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fxus66 kpqr 280520 
afdpqr

Area forecast discussion
National Weather Service Portland or
920 PM PST Mon Feb 27 2017

Synopsis...a trough of low pres will swing across Washington tonight
continuing cool showery weather. Flow will gradually change from
northerly to westerly Tue and Wed, keeping a chance for showers
going, but allowing temps to moderate a little. Late in the week a
couple of fronts will move through for more widespread precipitation
again.
&&

Short term...tonight through Thursday...a quick surface analysis
implies the surface trough associated with the upper shortwave
trough is about to come onshore. This seems fairly close to the
timing afforded by the 00z NAM and GFS. They show one last burst of
precip should fall by mainly 06z (the projected end of the winter
weather advisory), although some 1"/hr rates may continue over the
high Cascades for a couple hours beyond that. The 03z hrrr seems to
have a decent handle on the overall quantitative precipitation forecast this evening and is doing a
reasonable job of depicting the current radar trend. Models imply
there are marginal quantitative precipitation forecast amounts to snow ratios to perhaps justify
extending the advisory for the Cascades by a couple hours.
Unfortunately, the snotel network went down around 3 PM earlier
today so can only rely on the few northwest avalanche center
stations on Mt. Hood and/or the occasional passing car on pass
level odot cams to gauge if the snow rates will be truly easing as
modeled once the trough axis passes. Slightly cooler air will filter
onshore after the axis crosses and bring the accumulating snow level
back down to a broad average of 1500 feet tonight. The lowest 1500
feet will be largely isothermal so will expand the rain/snow mix
under the showers down to 1000 feet below the snow level but with
little and short-term accumulations expected. All told, will need to
wait until the last minute (10 PM tonight) before making the
decision to extend the advisory or allow it to expire. /Jbonk

Previous discussion follows: vort Max seen in water vapor pictures
was moving down Vancouver Island, and expected to swing across Washington
tonight. Radar showed fairly widespread showers across the forecast
area this afternoon in a westerly onshore flow, with heaviest
showers generally over the upslope terrain and some evidence of
rainshadowing in the Willamette Valley. Expect showers to continue
widespread through the evening as the shortwave moves across.
Although showers should decrease some behind the shortwave later
tonight, a NW onshore flow is expected to allow showers to persist,
but with more Reliance on orographic lift. Snow level early Mon
afternoon had generally lifted into the foothills and higher Coast
Range. Will continue with the Winter Weather Advisory for the
foothills and higher Cascades through the evening.

Models continue to show the upper flow transitioning from northerly
to westerly Tue and Wed, as upstream shortwaves begin to dig out a
trough over the eastern Gulf of Alaska. This will bring allow air
mass to modify some, with snow levels creeping up over the next
couple of days. Main focus of moist onshore flow and precipitation
will tend to shift slowly north with the rising upper heights, but
qpf fields still suggest that pops are likely persist for much of
the area through Wed. Wed night and Thu morning looks like the best
chance for a dry period, particularly for the southern half of the
forecast area, as a very Flat Ridge reaches its peak. Models remain
in good agreement in showing a shortwave ejecting out of the NE
Pacific Thu afternoon, increasing the onshore flow and brushing at
least the northern part of the forecast area with a weak warm front.
This will bring better chances for rain to the region by Thu
afternoon.

Long term...Thursday night through Monday...strong westerly
flow aloft gives way persists for much of the second half of the
week, until a broad upper trough pushes deeper into the Pacific
northwest during the by the weekend, providing for continued
precipitation. Snow levels will fall down to around 2,000 feet on
Saturday behind the front, which supports snow showers down into the
higher Cascade foothill elevations. Models remain in fair agreement
that deeper moisture will enhance rainfall again early Sunday as
another shortwave embedded in the flow rotates around the upper
trough and pushes through the region. Model solutions diverge a bit
more significantly by early next week, but there remains at least
some agreement that another round of steady moisture may again
approach. Cullen
&&

Aviation...expect a mix of MVFR and VFR conditions as showers
move through the area due to a disturbance moving through this
tonight evening, with much of the MVFR conditions. Conditions
will probably not change much or settle into MVFR tonight as the
showers and onshore flow continue tonight into Tuesday. May see
conditions lift into VFR Tuesday afternoon but there should still
be some showers around as onshore flow continues. The next
disturbance arrives Tuesday evening and will probably bring
MVFR ceilings primarily to the coast.

Kpdx and approaches...expect a mix of MVFR and VFR conditions
that will likely turn mostly to MVFR overnight into Tuesday
morning, possibly lifting to VFR cigs Tuesday afternoon. /Mh

&&

Marine...winds have eased over the waters this evening. There
are still a few gusts near 25 kt, but expect them to ease
shortly. The next disturbance moving through largely to our
north brings some more SW winds into the waters late Tuesday and
Tuesday night that will probably exceed Small Craft Advisory
thresholds in the northern waters. Those winds ease up on
Wednesday. The next batch of winds increase from the south on
Thursday and should be high end Small Craft Advisory winds or
possibly borderline gale force winds. Those winds turn westerly
and decrease Friday.

Seas remain 10 ft this evening and continue a bit above wave
model guidance, so have extended the advisory for hazardous seas
through 3 am. Seas rise up to just above 10 ft Wednesday into
early Thursday before subsiding again. The next batch of seas
above 10 ft is expected arrive Thursday night. Pt /mh

&&

Pqr watches/warnings/advisories...
or...Winter Weather Advisory until 10 PM PST this evening for
Cascade foothills in Lane County-northern Oregon Cascade
foothills-northern Oregon Cascades.

Washington...Winter Weather Advisory until 10 PM PST this evening for South
Washington Cascade foothills-South Washington Cascades.

Pz...Small Craft Advisory for winds from 6 PM Tuesday to 4 am PST
Wednesday for coastal waters from Cape Shoalwater Washington to
Cascade Head or out 60 nm.

Small Craft Advisory for hazardous seas until 2 am PST Tuesday
for coastal waters from Cape Shoalwater Washington to Florence or
out 60 nm.

Small Craft Advisory for rough Columbia River bar from 3 am to
8 am PST Tuesday.

Small Craft Advisory for rough Columbia River bar from 4 PM to
9 PM PST Tuesday.

&&

$$

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