidance indicates the possibly for a
more pronounced break in shower activity late this afternoon into
the evening for inland sites, roughly 23-05z, before another
frontal system arrives bringing an increase in rainfall and higher
probabilities (60-80%) for widespread MVFR CIGS. More stratiform
rainfall then breaks to post-frontal showers Monday morning, but
confidence in the exact timing of any categorical improvement
back towards VFR is low. Wind generally stay breezy out of the
south at 10-25 kts - highest coastal sites.

PDX AND APPROACHES...Currently MVFR with light rain but once this
shower band passes conditions are expected to bounce back to low-
end VFR. Will probably see a few more periods of MVFR again today
as waves of showers pass overhead. HREF is indicating MVFR
probabilities increasing again after a small lull this evening. So
will probably see MVFR prevailing overnight through about 14-16Z
Sunday. MVFR probabilities decrease again and precipitation
changes to a showery post-frontal regime afterward.
-Schuldt/MH

&&

.MARINE...
The active weather pattern continues into early this
week before a shift to generally calmer conditions and a more
summer-like pattern. The next frontal system arrives today
bringing another round of gusts up 30 kt as well as steep seas
across most of the waters through Monday morning. Following this
front high pressure over the northeast Pacific begins build over
the waters and bring breezy west to northwest winds (gusting 20-25
kt) across the waters later Monday into Tuesday. Also expect the
arrival of a pounced WNW 9-11 ft swell at near 12 seconds Monday
night. The high pressure shifts closer to the waters later next
week followed by a larger offshore component to the wind in the
Friday/Saturday timeframe. -MH/Schuldt

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...Winter Weather Advisory until 6 PM PDT this evening for
     ORZ126>128.
WA...None.
PZ...Small Craft Advisory from 5 PM this afternoon to 11 PM PDT
     this evening for PZZ210.
     Small Craft Advisory until 8 AM PDT Monday for PZZ251>253-
     271>273.
&&

$$

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