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Area forecast discussion 
National Weather Service Portland or
934 am PDT Tuesday Jul 29 2014

Synopsis...an upper level ridge remains anchored over the Great
Basin and northern rockies. Upper level low pressure lingers off the
West Coast of Canada...as a result the Pacific northwest remains
under south to southwesterly flow aloft. The result is very 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...mainly Lane County. Little change expected in this overall
pattern the next several days.

&&

Update...few changes were made to the forecast this morning.
Satellite imagery shows some altocumulus lifting into Lane County
from the south. This indicates a bit of middle-level moisture and
instability...and in-cloud lightning was detected in one of these
clouds east of Eugene earlier this morning. A similar altocumulus
buildup is headed toward Santiam Pass...and would not be surprised to
see a stray lightning strike out of this cell either. Thus we added a
slight chance of thunder for the Lane County Cascades early today as
well as this afternoon.

Believe the environment moving in aloft over east Lane County is
better characterized by the 12z Medford sounding than the Salem
sounding. Models do not seem to be handling the moisture aloft very
well. A weak dry slot is notable on water vapor imagery...headed
north from the or/California border. This could provide the trigger for more
numerous showers and thunderstorms to develop in the Oregon Cascades
as the day progresses. This was reflected well in the morning
forecast and made few changes.Weagle

/prev disc issued 307 am PDT Tuesday Jul 29 2014/

Short term...an upper low off the b.C. Coast and an upper level
ridge anchored inland over the western U.S. Will be the features
affecting the weather for the Pacific northwest this week. Light
mostly southwest flow aloft with high 500 mb heights. The marine
layer remains shallow under the subsidence aloft and is limited to
the coastal zones with a little seepage up the lower Columbia River.

Very similar conditions today. The main differences are lower 500mb
heights for a slightly cooler air mass along with an increase in the
low level onshore flow. This will keep the marine stratus along the
coast today. Later tonight into Wednesday morning an old frontal boundary
slides into the coastal waters and may bring some drizzle to the
coast.Marine stratus should seep further inland and try to extend
inland to near Portland but anything will be very brief and not have
much impact. Temperatures inland will continue to be very warm with little
change through Thursday.

As for thunderstorm activity...another batch of moisture arrives into
the southern zones this afternoon. Instability is limited for cloud
growth up to the -30c level and will likely need the higher terrain
of the Cascades to initiate convection. So expect any convection to
be isolated at best. However with the increased moisture a strong
cell could dump a fair amount of precipitation in a short time. The pattern
Wednesday and Thursday looks very similar to Tuesday...with a chance
of thunderstorms again over the Cascades in the afternoon and
evening. One change in the models...precipitable water values are
down to 0.6 inch from 1 inch over yesterdays run.



Long term...Thursday night through Monday. The ridge in place over
the western U.S. Will remain in place through the end of the week. A
series of shortwaves are forecast to travel up the west side of the
ridge...possibly impacting our area. These impulses bring chances
for thunderstorms...mainly in the Cascades...through the weekend.
Model uncertainty exists on the timing and strength of these
shortwaves as model resolution accuracy declines rapidly further in
the forecast period...especially for these smaller features. Have
decreased probability of precipitation further north on Friday as the models have converged
on a more southerly track for storm chances...and decreased
confidence of thunder chance over the weekend due to great model
uncertainty on timing and location. -McCoy

&&

Aviation...stratus and fog is hugging the coast and some of the
coastal valleys with IFR conditions. The stratus made it into
kkls this morning. Satellite imagery shows the stratus beginning
to clear inland...and expect VFR conditions for the coast between
19 and 21z. Gusty north winds can be expected for the coast this
afternoon and evening. Otherwise VFR conditions for the inland
airports. Some middle level clouds will move over the area...mainly
south of ksle today. These clouds will increase over the Cascades
this afternoon where there may be some thunderstorms. Tj

Kpdx and approaches...VFR prevailing next 24 hours with generally
light north-northwest surface winds. Cullen/tj

&&

Marine...there will be little change in marine weather this week.
Surface high pressure will remain over the NE Pacific as a
thermal trough holds over northern California. The thermal trough
will rise slightly north in the afternoon and evenings resulting
in a tighter pressure gradient over the Oregon waters. Gusty winds
can be expected at these times...occasionally reaching or
exceeding 25 knots. The gradient will be tightest south of Tillamook.
Have extended the Small Craft Advisory for this afternoon and
evening to include the waters beyond 10 nm south of Cascade Head.

The seas will primarily be driven by the local winds. Steep seas 3
to 5 feet will dominate through the week. There are some
background longer period seas that may occasionally show up on
the buoys...but these are very small...1 foot or less...and less
significant to the mariner than the 3 to 5 feet seas at 6 to 8
seconds. The steep seas will be a combination of wind wave and
fresh swell which is nearly impossible to distinguish and have
therefore combined the seas. The seas will be the steepest when
the winds are the greatest...during the afternoons and
evening...and especially south of Cascade Head and near shore. Tj

&&

Pqr watches/warnings/advisories...
or...none.
Washington...none.
Pz...Small Craft Advisory for winds from 2 PM this afternoon to 11
PM PDT this evening for coastal waters from Cape Shoalwater Washington
to Cascade Head or out 10 nm-coastal waters and from Cascade
Head or to Florence or out 60 nm.

&&

$$

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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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