Area forecast discussion
National Weather Service Portland or
200 PM PDT Sat may 25 2013
Synopsis...after a break in rain this afternoon...upper level low
pressure brings a wave of rain tonight...lingering into Sunday before
tapering to showers later Sunday afternoon. A stronger system should
spread more rain across the district on Memorial Day...possibly along
with a brief period of breezy conditions along the coast. Brisk
westerly flow will keep things unsettled with rain at times through
at least middle week. There is a hint that we may dry out by next
weekend.
&&
Short term... tonight through Tuesday...our main feature of interest
is the upper disturbance pushing inside 130 west around 46 north this
afternoon...as the upper low off Puget Sound sound remains stationary
through the evening. The main energy associated with this disturbance
will push south of the area...but enough will be left to bring a
round of widespread rain as these systems amalgamate into one mean
upper trough. Low to middle level warm advection in the SW flow ahead of
the low as increased the cloud shield offshore and into SW
Washington...while surface high pressure...now weakening...has kept
this system from making much inland progress elsewhere. Rain has now
finally pushed into Astoria.
The first low/disturbance will move onshore overnight tonight. Weak
flow pattern will result in minimal orographics...and little to no
rainshadowing for the Willamette Valley. Additionally...precipitable water values are
up around 0.9 in which is above the 75th percentile level for
may...or about 130-150% of normal. So again there could be a good bit
of steady rain in The Lowlands tonight into Sunday morning. Not
expecting the same kind of totals we experienced around Portland the
last few days...but this will probably be a heavier event for Salem
and Eugene who received less precipitation from the last couple of systems.
The upper low will very slowly progress through the area on
Sunday...thus expect lingering clouds and rain...with additional
energy associated with the low off the Puget Sound sound prognosticated to swing
through in the afternoon and evening. There is a bit of
question as to whether rain will be steady or more showery Sunday
afternoon as the associated upper low moves onshore. NAM bufr
soundings show a very moist air mass but little in the way of
instability. Regardless temperatures may again have a hard time getting out
of the 50s Sunday to near 60 in a cloudy and wet day...though they
may spike later in the afternoon in any sunbreaks...as overnight
lows will not drop much tonight given clouds spreading in. Kept the
middle-shifts trend to keep temperatures several degrees below guidance
but did nudge them up a touch. Anywhere that sees a few sunbreaks
will warm a bit more into the 60s with the warm late may sun...with
the best chance in Lee of the Coast Range and for areas southwest
where more ridging attempts to build in.
The upper trough will still be working its way through the northern
tier Sunday evening...thus some lingering rain/showers mainly before
midnight. Precipitation will taper off elsewhere as this system moves
east sun evening...as high pressure noses in from the southwest
through the afternoon. Should be a bit of break Sunday evening from
southwest to northeast...though some rain/showers may linger in the
northern tier as the upper low continues to pass through. The break
will unfortunately be short lived...as a more organized Pacific
frontal system to move onshore later Sunday night or Monday/Memorial
Day morning. Depending on the track of this system it may have a
brief punch of modest coastal wind with it...but this appears more
likely for southern Oregon or northern California where the best jet
dynamics will be. The surface pattern looks such that will remain
offshore for quite some time...keeping the strongest winds confined
to the coastal waters. As a result this system may try to split
somewhat as it moves onshore... but still should hold together enough
for another pretty good soaking Memorial Day. The upper trough
lingers in the area for Tuesday meaning cloudy and periods of
rain/showers. Kmd
Long term...Tuesday evening through Saturday...cool and showery
conditions will continue through much of the work week as several
shortwaves wrap around a broad upper level trough. An upper level
ridge attempts to build over the Pacific northwest by Friday and
early next weekend. This should dry out the conditions over the
region starting late Thursday night and Friday...allowing snow
levels to rise and temperatures back near normal. Granted there have
been decent model discrepancies concerning this potential
ridging...but seems like we could see ridging lingering through next
weekend. /27
&&
Aviation...VFR across the region will transition back to MVFR
this afternoon on the coast...and this evening further inland. This
due to increasing rain as yet another front now about 200 miles
offshore pushes inland. Widespread MVFR across region tonight and
into sun am... with local IFR conditions after 03z on the coast.
Cascades and higher terrain will be obscured in clouds and precipitation.
Kpdx and approaches...VFR and dry until later this afternoon.
Clouds will thicken after 22z...with occasional rain after 00z.
Still looks likely that ceilings will gradually lower. Expect mostly
MVFR ceilings...2000 to 2500 feet...after 04z. Little change expected
tonight with MVFR ceilings dominating. Rockey.
&&
Marine...front approaching...but gradient not impressive. Will
keep south winds 10 to 20 knots...but can not rule out a few gusts to
25 knots. Not enough to warrant any advisories. Front will push
onshore early sun am...with winds relaxing. Seas mostly 5 to 7 feet
though sun evening.
Another front arrives Sun night and Monday. Appears that solid small
craft winds of 20 to 30 knots. Seas may briefly clip 10 feet on Monday due
to combined seas...but drop back Monday night as wind eases. Rockey.
&&
Pqr watches/warnings/advisories...
or...none.
Washington...none.
Pz...Small Craft Advisory for rough Columbia bar conditions from
from 5 am to 9 am sun.
&&
$$
More weather information online at...
http://weather.Gov/Portland
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.
