Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Portland or
248 PM PDT Thursday Oct 8 2015
Synopsis...mild night ahead with passing high clouds will allow
another round of fog to develop overnight. Oho has transitioned into
a Post tropical low and will weaken as the remnants makes their
closest approach overnight. However...we will still see increasing
wind and seas affect our coastal waters over the next 48 to 72
hours. A cold front crosses Friday and spreads light rain mainly
across the northern and western portions of the forecast area.
Powerful southwest swells associated with oho and the incoming front
will start impacting the coast by Friday through the weekend
creating beach hazardous conditions. A stronger and slower moving
cold front will bring heavier rains and gusty winds to the coast and
interior Saturday followed by showers tapering off Sunday.
Short term...tonight through Sunday...low clouds and fog have
dissipated at this hour and will expect some modest clearing
overnight tonight. This and dew points in the 50s should result in
another batch of fog tonight and early tomorrow. Coastal areas will
see more fog and low clouds tonight as well as the ocean stratus
field gets pushed back onshore.
Oho remnants will weaken as they make their closest approach to our
area. The surface low pressure rises to around 986 mb near 138w off
the mouth of the Columbia. The extratropical low then strengthens
again reaching 981 mb by 12z at 55n 138w as they interact with the
broad upper trough over the Gulf of Alaska. The trailing surface
trough will drag close to the coast tomorrow morning and bring wind
gusts of 45-50 miles per hour to the coast. The offshore gradient will also
serve to weaken the incoming front from the advancing upper trough.
There will be plenty of tropical moisture left over the area to
still bring rains to a large part of the area although the central
Cascades may be left dry.
A stronger front with dynamic support form the upper trough will
cross the region Saturday and bring some of the best rains seen in
some time through Saturday night. The associated cold front will
also have tropical moisture from western Pacific typhoon remnants
entrained in the westerly Pacific flow. Upwards of a couple inches
will be possible ending Sunday morning for the Willapa Hills...north
Coast Range...and Montana St helens area extension of the South
Washington Cascades. The associated surface low will also pass north
of our area and allow north-S pressure gradients to open up with and
just behind the cold front crossing midday Saturday through the
early evening. Gusts 50-55 miles per hour at the coast and around 35 miles per hour for
the inland valleys will be likely although briefly higher gusts will
be possible during the frontal passage. Rain is followed by
orographic showers behind the front and continuing into Sunday.
Still have concerns for overall public safety this weekend on the
beaches and jetties as unusual swell combinations will affect our
coast. Please refer to the Special Weather Statement for additional
Long term...Sunday night through Thursday...weak upper level
ridging will keep most of the shower activity confided to the
northern portions of the forecast area late Sunday. Models continue
to show another front moving across the Pacific northwest on Monday
but are still struggling with some of the details. As of now...it
looks like most of the quantitative precipitation forecast will remain well to our north...with only
modest accumulations expected across northwest Oregon and SW Washington.
Upper level ridging returns Tuesday and looks to persist through
much of the upcoming week. This will bring the return of warmer and
drier weather...with temperatures remaining a few degrees above
Aviation...fog and low clouds remained over portions of the
interior lowlands into the early afternoon...but the latest
observations show widespread VFR conditions now across SW
Washington and northwest Oregon. Expect this continue through the rest of
the afternoon and most of the evening with broken high clouds
Dew points remain in the upper 50s to near 60 over much of the
interior today...so think we will again see fog and low clouds
develop later tonight with light winds expected and high clouds
likely to become scattered. Formation should occur earlier than
last night...with the tafs highlighting the 06z to 09z Friday time
frame for initial formation. An approaching frontal boundary will
bring increasing winds and middle level clouds tomorrow morning...which
should help to scatter out the low clouds and bring a return to
VFR between 15z and 18z Friday. The front will likely spread light
rain and MVFR ceilings into the coastal taf sites during this time as
Kpdx and approaches...VFR conditions expected through this evening
with broken high clouds overhead. Light winds and clearing skies
should allow IFR fog or low stratus to form between 06z and 09z.
An approaching front likely will mix out the low clouds between
15z and 18z Friday. Pyle
Marine...fairly benign conditions continue through this evening
with southerly winds of 10 to 20 knots and seas remaining at 3 to 5
feet. However...the extratropical remains of hurricane oho will
restrengthen and move to the west and north of the coastal waters
overnight and Friday. An associated warm front will move into the
waters early Friday...bringing likely gale force winds. Winds are
expected to increase to 25 to 30 knots with gusts 35 to 40 knots.
Decided to convert the gale watch for the near shore waters to a
Gale Warning with this forecast issuance...but still think the
strongest winds will be further offshore. It appears that the
strongest winds will be Friday morning and early afternoon.
There will be a relative lull in the winds later Friday and Friday
night...although expect wind speeds will still be at solid Small
Craft Advisory speeds. The trailing cold front will move through
the waters on Sat...and it is looking increasingly likely that
another round of gales will accompany this frontal passage. After
another lull Sat night and much of sun...a weaker front looks
likely to push in late sun or Monday...bringing another enhancement
to the winds.
In addition to the strong winds...seas are forecasted to be a
significant concern starting Friday and continuing through the
weekend. Seas will rapidly increase into the low teens Friday morning
in response to the expected gale force winds. A significant longer
period SW swell resulting from hurricane oho will then move into
the waters during the day during the day Friday and continue to
impact the waters through the weekend. Additionally...a strong
west-northwest swell is expected to arrive later in the weekend. This will
result in a period of confused seas combined with wave heights
running in the 13 to 15 feet range. Please see the marine weather
statement /pdxmwspqr/ for additional information about these
concerns. Seas do look to finally subside during the early part of
next week. Pyle
Pz...Gale Warning from 2 am to 5 PM PDT Friday for waters from Cape
Shoalwater Washington to Cascade Head or from 10 to 60 nm-waters
from Cascade Head to Florence or from 10 to 60 nm.
Gale Warning from 5 am to 5 PM PDT Friday for coastal waters
from Cape Shoalwater Washington to Cascade Head or out 10 nm-
coastal waters from Cascade Head to Florence or out 10 nm.
Small Craft Advisory for rough Columbia River bar from noon to
11 PM 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.