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Area forecast discussion 
National Weather Service Portland or
143 PM PST Tuesday Mar 3 2015

Synopsis...high pressure will dominate this week and maintain a dry
weather pattern. Weak offshore low-level flow will diminish this
afternoon. It will be cold again tonight...especially in the interior
valleys. Daytime temperatures will gradually warm each day later in
the week and over the weekend.
&&

Short term...tonight through Friday...high pressure and dry
conditions will prevail through the short term and beyond.
Large-scale features on water vapor loop include a high amplitude
ridge centered just inside 140w...an upper low near Baja California and an upper
level trough axis extending from the Dakotas to southern Nevada. Visible
imagery showing some small cumulus developing over the Cascade
foothills and Oregon Coast Range early this afternoon.

Early-afternoon temperatures running near or slightly higher than 24
hours ago. Thus...forecast maximum temperatures for today look to be on track.
Model 850 mb temperatures for today forecast to be just a touch higher than
yesterday. The 12z Monday NAM run appeared to be a little too strong
with the forecast offshore gradient through the Columbia Gorge. The
kttd-kdls peaked at -1.9 mb at 16z...about one mb lower than the 12z
Monday guidance. The gradient is weakening this afternoon...as expected.
Todays 12z run shows the gradient picking up just a bit tonight...
close to what it was this morning. Expecting just patchy fog tonight
in the usual locations. There may Abe enough offshore drift along the
immediate coast to prevent fog...but would not be surprised to see
some form near bays...such as ktmk and kast. Another cold night in
the interior valleys with min temperatures similar to this morning...
although slopes and ridges will warm just a bit as shallow
inversions develop.

NAM shows another verly weak upper-level disturbance moving acorss
the area late tonight and Wednesday morning for some high clouds. NAM
cross-section from kast through The Gorge not as supportive with high
clouds tonight compared to the past couple of runs. Other than
that...850 mb temperatures slowly warm through the short term. The upper
ridge will flatten Thursday as an upper trough moves inland north of
Washington. There may be some cirrus with this feature...but will
otherwise have little impact to the weather. NAM suggests that there
will be a wind reversal along the coast Thursday night. Resultant
onshore low-level flow will likely cool the coastal strip a couple
degrees Friday. Maximum temperatures Friday forecast to get into the lower 60s at many
inland lowland locations. Weishaar

Long term...Friday night through Tuesday...the upper ridge will
gradually shift from being centered near 130w late this week to then
expand eastward across the western Continental U.S. Through early next week.
Main change was to bump temperatures up another couple degrees for
the weekend bringing most valley locations into the middle 60s and the
coastal areas to the upper 50s to lower 60s. Not much surface
pressure gradient to speak of or else we could have easily hit the
magical 70 degree mark if we had offshore flow. As the upper ridge
expands eastward Monday...models are showing a somewhat aggressive
trough swing south from Alaska and drag a cutoff low out toward 150w
toward the pacnw coast. Have delayed any mention of rain until
Tuesday as medium range models are in modest agreement in bringing
the filling upper low close enough to the coast to spread rain well
inland Tuesday night. Am somewhat skeptical this will fully come to
fruition as the models indicate as we have seen this time and again
this winter at that time range. This one is a little different,
however, as the eastern half of the Continental U.S. Appears to be losing its
semi-persistent trough which could open the door as advertised.
Another factor is the timing of the Alaska trough. If it's much
faster or slower at swinging across the north then the timing will
be off and it may not gather the cutoff low and leave it well
offshore. /Jbonk
&&

Aviation...VFR continues this afternoon under dry and stable
northerly flow aloft with variable thin high clouds. Winds
continue to ease and will remain light tonight with weak surface
pressure gradient. With light winds and clear skies...expect
efficient radiational cooling again tonight and shallow patchy fog
will likely develop again late tonight. Best chances for fog
development are around kkls and the south Willamette Valley
including keug.

Kpdx and approaches...VFR continues through the period under north-northwest
flow aloft.

&&

Marine...high pressure firmly in control over the region today
and will remain through much of the week. Surface pressure
gradient remains fairly weak which will keep light northerly
winds at or below 15 knots for the next several days. The one
exception would be a few stronger gusts in the southern waters as
a weak thermal trough may build up the coast into southern or
central Oregon. Some models suggest a southerly wind reversal may
reach the central Oregon waters by late in the week. Seas
remain 4 to 6 feet and fairly short period. Seas will hold around 5
feet through the weekend...with the swell increasingly comprised of
numerous small swells at varying direction and period. Cullen

&&

Pqr watches/warnings/advisories...
or...none.
Washington...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. The area is
commonly referred to as the forecast area.

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