368
FXUS66 KPQR 251014
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
314 AM PDT Wed Sep 25 2024

.SYNOPSIS...Cold front is moving inland today which will bring
widespread rainfall and breezy winds. Thursday will bring a
brief lull in active weather, aside from lingering fog, Thursday
afternoon. Another weaker front arrives overnight into Friday
late Thursday into Friday morning. High pressure returns over
the weekend. More active weather expected mid-week.

&&

.SHORT TERM...Now through Friday...Satellite and radar show the
frontal boundary along -125W latitude this morning, and it is
very slowly moving eastward. This front is associated with a
cold air wrapped low that has dropped down from the Gulf of
Alaska. It is weakening as it fills, but ample cold air and
moisture accompanies it. The front will move inland late this
morning, with the coast seeing a few showers earlier. It will
then slowly move inland through the day becoming showery by
2200. Rainfall accumulations will be variable both by terrain and
latitude as the low is positioned to the north of the forecast
area. Because the area is sitting on the southern edge of the
front, a bulk of the precipitation will fall along the coast,
Coast Range, and the southern counties of Washington. A bit of a
rain shadow will keep the valley drier, then orographic lift
will aid heavier rain over the Cascades. This front is ushering
in air at around 4 degrees C at 850 mb (5500 ft). This, combined
with the orographic lift over the Cascades, southwesterly flow,
and warm temperatures from the previous day, will increase the
probability for thunderstorms. There is a 15-20% chance of
thunderstorms over Skamania, Cowlitz, Clark, Hood River,
Multnomah and northern half of Clackamas Counties.

As the front exits the area to the east, zonal flow will take
over with dry air shifting in. Will see moist conditions
overnight with clearing skies and radiational cooling. This will
make the environment suitable for fog formation Thursday
morning. Overnight Thursday though, yet another shortwave trough
will advect over the Pacific Northwest which is associated with
a 980 mb low at 51.2N -134.3W. This low will move northeast over
the Alaska Peninsula through Friday morning putting the forecast
area in the southern end of the front. Because of this,
precipitation will be reserved for the northern counties in
northwest Oregon and southern counties of Washington.
Accumulation will be minimal and will dry in the afternoon as
high pressure develops once again. -Muessle


.LONG TERM...Friday Night through Tuesday...For those with
weekend plans, prepare for another sunny and dry Saturday! High
pressure will reform late Friday into Saturday and persist
through late Monday/early Tuesday. Based on the pressure
gradient composition, winds will generally be diurnally driven
based on the position of the ridge and thermal trough axis.
Overnight, winds will be generally easterly/drainage then become
westerly/northerly in the afternoons. The pressure gradient is
around 4 mb during the day so the gustiest winds will occur in
the E-W aligned terrain like the Columbia River Gorge. Even
with this high pressure in place, not seeing significant
temperature rises as highs will remain near normal in much of
the long-term forecast.

Interestingly, on Sunday at the mid-levels, 500 mb vorticity
advection and heights are showing an energetic trough moving
through. However, it is not robust enough to bring rainfall to
the area that will overcome the ridge from Saturday. Instead, it
will be cooler than highs on Saturday. High pressure rebuilds on
Monday and Tuesday with it reaching it`s most amplified ridging.
Temperatures will rise on the "high side" of normal on these
days with the spread being around 5 degrees C between the 10th
and 90th percentile.

Late Tuesday yet another front nears the coast which could bring
another round of rainfall. Confidence is low at this time though
as the pattern is very unorganized and the system would have to
overcome the depth of dry air from the previous days.
-Muessle


&&

.AVIATION...Southwest flow aloft as a front moves across the area
today. LIFR to IFR fog/stratus at the coast likely continues
through 18z Wednesday. Stratus has deepened at KAST to at least
3500 MSL. Marine stratus continues to surge inland with high
chances (70-90%) for low-end MVFR CIGs between 11-18z this
morning. Rain is expected to begin along the coast by 15-18z and
spread inland this afternoon. Inland conditions likely to improve
to a mix of MVFR and VFR, becoming predominately VFR behind the
front after 00z Thursday. Coastal terminals likely remain IFR
(central) to MVFR (northern) after 00z Thu. Expect mountains to be
obscured by low clouds this afternoon through the evening. Also,
can expect an abrupt wind shift from southerly to westerly with
passage of the surface front, most likely between 19z at the coast
to 00z Thu inland.

PDX APPROACHES...Currently VFR and clear skies, but marine stratus
is expected to push into the terminal by 12z Wed (>90% chance).
Expect initial CIGs around 1000-2000 ft AGL. Rain chances with
the incoming front peak around 19-02z this afternoon. Southerly
winds expected to increase to around 6-10 kt before abruptly
shifting to the west later this afternoon, around 22z-00z. -DH

&&

.MARINE...A quick moving cold front will move across the coastal
waters today, pushing onshore this afternoon. Southerly winds
ahead of the front are expected to gust up to 25 kt during a brief
period of 2 to 4 hours north of Cape Foulweather. Seas are also
expected to build to around 8 to 10 ft, therefore have issued a
Small Craft Advisory for the combined potential impacts through
early this afternoon. Winds will shift to the northwest with the
frontal passage, becoming light overnight.

Winds turn southerly again on Thursday ahead of another frontal
system. Low pressure will rapidly intensify across the far NE
Pacific and move toward Haida Gwaii, while pushing a front toward
the coastal waters later Thursday. The strongest winds associated
with this system are expected across the northern waters. In fact,
model guidance continues to increase the probability of Gale
Force southerly wind gusts exceeding 33 kt to around 30-50%
Thursday afternoon and evening. The trailing cold front will
likely weaken as it moves across the coastal waters Thursday
night.

Then, a westerly swell moving in behind the front is expected to
build seas to around 12 to 14 ft on Friday. High pressure also
rebuilds on Friday, along with a strengthening thermal trough
along the southern Oregon coast. This will result in a return to
gusty northerly winds. Northerlies are likely to continue through
the weekend, while seas settle to around 8 to 10 ft.-DH


&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...None.
WA...None.
PZ...Dense Fog Advisory until 11 AM PDT this morning for PZZ210-
     251>253-271>273.
     Small Craft Advisory from 7 AM this morning to 1 PM PDT this
     afternoon for PZZ251-252-271-272.
&&

$$

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