Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Portland
234 PM PDT Sat Apr 25 2015
Synopsis...the upper low will continue weakening and move east by
early this evening as building high pressure brings a swift end to
any lingering showers. A warm front will mainly clip the northern
coastal areas with some light rain Sunday. Monday will be drier and
much warmer as a strong upper level ridge builds in from the south.
The next cold front looks to bring rain early Tuesday followed by
afternoon showers. More showers are possible for the rest of the
week as another upper level trough settles near the Pacific
Short term...tonight through Tuesday...convection has been less
than stellar in strength thus far today. Coverage has met
expectations though with some showers over the South Valley
producing a quick tenth or two of rain over the last couple hours.
Am not ready to completely give up on a thunder threat for the next
few hours but have certainly lost most of the excitement built over
the last couple days. Water vapor imagery is showing the leading
upper edge of the encroaching warm front about to cross the mouth of
the Columbia. This is further indication that middle level subsidence
is becoming dominant. As such...still expect showers will quickly
diminish this afternoon. For added confidence...the Storm Prediction Center storm scale
ensemble of opportunity members back this notion up and generally
cease all showers even over the Cascades by about 8 PM.
The clouds associated with the approaching warm front appear to stay
offshore enough to give decent clearing for a large part of the late
evening and overnight hours. Therefore added some patchy fog across
the rest of the lower elevations and including the coast. Forecasted
temperatures for tonight were already close to the dew points so
didn't see a need to lower the lows.
Sunday will stay dry for the bulk of the County Warning Area as models continue to
shunt the approaching warm front ever so gradually further north.
Still maintained around a 30 percent pop for the Coast Range during the
afternoon and about a 60 percent for the north coast Sunday evening but
feel like this is a case where its better to say there is a 70 percent
and 40 percent chance respectively it will not rain tomorrow. Would not
be surprised if following shifts lower probability of precipitation yet further based on
later model runs. Clearing skies will again allow for some fog
development late overnight Sunday and into early Monday daylight
The upper ridge over the region will be fairly transitory with
several models in rough agreement now showing a trough pushing down
the ridge with the attendant cold front bringing rain to the coast
by 12z/5am Tuesday. The lack of significant amplitude in the
downstream h500 pattern implies there is not much cold air to our
east to slow the movement of the ridge. The parent low center of
this trough will remain entrenched in the Gulf of Alaska and quickly
swing the cold front and trough axis across the area. Snow levels
ahead of the front on Tuesday will stay above 8000 feet and only
drop toward 6000 feet or so just ahead of the front. They do drop
significantly to around 3500 feet behind the front but there will be
little threat for significant Cascade snow accumulation from the
overall drier air mass. /Jbonk
Long term...Tuesday night through Saturday...some showers may
linger early Tuesday night behind the cold front but these will be
confined mostly to the Cascades before coming to an end by early
Wednesday. The weather pattern Wednesday through Friday will be
marked by a mean ridge over the central Continental U.S. With a mean trough
moving into the west. In general...this will lead to a more
progressive pattern but significant variation among operational
forecast models...and their ensemble members...keeps the confidence
in the particulars of the forecast below normal for the second half
of next week. Made some slight changes to probability of precipitation...drying out the
coast and valleys for Wednesday but maintaining chance probability of precipitation Thursday
and Friday. Slightly increased probability of precipitation Thursday night as the gefs mean
and European model (ecmwf) quantitative precipitation forecast forecasts seem to support enhanced shower
activity...though perhaps for different reasons /timing of shortwave
vs. Frontal passage/. Nonetheless...the timing of shortwaves and
when the upper trough shifts inland still create some considerable
uncertainty in the extended forecast period. Temperatures generally
trend near or slightly below seasonal normals through the second
half of next week. Cullen
Aviation...cool unsettled pattern continues. Showers decreasing
this evening...then should be dry as area will be between systems.
With clearing skies this evening...and cool air mass with wet
ground...will see patchy or areas of fog and IFR stratus form
later tonight. Next front offshore will arrive later sun...with
increasing middle and high clouds late tonight into sun am.
Kpdx and approaches...scattered to broken VFR clouds into early
this evening...along with an isolated shower or two. Skies will
clear this evening...with areas of fog/low stratus forming around
the Ops area after 08z...especially to west and north of kpdx. Will have
occasional IFR clouds into sun am...but not confident yet on how
long these persist due to increasing middle/high clouds very late
tonight and sun am. Rockey.
Marine...not a lot of change. West to northwest winds under 15 knots with
seas 5 to 7 feet tonight.
A warm front is then expected to move north across the waters sun.
Will have period of gusty S winds Sun afternoon into the evening
as the warm front lifts north and cold front approaches. Not all that
impressive...but enough to pop gusts to 25 knots. Will issue Small
Craft Advisory on all waters for Sun afternoon/evening. Seas will
push back up a bit...but remain below 10 feet.
Another break is expected on Monday before the next front arrives Monday
night or Tuesday. Front does not currently look all that impressive
in the forecast models...but another round of low end Small Craft
Advisory winds is currently being advertised. Forecast confidence
decreases for the latter half of next week...but no major weather
systems are currently expected. Rockpyle
Pz...Small Craft Advisory for winds on all waters for Sun afternoon
through sun evening.
Interact with US via social media
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.