100
FXUS66 KPQR 181112
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
412 AM PDT Fri Oct 18 2024

.SYNOPSIS...Generally dry today and Saturday, with higher rain
chances lingering over far southwest Washington. Temperatures
climbing back above normal by Saturday with warmer overnight
lows this weekend. More widespread rain returns on Sunday as a
weak atmospheric river moves south across the area.

&&

.SHORT TERM...Through Sunday Night...Northwest Oregon and
southwest Washington continue to dry out this morning as a
potent upper level trough rapidly departs into the Great Basin
and is replaced by low amplitude ridging building in from the
Pacific. Partial clearing and a lingering cold air mass have
allowed overnight temperatures to drop into the low 40s across
much of the area, with locations where clear skies have held on
a bit longer reporting temperatures in the mid to upper 30s. A
Freeze Warning remains in effect for the Upper Hood River Valley
until 9 AM this morning, with a Frost Advisory in place for the
south Willamette Valley. Thickening mid and high level cloud
cover associated with a warm front well off the Washington coast
this morning should help locations from Salem up through the
Portland/Vancouver metro remain just warm enough to avoid
widespread frost/freeze concerns, but certainly could still see
some patchy frost in colder outlying parts of these areas
through daybreak.

Expect mostly dry conditions across the area today as mid and
high clouds continue to spread into the region atop the
relatively flat upper level ridge. Temperatures aloft will only
slowly rebound through the day from around +1 C currently to
5-6 C by this afternoon, with high temperatures struggling to
get out of the upper 50s in most locations today as a result.
Although dry weather will prevail across most of the area, the
aforementioned warm front will move closer to the Washington
coast by later this afternoon and likely bring some rain to
parts of far southwest Washington today. Any rain totaling more
than a few hundredths of an inch will likely be confined to the
Washington Coast and the Willapa Hills, but cannot rule out
some periods of light rain farther inland along the Lower
Columbia River Valley through this evening. Still, most of these
areas away from the coast, including the Portland metro, will
have around an 80 percent chance to remain dry.

Temperatures will jump back above normal on Saturday as the air
mass continues to moderate with the building ridge. This should
keep Saturday morning lows warm enough to avoid any
frost/freeze concerns, with afternoon highs trending a bit
warmer to around 70 degrees for many inland valley locations.
Models continue to depict a weak to moderate atmospheric river
setting up over Vancouver Island and the Olympic Peninsula
tonight into Saturday. This will maintain a sharp precipitation
gradient across the far northern extent of the area through
Saturday. Much like today, expect any rainfall more than a few
hundredths of an inch to be confined to the coastal parts of
southwest Washington as the primary IVT plume remains focused
north of the area.

Widespread rainfall will return to area on Sunday as the
atmospheric river weakens and shifts south across western
Oregon. Latest model guidance has trended a bit slower with this
progression, with rain now looking to hold off until late
Sunday afternoon into Sunday night across much of the area.
Could see some brief periods of heavier rain later in the day
Sunday as IVT values continue to max out around 500-700 kg/m/s
across northwest Oregon Sunday afternoon. Probabilistic guidance
continues to indicate a greater than 90% chance for at least a
half inch of rain for coastal areas and around a 70% chance
across the rest of the area through Sunday night. Hydro concerns
remain minimal given the progressive nature of the system, with
area rivers currently remaining near seasonal low levels
despite the recent rainfall this week. /CB

.LONG TERM...Monday through Thursday...The atmospheric river
will be out of the picture by Monday, with an elongated,
positively tilted upper level trough spanning from western
Canada well into the Pacific to begin the week. This will keep
a lingering chance of showers across the area on Monday in the
presence of cyclonic flow aloft, but with minimal QPF. The
forecast trends drier Tuesday as split flow develops, with
ensemble clusters in good agreement on the western extent of
the trough evolving into a deep cutoff low over the NE Pacific
while broad upper level ridging builds over the western CONUS.
Therefore expect mild and pleasant weather with daytime highs
in the low 60s and chilly overnight lows potentially leading to
renewed frost/freeze concerns into the middle of next week.
Precipitation chances increase again later next week as the
ridge shifts eastward and the low over the Pacific approaches
the West Coast. /CB

&&

.AVIATION...Brief high pressure over the area this morning before
a warm front arrives this afternoon. This high pressure stable
air mass does provide favorable conditions for patchy fog this
morning. However, with the incoming front bringing some cloud
cover, it looks like fog will struggle to form.
Probabilistically, chances of MVFR VIS are around 10% at most
terminals until 18Z Friday, with the exception of a 20-30%
chance at KHIO and KTTD. With these factors considered, expect
mostly VFR conditions with infrequent MVFR VIS at a few
terminals until 17-19Z Friday.

Looking ahead, the front will arrive at the coast around 13-15Z
Friday and inland around 17-19Z Friday, bringing showers to
inland areas north of KSLE and coast areas north of KONP. This
will lower CIGs to around FL045 to FL060 through the rest of the
TAF period (10-20% chance to lower to MVFR). One exception will
be KAST, with a 30-50% chance of MVFR condition due to
encountering the front first and passing heavy showers
temporarily lowering VIS to MVFR.

PDX APPROACHES...Prevailing VFR conditions with a 10-20% chance of
MVFR CIGS from 18Z Friday until the end of the TAF period.
Southeasterly winds well under 10 kt.
~Hall

&&

.MARINE...A low pressure system will move towards the central
British Columbia coast this morning which will swing a warm
front toward the Pacific Northwest coast. The surface pressure
gradient is expected to tighten along the WA coast and along the
OR coast north of Cape Foulweather. This will bring Small Craft
Advisory gusts of 20 to 30 knots from Cape Foulweather to Cape
Falcon. Stronger, low end Gales are likely from Cape Falcon, OR
to Cape Shoalwater, WA with gusts around 35 kt. Seas will also
build to between 10 and 12 feet at 10 to 11 seconds, producing
steep seas. As a result, have issued Small Craft Advisories and
Gale Warnings which will be in effect from this morning until
early Saturday morning.

A very strong ebb is expected this evening for the Columbia River
Bar. With winds expected to reach Gale Warning criteria, have
issued a Gale Warning that will be in effect from this morning
until Saturday morning.
~Hall/Batz

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...Frost Advisory until 9 AM PDT this morning for ORZ116>118-124-
     125.
     Freeze Warning until 9 AM PDT this morning for ORZ121.
WA...None.
PZ...Small Craft Advisory from 10 AM this morning to 2 PM PDT this
     afternoon for PZZ210-251.
     Gale Watch from this afternoon through late tonight for PZZ210-
     251.
     Small Craft Advisory from 10 AM this morning to 5 AM PDT
     Saturday for PZZ252-272.
     Small Craft Advisory from 7 AM to 11 AM PDT this morning for
     PZZ271.
     Gale Watch from 11 AM PDT this morning through this evening
     for PZZ271.
&&

$$

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