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fxus66 kpqr 051035 
afdpqr

Area forecast discussion
National Weather Service Portland or
233 am PST Thu Dec 5 2019

Synopsis...an upper level ridge will reside over the area Thursday
and then shift east Thursday night. A weakening frontal system moves
inland Friday afternoon or evening, followed by an upper level trough
Friday night and Saturday. High pressure returns early next week.
Another weak system may impact the area late next Tuesday or early
Wednesday.

&&

Short term...Thursday through Saturday...the large-scale weather
pattern early this morning is comprised of a deepening upper level
low pressure area near 40n 140w and a transitory 500 mb ridge along
the pac northwest coastline. Main weather feature tonight is the valley fog.
Conditions seem to have improved a little since 06z, with localized
dense fog noted at khio and between k77s and k61s. Strong inversions
are expected to continue through Friday, which will maintain air
quality issues for the interior valleys.

The kttd-kdls gradient strengthens this afternoon through Thursday
night and should peak around -5 to -6 mb. This will certainly help
air quality in the north Willamette Valley, but will not have much
impact for the central and southern sections or the lower Columbia in
SW Washington. The upper low out near 40n 140w early this morning
drifts east through Friday night, moving inside 130w by 12z Sat. A
weakening occluded front reaches the coast around 18z Fri. This
feature will become parallel to the mid and upper level flow and
likely slow down as it nears the coast. Inland areas east of the
Oregon Coast Range and also the SW Washington interior may remain dry
through much of Friday.

The upper low ejects east to northeast and weakens as it moves inland
Sat. Latest operational GFS, NAM and European model (ecmwf) show a 1000 mb surface low
just off the central or south Oregon coast by 21z Sat. The
operational GFS has the low sitting just west of konp, while the NAM
and European model (ecmwf) have it closer to the Oregon and California border. The
heaviest quantitative precipitation forecast Fri night through Sat will be directed at SW Oregon and
northern California. Opted to include a slight chance of thunder for
the far south part of the central Oregon coastal waters and also the
coastline roughly south of Heceta Head Sat morning as the nbm hints
at this and to mesh with kmfr. Weishaar

Long term...Saturday night through Thursday... an elongated
northeast-to-southwest oriented 500 mb trough will be over the region
Sat night and then shifts to the southeast sun. Models in good
agreement depicting some lingering precip over the area through sun,
primarily in the morning. A sharply amplified 500 mb ridge axis
develops near 135w sun and this will become the dominant feature for
the first part of next week. The European model (ecmwf) 500 mb ensembles and 500 mb
total mean cluster analysis provide moderate to good forecast
confidence in this ridging pattern. Would expect strong valley
inversions to return with minimal (2 mb or less) offshore low-level
gradient through The Gorge. Would also anticipate stubborn fog and
low clouds to become more prevalent in the central and south
Willamette Valley. Models show the upper ridge migrating east late
Tue and Tue night as another weak system approaches the coast. The
operational GFS and European model (ecmwf) differ on timing, with the European model (ecmwf) a little
faster. There is lower confidence in the Tue night-Thu time frame, as
shown by the European model (ecmwf) ensembles and relative measure of predictability
(rmop). Wpc favors a general zonal pattern 12z Wed. In any event, it
appears the pattern of occasional weak weather systems will continue.
Do not see any typical December rain events and additional mountain
snow will be minimal. Weishaar

&&

Aviation...a mix of flight conditions ranging from LIFR in fog
and low clouds, to VFR continued early thursay morning. With
multiple cloud layers, it appears likely that areas of MVFR to IFR
ceilings and visibilities will persist through 22z, most likely in
inland valleys away from the Columbia River gorge that include khio,
ksle and keug. After 22z expect mainly VFR conditions except in the
central and south Willamette Valley including ksle and keug however
will retain a good chance for seeing lower conditions with areas of
fog, mainly after 02z.

Kpdx and approaches...mainly VFR conditions expected next 24 hours.
There is a small chance for seeing some fog or low clouds developing
for a few hours this morning between 14z and 20z.

&&

Marine...a low pressure system approaching the south Oregon and
north California coastal waters from the west will bring
strengthening south to southeast winds today through Friday. The
strongest winds will tend to be over the outer waters beyond 20 nm,
with gusts likely reaching the range of 20 to 30 kt Friday. In
response to the southerly winds, a mix of wind waves and fresh swell
will create steep short period seas building as high as 10 to 14
feet over the outer waters Friday.

Friday night through Saturday night will see winds and seas slowly
decreasing again as the low heads towards the north California
coast. Winds are then expected to remain under 20 kt and seas under
8 ft, until another frontal system approaches the region around late
Tuesday.

&&

Pqr watches/warnings/advisories...
or...air stagnation advisory until 4 PM PST Friday for Cascade
foothills in Lane County-central Willamette Valley-greater
Portland Metro area-lower Columbia-northern Oregon Cascade
foothills-south Willamette Valley.

Dense fog advisory until noon PST today for south Willamette
Valley.

Washington...air stagnation advisory until 4 PM PST Friday for Greater
Vancouver area-I-5 corridor in Cowlitz County-South
Washington Cascade foothills.

Pz...none.
&&

$$



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