Scientific Forecaster Discussion
fxus66 kpqr 121818
Area forecast discussion
National Weather Service Portland or
1018 am PST Tue Dec 12 2017
Synopsis...a ridge of high pressure remains over the region much
of the week, although a weak disturbance will move across the top of
the ridge today. Otherwise, little change is expected to the pattern
until late in the week when a front will bring a return of onshore
flow and chances for rain beginning Thu night and Fri.
Short term...today through Thursday...only a few minor adjustments
to the sky cover and temperature forecast for today and tonight to
better match prevailing trends. Otherwise the forecast remains on
track and the previous short term discussion follows below.
With weak dynamics running into the strong ridge, and a lack of
deeper moisture, will keep forecast dry for the NW part of the
forecast area today. Some mid and high clouds however will likely
accompany the disturbance moving across the region today. Elsewhere
things remain pretty much the status quo, although the offshore
pressure gradients through The Gorge have weakened by 3 to 4
mb in the past 24 hours. The weakened offshore flow is expected to
continue through Wed, before picking up a little again Wed night and
Thu as the reinvigorated upper ridge shifts back onshore. The low
level subsidence inversion will keep the inland valleys cool with
patchy fog and low clouds in the wind protected areas, while warmer
temps, a bit modified from previous days, continue above the
inversion in the foothills.
Long term...Thursday night through Monday...no changes. Previous
discussion follows. Rain moves in late Thursday night or early
Friday. Models are still not converging on timing of this system.
The timing could be critical, with a chance for a short period of
freezing rain at onset in the central Columbia River gorge and
around Hood River. Amounts should be light if any, but could be
impactful to travel in this area for a few hours. For the rest of
southwest Washington and northwest Oregon, temperatures should be
above freezing when rain starts and rain amounts should be
relatively light. Snow levels will be falling through this event,
bringing snow down to around 3000-4000 ft by Saturday morning.
Precipitation should come to an end early Saturday, with a break in
the weather under a shortwave ridge of high pressure. Forecast gets
more uncertain later in the weekend going into early next week as
there has been some run-to-run differences in how far north the
ridge of high pressure will push the jet stream/storm track back up
on Sunday into Monday. Previous runs showed US staying wet through
the weekend into early next week, but today's model data suggests
the storm track shifting farther north. This keeps the southern half
of our area dry the rest of Saturday through early next week, with
precip only clipping far northern Oregon and southern Washington.
Right now have left a chance for rain across our southern counties,
but if this trend continues, will probably take pops out of these
areas. In any case, do not see any significantly impactful weather
through early next week. -McCoy
Aviation...no significant changes.Preivious forecast
follows...offshore under high pressure aloft will continue to
bring predominantly VFR conditions areawide. The main exception
to this will be across the southern half of the Willamette Valley
where fog will likely bring LIFR conditions to keug between
through 20z Tuesday and return again tonight.
Pdx and approaches...VFR prevailing next 24 hours. Weaker wind
over eastern approaches today, then increasing again tonight.
Marine...weak front dissipating over the coastal waters today
then offshore flow increases tonight. Winds remain below 20 kt
through much of the week. A front looks to move into the area on
Friday. Models do show the Friday front weakening, will continue
to monitor for potential Small Craft Advisory conditions.
Additional weak fronts occur during the weekend. However, model
solutions show the fronts primarily affecting the South
Washington and north Oregon waters.
A couple swells moving in from the southwest and west will
generally result in seas hovering between 6-8 ft for the rest of
the work week.
or...air stagnation advisory until 2 PM PST Thursday for central
Willamette Valley-lower Columbia-south Willamette Valley.
Washington...air stagnation advisory until 2 PM PST Thursday for Greater
Vancouver area-I-5 corridor in Cowlitz County.
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