Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Portland or
237 am PDT Thursday Jul 24 2014
Synopsis...an upper level low over that was over western Washington early
Thursday morning will move quickly east today. After the low departs...an
upper level ridge will slowly build over the western states...
resulting in a return to dry weather with warming temperatures through the
rest of the week and into early next week.
Short term...water vapor pictures showed an upper level low over
western Washington early this morning with a vorticity maximum wrapping around the
base of the low pushing into northwest Oregon. Most of the remaining rain
showers were ahead of the vorticity lobe over northwest Oregon and SW Washington. There
is some potential for showers lingering today in the wrap around
moisture on the backside of the low today...but overall expect
showers to decrease quickly this morning as the low accelerates
eastward. Temperatures again will be below seasonal normals today...with
models generally showing modest warming at 800 mb to around 8 degree c by
A more climatologically normal pattern returns tonight and in
general remains through Sat as high pressure offshore turns the low
level flow back to the northwest...and the upper level flow GOES from west
tonight to SW by Sat with slowly building heights in response to a
slowly strengthening upper ridge off to the east. Marine clouds will
tend towards a pattern of retreating to the coast in the afternoons
Friday and Sat...and locally pushing inland especially up the lower Columbia
during the late night and morning hours. The only exception to the
dry weather depicted by the models is some light quantitative precipitation forecast seen in NAM and
to a lesser extent the GFS late tonight and early Friday. This appears
to come from some weak ooist isentropic lift depicted along the 290k
isentrope in the marine layer of those models 12z Friday morning.
Although weak looking...since both NAM and GFS indicate this...will
add chance for some drizzle but with just 10 percent probability of precipitation for the
far north part of the coast Friday morning. With rising heights aloft and
shallower marine influence...will see daytime temperatures climb above
normal by Sat.
Long term...no changes. Previous discussion follows...high pressure
will continue to build over the Pacific northwest ahead of a
shortwave trough...forecast to come onshore sometime late Sunday
night or Monday morning. There is still uncertainty on the time this
system will move into our area and the location of any associated
precipitation.The wave is unresolved in the mean 500-mb height
field from gefs and naefs...but the GFS and European model (ecmwf) both have it moving
through Sunday night or early Monday morning. Behind this
system...the large upper-level ridge is forecast to continue to build
in amplitude and westward and will become centered over The Rockies
by mid-week. Southwesterly upper-level flow and clear skies inland
will keep temperatures fairly warm over our area Tuesday and
Aviation...VFR across the region...but MVFR ceilings along the coast
with pockets of MVFR inland north of a kmmv to kpdx line...though
worst conditions over inland SW Washington. Several bands of
showers rotating inland north of a konp to ksle line. These showers
will decrease after 13z or 14z. By 19z...will see VFR conditions
with increasing breaks between clouds and only a few showers.
Should see widespread clearing after 03z as air mass stabilizes.
Kpdx and approaches...VFR conditions today...with showers
through 15z. Ceilings will occasionally lower to 2500 feet during that
time. Clouds breaking up significantly after 20z. Rockey.
Marine...low pressure continues to work inland over West Washington.
This maintaining gusty S to SW winds on the coastal waters north of
Cascade Head...with gusts 25 knots through 9 am. As the low moves
further inland...winds will ease and turn more westerly later this
am into this afternoon.
Seas generally 7 to 9 feet on the waters north of Cascade Head this
am...with seas 5 to 7 feet to the S. Periods still short...generally
around 8 second. So will maintain Small Craft Advisory for steep
choppy seas on waters between Cape Shoalwater and Cascade Head
until 9 am. Later today and tonight...seas will subside...with 4
to 5 feet by late evening on most waters.
High pressure will then build over NE Pacific tonight and Friday...with
thermal low pressure building over northwest calif. This will bring a more
typical summertime pattern of gusty northerly winds...and
potentially steep seas...especially S of Newport. Rockey.
Pz...Small Craft Advisory for winds this morning on all
coastal waters from Cape Shoalwater to Cascade Head.
Small Craft Advisory for hazardous seas until 9 am today
on coastal waters from Cape Shoalwater to Cascade Head.
Small Craft Advisory for rough Columbia bar conditions
until 6 am today.
More weather information online at...http://weather.Gov/Portland
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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.