Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Portland or
914 am PDT Tuesday Jul 22 2014
Synopsis...a low pressure system will bring an increasing threat
of showers to the area Tuesday and especially Wednesday. A few
thunderstorms will be possible over the Cascades mainly late this
afternoon and for this evening. Wednesday will likely be the coolest
day of the week...however...thunder will be possible with any of the
heavier showers as the upper low crosses overhead. Drier and more
Summer-like temperatures are expected to arrive late in the work week
and continue into the weekend.
Update...where to start...water vapor satellite imagery shows an
elongated longwave trough over the eastern Pacific with a number of
features playing into our current and anticipated weather over the
next 48 hours. First...a shortwave trough digging southward near 135w
and 35n has allow a middle level baroclinic zone to shift northward
overnight across the area. Frontogenesis along this boundary
overnight helped produce between a quarter and a half inch of rain
for the northeastern part of the Willamette Valley...and the South
Washington Cascades. This area of precipitation will shift northeast
of our forecast area in the next couple of hours.
Visible satellite imagery this morning reveals plenty of middle level
clouds once again streaming northeastward into southwest Oregon.
Precipitation generally remains spotty and light in nature underneath
this band with Grants Pass...Roseburg and Eugene all tracing in the
last couple of hours. Anyway...this band of moisture is centered
along another middle level baroclinic zone that is forecast to shift
northward as today progresses. Hit or miss...more nuisance type
showers are anticipated to spread northward across the forecast area
today...but no large areas of organized precipitation are expected to
develop underneath this band.
As this area of clouds shifts northward...our southern Cascade zones
will see increasing sunshine and begin to destabilize. With 40-45 kts
of bulk shear and relatively fat cape...more organized
supercellular type storms capable of producing large hail and
damaging winds will be possible. However...and this is a big
however...we are still concerned that this surface based convection
will remain capped to the west of the crest this afternoon and
thunderstorms will not get going until shifting east of the
Cascades. Further a distinct vorticity maximum...apparent in water vapor
imagery...currently lifting into the Bay area is forecast to begin
lifting more northeastward than northward today. This would end up
placing the best middle level support for thunderstorms just east of
our forecast area late this afternoon and evening. Plus...the south-southwest
to SW flow at 700mb will push any developing convection east of the
crest quickly. Nonetheless...there is a decent chance towards the
evening hours that convection developing in eastern Douglas and
Klamath counties will become more elevated and clip eastern Lane and
Linn counties when the middle level flow is most southerly. As a
result...continued the scattered wording of thunder in this region
with at least a mention of gusty winds and small hail in the grids.
Finally...a larger and more organized shortwave trough currently due
west of Oregon near 135 west will begin to elongate and shift
northeastward across our northern zones Wednesday. Low to middle level
frontogenesis ahead of this system should produce a north to south
oriented band of steady rain that will shift eastward across the
area Wednesday morning into early afternoon. A quarter to half inch
of rain over a good portion of the forecast area is not out of the
question...especially north of Salem. Clearing behind this feature
should allow the atmosphere to destabilize in the middle to late
afternoon and produce scattered showers with isolated very short
lived thunderstorms...more reminiscent of Spring.
Otherwise...the drying trend Wednesday night into Thursday looks on
Long term...no changes...previous discussion follows. An upper-
level ridge is forecast to build back in over the Pacific northwest
bringing high pressure over our area. Southwesterly winds and
generally sunny skies will cause temperatures to increase by early
this weekend. Temperatures are forecast to get back into the middle 80s
by Friday and possibly reach the low 90s in some areas inland as
early as Saturday. -McCoy
Aviation...a band of rain continues to slide north across the
region bringing a mix of MVFR and VFR conditions to the Portland
metropolitan area terminals.Another line of rain is moving into the S
Willamette Valley and Cascades and will slide north today. Temporary
reductions to MVFR...or possibly high end IFR...possible at times
in heavier rain. Otherwise...expect overcast skies with ceilings around
4000 feet. Higher terrain and mountains obscured today. Another
round of rain and lowering ceilings expected tonight after 06z at the
coast...and around 10z-12z across the interior...with MVFR
conditions likely at this time.
Kpdx and approaches...line of rain should clear the terminal by
17z with ceilings gradually lifting to around 4000-5000 feet by 18z.
Expect VFR conditions to generally continue through 12z...though
heavier rain at times may bring temporary reductions to MVFR.
Cannot rule out some thunder development over eastern approaches this
afternoon/evening but not expected at terminal at this time. Cullen
Marine...relatively quiet conditions over the waters continue
today as the surface gradient remains weak in between
systems...keeping winds 15 knots or less. Seas remain 3-4 feet today
with a continued small west-northwest swell.
A low pressure system will pass through the waters late Tuesday into Wednesday.
Models have trended to indicate the potential for marginal small
craft winds on Wednesday with this...but confidence is low at this time
due to uncertainty in the track and strength of the low. Later
this week...high pressure redevelops over NE Pacific as thermal low pressure
builds over north California. This will bring a return to gusty north
winds and steep wind driven seas...particularly in the late
afternoon and evening hours...Friday through the weekend. Cullen
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.
Commonly referred to as the forecast area.