185
FXUS66 KPQR 161720
AFDPQR

Area Forecast Discussion...UPDATED
National Weather Service Portland OR
1018 AM PDT Thu May 16 2024

Updated Aviation discussion

.SYNOPSIS...A weak cool front sits off the coast and is slowly
moving inland this afternoon. Some light showers on the leading
edge before more persistent rainfall through Friday morning.
Zonal flow through Saturday before another long wave trough
arrives on Sunday. Higher confidence in troughing pattern with
rain through mid-week.

&&

.SHORT TERM...Now through Saturday Night...Satellite shows the
pervasive marine stratus this morning with the cold front still
sitting offshore. Upper level troughs are digging southward with
the axis positioned right over the forecast area by early Friday
morning. Today will be more of a transition day with stratus
spilling over the Pacific Northwest, and onshore flow advecting
low-level moisture into the region. The addition of the moisture
near the surface will aid in setting up an environment where
rain has a better chance of actually reaching the ground. Will
see minimal, if any, rain today until late tonight into very
early Friday morning when the trough really takes hold over the
area.

In the upper levels, the main energy of the trough hangs just
to the north which coincides with the bulk of the precipitation
falling over Washington state. On the convergent side of the
trough we are going to see a jet max with winds around 100 kt
set up right over Northwest Oregon north of Marion County. This
jet will support rising motion and thus low pressure at the
surface. The pressure gradient will cause winds to increase,
especially in the Columbia River Gorge. The pressure gradient
between Troutdale and the Dalles will range between 4-6 mb this
afternoon so have increased westerly wind speeds within the
Upper Hood River Valley and near Cascade Locks. Could see
gustier conditions as well along the leading edge of the front
as mixing will be amplified.

On Friday, the combination of these features will provide
enough energy to intensify the precipitation at the surface.
One challenge will be the rain shadow possibility within the
Willamette Valley. Confidence is high that rain will be heavier
along the coast, Coast Range, and Cascades but some models
(especially the NAM) are showing the signs of some rain
shadowing east of the Coast Range. Because of this feature,
precipitation will be even less in the interior lowlands. The
NAM has a tendency to really capture these local effects well.
Looking at the broad ensembles, the ECMWF is slightly drier than
the GFS, and the HREF (combination of higher resolution models)
is showing only a 10% chance for measurable precipitation
around Portland. In contrast, the coast has a higher probability
of seeing measurable rain. In Astoria, the 24 hour forecast
ending at 4 PM Friday is showing around a 30% chance of 0.05-0.1
inch, and a 15% chance of up to 0.2 inch. The NBM in contrast
is showing less than a 5% chance of QPF up to 0.2 inch. While
this is more precipitation than we have seen over the last few
days, it is still fairly light and more of a high PoP, low QPF
scenario.

By Saturday, zonal flow takes over with some residual rain
present - especially along the coast. There is a shortwave
trough passing through the mainly zonal flow aloft which will
act as an encouraging feature for potentially more light rain
once again. This remains a high PoP, low QPF pattern with
rainfall totals being very localized.
  -Muessle


.LONG TERM...Sunday through Wednesday...Minimal change in the
forecast for Sunday and Monday as mild temperatures and a few
sprinkles remain. Monday appears to be warmer than Sunday as
the NBM shows ranges between 66-74 degrees F (the 25th-75th
percentile) in Portland, 59-62 degree F along the coast, and
64-71 degrees F in Hood River. Any rain that does fall on these
two days will be light and showery. Long term ensembles have
narrowed in on a pattern finally where as yesterday there was
still quite a bit of variation between them. On Tuesday, there
is a trend towards troughing but the depth of that trough still
remains unknown. Given that that is 6 days out, there is still
time for it to shift. Have trended in the route of the grand
ensemble which is showing moderate troughing, rain, northerly
winds and moderate temperatures. The long-term forecast is
unremarkable but there remains high uncertainty on Tuesday and
Wednesday in regards to temperatures and precipitation
accumulation. -Muessle


&&

.AVIATION...Satellite and surface weather observations show marine
stratus deck along the coast with widespread IFR from earlier now
lifting to a MVFR deck as of 17Z Thu.  MVFR stratus reached southwest
WA and northern and central Willamette Valley affecting areas from
KSLE northward.  This stratus is breaking up, but with daytime
heating will see stratocumulus deck forming in the same area. Trend
in models have cigs lifting, breaking up to VFR after 20Z Thu or so.

Weak frontal boundary to move to the north coast 20-22Z Thu to bring
IFR and LIFR to KAST-KTMK area, then to KONP area around 00Z Fri.
Should then see conditions improving to a mix of MVFR and VFR after
05-07Z Fri.  The frontal boundary breaks apart as it moves inland
this evening. HREF shows 30-40% chance for MVFR cigs KSLE northward
04-07Z Fri, and 30-50% chance MVFR south of KSLE 05-10Z Fri.

PDX AND APPROACHES...Cigs lifting above 2000 ft in the area and is
expected to continue to lift through the afternoon, likely reaching
VFR after 21Z Thu. The aforementioned frontal boundary breaks apart
as it moves over the area this evening, to bring a 30-40% chance for
MVFR cigs 04-07Z Fri.  Expect northwest surface winds around 8 to 12
kt 21Z Thu to 10Z Fri.
/mh -TK

&&

.MARINE...A weak front will move over the coastal waters during
the late morning and early afternoon hours on Thursday, bringing a
shift from northerly winds to northwesterly winds Thursday
afternoon through Thursday night. Northwest winds behind the front
are still expected to increase to around 20-25 kt over the waters
(90-95% chance of wind gusts in excess of 21 kt). As such, the
Small Craft Advisory that was initially in effect for only the
northern waters has been expanded to include the central and
southern waters as well (beginning Thursday afternoon over the
northern waters and Thursday evening over the central/southern
waters). Beyond Thursday, the return of high pressure will result
in a typical summertime northerly flow regime Friday through the
upcoming weekend with wind gusts peaking in strength during the
afternoon/evening hours each day. The strongest winds will occur
over the southern waters where small craft advisory level wind
gusts to 25 kt are likely to occur again.

In regards to seas, buoy observations and the latest guidance
continues to suggest significant wave heights remaining between 5
to 9 feet over the next several days as a primary northwest swell
interacts with northerly wind waves.  -TK

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...None.
WA...None.
PZ...Small Craft Advisory from 2 PM this afternoon to 8 PM PDT
     Friday for PZZ251-271.
     Small Craft Advisory from 7 PM this evening to 8 PM PDT Friday
     for PZZ252-253-272-273.
     Small Craft Advisory until 5 AM PDT early this morning for
     PZZ253-273.
&&

$$

www.weather.gov/portland

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