Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Portland or
821 PM PDT Tuesday Aug 26 2014
Synopsis...little change through Wednesday as will have one more rather
warm day. Then onshore flow increases Wednesday night and Thursday...bringing a
gradual cooling trend. Highs only in the lower 70s inland this
weekend. Will have also have a threat of showers this weekend but
appears will likely have more dry periods than showers.
Short term...tonight through Friday...
not much change...as strong high pressure remains over the region.
Thermal low pressure trough sits on the east slopes of the Coast Range
this evening. To the west...extensive low clouds and cool conditions
on the coast. To the east of the thermal trough lies warm and dry
conditions. The trough will move little tonight and early Wednesday...then
push across the Willamette Valley on Wednesday and Wednesday evening. As a
result...inland areas will see another mild night with one more
rather warm day. Once the trough shifts east of the region...onshore
flow will increase. This will bring a deepening marine layer and
cooler temperatures into the interior for later Wednesday night and Thursday.
Westerly flow aloft will be increasing on Thursday and Friday...allowing the
ridge to flatten. This will bring a few weak disturbances into the
Pacific northwest...but mainly affecting Washington and southern British
Columbia. As marine layer deepens late Wednesday night and Thursday am...one
weak disturbance will be close enough to provide weak lift north of a
Newport to Portland to Montana Adams line...resulting in areas of
drizzle...especially over SW Washington and north Oregon coast.
Either way...onshore flow Thursday and Friday will cool air mass...with
interior high still low to middle 80s on Thursday...then near 80 on Friday. But
Friday highs may be still too warm if clouds persists. Would not be
surprised if guidance is correct and highs are only in the lower to
middle 70s on Friday. Will make no changes this evening...and will let middle
shift Ponder whether or not to cool Friday highs further. Rockey.
Long term...a trough approaches the West Coast on Friday which will
bring a chance of rain over the weekend. The 12z European model (ecmwf) and GFS both
bring the system onshore late Friday night. Northwesterly flow will
become dominant over our area behind the initial cold front...with
multiple impulses bringing chances of rain through Sunday evening.
12z GFS seems a little strong early next week bringing rain and cloud
cover over our area after two days of northwesterly flow...so opted
to lean more towards European model (ecmwf) with slight chance of rain mainly along
South Washington Cascade and coastal ranges. -McCoy
Aviation...VFR conditions inland tonight and Wednesday...except some
MVFR marine stratus pushing inland to about Kelso and Scappoose
by 15z. On the coast...IFR or LIFR ceilings return tonight with
persistent IFR visibility along the central Oregon coast. Some localized
smoke from the fires near Oakridge in eastern Lane County may reduce
visibility in that area.
Kpdx and approaches...VFR conditions tonight and Wednesday.
Marine...persistent pattern continues with high pressure over
the NE Pacific and thermal low over northern California. Winds
will be below 20 kts tonight...so have cancelled the Small Craft
Advisory about an hour earlier than it was set to expire. Seas
remain choppy tonight with short period wind waves dominating.
High pressure sinks southward through the week...making way for
our next low pressure system to approach Friday night with increasing
swell late Saturday. Winds turn southerly over the nearshore
waters on the back side of the low Sunday with some disagreement
among models as to timing of shift back to north and northwest but winds
will remain below 10 knots during the period of uncertainty...so
impact should be minimal. Tw
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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.