Area Forecast Discussion
Issued by NWS Portland, OR

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FXUS66 KPQR 070543 AAA

Area Forecast Discussion...UPDATED
National Weather Service Portland OR
942 PM PST Wed Dec 6 2023

Updated Aviation discussion.

.SYNOPSIS..Steady rain is starting to transition into light to
moderate showers as the front exits the area. River, urban, and small
stream flooding is still a concern through this evening, however, the
threat will subside tonight. A colder weather system will arrive
Thursday, bringing additional rain and Cascades snow. There will be a
brief break in the weather Friday before another warmer yet weaker
weather system arrives over the weekend. Dry weather expected to
return early next week.


.SHORT TERM...Wednesday through Thursday night...Current radar
imagery and observations as of 1pm PST show widespread, moderate rain
across northwest Oregon and southwest Washington as a once-stationary
front slowly begins moving eastward. There are signs that this front
is finally moving, as steady rain has subsided significantly in
portions of coastal southwest Washington (Pacific County) and far
northwest Oregon (Astoria). In general, the most recently observed
rainfall rates are around 0.05-0.1 in/hour, which is now lower than
what it was earlier this morning. HREF guidance suggests that we`ll
maintain these rain rates through the remainder of the frontal
passage this afternoon/evening.

Over the last the 48 hours (1pm Mon-1pm Wed), about 1-3 inches of
rain have fallen along the I-5 corridor from Lane to Cowlitz County,
3-5 inches along the coast, and 4-9 inches in higher terrain of the
Coast Range, Willapa Hills, south Washington Cascades, and Mt. Hood
area. Meanwhile, the Hood River Valley, most of the north Oregon
Cascades, and Lane County Cascades received only 0.5-1 inch of rain.

Most high-resolution model guidance suggest that rain will gradually
subside and become more showery in nature this evening as the front
slowly exits our area. However, the threat for urban, small stream,
and river flooding still remain. Flood warnings continue for the
Grays, Cowlitz, Wilson, Luckiamute, and Siletz rivers, but river
levels are forecast to drop below flood stage this evening. In
addition, landslides could occur, so anyone near steep hillsides
should remain alert for changes in conditions of soil and vegetation.

To view current and forecast river stages, visit

Tonight into Thursday, an upper level shortwave trough will move into
the region, bringing additional light rain and a return of snow in
the Cascades. Forecast rain amounts are likely not going to cause
concerns for additional river flooding. Current guidance suggests
only a 5-10% chance of coastal rivers rising again to minor flood
stage Thursday. Snow levels are forecast to fall to 2500-3000 ft by
late Thursday night. Expect around 6-12 inches of snow between early
Thursday morning and early Friday morning (24 hr period) for
elevations above 2500 ft. These amounts currently fall below advisory
level criteria. Accumulation to significant amounts may still be
difficult due to relatively warmer roads and surfaces, however,
travelers to the Cascades should still prepare for winter weather

On a separate note, since this will be a colder system, cold air
aloft will create a more unstable environment along the coast and the
Willamette Valley. Therefore the current forecast also reflects a
10-30% chance of thunderstorms tonight through tomorrow night for
these locations.       -Alviz

.LONG TERM...Friday through Tuesday...Drier and colder weather
expected on Friday as ensemble guidance continues to indicate a
brief break between weather systems. The next frontal system
is slated to bring another round of precipitation on Saturday
into early Sunday. GEFS/EPS continue to indicate an 80-90%
probability of a plume of moisture with increased IVT values
likely to be associated with this front. However, uncertainty
still remains with exactly where the main plume of moisture with
this AR will fall as well as in the timing of heaviest rainfall,
though confidence is pretty high that this will not be as
impactful as previous ARs. Latest guidance has shifted the main
plume of moisture north into central and northern Oregon with
rainfall totals of 1-2.5 inches over the Coast Range and
Casacdes and 0.5-1.25 inches for inland lowlands. Snow levels
are expected to rise again on Saturday from around 3000 ft to
6000-7000 ft.

WPC cluster analysis shows upper level ridging building over
the Pacific coast early next week bringing a couple of dry days
to the region. However, there are indications that another
frontal system could impact the region sometime mid to late
week, though uncertainty is high in precipitation amounts at
this time.   -Alviz/HEC


.AVIATION...MVFR cigs with the occluded front presently over the
north Oregon coast range extending southeast into the Cascades.
Expect a mix of MVFR and VFR overnight into Thursday. Current HREF
indicating VFR more likely than MVFR 10-16Z Thu. Buth MVFR more
likely later Thursday morning into the afternoon as an upper
trough transits west to east across the area.

A weakening surface low forecast to approach the north coast
around 12-15Z Thu, so will see southerly wind increase with gusts
25-30 kt along the coast and 15-20 kt for inland TAF sites through
about 20Z.

Westerly boundary layer winds will enhance mountain obscurement
across the Cascades and parts of the Coast Range.

*Please note that the automated weather station at KONP (Newport) is
currently out of service. This site is not maintained by the
National Weather Service. Because of this, the TAF produced for KONP
is based on model data without the input of observations, and may
not be representative of current conditions.*

PDX AND APPROACHES...Expect a mix of VFR and MVFR conditions
through Thursday. HREF indicating less than 15% chance for MVFR
conditions 10-16Z Thu. This may occur with increasing winds during
that time to help mix the lower layers, but forecast confidence
is not very high at this time. /mh


.MARINE...A dynamic pattern will maintain active weather across
the waters through the weekend. Seas remain in the 14-15 ft range
around 16 seconds as of 2 PM. Winds and seas will diminish going
into this evening but will likely remain above 13 ft.

The next frontal system, currently near buoy 46005, is fast
approaching. The low is expected to cross the coastal waters
tonight into early Thursday. Model guidance is showing seas
building in the southwest sector of the low for a brief period up
to 17 ft within the region of strongest winds. Wave steepness will
rapidly increase and therefore have extended the Hazardous Seas
Warning to encompass this timeframe. Gale Force wind gusts are
possible but they`re not all that widespread. The most likely area
of Gales will be in PZZ272 but depending on where the low tracks
this could shift.

High pressure will build across the waters bringing weaker
northwesterly winds with a westerly swell between 10-12 ft
remaining. An upper level trough will send its attendant surface
low towards the coast Friday night into Saturday bringing
southerly winds back across the waters. Gale Force wind gusts
likely ahead of the cold front which is expected to cross the
waters Saturday evening.  -BMuhlestein



PZ...Hazardous Seas Warning until 1 PM PST Thursday for Coastal
     waters from Cape Shoalwater WA to Florence OR out 60 NM.

     Small Craft Advisory until 1 PM PST Thursday for Columbia River


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