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fxus66 kpqr 300317 

Area forecast discussion
National Weather Service Portland or
817 PM PDT Mon may 29 2017

Synopsis...a cooler weather pattern is in store for the rest of the
week, with periodic chances for showers as a large trough of low
pressure will remain over the NE Pacific, and a series of
disturbances lift NE across the Pacific northwest. Expect a little higher
pops Tuesday, and then especially Wednesday night and Thursday

Short term...tonight through Thursday...strong and relatively deep
onshore flow remains over the Pacific northwest this evening, and the
clouds that cleared back to the coastal slopes of the Coast Range
will spread onshore or reform again tonight and Tuesday morning
inland into the west slopes of the Cascades. Convection did develop
over our Oregon Cascade zones this afternoon and evening and a threat
will continue a few more hours. There is still some lightning to our
south in southwest Oregon. The model soundings and the models suggest
that the threat of thunder will diminish overnight as this is not
like our typical elevated convection scenario in the main part of
Summer following a hot day across the area, but nevertheless there
could be a few showers spreading out to the northwest over the
valleys and the coastal mountains overnight tonight into early
Tuesday. Will stay with that idea for now.

A broad low over the NE Pacific will then push a decent short wave
across the forecast area mainly the second half of Tuesday. Believe
there is a better chance of showers at that point. Still, the main
low level front stays offshore and it is the upper level part of the
system that moves through. This disturbance will reinforce the
onshore flow, and pressure gradients through The Gorge will be
stronger than those today. As a result, it will be quite windy
through The Gorge and over the Cascade ridges Tuesday afternoon and
evening. This will reduce the threat of any thunder Tuesday, though
there is a small chance down toward Willamette Pass and north of
Mount Adams Tuesday afternoon near the crest.

Forcing for any showers in our forecast area for Wednesday is
relatively weak. The next significant disturbance for our forecast
area is Wednesday night into Thursday morning, and have bumped pops
into the likely category for much of the area for that time period.
The models indicate that there may be a weak frontal boundary moving
through with that disturbance. Showers should start to ease late
Thursday and Thursday evening. Tolleson

Long changes. Previous discussion follows...
Thursday night through Monday...models in general agreement on
swinging another shortwave through early Friday, as a longwave trough
remains offshore. Wave is weak, and moisture is mainly low level, so
will keep chances for showers with this feature limited to chance
category, and will show a decreasing pop trend in the afternoon.
Models remain in decent agreement showing shortwave ridging Fri night
and early Sat for a bit of a break in the chances for showers. The
next shortwave approaches Sat night and sun, but models begin to
diverge in timing and intensity of the wave. GFS digs a deeper trough
over the Pacific northwest sun, but this appears to be an anonymously deep
and cool trough compared to gefs ensembles. Similarly, the ec
operational run appears to be weaker and faster than the ec ensemble
mean, so overall believe something in between the two operational
runs appears most likely. Will add in a slight chance for showers
over the northern part of the forecast area from late sun through
early Mon for the passing trough.

Aviation...typical late may/early June pattern this evening. Coast
is under IFR/MVFR stratus while inland valleys are clear, and showers
and few thunderstorms rumble over the high Cascades. Weak south to
southeast flow tonight may allow for a few showers to drift
northwestward away from the Cascades, but any threat of thunderstorms
looks to remain over the crest and eastward.

Now, with not much change to the marine layer that lies west of the
Cascades, will expect stratus to spread over most areas again
tonight, though think will be mostly MVFR inland. Can not rule out
occasional IFR with drizzle on the coast. Like Mon am, clouds will
take until midday to early afternoon to break apart over the

Kpdx and approaches...VFR under clear skies this evening. But, will
see increasing low MVFR stratus again late tonight between 08z and
11z. Like Mon am, will take until early Tue afternoon for stratus to
break out to VFR conditions.

Though 20% of a shower later tonight, will keep out of tafs. However,
will showers and evening thunderstorms will continue in eastern
approaches east of Hood River until 07z. Rockey.

Marine...quiet conditions continue with winds below 20 kt through
Friday. Weak front moves through late Wednesday and another on
Friday, doing little more than shifting winds to southerly starting
Tuesday afternoon and keeping generally 5 to 15 kt southerly winds
through Friday. Seas through Wednesday morning stay around 3 to 4 ft,
building to 5 to 6 ft by Wednesday afternoon. Seas stay up around 5
to 6 ft through Friday.

Next chance for winds above 20 kt will be next weekend as a broad
upper-level ridge builds over the NE Pacific and a thermal low sets
up over northern California, generating gusty northerly wind pattern
over our waters through next weekend. With gusty winds, may also get
steep seas next weekend. -McCoy

Pqr watches/warnings/advisories...


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