946
FXUS66 KPQR 051046
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
345 AM PDT Sun May 5 2024

.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...Today through Tuesday...Latest radar imagery, webcams,
and surface observations show precipitation has largely decreased
across SW Washington and NW Oregon overnight, though light to
moderate snow persists in the Cascades above 3500-4000 ft. For a
stratiform rain in May, rainfall totals have been impressive over the
past 24-36 hours with just about the whole forecast area showing over
an inch of rain. The main exception has been the Hood River Valley
which has received 0.5-1.0 inch. The heaviest precipitation has been
Salem southward, where most locations have received 1.5-2.5 inches
since Friday afternoon. The rain has been enough to bring noticeable
rises in area drainages, but not enough to threaten flooding. In the
Cascades, the primary concern has been snow since snow levels fell
below pass level Saturday afternoon. In general, 5-10 inches of snow
have accumulated in the Oregon Cascades above 4500 ft, with
considerably less in the S WA Cascades and below 4500 ft.

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 the past hour
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. An educated guess of what is going on
is that a lobe of enhanced Q-vector convergence associated with the
broader area of deformation precip across NE Oregon is passing over
eastern portions of the PDX metro area. If this is indeed what is
causing the enhanced precip, it will likely pivot over the north
Oregon Cascades over the coming hours while precip decreases over the
PDX metro area. 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.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...Area still under the effect of the upper low, through
less than 24 hours ago as the mid level low center is now over
southeast Oregon. Wrap around moisture continues into western OR
on the back side of the low. Generally seeing VFR conditions
across the areas terminals, except MVFR along the coast. HREF
guidance does show lower probabilities for MVFR through about 15Z
for inland terminals, but then probabilities for MVFR increase,
the decrease again later in the afternoon. It`s worth noting there
are chances(10-20%) for a period of IFR cigs 12-18z Sun, but
coverage is not widespread. Wind generally stay less than 10 kts
inland but coastal areas appear a little breezier with
southwesterly winds 10-20 kt in the afternoon.

PDX AND APPROACHES...Currently VFR with light rain, but radar
showing developing area of rain heading south from WA. Will
probably bring periods of MVFR again to the terminal area. HREF is
indicating a MVFR probabilities increasing again after a lull
overnight. So will probably see MVFR prevailing again through
about 22Z Sun when MVFR probabilities decrease again. This seems
reasonable given the low and its effect will be waning.
/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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