-- Discussion containing changed information from previous version are highlighted. --
988
FXUS66 KPQR 190956
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
256 AM PDT Sun May 19 2024

.SYNOPSIS...
-- Changed Discussion --
Scattered showers return this morning and
afternoon across the region with a brief period of dry weather
Monday. Expect morning stratus both days. A weather system will
bring more widespread but generally light rain across the region
Tuesday with showers continuing Tuesday night into Wednesday.
Dry Thursday with the pattern becoming more uncertain beyond
Thursday with low chances of scattered showers into the weekend.
Temperatures remain mild all week with generally 60s in the
inland valleys and 50s across the coast, except for Monday when
temperatures could reach right around 70 degrees for inland
valleys.
-- End Changed Discussion --

&&

.SHORT TERM...
-- Changed Discussion --
Sunday through Tuesday...Radar imagery early
Sunday morning shows isolated showers occurring in far northern
Oregon and SW Washington, mainly along the coast and over the
waters. Ensemble guidance indicates another upper level
shortwave trough will move through the region late this morning
and afternoon. Scattered showers will increase in number across
the region from west to east as the trough approaches and moves
through this morning and afternoon. With less moisture (PWATs
around 0.5 inch or less) and a much weaker jet stream
associated with this wave, not expecting showers to perform as
well today as they did yesterday afternoon with QPF amounts in
showers generally less than 0.1 inch. However, can`t rule out a
stray heavier shower or two, especially over the foothills and
Cascades with orographics enhancing shower growth. Onshore flow
is continuing today, leaving daytime temperatures very similar
to yesterday with low to mid 60s in the interior lowlands and
50s along the coast. Snow levels with this colder air will
remain around 3500-4000 feet through the morning before rising
above 4000-4500 feet in the afternoon. A dusting of snow is
possible with any shower, mainly above pass level.

Will still be keeping an eye on frost potential in the Upper
Hood River Valley the next few mornings. For this morning, both
NBM and HREF probabilities are down to less than 5% probability
of temperatures at or below 35 degrees for Parkdale and north,
which seems reasonable as temperatures at 3 AM are still in the
low to mid 40s. These probabilities are similar for Monday and
Tuesday mornings, so it looks like the frost threat is
decreasing.

Monday will be dry and slightly warmer as ensembles indicate
shortwave ridging over the PacNW. Will see mostly sunny skies
except for some marine stratus in the morning hours due to
onshore flow. NBM probabilities show a 40-60% chance of
temperatures reaching or exceeding 70 degrees in the northern
Willamette Valley stretching north along the southern Washington
lowlands through Longview. In the central and southern
Willamette Valleys, probabilities drop to 15-25% with 85-95%
probability of temperatures above 65 degrees.

Enjoy the warmer and dryer day on Monday because cooler and
wetter conditions return Tuesday. Ensemble guidance indicates
good confidence in a closed upper low pushing south along the
British Columbia coast Tuesday with a shortwave ahead of this
parent low moving through the PacNW. The shortwave will have
more of a west to east progression with an associated surface
frontal system, which will bring a period of stratiform
precipitation to the region from the northwest to southeast. The
Willamette Valley will likely be rain shadowed due to the
direction of the flow with higher precipitation amounts in SW
Washington (including lowlands), the Coast Range, and Cascades.
NBM indicates only a 20-40% probability of 0.25 inch or higher
rain amounts in 24 hours in the central and southern Willamette
Valley ending 5am Wednesday. These probabilities increase to
50-60% in the Portland/Vancouver metro area, and 75-90% in the
lowlands north of Clark County. The Coast Range and Cascades
north of a line from Tillamook to Government Camp are forecast
to receive the highest QPF, anywhere from 0.75-1.25 inch. NBM
indicates 50-70% probability of QPF greater than 0.75 inches for
these areas through Tuesday night. Snow levels will remain above
5500-6000 feet through Tuesday, so snow at or below pass level
is not expected. -HEC

.LONG TERM...Wednesday through Friday...Ensemble guidance has
been in better agreement of the track of the closed low as it
moves south from British Columbia into the PacNW on Wednesday.
85% of the 00Z Sunday WPC 500mb clusters, along with ensemble
means, indicate the low will move through NW Washington to the
central/east central Washington/Oregon border, then southeast
through eastern Oregon. This pattern will continue onshore flow
with cooler and more moist conditions. The other 15% of the
clusters track the low much farther east through eastern
Washington into Idaho in a more "inside slider" pattern. This
would bring more of a breezier northerly wind pattern and dryer
conditions. NBM is hedging towards the first pattern, as well,
with showers continuing into Wednesday with thunderstorm chances
as colder air aloft moves over the region.

Ridging builds again over the eastern Pacific Thursday with dry
conditions returning. Beyond Thursday, ensemble guidance turns
more uncertain. WPC clusters indicate general troughing moving
back over the region sometime Friday into Saturday, but timing,
location, and strength of the trough are all uncertain. NBM
forecast indicates daytime temperatures remaining fairly steady
in the 60s across the lowlands and 50s across the coast through
the weekend with precipitation chances of 15-30% across the
lowlands Friday and Saturday and up to 40-50% across the higher
elevations. -HEC
-- End Changed Discussion --

&&

.AVIATION...
-- Changed Discussion --
Predominately VFR conditions through 12z Monday. A
surface high centered over the northeast Pacific continues to
extend into western WA/OR. Moist northwest flow continues aloft,
resulting in persistent cloud cover over the mountains. Satellite
and surface weather observations from 0930z Sunday also showed
cloud cover expanding over the lowlands, however cigs remained VFR
at all terminals (except at KAST where high end MVFR cigs were
being observed). Expect cloud cover to continue expanding Sunday
morning before clouds begin scattering out Sunday afternoon. Cigs
are very likely to remain VFR at all terminals through 12z Monday,
except at KAST as previously mentioned; suspect MVFR cigs will
become more persistent at KAST after 09z Monday when
probabilities for cigs below 3000 ft increase to around 90%.

Similar to yesterday, an upper level disturbance will bring
increasing chances for rain showers Sunday afternoon, especially
north of KEUG. That being said, minimal impacts are expected as
cigs/visibilities should generally remain VFR even with passing
showers. Light winds Sunday morning will give way to increasing
northwest winds during the afternoon, sustained between 5-10 kt
inland and 10-15 kt along the coast.

PDX AND APPROACHES...Expect VFR conditions through 12z Monday. Winds
will generally remain out of the northwest sustained between 5-10
kt during the afternoon/evening hours. There is around a 20%
chance of rain showers at the terminal Sunday afternoon, however
minimal impacts are expected as cigs/visibilities will likely
remain VFR even with passing showers. Brief wind gusts up to 20 kt
or so are possible near convective outflows associated with
passing rain showers. -TK
-- End Changed Discussion --

&&

.MARINE...
-- Changed Discussion --
Buoy observations from early Sunday morning showed seas
between 4 and 7 ft with a dominant wave period around 8 seconds.
Winds were out of the northwest around 10-15 kt with gusts up to
20 kt. Gusts to 25 kt are likely occurring over the far outer
waters. Given both winds and seas are flirting with small craft
advisory criteria, have decided to maintain the ongoing Small
Craft Advisory over the outer waters while also extending it
through Monday morning. In addition, a Small Craft Advisory
remains in effect for the inner waters Sunday afternoon through
Sunday night.

The next frontal passage of any significance is set to arrive
late Monday night into Tuesday morning, bringing rain and a shift
to westerly winds. However, wind gusts look to generally stay
around or below 20 kt as the pressure gradient associated with
this frontal passage appears rather weak. The exception to that is
over the northern outer waters where isolated small craft
advisory level gusts to 25 kt are now likely to occur (80-90%
chance) late Tuesday into Tuesday night. -TK
-- End Changed Discussion --

&&

.PQR WATCHES/WARNINGS/ADVISORIES...

-- Changed Discussion --
OR...None.
WA...None.
PZ...Small Craft Advisory from 11 AM this morning to 5 AM PDT Monday
     for PZZ251>253.

     Small Craft Advisory until 11 AM PDT Monday for PZZ271>273.
-- End Changed Discussion --
&&


$$

www.weather.gov/portland

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