Heavy Rain in Washington, September 23 to 25, 2020

Area Forecast Discussion...UPDATED
National Weather Service Portland OR
1010 AM PDT Wed Sep 23 2020

Updated short term and aviation discussion.

.SYNOPSIS...A cold front will produce rain and breezy winds across
the region today as it moves from the coast this morning to the
Cascades late this afternoon. Cooler conditions with showers moves
in behind the front tonight through Thursday with the possibility
for thunderstorms across extreme northwest Oregon and southwest
Washington. Another frontal system will continue the chance of rain
Thursday night and Friday. An upper level ridge will build across
the Pacific Northwest late this weekend and early next week, marking
a temporary return to drier and warmer weather.

&&

.SHORT TERM...Today through Thursday night...The first front
of Fall is starting to move inland this morning increasing concerns
for rain and wind today. Winds are beginning to shift southerly
along the coast with gusts beginning to increase. Coastal
observations are showing gusts ramping up, especially along the
central Oregon coast, and beaches and headlands. Winds inland will
become south/southwest later this morning into the afternoon. Breezy
south/southwesterly winds are expected through the Willamette Valley
and along the Cascades today. Strongest winds expected along higher
terrain of the Coast and Cascade Ranges as winds at 850 mb are
reaching near 40-50 mph.

Satellite derived Total Precipitable Water (TPW) shows values around
1.5 inch with the front, and shows a narrow plume of deeper moisture
that extends from just offshore the Pacific NW to around 700 miles
NW of the Hawaiian Islands. This is an impressive amount of moisture
and is about 150% of the normal TPW for this time of year. GFS
initializes well with the satellite TPW and forecasts 1.5 inch TPW
across the Willamette Valley late this afternoon. This front and
associated low have ample upper level support as it sits in the
right rear quadrant of a 140+ kt jet streak at 250 mb. As this jet
streak moves over NW Oregon the high moisture moves inland this
afternoon. There should be enough lift to tap into the moisture
aloft, and generate a decent early season rain event. Heavier rain
is expected to move across the coast and coast range through the
early afternoon, the Willamette Valley later this afternoon and
evening, and the Cascades this evening and tonight. The front is
trending towards a slight stall over the coast range this morning so
accumulations there may be higher. High rain rates may result in
localized flooding in areas with poor drainage as well as localized
ponding, and reduced visibilities. The heavy rain may cause
localized mud or debris slides in the higher terrain and near burn
scars. We will keep an eye out on areas recently burned by the
wildfires for the potential of impacts, but am not confident that
rain rates will be high enough to cause major problems there.

Satellite imagery shows open cell cumulus behind the front
indicative of colder air in the wake of the front. Lightning has
been detected within these clouds in the cold pool behind the front.
This is enhancing forecast confidence in the possibility for
convective showers behind the front tonight and Thursday morning, as
well as a possibility for thunderstorms across extreme southwest
Washington. Thursday afternoon will be cooler with temperatures not
likely making it to the 70s. Showers decrease Thursday night, but
the rain will not fully stop as a weak warm front will move south to
north across the area late Thursday night. -Muessle/TJ

&&

.LONG TERM...No Changes. Previous discussion follows...Friday through
Monday night...As the aforementioned next upstream impulse moves
quickly toward the Pacific Northwest coast on Friday, the front will
lift northward as a warm front across western Oregon, along with
increasing low level moisture transport into the active frontal zone.
 While the strongest synoptic scale support for dynamic forced ascent
should be consolidating just to our north over western Washington and
adjacent coastal waters Friday afternoon, there still should be more
than sufficient forcing to aid the continued development of
additional rain across most areas on Friday with the northward
lifting warm frontal zone.  Flow just off the surface will back to a
more WSW direction during this time period, thus altering the favored
orographic areas most at risk for enhanced rainfall rates.

Beyond Friday night, the main jet axis looks to shift northward just
far enough to take most of the forcing and best chances for precip
with it, with a bit stronger upper level ridging then subsequently
building into the region later in the weekend and early next week.
Thus, a drying trend is anticipated (at least based on current
medium-range deterministic and ensemble guidance), along with
temperatures climbing back into the 70s in most locations, and
perhaps a few lower 80s. - Miller

&&

.AVIATION...A strong cold front will move across the area this
evening bringing wet and gusty conditions for the next 24 hrs. Rain
and winds are begin to ramp up across the area as the warm sector of
the storm continues to move northeast. Strong southerly winds are
expected along the coast through this evening. Gusty winds are
expected to be between 20-25 kts for most of the afternoon.
Southwesterly LLWS around 020 ft is expected west of the Coast
Range. Wind shear is expected to reach around 40-50 kts this
afternoon ahead of the front. Expect conditions along the coast to
deteriorate between IFR to LIFR ahead of the cold front which is
expected to arrive around 00Z Thurs. Improvement to MVFR is expected
after 03Z.

Inland locations should expect conditions to drop down into MVFR
after 20Z. Intermittent periods of IFR visibility likely between 00-
03Z with increasing showers throughout the day.
LLWS is not expected to be a concern for locations east of the Coast
Range. Expect conditions to improve to VFR after 06Z in the post
cold frontal airmass. Gusty winds with intermittent showers will
continue into tomorrow morning. -BPhillips

Refer to http://weather.gov/zse for detailed regional aviation
weather and hazard information.

KPDX AND APPROACHES...VFR conditions are expected to deteriorate to
MVFR after 00Z as rain moves northeast across the terminal.
Intermittent periods of IFR visibility are likely between 00-03Z
with increasing showers. Southwest surface winds expected to be
between 25 to 30 kt with strong southwesterly winds aloft around 50
kt at 035 ft are expected with the passage of the front. -BPhillips

&&

.MARINE...Strong southerly winds will continue through this
evening as a cold front moves across the waters. Winds have been
gusting to around 35 to 40 kt the past few hours over the outer
waters. Expect these winds to move into the inner waters by sunrise.
The strongest winds are expected later this morning, with gusts to
45 kt possible between 8 AM and 6 PM. Guidance continues to suggest
there may be an outside chance for some coastal jetting this
afternoon with 925 mb winds forecast to be around 50 to 60 kt. The
Gale Warning remains in effect through this evening, but could see a
few hours of isolated Storm gusts between 11 AM and 4 PM. Seas will
continue to build in response to the winds with combined seas around
12 to 16 ft at 13 seconds expected this afternoon.

The broad area of low pressure will remain over the Gulf of Alaska
through Friday as a weaker front move across the waters.
Winds will ease behind the first front tonight but expected to
remain rather breezy as southerly winds continue with gusts to 25 kt
Thursday. Another front is expected to arrive Friday as breezy
southerly winds continue. Seas also remain elevated through the end
of the week as a large westerly swell moves across the waters.
Expect seas around 12 to 16 ft at 15 seconds Thursday and Friday.
Conditions should start to relax over the weekend as high pressure
builds into the NE Pacific. /64/DDH

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...None.
WA...None.
PZ...Gale Warning until 9 PM PDT this evening for coastal waters
     from Cape Shoalwater WA to Florence OR out 60 NM.

     Small Craft Advisory from 2 PM this afternoon to 5 PM PDT
     Thursday for Columbia River Bar.
&&

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