Area forecast discussion
National Weather Service Portland or
230 PM PDT Thursday may 23 2013
Synopsis...an unseasonably cold upper level closed low will remain
over the Pacific northwest into Friday...continuing to wrap moisture around much
of southwest Washington and northwest Oregon. The low will begin to
move east Friday and the precipitation will break up into a more
showery pattern. Temperatures will warm a bit over the weekend...but a
chance for showers will remain as weak energy moves through. A
couple of systems are expected next week for more precipitation
chances.
&&
Short term...the upper low that has been over the Pacific northwest the
past couple of days was still over western Washington early Thursday afternoon. A
short wave rotating around the low was pushing SW off the North Washington
coast...with radar showing a large area of rain ahead of the
shortwave extending from off the Washington coast into northwest Oregon. As this
wave swings around the low and through Oregon overnight...should see
rain increase again over most of the area overnight. Models with
this system have tended to underestimate rainfall amounts...but seem
to have have adjusted somewhat with the latest run.
Once this wave moves through...models indicating the the main part
of the upper low lifts NE towards southern Alberta Friday. To varying
degrees models suggest a weaker shortwave still on the backside of
the low Friday...so will leave likely probability of precipitation in through Friday. After
that...a few more shortwaves still expected Sat into sun...the most
prominent of which pushes inland S of the forecast area Sun morning.
A modest onshore flow persists through the period...necessitating
the includion of probability of precipitation through the weekend.
Long term...westerly flow aloft likely to be persist through the
early part of the week. This will keep region under a persistent
chance for rain as a series of shortwaves moves through. The
westerly flow off the Pacific will keep temperatures
moderated...likely a little below seasonal averages. Models tend to
diverge some late in the week with GFS and Canadian cutting off an
upper low offshore. Bottom line though is for a continuation chance
of rain.
&&
Aviation...ceilings have improved a bit this afternoon...with MVFR
mostly along the coast and far north interior and VFR elsewhere. However
as upper low of Washington coast drifts a bit to the S and SW
tonight...should still see rain and MVFR conditions spread across the
region this evening and tonight. Steady precipitation ends later tonight
but moist air mass still supporting quite a bit of MVFR ceilings. Will
see conditions become VFR over most areas on Friday with scattered
showers. Cascades frequently obscured through tonight.
Kpdx and approaches...mix of VFR and MVFR until 00z or 01z...then
will have increasing MVFR ceilings as rain increases. Ceilings improve a bit
later tonight...but still higher end MVFR. Rockey.
&&
Marine...seas were still bouncing between 9 and 10 feet...but will
continue to subside tonight and Friday. Broad low pressure stalled off
the Washington coast and weak pressure gradients...so winds mostly 15 knots or
less. Low pressure will move inland and weaken Friday and Sat.
The next system could increase winds/seas later sun or Monday...but
still uncertainty on the track and strength of this system. No
changes for now...as looks to be decent potential for 20 to 25 knots
southerly winds with that front. Rockey.
&&
Pqr watches/warnings/advisories...
or...none.
Washington...none.
Pz...Small Craft Advisory for rough Columbia bar conditions until
6 PM today...and again between 3 am and 8 am Friday.
&&
$$
More weather information online at...
http://weather.Gov/Portland
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.
