Area Forecast Discussion
Issued by NWS Portland, OR

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FXUS66 KPQR 270451

Area Forecast Discussion
Weather Service Portland OR
940 PM PDT Sun May 26 2019

.SYNOPSIS...Low level onshore flow will keep temperatures near to
slightly above average this week with hit or miss showers and
possibly a thunderstorm or two each afternoon and evening.


.SHORT TERM...Tonight through Wednesday...Water vapor satellite
imagery this evening reveals a closed low pressure spinning over the
southwestern US. A narrow and elongated shortwave ridge is located
over the northeast Pacific, which has placed the Pacific Northwest in
deep northerly-ish flow and still under the influence of the upper
level trough. After the gradual dissipation of morning clouds earlier
today, this setup eventually helped produce a round of showers and
some isolated thunderstorms this afternoon and evening. Given the
weak flow aloft, heavy rain and lightning were the primary threats
with these showers and thunderstorms.

Surface based instability is quickly waning and the couple showers
hanging tough across Marion and Linn Counties should continue to
weaken over the next couple of hours. Aside from the remnants of
convection currently over eastern Oregon and Washington shifting into
the Cascades from the east to northeast, not expecting much in the
way of wet weather after midnight across northwest Oregon and
southwest Washington.

Visible satellite imagery this evening also revealed marine clouds
quickly congealing over our waters and along the central Oregon
coast. A modest southwesterly pressure gradient should allow these
marine clouds to spread along much of the coast and into at least the
southern half of the Willamette Valley by daybreak. This should keep
high temperatures in the upper 60s to low 70s for places like Salem
and Eugene. Farther north, the marine cloud cover forecast is more
uncertain, but marine clouds may have a hard time filling in all of
the Willamette and lower Columbia River valleys. As a
result, the high temperature forecast for Monday is a bit more
uncertain for areas farther north such as Kelso/Longview and the
Portland/Vancouver metros, but these locations should end up at least
a couple degrees warmer than places like Salem and Eugene.

Instability should once again develop over the Cascades Monday
afternoon, particularly across the south Washington and north Oregon
Cascades and lead to scattered showers and possibly a few
thunderstorms. Some models do suggest enough instability may be
present to allow a few storms to fire over the east slopes of the
Coast Range in Columbia, Washington and Yamhill Counties Monday
afternoon. An advancing sea breeze front may be a lifting a mechanism
and allow the storms to propagate into the more populated portions of
these counties as well. Concurrently, storms developing over the
south Washington Cascades should have a tendency to drift
southwestward and may work their way into the Portland/Vancouver
metro from the northeast. Increased PoPs a bit in the aforementioned
areas given the activity today along the east slopes of the Coast
Range and the higher model consensus in storms firing and spreading
into at least portions of Washington, Multnomah, Clackamas and Clark
Counties late Monday afternoon and early evening. While the steering
flow tomorrow will be a touch higher tomorrow, heavy rain and
lightning will likely be the primary threats for any storms. Some
small hail could also accompany some storms as well. It should be
noted that the major wrench that could ruin shower and thunderstorm
chances for the aforementioned areas is if the marine clouds spread
into the entire valley late tonight and Monday morning. This could
keep temperatures down a couple degrees and prevent the atmosphere
from destabilizing enough to allow convection over the higher terrain
to hold together as it spreads into the lower elevations.

Otherwise, onshore pressure gradients look to remain favorable for
marine clouds to congeal each late afternoon and evening along the
coast before pushing inland Tuesday and Wednesday. Cloud cover was
adjusted upward along the coast and in the Coast Range gaps to
account for this scenario. In addition, hit or miss showers and
possibly a thunderstorm or two will also be a threat near the Cascade
crest each day and the inherited forecast captures this well.
Otherwise, expect near to slightly above average temperatures through
midweek. /Neuman

.LONG TERM...Wednesday night through Sunday...Ridge of high pressure
to our north midweek keeps dry, mild weather in place. We may see a
stray shower or thunderstorm near the crest of the Lane County
Cascades Wednesday night, or a few showers in the Oregon Cascades on
Thursday as we get clipped by shortwaves passing mostly to our
south. Weak onshore flow will keep temperatures fairly close to
normal for this time of year. Next weekend should start out dry and
mild as high pressure sits over the area. Sunday, a shortwave trough
moves through which will increase cloud cover and may trigger a few
showers, mostly along the Coast, Coast Range, and in the Cascades.


.AVIATION...Low MVFR marine stratus is pushing up the coast this
evening and will push into the southern Coast Range gaps
overnight. Expect MVFR stratus to spill into KEUG and then
attempt to spread northward through the valley, initially along
the eastern valley against the Cascade foothills. Models now shoe
the stratus mostly impacting areas near the foothills and not the
main valley terminals as much, except KEUG. While the coast will
have trouble getting out of MVFR Monday, any inland areas with
low CIG should improve to VFR by around 18Z Monday.

KPDX AND APPROACHES...Models now showing VFR conditions sticking
around through the next 24 hours. If MVFR stratus does develop at
the terminal, most likely time period would be 15Z-18Z, but it
seems to be a low chance. Areas east of the terminal are more
likely to see MVFR stratus Monday morning. Bowen


.MARINE...No changes. Previous discussion follows...Seas are
holding at 6 to 8 feet across the coastal waters this afternoon,
but remain rather steep with dominant periods around 9 seconds.
Meanwhile, only a few localized gusts to 20-25 kt across the far
southern/outer waters. Light winds and smaller seas will then
develop across the waters for the first half of the week as high
pressure develops over the waters. Gusts in the afternoon hours
on Tuesday and Wednesday may being to approach 20 kt, however, as
a thermal trough begins to build up the southern portions of the
Oregon coast. Seas will remain 4 to 6 feet through at least
midweek, but will be made up of 2-3 different swells, each around
3 feet. Cullen





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This discussion is for Northwest Oregon and Southwest Washington
from the Cascade crest to 60 nautical miles offshore. This area is
commonly referred to as the forecast area. is the U.S. government's official web portal to all federal, state and local government web resources and services.