Heavy Rain in Oregon, June 10 to 11, 2022

Area Forecast Discussion
National Weather Service Portland OR
331 AM PDT Fri Jun 10 2022

.SYNOPSIS...A late season atmospheric river will impact
portions of the Pacific Northwest through Saturday with periods
of rain and humid conditions. Cool and showery weather appears
likely to follow for at least Sunday and Monday before a day or two
of drier weather follows midweek.

&&

.SHORT TERM...Today through Sunday...Water vapor satellite imagery
early this morning continues to show a broad longwave trough over the
northeast Pacific and Gulf of Alaska. An atmospheric river on the
southeast periphery of this trough remains pointed at the Pacific
Northwest. Most 18-00z model initializations indicate IVT values
climbed up to 1000-1200 kg/ms as the atmospheric river pushed into
western Washington yesterday.

The main band of rain associated with the atmospheric river has since
dropped southward and lies across much of the CWA. Models and their
ensembles are in good agreement it will continue to shift southward
ever so slightly this morning while weakening further. However, a
shortwave trough rounding the base of the upper level trough near
140W will swing northeastward later today and result in the band of
rain re-intensifying over the area late this afternoon into tonight
across much of Oregon and the south Washington Cascades. NBM4.1's
continues to suggest there is a 50% chance that rainfall amounts
will end up between 1/4-3/4" across the lowlands and 1-3" across the
Cascades between Friday PM and Saturday AM. The odds of rain totals
exceeding 1.5" at any given location in the lowlands and 4" in the
Cascades is less than 5% so confidence that any significant hydro
impacts will occur with this second round of rain remain very low.
Nonetheless, it should be noted that all of the snowmelt and rain
across the greater Pacific Northwest will result in the Columbia
River nearing minor flood stage late in the weekend into early next
week between Vancouver and Kelso, but at this point, it is
forecasted to fall short of actual minor flood stage. Either way,
the Columbia River will be plenty cold enough for cold water shock
and will certainly have stronger currents than normal for much of
the next week or two. In addition, Interstate 5 bridge lifts are
more frequent during periods of higher river levels.

Models and their ensembles are coming into better agreement on the
timing and path of a shortwave trough currently located south of the
Aleutian Islands. It too will move eastward along the base of the
broad longwave trough before sliding directly into western Oregon
Sunday. Expect another day with widespread showers, particularly
across western Oregon with temperatures cooling below average and
snow levels lowering to near 6000 feet. /Neuman

.LONG TERM...Monday through Thursday...Ensemble systems indicate that
the Sunday shortwave trough will shift east of the region Monday
into Tuesday. Nonetheless, there should be enough moisture and cool
air aloft to keep showers going on Monday and perhaps even into
Tuesday. Cannot rule out a brief thunderstorm or two near the Oregon
Cascade foothills Monday afternoon. Shortwave ridging should at
least briefly shift across the Pacific Northwest Tuesday and/or
Wednesday. This will likely bring at least a 24-48 hour period of
drier weather and allow temperatures to moderate. However, most
members in the EPS and a sizable number of the GEFS and CMCE members
suggest another shortwave trough will push towards the Pacific
Northwest Thursday...bringing with it another round of rain to the
region late next week. /Neuman

&&

.AVIATION...12Z TAFs: Messy aviation forecast early this morning.
A late-season atmospheric river continues to result in a wide
range of flight conditions early this morning, but primarily MVFR
or lower. The first wave gradually weakens and through mid-
morning. Model guidance shows this weakening frontal boundary
becoming reinforced around midday, with increasing rainfall rates
Friday afternoon through early Saturday. Would expect predominant
MVFR
or lower conditions through 12Z Saturday. The south Washington and
far north Oregon coast may see some improvement between 06Z and
12Z Saturday as the frontal boundary drifts southeast. Expect
frequent mountain obscurement through the 12Z forecast period.

For detailed Pac NW aviation weather information, go online to:
https://weather.gov/zse

KPDX AND APPROACHES...Predominant MVFR cigs are expected at the
terminal and vicinity through much of the 12Z forecast period.
There may be some brief periods of VFR Friday morning, but expect
these to become more the exception than the rule Friday afternoon
through 12Z Saturday as rainfall intensifies, resulting in
lowering cigs and/or vis. Weishaar

&&

.MARINE...As of 0930Z, wind speeds continue to slowly diminish.
Buoy observations show generally wind speeds to 10 kt with gusts
to 15 kt. Still getting a few gusts to 20 kt near shore, such as
the Newport jetty. Otherwise, latest model guidance suggests wind
speeds will be generally 10 kt or less Friday evening through at
least 12Z Sunday. Wind speeds pick up a little bit Sunday with a
return to more typical northwest to north wind direction, but not
anticipating small craft advisory speeds. The northwest to north
wind pattern then continues the first half of next week. There is
a small chance of 20-25 kt gusts from Heceta Head to Florence
Tuesday afternoon with the presence of a surface thermal trough
along the south Oregon coast.

At 09Z, wave heights were hovering around 9 ft, with periods in
the 8-10 second range. Have extended to small craft advisory
through 13Z to cover these steep and choppy conditions. Guidance
suggests wave heights subside to around 5-6 ft by Friday
afternoon and then remain in the 4-6 ft range well into next week.

Unseasonably high moisture content with the atmospheric river
impacting the waters through early Saturday will result in areas
of fog over the waters, with visibilities below 1 statute miles
at times. Weishaar

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...None.
WA...None.
PZ...Small Craft Advisory until 6 AM PDT early this morning for
     coastal waters from Cape Shoalwater WA to Florence OR out
     60 nm.

&&

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