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Area forecast discussion 
National Weather Service Portland
327 am PDT Monday Apr 27 2015

Synopsis...the remnant moisture from a warm front continues to bring
areas of fog and drizzle to southwest Washington and northwest Oregon
this morning. Skies should gradually clear as the morning
progresses...with plenty of sunshine and warmer temperatures during
the afternoon. A cold front will spread more light rain inland
Tuesday...with temperatures falling back to near or slightly below
normal. Showers continue into Wednesday as another disturbance moves
onshore...especially Salem northward. Gradually drier and milder
weather is expected later in the week as a Flat Ridge of high
pressure builds across California and southern Oregon. However weak
disturbances may clip the northern half of the forecast area with
clouds and a few showers at times through next weekend.

&&

Short term...today through Wednesday...stubborn low-level moisture
left behind by yesterdays warm front continues to bring areas of fog
and drizzle to SW Washington and northwest Oregon early this morning. Areas
along the coast and higher terrain north of Portland continue to
receive a couple hundredths of an inch of -ra/dz per hour...and
klgx/krtx Doppler radars continue to pick up on some very light
echoes moving onshore. Decided to hold clouds/probability of precipitation in the forecast a
bit longer North Portland northward...though it is doubtful the pdx
metropolitan will see any additional measurable precipitation...just some areas of
light drizzle mainly near the Cascade foothills. Meanwhile fog is
reducing visibility to one half mile or less in Corvallis and Eugene.

Moist air with surface dewpoints around 50 degree f is the main culprit
behind all the fog...drizzle...and low clouds. With high pressure and
light wind across the forecast area these clouds will be in no hurry
to clear out this morning. However the strong April sun will
eventually do its work...initially lifting the low clouds into a
stratocumulus deck then clearing it out by early afternoon. The
abundant cloud cover will likely delay todays Big Warm up...but temperatures
should jump quickly once skies clear. 06z NAM shows 850 mb temperatures
around +12 to +14 degree c by 21z.

Significant offshore flow would help maximize the warmth this
afternoon...and most models seem to be indicating potential for
offshore flow to occur. 00z met/mav guidance forecast east winds of 10 to
15 knots at pdx this afternoon...but oddly keep dewpoints around 50 degree
f despite the drying effect of offshore flow. Something seems a bit
fishy about this solution...especially considering gradients remain
flat or slightly onshore through the Columbia Gorge early this
morning. Gut feeling is that any offshore flow should be too weak to
have a dramatic impact on temperatures. Therefore the persistent clouds and
moist air mass may keep most locations in the 70s...with Eugene
having the best shot of reaching 80 degrees today. Therefore we
bumped forecast highs down a few degrees north of Salem. There will be a
touch of humidity to the air due to the lingering low level moisture.

The next Pacific front is already near 140w and will cause clouds to
increase tonight. While this front involves the remnants of a Kona
low which was lingering near Hawaii for the past several days...most
of the associated dynamics and moisture appear to be headed for the
British Columbia coast with the tail end of the front dragging across the Pacific northwest
Tuesday. There should still be enough dynamics/moisture in place for some
light rain across most of the district with the heaviest
amounts...only 0.25-0.50 inch...likely along the north coast and
Coast Range. There is a decent thermal signature to this cold
front...with 00z European model (ecmwf) dropping 850 mb temperatures back down to around +4
degree c across our area by midday Tuesday. With plenty of clouds and
moisture around...many locations may struggle to climb above 60
degrees Tuesday...so enjoy the warm weather while its here today.

A following disturbance will push through the forecast area Wednesday. This
system is presently just south of the Aleutians near 160w. Models
show a decent cold pocket aloft with this system...with plenty of
shallow instability. Therefore showers are a good bet for Wednesday...and
snow levels may again be down near the Cascade passes. Depending on
the magnitude of cold air aloft there is an off chance of thunder for
our northern zones Wednesday...but confidence is not quite high enough to
include in the forecast yet. Weagle

Long term...no changes. Previous discussion follows...
Wednesday night through Sunday... residual showers may linger in the
northern zones Wednesday night. Then generally zonal flow aloft will
likely maintain over the region for the second half of the week. A
few weak disturbances may bring occasional light rain at times but
not possible to pinpoint the strength of these impulses and timing of
their arrival at this time. Thus while confidence is improving in the
general pattern and trend...the particulars reflect a lower
confidence forecast. That said...did cut back probability of precipitation a bit for Thursday
Onward given the recent trending of the models...but given some
uncertainty did keep slight chance for the northern
zones...Cascades...and portions of the coast/Coast Range at times.
Despite this...suspect that many locations...including the
central/south Willamette Valley...likely remain largely dry from
Friday into early next next. Temperatures through the extended period
look to remain near normal through early next week. Cullen

&&

Aviation...a slow moving warm front brought widespread MVFR and
pockets of IFR conditions to the region late yesterday. The MVFR
cloud deck is dissipating from south to north this morning likely
due to subsidence behind the front. However with dew points
around 50 degrees...IFR fog and low stratus is forming fairly
quickly after the MVFR cloud deck clears. Expect this general
trend to continue for the next several hours...with the MVFR
stratus completely gone by 15z to 18z. Any fog and low level
stratus that forms should also dissipate by middle morning. Then
expect mostly VFR conditions late this morning and this afternoon
as high pressure and weak offshore flow develops. IFR stratus should
return to the coast this evening as winds turn southerly ahead of
an approaching front.

Kpdx and approaches...MVFR stratus associated with a warm front
should dissipate by around 15z. Then expect VFR through the rest of
the day and tonight. Breezy easterly winds may develop near the
western end of the Columbia River gorge this afternoon and early
evening...but confidence is low that these winds will impact kpdx
directly. Pyle

&&

Marine...fairly benign conditions are expected today with weak
high pressure over the waters. Expect southerly winds of 5 to 10 knots
and seas at 5 to 7 feet. Southerly winds will increase again late
this evening ahead of the next approaching front. Expect a period
of 15 to 20 knots winds with gusts to 25 knots lasting into Tuesday morning
as the front moves through the waters. Winds will turn weak and
turn westerly behind the front Tuesday afternoon. Expect increasing
westerly swell to build in behind the front later Tuesday...likely
pushing seas back near or just above 10 feet through Wednesday.

High pressure returns to the waters for the remainder of the
week...with thermal surface low pressure along the northern
California coast. This commonly brings advisory level wind gusts
to the central Oregon waters during the afternoons and evenings.
Pyle

&&

Pqr watches/warnings/advisories...
or...none.
Washington...none.
Pz...Small Craft Advisory for winds from 11 PM this evening to 11 am
PDT Tuesday for coastal waters from Cape Shoalwater Washington to
Florence or out 60 nm.

&&

$$

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

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