Area Forecast Discussion
Issued by NWS Portland, OR

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FXUS66 KPQR 250423

Area Forecast Discussion
National Weather Service Portland OR
921 PM PDT Sat Mar 24 2018

.SYNOPSIS...A low off the central Oregon coast will move onshore
into southern Oregon and northern California tonight and weaken.
Snow levels will lower to 500 to 1000 feet tonight and early Sunday,
rising a bit in the afternoon. Showers will continue Sunday into
Sunday night in the wake of the departing low. Another modest front
will move through the forecast area Sunday night into early Monday.
An upper ridge will strengthen in the Gulf of Alaska, but some
moisture and energy will ride over the upper ridge and spread some
light precipitation at times into the Pacific Northwest during the
first part of next week before finally drying out for the second
part of the week. The next system may reach the forecast area next


.SHORT TERM...Tonight through Tuesday...Shower are becoming more
sparse this evening as the upper low weakens. Infrared satellite
loops shows clouds tops warming through the evening and becoming more
disorganized.  Some showers should linger overnight will keep a
chance of showers through the night. Snow levels will drop again
tonight down to the 500 to 1000 ft level but showers will become
increasingly sparse.

Showers will continue on Sunday, especially in the afternoon, in the
residual onshore flow in the wake of the departing low. Another
modest front is forecast to drop down Sunday night into Monday for
more rain and snow. 00Z models show an increase in rain Monday so
have increased PoPs and QPF.  Snow levels will still be low in the
1500 to 2000` range.  Additional moisture and energy will spread into
the Pacific Northwest through the day Tuesday in the
wake of the front.  During the Tuesday time period it appears
precipitation will be limited to the coast, mountains and
northern half of the interior lowlands with snow levels around 2000`
Monday, rising to 4000 to 5000` Tuesday afternoon.  /mh

.LONG TERM...Tuesday night through Saturday...Moist northwesterly
flow aloft will continue to bring periods of rain or showers through
at least Wednesday morning as a weak disturbance rides along the
backside of the upper level ridge. Models then start to diverge a
bit beyond this. The ECMWF maintains a stronger ridge over the Pac
NW through Thursday which should keep conditions dry. However, the
GFS shows the ridge breaking down as another shortwave disturbance
moves across the region. Decided to trend the forecast towards the
drier ECMWF on Thursday since the shortwave in the GFS is relatively
new. Beyond this ridge, models are in general agreement that the
longwave trough will re-establish itself across the Pac NW. This
should bring the return of cooler weather going into next weekend.
Limited moisture should keep things mostly dry late next week.


.AVIATION...Mixture of MVFR and VFR conditions this evening as a
weak low continues to pivot towards the area. Showers will become
less numerous over the next several hours. Have tried to show
this trend with the TAFS this evening. Expect more MVFR than VFR
conditions through the middle of tomorrow morning at which time
more VFR conditions will likely become apparent. Some locations
will likely clear earlier than others and it is hard to tell
where those locations may be. Have given a broad brush
improvement from MVFR to VFR at most TAF sites between late
tomorrow morning and mid afternoon.

PDX AND APPROACHES...Mostly MVFR ceilings through tomorrow
morning with improving conditions to VFR tomorrow afternoon. Do
not expect periodic IFR conditions to last long. /Bentley


.MARINE...There will be a period of very light winds tonight
through the day Sunday as a weak ridge of high pressure moves
over the waters. A quick moving short-wave trough will move
through Sun night, and is looking increasingly likely to bring a
brief period of gusts to around 25 kt. Then higher pres will
settle over the NE Pac for most of next week. Thermal low pres
developing over northern Calif will lead to the development of NW
winds over the Oregon and Washington waters. Seas will continue
to subside through Sunday, reaching around 5 ft by Sunday night.
Seas should then remain well below 10 ft for most of next week.





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This discussion is for Northwest Oregon and Southwest Washington
from the Cascade crest to 60 nautical miles offshore. This area is
commonly referred to as the CWA or forecast area. is the U.S. government's official web portal to all federal, state and local government web resources and services.