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Area forecast discussion
National Weather Service Portland Oregon
839 PM PST Mon Jan 23 2017

Synopsis...low pressure is moving southward along the California
coast, with skies clearing across southwest Washington and northwest
Oregon this evening. High pressure will build into the pac northwest
tonight, bringing mainly clear skies and light winds. This will set
the stage for some areas of fog and perhaps some frosty roads later
tonight and early Tuesday. A weak disturbance is expected to clip the
pac northwest later Tue, bringing the possibility of a couple sprinkles or
brief showers...primarily in the higher terrain. A decaying front
will attempt to move onshore late Wed into Thu, bringing another
chance for some light rain. High pressure then strengthens over the
pac later Thu through the first half of the weekend. This will bring
dry weather and a low level inversion, with clouds and cooler temps
in the valley and sun and warmer temps in the mountains. Another weak
front will move in sun, bringing the next chance for rain.

&&

Short term...tonight through Thursday...the low pressure system
which looked quite impressive off the pac northwest coast this weekend is
now headed down the California coast. As of 8 PM, the low was
centered about 200 miles west of Eureka...and continues to fill as it
continues south along the California coast. High pressure is taking charge
across the pac nw; the resulting clearing and calm conditions will
likely allow some fog to form in the valleys tonight/Tue morning.
Another concern is the potential for some frosty/icy roads in spots
as temperatures fall just below freezing in many locations tonight.
Dewpoints started this evening well up into the 30s, indicating some
low-level moisture left behind. The ground also remains damp from
recent rains, so it is possible frost may try to form on some
roads/sidewalks by the Tue morning commute.

Still having a tough time believing that much in the way of shower
activity is going to occur later Tue, as a very weak upper level
disturbance presently over British Columbia clips by the northeastern portion of
our County Warning Area. This disturbance is currently very difficult to pick out,
but it appears it is shearing off from a somewhat larger shortwave
crossing the northwest territories. However, 00z GFS/NAM continue to
suggest some quantitative precipitation forecast is possible across our County Warning Area north of Salem Tue aftn
through Wed morning. Trimmed back pops a little, but kept the
occasional sprinkles/flurries wording in the forecast. Suspect the
better chance for measurable rain will be in the higher terrain,
particularly in the Cascades north of Mount Hood...where the best
cyclonic flow is. Some cloud cover should linger into Wed morning,
which could limit fog and hold low temps up a bit. Adjusted the
forecast slightly to reflect this. Otherwise, the inherited forecast
appears to be in good shape, and we only made minimal changes on this
shift.Weagle

Previous discussion from 245 PM Mon Jan 23 below...

The low pres that has been lingering offshore for the past few days
continues to drift southward today. Based on circulation on
satellite, the center now appears to be about 200 miles west of Port
Orford on the southern Oregon coast. The light precip over portions
of SW Washington and northwest Oregon this morning appears to be finished,
with clouds also decreasing as the low and associated southerly flow
move further away. Expect only a few high clouds around for the late
afternoon and evening. Temps are also mild today, with upper 40s and
low 50s over The Lowlands providing a pretty nice day.

A ridge of high pres will build toward the pac northwest tonight, which
combined with the clearing skies and light winds, will likely lead to
extensive low clouds and fog tonight for The Lowlands and the coast.
The low level temperature inversion won't be all that strong
tomorrow, so think the low clouds and fog will clear out by late
morning or early afternoon. The fcst models continue to show a weak
shortwave rotating through the fcst area from the north tomorrow,
with occasional light showers or sprinkles being wrung out starting
in the afternoon. Otherwise the main impact will be some increased
cloud cover and temps a little cooler than today.

Some light showers may linger into Tue night as the upper flow
remains cyclonic on the backside of the shortwave, with the best
chances over the Cascades. Then expect Wed will be mainly dry for
most of the day as the shortwave offshore ridge moves overhead. Temps
will be seasonal and think we may again deal with some lowland fog or
low stratus to start the day. The next front will come onshore late
Wed into Thu. This system can currently be seen on satellite imagery
over the Gulf of Alaska and is fairly well organized. However, the
fcst models indicate the front will decay considerably before it
moves onto the pac northwest coast. Think it will not be strong enough to
produce widespread precip over the region, especially east of the
Coast Range. Still there is enough quantitative precipitation forecast in the models to keep a low
end chance for most of the region late Wed and early Thu. Expect any
lingering showers and clouds to begin to clear out Thu afternoon as
the upper ridge begins to strengthen over the pac northwest. Pyle

Long term...Thursday night through Sunday...expect dry conditions
from Thu night through at least Sat as an upper level ridge
strengthens directly overhead. This will bring a fairly strong low
level temperature inversion. So expect plenty of low clouds and
cooler temps in the interior valleys, with warmer temps and sun in
the mountains. It is not a very cold airmass, but think we will see
highs top out in the upper 30s to low 40s for The Lowlands Fri and
Sat. The Coast Range and Cascade foothills will be in the upper 40s
to low 50s. Another weak front is modeled to arrive on sun, likely
bringing a period of light rain to the region and stirring up the low
level inversion somewhat. Pyle

&&

Aviation...VFR conditions across the forecast area as of 04z Tue.
Areas of IFR expected to develop in the interior valleys overnight
due to generally clear skies and minimal surface pressure
gradients. There is a good chance that local IFR conditions will
occur along the coast late tonight through about 17z Tue morning.
Model soundings for the valley suggest MVFR cigs to persist
through the afternoon due to a very weak disturbance moving
across the area Tue morning. This will result in light onshore
low-level flow. VFR conditions along the coast through Tue
evening, with the exception of any local morning IFR conditions
that do develop.

Pdx and approaches...VFR through at least 13z Tue. Conditions
likely to fall into IFR or low-end MVFR around 13z due to stratus
or freezing fog, especially in the Tualatin valley. MVFR cigs
expected late Tue morning through the afternoon. Weishaar

&&

Marine...near 1000 mb surface low near 40n 128w at 04z will
continue to dive south and east tonight. Wind speeds have
diminished below 15 kt and this will prevail through at least Wed
morning. Weakening system moving through the waters late Wed and
Wed night may result in 20-25 kt gusts, but feel strongest wind
gusts will end up below 20 kt. Beyond Wed night, no other
significant wind issues through the weekend.

Seas continue to subside this evening. Will maintain the Small
Craft Advisory for hazardous seas through 12z Tue. Northern waters
may fall below 10 ft a little sooner, but will let next shift
decide if the advisory can be dropped a little sooner. Seas lower
to around 6 to 7 feet Tue evening. Latest guidance shows another
series of larger and very long period swell reaching the waters
late Wed or Wed nights, peaking around 16 ft but with 18 to 20
second periods. Weishaar

&&

Pqr watches/warnings/advisories...
or...none.
Washington...none.
Pz...Small Craft Advisory for hazardous seas until 4 am PST Tuesday
for coastal waters from Cape Shoalwater Washington to Florence or
out 60 nm.

Small Craft Advisory for rough Columbia River bar until 4 am
PST Tuesday.

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

&&

$$

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