Area Forecast Discussion
Issued by NWS Portland, OR

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FXUS66 KPQR 260429
AFDPQR

Area Forecast Discussion
Weather Service Portland OR
919 PM PDT Sat May 25 2019


.SYNOPSIS...Low level onshore flow will keep temperatures near
average with a risk for a shower or thunderstorm for inland locations
through the first half of the week.


&&


.SHORT TERM...Tonight through Tuesday...Water vapor satellite imagery
this evening reveals a mid to upper level low pressure system
dropping southward just off the Oregon coast. An area of mid level
deformation led to widespread rain late this afternoon and early
this evening across northwest Oregon and southwest Washington. The
forcing driving this precipitation is quickly dropping southward with
the low pressure and should result in a gradual decrease in rain from
north to south across the area overnight.

As the low pressure continues to drop southward towards California
overnight, increasing easterly flow will develop above 925mb. This
should result in a lot of the cloud cover departing the region as
well. However, there may be enough northwesterly low level flow to
keep at least some low clouds entrenched across portions of the area
into Sunday morning. Any low clouds lingering into Sunday morning
should dissipate rather quickly with daytime heating, though.

With warmer temperatures aloft and increased sunshine, high
temperatures should warm into the 70s for the Willamette and lower
Columbia River valleys Sunday through Tuesday.

The big forecast uncertainty revolves around when and where shower
and thunderstorm chances are highest during this time period. At this
point, it appears the Lane and Linn County Cascades appear to be the
safest bet to have at least a thunderstorm or two develop Sunday
afternoon per the latest round of model guidance.

Pattern recognition suggests the threat should be at least as high
across the south Washington Cascades, too. The problem with that
idea is that an area of widespread showers over west-central Idaho
associated with an area of mid level deformation and a weak 700mb
low pressure will move northwestward and then eventually westward
across Washington later tonight and Sunday. There may be enough
residual cloud cover to limit solar insolation across the south
Washington Cascades Sunday to inhibit convection. Nonetheless, some
model scenarios weaken this feature before it arrives while others
keep it farther north over the central Washington Cascades. As a
result, the inherited thunderstorm forecast captures this
uncertainty well so few changes were made to the southwest
Washington thunder forecast. It should be noted that the
thunderstorm threat for the Portland/Vancouver metro is very much
dependent on whether or not thunderstorms develop over the south
Washington Cascades Sunday afternoon and then subsequently spread
southwestward towards the metro.

Elsewhere, did expand the mention of thunder into the east slopes of
the Coast Range late Sunday afternoon and early evening. This is
primarily due to some models suggesting there may be enough
instability for a surface based storm or two to pop late Sunday
afternoon and evening. These same model dewpoint fields seem a bit
high so am somewhat skeptical there will be enough low level
moisture to make it happen, but cannot ignore the possibility
either. While some models suggest this instability will be equally
impressive across the central and southern Willamette Valley Sunday
afternoon, any shower and thunderstorm development will likely be
focused over the terrain initially given the light steering flow. It
will take some time for outflows to converge and realize the
potential for the center of the south and central Willamette Valley
so have held off on the mention of thunder until the early evening
hours for these areas.

Otherwise, expect another round of showers and possibly a
thunderstorm or two to develop near and across the Cascades on
Monday and Tuesday, but pinpointing who will or will not see wet
weather remains a low confidence forecast at best. /Neuman


.LONG TERM...Tuesday night through Saturday...Ridge of high pressure
remains in place over western Canada. The high is far enough north
to allow for shortwave disturbances to slide south and clip our area
with a few chances for showers, mainly in the Linn and Lane County
Cascades. Otherwise, expect fairly nice weather for the rest of the
forecast area through midweek, with marine clouds on the coast and
partly cloudy skies inland. Temperatures will be near seasonal for
this time of year, only a few degrees above normal inland. Next
weekend a more zonal flow pattern sets up over the Pacific, but high
pressure remains fairly dominant over the Pacific Northwest into
next weekend. -McCoy


&&


.AVIATION...Mostly VFR conditions this evening with light rain
lingering. Expect rain to slowly taper off from north to south
overnight with MVFR becoming more widespread as rain dissipates.
VFR conditions should return Monday afternoon with convection
over the Cascades. Convection, including the possibility of
thunderstorms, will drift west into Sunday evening, encroaching
on valley terminals.

KPDX AND APPROACHES...Rain decreases overnight and expect CIG to
drop to MVFR as rain ends. CIG rises to VFR Sunday afternoon, but
convection, including thunderstorms, may move in from the east
late Sunday afternoon and Sunday evening. Bowen


&&


.MARINE...Seas are holding at 10 to 12 feet this afternoon, with
the influence of a fresh swell holding conditions higher.
Meanwhile, winds across the outer portions of the coastal waters
continue to gust to 20-25 kt this afternoon. Have maintained the
Small Craft Advisory for winds through 7 PM as a result. These
stronger N-NW winds will likely keep seas above 10 ft into this
evening, but gradually easing to 9 feet late. However, dominant
periods will likely remain at 9 to 10 seconds, so opted to extend
the advisory for hazardous seas a little longer, through 11 PM
for the inner waters and 5 AM for the outer zones. Weaker
gradients Sun night through early next week will generally result
in wind speeds 15 kt or less. Seas eventually fall to around 5
ft Sun night or Mon as winds remain moderate Monday and Tuesday.


&&


.PQR WATCHES/WARNINGS/ADVISORIES...
OR...None.
WA...None.
PZ...Small Craft Advisory for hazardous seas until 5 AM PDT Sunday
     for Waters from Cape Shoalwater WA to Florence OR from 10
     to 60 NM.

     Small Craft Advisory for hazardous seas until 11 PM PDT this
     evening for Coastal Waters from Cape Shoalwater WA to
     Florence OR out 10 NM.

&&


$$

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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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