Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Portland
841 am PDT Tuesday may 5 2015
Synopsis...a cold upper level low will result in unsettled
conditions through Wednesday. There is a risk of afternoon and
evening thunderstorms today and Wednesday. High pressure starts to
return Thursday as the upper low shifts further southeast. Dry and
seasonal conditions are expected Friday through the weekend.
Short term...krtx dual-pol Doppler radar this morning showing shower
activity focused over the Oregon Coast Range...S Washington and north Oregon
Cascades. Cascade web cams indicating snow levels down to around 5000
feet. Hoodoo cam...further to the south...showing wet snow at 4700
feet. New dusting of snow at Montana. Hood Meadows. 12z ksle freezing level
was at 4800 feet. Water vapor imagery indicates tkhe upper low centered
over Vancouver Island at 15z. A couple of short-waves rotating around
the low expected to move across the northern zones today.
Models show the upper low migrating east-southeast today...but not a lot of
southward progression. The main cold pool aloft will drift over
western Washington today...with the air mass across our County Warning Area gradually
destabilizing as this cool pool pushes inland. GFS Li forecast has the
best instability this afternoon generally north of a ktmk-ksle-Mt.
Jefferson line. Namm model soundings for kpdx and in north Oregon
Cascades show surface-based cape around 250 j/kg and wet-bulb zero
heights around 4300 feet. Unidirectional flow throughout the air
mass...with some speed shear between 850 and 650 mb. Rather low
web-bulb zero heights point to small hail threat with heavier
showers. Although snow levels will be at or near the Cascade
passes...do not anticipate any impacts on major roadways.
Models show good agreement in the main upper low digging south into
Oregon Wednesday...which would likely promote better instability due
to colder air aloft. 12z GFS has 500 mb temperatures around -28c Wednesday across
much of the County Warning Area Wednesday afternoon. With the strong may sunshine likely
warming surface temperatures to around 60 degree c in the Willamette Valley...
lifted indices expected to be around -1 to -2 degree c. As a result both
the NAM/GFS show Wednesday afternoon as the best cape day with
values 200-500 j/kg across most of our inland zones. Additionally the
overall setup is similar to past may events...such as 26th of 2012
when showers/thunderstorms developed in the S Washington Cascades then drifted
southward into the east Vancouver/Portland metropolitan due to northerly
flow aloft. Showers eventually diminish Wednesday night as the air mass
warms aloft with decreasing instability.
Models show the upper low gradually sinking south over the northern
sierras by 12z Thursday. Expect wrap-around flow to continue a threat of
showers over far southeast part of the forecast area. So will maintain some
minor threat of showers over the central Oregon Cascades and possibly
as far west as the south Willamette Valley. Otherwise upper level
ridging amplifies around 135w resulting in drier north-NE flow aloft over
rest of the area. Expect the coast to have the most sunshine Thursday.
Daytime temperatures will be a little below normal today and Wednesday...but
rebound to slightly above normal Thursday.Weishaar
Long term...Thursday night through Sunday. Showers will come to an
end over the central Oregon Cascades by late Thursday night as an
upper level ridge of high pressure begins to build over the Pacific
northwest. This ridge will keep dry conditions over the area along
with a return to mild daytime temperatures for Friday...and perhaps
Saturday as well. Model agreement has improved with the 12z
cycle...and thus with increased confidence have removed mention of
probability of precipitation for Sat and Sat night. On sun...the upper ridge axis will push
eastward as a baroclinic zone associated with a large upper low
approaching Vancouver Island will bring cooling temperatures and
increased clouds with a push of marine air and a return to
precipitation chances for later Sunday and into early next
week. While models vary in the path of this upper low...there is
good agreement that it will slowly meander over the region early
next week keeping precipitation chances around until a larger upper
low slides south from the Gulf of Alaska around Monday or Tuesday.
Aviation...expect mainly VFR conditions through 18z Wednesday as
an upper level low pressure brings scattered showers to the area.
An occasional MVFR flight restriction will be possible with any
heavier shower. There is a slight chance of a thunderstorm
impacting any one terminal...primarily between 21z Tuesday and
03z Wednesday. Although...the chances are slightly higher for the
more northern interior taf sites.
Kpdx and approaches...an upper level low pressure will generally
bring VFR conditions through 18z Wednesday. Scattered
showers...mainly during the afternoon and evening hours...will likely
impact the terminal with a chance of brief MVFR conditions possible.
There is a slight chance of a thunderstorm between 21z Tuesday and
03z Wednesday. /Neuman
Marine...an upper level trough will remain over the Pacific
northwest through Wednesday. Aside from an occasional gust to 25
knots from a heavier shower...winds will remain below advisory
criteria. A westerly swell currently bringing seas to around 10 feet
across the northern waters will subside gradually through the
High pressure will build over the northeast Pacific and thermal
low pressure over northern California will lead a Summer-like
northerly wind pattern for the second half of the work week.
Expect periods of gusty 25 to 30 knots winds...particularly south of
Tillamook. Models continue to hint there is a chance northerly
winds may increase and become more widespread on Sunday. There is
a chance seas could become steep and choppy Sunday or Monday if
the stronger northerly winds mentioned earlier materialize.
Otherwise...a weak system approaching the region early next week
does not appear likely to bring significant marine impacts. /Neuman
Pz...Small Craft Advisory for hazardous seas until 5 PM PDT this
afternoon for coastal waters from Cape Shoalwater Washington to Cascade
Head or out 60 nm.
Small Craft Advisory for rough Columbia River bar until 9 am
PDT this morning.
Small Craft Advisory for rough Columbia River bar from 5 PM
this afternoon to 8 PM PDT this evening.
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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.