National Weather Service Weather Forecast Office

• Home
• Site Map
• News
• Organization
• Search for: NWS All NOAA 

000
FXUS66 KPQR 121011
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
311 AM PDT Fri Apr 12 2024

.SYNOPSIS...Low pressure sits over the northeast Pacific
producing two waves of fronts which will move inland through
the day. Light showers will persist through the weekend with
increased chances for thunderstorms in Lane County Friday and
Saturday afternoons. High pressure builds in once again late
next week.

&&

.SHORT TERM...NOW THROUGH SUNDAY...Minimal change in sensible
weather in the short term forecast. Satellite imagery shows the
low pressure center sitting around 44.04N -134.9W. This low has
produced two separate fronts - though both are decaying
quickly. The first front is draped over nearly all of Washington
and western Oregon this morning, with the second still over the
ocean. As the low shifts southward through the day, the jet
stream will become negatively tilted and the jet streak on the
western side of the low will intensify. There is a weak "tongue"
of the jet that will sit right over Lane County this afternoon.
This low transition will cause winds to become more north-
northeasterly though speeds will not be high. However, this may
be enough to encourage orographic lift over the Cascades and
could tap into some of the instability aloft. With the support
of the section of the jet stream, cannot rule out some
thunderstorms in the afternoon/evening. However, with drier air
at the surface and ample cloud cover during the day,
thunderstorms may struggle to form. Will say though that high-
resolution lightning models are showing a few outcomes bringing
lightning to the southern Willamette Valley and Lane County
Coast Range this afternoon. Sensible weather wise, temperatures
will see little change but could be a degree or two warmer.
Temperatures on Thursday were at or above the 75th percentile
for the NBM and other higher resolution models so have trended
slightly warmer than what models are suggesting in order to
account for a similar outcome.

Saturday is still trending on the warmer side and will be the
warmest day of the weekend as an inverted trough forms over the
area. Temperatures will vary with the warmest temperatures in
the northern Willamette Valley and slightly cooler in the south.
The coast will remain moderate, and could even be slightly
cooler than Friday`s highs due to an onshore wind. With showers
still a possibility and more of a westerly flow in the evening,
cannot rule out the chances for thunderstorms along the
Cascades. Currently sitting around 20% chances within the
Willamette Natl Forest and associated foothills. Have extended
the changes slightly more westward to include cities like Lowell
due to a slight veering profile and increased projected CAPE
values. Confidence is low, and this is not looking to be a
widespread thunderstorm event. High pressure builds on Sunday.



.LONG TERM...Saturday Night through Thursday...Onshore flow
returns on Monday as an area of high pressure develops well off
the coast in the northeast Pacific. The location of this low
will usher in cooler air aloft (generally around -6 degrees C)
by Tuesday early morning. Overall high temperatures will cool a
few degrees on Monday, and lows overnight (into Tuesday) too
will cool. The NBM is showing low temperatures around 38-42
degrees F (25th-75th percentile), while around 35% of the other
long range models are showing temperatures less than 40 degrees
from McMinnville Eastward. Confidence is quite low given this
spread, but if these cold temperatures do manifest cannot rule
out a round of frost.

On Tuesday, the high pressure ridge over the Pacific will
build, but there is a slight shift as a broad area of low
pressure begins a slow decent southward from Canada over the
Rockies. Looking at the long term model clusters, each are
showing this overall pattern and could even take on the synoptic
shape of an omega block. By Wednesday, easterly winds will
develop which will bring in drier and potentially cooler air
from east of the Cascades. There is quite a difference in
modeled overnight temperatures on Wednesday and Thursday.
Ultimately it will depend on how far south the trough gets over
the Rockies, how much clearing we experience at night, and how
cold the air is east of the Cascades. Cannot rule out frost
again. One thing to note with a pattern like this is that
commonly this is one of our windier pattern. It is not very
strong at this point so climatologically anomalous winds are
not expected. -Muessle


&&

.AVIATION...Early this morning the weak frontal boundary over the
region has largely fallen apart with light precipitation chances
waining as we headed into the midday hours. VFR conditions over
the inland sites will continue to prevail through the forecast
period with guidance maintaining a less than 10-20% chance for
MVFR CIGS. A mix of MVFR and IFR conditions along the coast likely
sees some improvement by midday, although confidence in the exact
timing of said rising CIGs/VIS is low at this time. Otherwise,
we`ll have to keep an eye on wrap-around shower and isolated
thunderstorm activity later this afternoon through Friday evening
across the southern portion of the forecast area - mainly in the
vicinity of KEUG.

PDX AND APPROACHES...Expect predominately VFR with lowering CIGS into
Friday morning although high resolution guidance keeps the
probability for MVFR CIGS rather low at around 5-10% locally. A
few scattered light showers may wander in the vicinity of the
terminal the rest of the morning but expect dry weather to prevail
through the forecast period. West winds stay light and variable
much of this morning, then winds pick up out of the north-
northwest near midday into the afternoon. -Schuldt

&&

.MARINE...A rapidly weakening frontal boundary has passed
overhead early this morning as the parent upper-level low
pressure offshore begins to dive southward. This will allow
northerly winds to increase as Friday progresses with gusts likely
in the 20 to 30 knot range come after afternoon hours. Confidence
higher this forecast cycle that these winds will work into not
only the outer waters but also in the inner waters as well and
have thus expanded the coverage of the Small Craft Advisory for
today and tonight. It`s worth noting the Columbia River Bar is
under a Small Craft Advisory this early morning for strong ebb
currents and 7-8ft seas before transitioning to 22-27 knot gusts
midday into the afternoon. Going forward N-NW winds prevail
through the weekend and while the inner water likely see a brief
reprieve from Small Craft Advisory conditions Saturday morning
winds increase again late Saturday into Sunday. A NW swell at 7-9
feet persists into Tuesday before relaxing to 4-6 feet around the
middle of next week. -Schuldt

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...None.
WA...None.
PZ...Small Craft Advisory until 2 AM PDT Saturday for PZZ210.
     Small Craft Advisory from 1 PM this afternoon to 5 AM PDT
     Saturday for PZZ251>253.
     Small Craft Advisory from 8 AM this morning to 5 PM PDT
     Saturday for PZZ271>273.
&&

$$

www.weather.gov/portland

Interact with us via social media:
www.facebook.com/NWSPortland
www.twitter.com/NWSPortland