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Area forecast discussion 
National Weather Service Portland
853 PM PDT Tuesday may 5 2015

Synopsis...a cold upper level low over Washington will drift south
over Oregon Wednesday...keeping our weather cool and unsettled.
Scattered showers will continue north tonight...then spread south
across most of the forecast area Wednesday. Enough instability will
likely exist for isolated thunderstorms Wednesday afternoon and
evening...some with small hail. High pressure starts to return
Thursday as the upper low continues southward into California and
Nevada. Dry and warmer conditions are expected Friday through the
weekend as the upper level ridge builds across the Pacific northwest.

&&

Evening update...much of the shallow convection which developed this
afternoon is collapsing with the loss of daytime heating this
evening. The exception is across SW Washington...mainly near the
coast...where an uptick in shower activity is occurring due to a lobe
of cold air aloft digging S-southeast along the Washington coast. Expect this to
continue overnight...possibly bringing more clouds and showers into
the pdx metropolitan later on as the vortmax and cool air aloft swing
onshore.

Made some changes to the forecast...generally lowering sky cover and
probability of precipitation through Thursday. Kept Wednesday morning mostly cloudy as it the top
of the boundary layer may cool enough to allow a stratocumulus deck
to form overnight/early Wednesday morning. If the ceiling does develop...
mixing will likely allow some sunbreaks to form by middle-late morning.
After that Wednesday should be very similar to today...unsettled with
a mix of showers and sunbreaks.

Models still suggest a better chance of deeper convection Wednesday
afternoon than today...mainly due to cooler temperatures aloft promoting
slightly steeper lapse rates. With 500 mb temperatures nearing -30 degree c and
surface temperatures around 60 degree f...cloud tops could reach 20-25 kft. Given
the cool air mass this would give plenty of room for charge
separation and lightning. Heavier showers and/or thunderstorms could
also produce small hail. Cape is still very skinny on NAM bufr
soundings...so decided to keep coverage isolated. Weagle

/prev disc issued 227 PM PDT Tuesday may 5 2015/

Short term...tonight through Friday...early afternoon water vapor
imagery shows the closed upper low centered over extreme southwestern British
Columbia. Moisture seems to be concentrated close to the low
center...focused over western Washington. Water vapor also shows a dry air tongue
rounding the trough base entering western Oregon. Shower activity not all
that impressive as of yet. Krtx dual-pol Doppler radar shows the
majority of the activity over the northern half of the County Warning Area...favored over
higher terrain due to the westerly orographic flow. Cascade web cams and
visible satellite imagery show a decreasing cloud trend over the
higher Lane County Cascades...in the core of the dry air aloft as
seen in water vapor. Snow levels to remain near the Cascade passes
tonight. Accumulations to be in the 1 to 3 inch range...but will have
little to no impact on major roadways.

Models continue to show the upper low migrating east-southeast tonight...then
splitting into two pieces by Wednesday. Latest European model (ecmwf) and GFS valid 18z Wednesday
show one piece over southern Alberta and the main energy over
north-central Oregon. 12z GFS has 500 mb temperatures around -28c Wednesday across
much of the County Warning Area Wednesday afternoon. The strong may sunshine will likely
boost surface temperatures to around 60 degree c in the Willamette Valley...above
the forecast convective temperature. Model lifted indices expected to
be around -1 to -2 degree c from the Coast Range eastward. NAM model
sounding for kpdx has almost 500 j/kg surface-based cape Wednesday
afternoon...compared to 250-300 j/kg on the GFS. Wednesday should have
higher potential for thunderstorms with the upper low settling over
central Oregon. Wet-bulb zero heights Wednesday remain around 4000 feet for a
continued threat of small hail in heavier showers. The upper low
continues its dive south Wednesday night...ending up over north-central
California by 12z Thursday. Will need to leave some low-end probability of precipitation over the
Lane County Cascades Wednesday night with wrap-around moisture still a
threat.

Decided to pull all mention of probability of precipitation Thursday as the upper low sinks
further south. Upper level ridging centered near 135w amplifies and
nudges closer to the coast Thursday and Thursday night...resulting in drier
north-NE flow aloft. Daytime temperatures rise from slightly below normal
Wednesday to a few degrees above normal Thursday. GFS 850 mb temperatures go from +8 to
+10c Thursday to +12 to +13c Friday. European model (ecmwf) values look to be 1-2c
cooler...but these values would yield maximum temperatures at least to the middle
70s inland. Weishaar

Long term...Friday night through Monday. An upper level ridge will
dominate Friday and Saturday for dry and warm weather. Inland
temperatures will warm to the upper 70s Friday and Saturday. Gusty
north winds along the coast will keep coastal temperatures in the low
60s. Sunday will be slightly cooler as an upper level trough
approaches. A weak cold front may bring some light rain Sunday night
and Monday morning. An upper low will move over the area Monday
continuing the threat for showers. The upper trough/low will result
in cooler temperatures Monday and Tuesday with highs in the 60s and
lows in the middle 40s. Tj

&&

Aviation...expect mainly VFR conditions through 06z Thursday as an
upper level low pressure tracks across the area tonight and Thursday.
Scattered showers moving ashore tonight as shortwave moves land
and should mainly affect areas north of ksle. There may be some
MVFR cits developing between 13z and 18z Wednesday in the area but
confidence not high enough to include at taf sites. Shower
intensity increase Wednesday afternoon. Isolated thunderstorms are also
expected Wednesday afternoon.

Kpdx and approaches...VFR conditions prevailing through 06z Thursday
with low chances for MVFR cits around 2500-3000 feet in the area
between 14z and 18z Wednesday. Showers become more active Wednesday
afternoon with a slight chance of a thunderstorm impacting the
terminal. /26

&&

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 around 8 feet will gradually subside to
near 6 feet on Wednesday.

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 suggest there is a chance northerly
winds may increase and become more widespread on Sunday. There is
a chance seas could become steep and choppy on Sunday or Monday
if the gustier northerly winds materialize.

Otherwise...a weak system approaching the region early next week
does not appear likely to bring significant marine impacts. /Neuman

&&

Pqr watches/warnings/advisories...
or...none.
Washington...none.
Pz...Small Craft Advisory for rough Columbia River bar from
5 am to 9 am PDT Wednesday.

&&

$$

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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.

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