Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Portland or
258 am PDT Thursday Jul 31 2014
Synopsis...an upper level ridge over The Rockies and Great Basin
remains our most dominant weather feature...keeping temperatures
several degrees above normal inland. This ridge will very slowly
weaken through the middle of next week...eventually allowing more
onshore flow and slightly cooler temperatures by early next week. In
the meantime south to southwesterly flow aloft will continue to guide
weak disturbances into our area for occasional thunderstorm chances
in the Cascades. One such disturbance should promote another day of
enhanced Cascade thunderstorm activity Thursday...with another
Short term...the pattern of the last few days with an upper level
ridge anchored inland over the western U.S. And an upper low off the
b.C. Coast will continue for some time. This will result in a light
mostly southwest wind flow aloft...with the threat of thunderstorms
in the Cascades as weak disturbances move through. This will also
keep temperatures inland above normal.
The marine clouds that had been along the coast the last couple of
days cleared Wednesday afternoon...and only expect some patchy fog or
low clouds along the coast in the northerly upwelling flow. Do not
expect much if any inland either today. As a result...temperatures today
will be a bit warmer along the coast...and inland should be about the
same as on Wednesday or even up a degree or two.
There is an upper level disturbance along moving in from the
southwest today as evident especially on water vapor satellite
imagery. This looks to be the trigger for thunderstorms again today.
The upper flow tends to stay from the southwest today...allowing this
feature to interact with moisture being pulled up from the south.
Model soundings suggest that the best threat of convection will be
over the elevated terrain such as the Oregon Cascades and possibly
the Oregon Cascade foothills...while farther west looks more stable
with an inversion. There is some moisture brushing the central
Oregon Cascades this morning so cannot rule out a thunderstorm there
early but the GFS model looks too bullish taking this too far
northward this morning based on the current upper wind flow.
However...the better chances look to be this afternoon and evening.
The upper level short wave makes slow progress through our area today
and tonight...and will keep the convection going quite a while
tonight. This feature should also help spread the convection farther
north toward the Columbia River overnight tonight. The models also
suggest the marine clouds should fill in along the coast better
tonight as this upper short wave moves in...however inland
penetration of the low clouds will still be limited
Friday morning...possibly reaching Kelso along the Columbia River.
The upper short wave is still over our forecast area on Friday... and
expect more thunderstorms on Friday over the Cascades...possibly
reaching the South Washington Cascades. The models all suggest the
thunderstorms will stay east of the valleys.
There is not much change on Saturday...as the short wave either
lingers over the area or a piece of trailing energy moves in and
slows its progress. It looks like there is another chance of some
thunderstorms on Saturday...with some left over nocturnal activity
possible early in the day. The marine layer continues to be more
solid at the coast...and expect a bit better penetration inland
along the Columbia River Saturday morning...that may shave a couple
of degrees off especially in the North Valley areas. Tolleson
Long term...no changes. Previous discussion follows...
Saturday night through Wednesday...fairly good consensus between the
medium range models that the upper ridge to our east will gradually
erode through early next week. This may allow a greater westerly and
onshore component to the flow...perhaps cooling inland areas by a few
degrees and down closer to normal. As onshore flow becomes a bit
stronger...expect the thunder risk to decrease on our side of the
Cascades. Also the marine layer should deepen a little...pushing a
bit further inland as the thermal low over the middle-Columbia becomes
more dominant and increases the onshore pressure gradient/flow. No
significant widespread rain events on the horizon going well into
next week. Weagle
Aviation...high pressure and weak onshore flow continue to keep VFR
conditions across the region. A few patches of local IFR may develop
along the coast from ktmk south after 11z...but widespread stratus
not expected to form. Any that does develop should clear by
16z...giving way to VFR conditions. Elsewhere...VFR prevailing across
the interior next 24 hours.
Kpdx and approaches...high pressure and northwest flow will maintain VFR
conditions through next 24 hours. Cullen
Marine...only subtle day to day variations expected in the weather
pattern through the weekend as high pressure remains over NE Pacific.
This will maintain a typical summertime pattern of north winds...
generally increasing each afternoon through around midnight. As a
result...issued a Small Craft Advisory for winds for this afternoon
and evening for the inner waters and outer waters S of Cascade
Head...with gusts to 25 knots likely. Expect the conditions today to be
largely similar to yesterday. That said...while observation from buoy 46050
were running a bit below criteria yesterday evening...an ascat pass
indicated that winds may have been closer to 20 knots over much of the
area. This pattern will continue to result in locally generated wind
waves dominating the seas through the weekend....leading to steep
conditions but with total wave heights at or below 6 feet. Cullen
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
from Cascade Head or to Florence or out 60 nm.
More weather information online at...http://weather.Gov/Portland
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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.