Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Portland or
957 am PDT Wednesday Jul 30 2014
Synopsis...an upper level ridge remains anchored over the Great
Basin and northern rockies while upper level low pressure lingers
off the West Coast of Canada leaving the Pacific northwest under
south to southwesterly flow aloft. Surface low pressure east of the
Cascades brings weak onshore flow...enough to affect the immediate
coastal valleys while the interior remains very warm. Weak
disturbances lifting north from California will bring a chance for
thunderstorms at times to the Cascades...especially Lane County.
Little change expected in this overall pattern the next several days.
Update...very little in the way of change to the forecast package
this morning. Today still appears to be a relatively down day in
terms of Cascade convection...at least compared to yesterday and what
is expected for Thursday/Thursday night. Model soundings show the batch of
moisture showing up on water vapor off the Oregon coast is mainly
above 400 mb...so expect mainly cirrus from that as it moves through
today. More significant players appear to be the upper trough
developing off the California coast near 140w and the remnant
moisture from Post-tropical cyclone Hernan beginning to move
northward into California. These two players will likely team up for
greater coverage of Cascade thunderstorms Thursday...and possibly a bit further
west than yesterday. However flow aloft never really turns east of
south...so at this point it appears doubtful this convection would
affect the Willamette Valley. Temperatures remain 5-10 degree f above normal
inland with valley highs in the upper 80s to middle 90s. Coast will
remain seasonably cool with temperatures in the 60s and lower 70s on weak
onshore flow and areas of low clouds and fog the next few days.
/prev disc issued 305 am PDT Wednesday Jul 30 2014/
Short term...through Thursday night...the upper ridge axis extends
from Idaho to Arizona with an upper low near the b.C. Coast. The
pattern changes little during the short term period. As a result
very warm weather will continue through with inland high temperatures
running about 5 to 10 degrees above normal. While coastal
areas remain near normal with the strong influence of the marine
Thunderstorm threat for the Oregon Cascades will vary over the next
few days as subtle changes in instability and moisture occur over.
Thunderstorm activity will be lower today as moisture is lacking and
instability below 500 mb is low. The Lane County Cascades appear to
have the best chances for convection today and this evening as vorticity
maximum clips the area. Better chances for Thursday/Thursday night as the remnant
moisture of Post-tropical cyclone Hernan lift north and possibly
combine with one of the weak shortwaves offshore of the California
coast. NAM vertical moisture has been more inline with observations
so will lean toward its high moisture values. The high Oregon
Cascades are the most likely area to get the thunderstorms. However
it is not out of the question areas further west could be affected if
the shortwave becomes strong enough to pull in some southeast flow aloft.
For now...just increased probability of precipitation a bit along the crest of the Oregon
Cascades Thursday/Thursday night. Similar threat for thunderstorms for the
Oregon Cascades return Friday afternoon. Weagle/mh
Long term...Friday night through Tuesday. Our area is on the west
side of an upper-level ridge over the western Continental U.S.. several
shortwaves are forecast to move up this ridge and bring chances for
thunderstorms along the Cascade crest. This ridge will remain
stationary through Tuesday. Uncertainty remains for timing and
location of thunderstorms...but looks like best chance will stay
along the Cascade crest. The stationary nature of this weather
pattern will keep high and low temperatures fairly persistent until
this upper-level ridge either breaks down or moves off to the east.
Aviation...IFR stratus along the coast and MVFR stratus around
the Portland metropolitan area this morning will generally give way to
VFR conditions by late morning. High pressure and weak onshore
flow will generally keep conditions VFR through 18z Thursday.
There is a chance IFR conditions will redevelop along the coast
towards 09z to 12z Thursday...but confidence in this scenario
panning out is low at this point.
Kpdx and approaches...MVFR conditions will give way to VFR
conditions between 17z and 18z this morning. VFR conditions should
then prevail through 18z Thursday. /Neuman
Marine...little change to the overall pattern through the
upcoming weekend as high pressure lingers over the northeast
Pacific and thermal low pressure remains over northern California.
This will produce a fairly typical summertime pattern of gusty
northerly winds at times. The gustiest winds will be primarily
centered over the central Oregon waters for the next couple of
days...before the ridge axis shifts slightly further south...and
perhaps keeps small craft winds from materializing across all of
our waters towards the weekend.
Continued the combined seas wording in the coastal waters forecast
due to the waves being primarily a mixture of wind wave and fresh
swell. In general...expect seas to be fairly choppy with heights
fluctuating between 4 and 6 feet through the weekend. /Neuman
Pz...Small Craft Advisory for winds until 11 PM PDT this evening for
coastal waters from Cape Shoalwater Washington to Cascade Head or
out 10 nm-coastal waters from Cascade Head or to Florence or
out 60 nm.
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.