Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Portland or
237 am PDT Thursday Oct 30 2014
Synopsis...after a few light showers today...steadier rain will
shift over the region this evening and linger into Friday morning as
a cold front slowly pushes through the region. Expect showers
containing brief heavy rain...small hail and possibly a clap or two
of thunder to bubble up Friday afternoon. Showers should quickly
become isolated during the evening hours for those trick or treating.
Hit or miss showers may linger into Saturday...before light rain
spreads back across our northern zones Sunday. Another wet system
appears on tap for late Monday and Tuesday of next week.
Short term...today through Sunday...water vapor satellite imagery
reveals a large digging shortwave trough near 140w. Ahead of this
system...an old baroclinic zone is being re-energized and lifting
back northward across the area in the form of a warm front. This
system has tapped into precipitable water values in excess of 1 to
1.25 inches. Light rain showers have broken out across western
Oregon and Washington early this morning...but have only resulted in
a couple hundredths of an inch of rain or less early this morning.
With cloud tops continuing to cool to our south...expect off and on
light showers to continue across most of the area today with perhaps
a more extended break in the rain towards Lane and Linn counties
later this morning. A more consolidated region of rain is currently
lifting northward off the north Oregon and South Washington
coast...a prelude of things to come tonight.
A cold front currently near 130w will push eastward towards the coast
this afternoon and evening. With good low and middle level
support...expect a south-southwest to north-northeast band of rain to
shift eastward towards the coast today...before pushing into the
interior this evening and overnight. One to two inches of rain
appear likely along the coast and across the higher terrain...with
rain totals in the 0.50 to 1.00 range for many valley locations.
This front may have a tendency to slow down late tonight into Friday
morning more than current model guidance suggests due to a 120 knots
jet digging down the backside of the upper level trough.
Once the steadier rain shifts east of the region Friday...a much
cooler and unstable airmass will spread into the region. Expect
showers to pop in the afternoon hours...although this will be
somewhat dependent on how quickly the aforementioned front shifts
east of the area. Brief heavy rain and small hail will be the main
threats with any showers that bubble up. Ocean temperatures certainly
support thunder near the coast...and depending on clearing over the
interior...a couple rumbles of thunder will be possible in the
afternoon. Any shower activity over the interior should become much
more isolated Friday evening as the lower atmosphere stabilizes.
This should result in most trick or treaters staying dry Friday
The aforementioned cold front will allow snow levels to lower into
the 4500 to 5000 feet range late Friday into Saturday. As the upper
level trough digs...low level flow will weaken with this system.
This should result in limited orographic precipitation enhancement
Friday night into Saturday. As a result...snow amounts are expected
to stay under Winter Weather Advisory criteria. Nonetheless...higher
passes like Santiam Pass and Willamette passes should still see
enough snow to at least impact travel off and on Friday evening into
early Saturday...even if accumulations remain relatively light.
A weak warm front spreading eastward may bring light rain to our
forecast area late Saturday night into Sunday...especially across our
northwestern zones. /Neuman
Long term...Sunday night through Wednesday...little in the way of
change in the extended...models seem to converging on the idea of a
consolidated...but transient front bringing a good shot of rain to
the area late Monday into Tuesday. Low end categorical to likely probability of precipitation
were maintained for this period. However...the brunt of the quantitative precipitation forecast looks
to remain across the olympic peninsula and points northward. With
zonal flow to follow...probability of precipitation were kept a bit above climatology in the far
Aviation...overall VFR conditions early this morning will
gradually lower the next few hours as rain moves north over the
area. The only exception is for local IFR/LIFR visible and ceilings in
the central and south Willamette Valley through 14-16z this
morning. Local MVFR/IFR conditions are possible for the north
Willamette Valley 12z-16z today as well. Surface low pressure
deepens off the north Oregon coast and will induce a moderate
pressure gradient across the Cascades with east wind gusts 25 to
30 knots possible at kttd through 21z. Expect lighter easterly winds
Kpdx and approaches...VFR conditions persist until a warm front
nears the terminal by 12z. MVFR/IFR ceilings around 1500-2500 feet will
be possible through 16-17z this morning. Breezy east winds
increase after 14z...with gusts 15-20 knots possible. /27
Marine...a warm front will push across the coastal waters this
morning...quickly followed by the associated cold front tonight.
Southeast winds will gradually back to southerly later this morning and
increase with gusts 25 to 30 knots expected. The Small Craft Advisory for winds
remains in effect through early this evening. Winds quiet down
behind the front by Friday...turning northerly by Saturday.
Winds remain below Small Craft Advisory criteria into the weekend.
Combined seas may briefly touch near 10 feet as winds peak this
morning but feel it will be brief and more localized in nature.
The next upper low may send another push of seas above Friday
afternoon and evening and perhaps a stronger system Tue/Wed.
Pz...Small Craft Advisory for winds until 7 PM PDT this evening for
waters from Cape Shoalwater Washington to Florence or from 10 to
Small Craft Advisory for winds from 7 am this morning to 7 PM
PDT this evening for coastal waters from Cape Shoalwater Washington
to Florence or out 10 nm.
Small Craft Advisory for rough Columbia River bar from 9 PM
this evening to 1 am PDT Friday.
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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.