Area Forecast Discussion
Issued by NWS Portland, OR

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FXUS66 KPQR 251709
AFDPQR

Area Forecast Discussion
Weather Service Portland OR
945 AM PDT Tue Jun 25 2019

.SYNOPSIS...An upper level low pressure will bring cooler
temperatures, periods of showers and a slight chance of
thunderstorms each day this week before temperatures moderate and
rain chances decrease substantially over the weekend.


&&


.SHORT TERM...Today through Thursday...Water vapor satellite imagery
this morning reveals a large upper level low pressure system and
shortwave trough over the northeast Pacific slowly drifting east-
southeastward towards the region. An area of upper level diffluence
coupled with cooling temperatures aloft have resulted in an area of
showers spreading north-northeastward from a line extending between
Florence/Newport and the Portland metro this morning. As our upper
level flow turns more southerly this morning, this area of showers
should shift more towards north Oregon coast range and lower
Columbia River valley including Kelso/Longview before dissipating.
However, all models are struggling to resolve this area of
instability so uncertainty is rather high with regards to how this
situation will unfold.

Nonetheless, visible satellite imagery shows mostly sunny to partly
cloudy skies are in place to the south and east of this band of
showers. This should allow the atmosphere to destabilize somewhat
this afternoon. The pattern is a bit odd for deeper convection to
develop in our CWA this afternoon and evening, but model soundings,
SREF thunder probabilities and a few of the HREF members suggest the
chance is worth including in the forecast. At this point, it appears
the highest chances for thunderstorms appears to be across the
Oregon Cascades where surface heating will be greatest based on the
latest satellite trends. However, the modest southwesterly steering
flow will likely result in any storms that do develop to shift east
of the Cascade crest rather quickly.

In addition, some models suggest the east slopes of the north Oregon
Coast Range extending northeastward towards Cowlitz County is
another area ripe for thunderstorms this afternoon. However, the
ongoing showers and clouds this morning could put a damper on these
chances. Nonetheless, we opted to include this area in the mention
of thunderstorms given the uncertainty. It should be noted that
there is a bit more wind shear to play with than usual across our
CWA today so storms that do get going could organize a bit.

Expect rain chances to increase Wednesday and particularly Thursday
as the upper level low pressure and associated cold pool aloft
approaches the Pacific Northwest and subsequently sits over the
region for 24+ hours. Expects showers and particularly thunderstorms
to be most prevalent during the afternoon and early evening hours
each day. Nonetheless, shower chances appear they will persist for
much Wednesday night and Thursday morning as the main mid to upper
level low pressure moves towards the region. Rain chances should
start to trend downward Friday, but it appears likely there will be
enough lingering instability across the northern half of the CWA,
that another round of afternoon and evening showers will likely
develop. /Neuman


.LONG TERM...Thursday night through Sunday...No changes.
Previous discussion follows. Models are in surprisingly good
agreement the closed low pressure impacting the region this week
will shift northeast of the region late Friday into Saturday. This
means we should see another round of cool temperatures and showers
on Friday before conditions gradually improve over the weekend.
While a weak trough will likely linger over the Pacific Northwest
into Sunday, building 500mb heights should result less clouds and
showers chances declining below mentionable thresholds.
Nonetheless, the operational models depict another closed low
pressure developing off the Pacific Northwest Coast early next week,
and that it should remain far enough the coast to initially keep rain
chances minimal. /Neuman


&&


.AVIATION...Shower activity continues along a line from about ONP
to KLS with brief MVFR VIS at impacted inland sites and IFR along
the coast. Expect these showers to break up by about 19Z. Then
will see some showers and possible thunderstorms develop in the
afternoon and evening hours. Highest chance for thunder appears
to be from about MMV north and east through SW Washington and
along the Cascade crest. May see some MVFR CIG and/or VIS with
showers ~21Z-04Z. Low MVFR or high IFR stratus along the coast
tonight, but VFR inland overnight.

KPDX AND APPROACHES...Most likely scenario is VFR next 24 hours.
Could see a shower or two this afternoon and evening with a low
chance of brief associated MVFR CIG and/or VIS. Bowen


&&


.MARINE...No significant changes made. Previous discussion
largely follows with minimal modifications. Weak high pressure
over the coastal waters will shift inland today. Northerly winds
continue to weaken, turning south to southeasterly on this
morning. Still, gradients remain weak, with mostly 10 to 15 kt
winds.

Broad area of upper low pressure will settle over the offshore
waters for Wed through Thu. This will maintain a southerly wind
but still 10 to 20 kt at most during that time frame. Can not
rule out a brief burst of 25 kt winds on Wed, and possibly again
later Thu, but not confident such winds will materialize. So,
will trend to keep winds just below any advisory criteria.

Seas generally 4 to 5 ft tonight into Tue, then ease down a tad.
Seas will stay in the 3 to 5 ft through Sat. JBonk/Rockey


&&


.PQR WATCHES/WARNINGS/ADVISORIES...
OR...None.
WA...None.
PZ...None.


&&


$$

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This discussion is for Northwest Oregon and Southwest Washington
from the Cascade crest to 60 nautical miles offshore. This area is
commonly referred to as the forecast area.


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