Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Portland or
842 PM PDT Monday Mar 10 2014
Synopsis...showers will quickly decrease this evening as high
pressure begins building into the region. Patchy valley fog and
frost seem likely across many of the normally colder valley
locations. Mild and dry weather will be the norm Tuesday through
Thursday...before the next major threat of rain appears on tap for
Friday. High pressure looks to bring at least another day...possibly
two...of dry weather over the weekend.
Evening update...overall forecast appears to be on track with a
general trend of decreasing clouds and showers across the district.
Showers are becoming confined to the mountains...mostly the
Cascades...where the air mass aloft remains cooler and there remains
a little orographic lift. As mentioned in the previous discussion...
warmer and drier air aloft continues to push onshore...capping any
shower activity that tries to get going. Also with the loss of solar
heating...the air mass will become increasingly stable as tonight
GOES on. Expect any lingering shower activity to end by sunrise.
Main changes to the forecast package were to tweak the sky cover a
bit...adding a few lingering clouds to the Cascades Tuesday morning.
Any valley low clouds and fog which develop due to a strengthening
inversion tonight will be stubborn in the south Willamette Valley...
so due to the expected cloud cover we took forecast temperatures down a few
degrees for the Eugene vicinity Tuesday. Will need to watch the
extent of fog/low clouds Tuesday night/Wednesday...with saturated ground
and a strong inversion...our current forecast may be slightly too
optimistic for warm sunshine in the Willamette Valley. However
easterly flow will be increasing...which is what we are banking on to
inhibit widespread valley fog/low clouds Tuesday night/Wednesday which would
tend to hold temperatures down in the valleys Wednesday. Weagle
Short term...Doppler radar this afternoon shows numerous showers
streaming east southeastward across the Pacific northwest. A few
reports of small hail are trickling in...but cloud tops are
generally remaining just a bit too low for thunder. Although...as I
was typing this...a former employee reported thunder near Eagle
Creek and vaisala lightning data verified the report. Laps data
suggests we are working with 300 to 400 j/kg of cape with surface
temperatures in the low 50s. We will leave the mention of thunder in
the forecast through 00z...but will continue with the idea that the
threat will diminish quickly after that point.
There is not a whole lot of veering in the low level wind field
today...but due to the relatively strong instability in the lower
levels...we will continue to monitor the radar for cold core
While plenty of snow showers remain across the Cascades...and will
likely continue into the early evening hours...Road surfaces are
becoming primarily wet. With additional accumulations likely to be
another inch or two at most...primarily above pass level...will drop
the Winter Weather Advisory as the impacts seem to be diminishing.
Based on the sudden drop off in shower activity just off the
coast...it appears the stable air aloft...600mb and above...is on
track to arrive late this afternoon and evening. Models generally
agree cloud cover will dissipate this evening and skies will
clear...especially away from the Cascades. Based on current
dewpoints...and surface high pressure over the region
tonight...areas of valley fog should develop tonight.
Strengthening thermal low pressure along the southern Oregon coast
late tonight into Tuesday should create enough of a pressure gradient
that northerly and easterly winds will mix out any fog and low clouds
by Tuesday afternoon...and a pleasant day appears on tap.
Thermal low pressure will shift over the Willamette Valley
Wednesday...likely bringing temperatures into the warmest day of the
year category...or very close...for many locations. Thermal low
pressure should then shift east of the Cascades Thursday. This
should result in slightly cooler temperatures on Thursday...but
temperatures will likely remain several degrees above normal. Models
continue to advertise a dying front will try to push into the region
late Thursday...but moisture appears very limited so the forecast
was trended towards a bit more sunshine...even for southwest
Long term...Thursday night through Monday...the ridge of high
pressure will gradually be flattened by a frontal system moving into
British Columbia early Friday...bringing the next threat for
precipitation to the region. Both the European model (ecmwf) and GFS show building
upper level heights for Saturday...bringing a day of dry weather
before another system clips the forecast area with showers by
Sunday. This pattern of intermittent showery and dry days looks to
continue into early next week as seasonal temperatures prevail. /27
Aviation...showers generally confined to the Cascades where
terrain likely obscured at times through 12z Tuesday. For the
remainder of the area VFR conditions are prevailing as cloud
cover gradually decreases. Clearing skies and light winds should
allow for areas of fog to develop between 09z and 12z. Many spots
will likely see IFR conditions or lower. Increasing easterly low
level flow should quickly clear away any fog between 15z and 20z
with VFR conditions through the rest of the day.
Kpdx and approaches...clearing skies will likely allow IFR or
lower fog to develop near the terminal between 09z and 12z.
Increasing easterly flow will improve conditions between 15z and
18z and VFR will remain through the rest of the day. Pyle/mh
Marine...a relatively quiet period over the waters for the next
several days. High pressure builds over the waters tonight and inland
Tuesday. Forecast models indicate that a thermally-induced area of low
pressure will form along the S Oregon coast early Tuesday and then expand
north Tuesday afternoon. This will create a Summer like northerly flow
pattern. The latest forecast model winds indicate that a few gusts 20
to 25 knots are possible...particularly S of Newport Tuesday afternoon
and evening. The thermal trough shifts inland Wednesday...but high pressure
Expect seas will diminish a bit more later today...reaching near
7 feet by this evening. An incoming west swell around 10 to 12 feet
still looks on tap for Wednesday with seas quieting down once
again late in the week...until the next system moves into the
waters on Sat. A more significant westerly swell may bring seas
back into the middle teens over the weekend. Pyle
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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.