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Area forecast discussion 
National Weather Service Portland or
202 PM PDT Tuesday Jul 22 2014

Synopsis...a low pressure system will bring an increasing threat
of rain showers and even a few thunderstorms to the area tonight and
especially Wednesday. After a cloudy and wet start to
Wednesday...expect a mix of sunshine and showers in the afternoon.
Drier and more Summer-like temperatures are expected to arrive late
Thursday and Friday...before temperatures push 90f late in the
weekend.

&&

Short term...this evening...a vorticity maximum...apparent in water vapor
satellite imagery...near the Bay area will lift northeastward this
afternoon and evening into south central Oregon. Visible satellite
imagery shows fairly extensive cloud cover over the eastern half of
the forecast area with a few breaks creeping into the Cascades. A
destabilizing lower atmosphere and lift ahead of the aforementioned
vorticity maximum should allow thunderstorms to develop across south central
Oregon this afternoon. In fact...convection is beginning to fire
east of the central Oregon Cascades near Bend. With additional
heating this afternoon...expect some of this convection to begin
developing just to the west of the crest...but there still remains
uncertainty in how much...if any...will develop. The latest rap
guidance suggests a few thunderstorms will be possible late this
afternoon over the Lane and Linn County Cascades...but if the
current middle level clouds linger too much longer this
afternoon...surface based convection will likely stay entirely east
of our forecast area. The latest hrrr supports the latter idea. It
should be noted that if any storms do break the cap on our side of
the crest...40-45 knots of 0-6km bulk shear and cape values between 500-
750 j/kg will aid storm organization. Frequent lightning...and
possibly marginally severe hail and damaging winds would be the
primary threats of concern if storms were to materialize.
Again...the most likely area to experience such storms is primarily
near the Cascade crest...south of Mount Jefferson.

For tonight...the large shortwave trough currently near 135w...due
west of Oregon...will elongate further tonight...turning the middle
level flow over northwest Oregon slightly more southerly. The NAM
brings a piece of the vorticity maximum/pv anomaly near 130w and 35n across
the area late tonight. With negative thetae lapse rates between
700-500mb advecting into the region...and region being in the right
entrance/left exit region of two upper level jets...we may see
showers and even a few elevated thunderstorms spread northward across
western Oregon late tonight. However...confidence is low so the
thunder mention was limited to a slight chance. The latest hrrr
keeps the bulk of the activity right along the Cascade crest and
points east...but given the pattern will keep the mention in the
forecast.

The previously mentioned elongating shortwave trough near 135w will
begin to push east-northeastward across the area Wednesday. Strong
middle level frontogenesis should lead to a fairly organized north-south
oriented band of rain moving eastward into the Oregon coast Wednesday
morning. Some modest middle level instability coupled with the dynamics
may lead to a few lightning strikes along the front...particularly
over the coastal waters and near the coast late tonight into
Wednesday morning.

The band of rain should push eastward across the area Wednesday
morning. Subsidence behind the middle level front may lead to a rather
abrupt end to the rain/clouds early to middle Wednesday afternoon.
However...any clearing will not last long...as the lower atmosphere
should destabilize quickly...and a Spring-like pattern of scattered
showers and isolated thunderstorms will likely ensue late Wednesday
afternoon and early evening. The greatest instability will be over
the northern half of the forecast area so the mention of thunder was
increased to chance wording.

With a surface low pressure lingering just off the southwest
Washington coast Wednesday there may be a tendency for surface winds
in the Willamette Valley and along the lower Columbia River to have a
bit more of a S to south-southeast component than otherwise expected. Combined
with west-southwest winds of 25 knots at 900mb and 40kt at 500mb...a rapidly
strengthening updraft could rotate briefly. However...confidence is
very low in this actually occurring somewhere within the County Warning Area...but
it will still be Worth monitoring. The more expected hazards...
gusty winds and small hail were added to the thunder wording in the
grids.

On the fire front...most locations across the forecast area should
see wetting rains of a quarter to half inch with a few locations
likely pushing an inch of rain...particularly across southwest
Washington. It should be noted wetting rains are less likely towards
Lane County...but certainly appear possible Wednesday.

Expect a drying trend Wednesday night into Thursday before more
seasonable weather returns Friday. /Neuman



Long term...high pressure will build westward across the Continental U.S.
Amplifying an upper-level ridge that will set up over the
northwestern U.S. This will increase temperatures back to above
normal over the weekend. There is a possible disturbance that could
move through as early as Sunday night...but much uncertainty exists
in the timing of this system and location. -McCoy



&&



Aviation...generally VFR this afternoon with mostly clear skies north
of a k6s2-kkls line. S of this line is also VFR but with scattered-broken
cloud deck generally above 6000 feet. The exceptions are a few local
MVFR pockets associated with showers...and along the immediate coast
where LIFR marine stratus has surged north from k6s2 to just north of konp.
Expect trends to persist until tonight when rain associated with
approaching low pressure system moves into the region starting
around 10z. Expect ceilings to lower after 12z...with MVFR ceilings around
2000 feet possible. Local pockets of IFR possible in heavier
rain...and along the coast. Higher terrain and mountains obscured
tonight through middle morning Wednesday.

Kpdx and approaches...expect VFR to continue through this evening
with scattered clouds around 3000 feet and an overcast deck above 5000
feet. Some light rain possible...but expect rain to increase after 12z
with ceilings lowering to low VFR to higher MVFR. Cannot rule out some
thunder development over eastern approaches this evening...or over much
of the Ops area tomorrow afternoon...but not enough confidence to
include in taf for now. Cullen



&&



Marine...relatively quiet conditions over the waters continue
tonight 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 tonight into
Wednesday. Models have trended to indicate the potential for marginal
small craft winds tomorrow afternoon...both S winds along the
coastal inner waters and also over much of the waters as the surface
low pushes across. Confidence remains somewhat low at this time
however so will let next shift evaluate next model cycle before
potentially raising the Small Craft Advisory. 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



&&



Pqr watches/warnings/advisories...
or...WA...none.
Pz...none.



&&



$$



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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.

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