Area Forecast Discussion
Issued by NWS Portland, OR

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FXUS66 KPQR 231028
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
328 AM PDT Sat Mar 23 2019

.SYNOPSIS...An upper level trough of low pressure will lift
northeast across the forecast area today bringing more showers and a
slight chance of thunderstorms. A brief period under high pressure
Sunday will bring a dry ans slightly warmer day, before another
series of frontal systems brings more rain next week.

&&

.SHORT TERM...Today through Monday...Another shortwave associated
with the large upper low over the Gulf of Alaska was seen in water
vapor pictures approaching 130W early this morning. Air mass remains
unstable this morning, but instability is expected to deepen later
today with surface heating and the arrival of the shortwave. With
some dynamic forcing with the shortwave, expect showers to increase
in coverage today, with a slight chance of thunderstorms as the
upper trough passes. With the showers and cool trough, expect
daytime temepratures are likely to be a little below normal again
today.

The shortwave finally passes tonight, bringing an end to the chance
for showers. Models remain in good agreement showing a shortwave
ridge of high pressure moving across the region Sunday and Sunday
night, for a period of dry weather. Surface ridging Sunday morning
along with residual low level moisture suggests some patchy fog
likely.

The next shortwave kicking out of the Pacific low is expected to
push a cold front into the forecast area Monday. There are some
minor timing issues with the cold front, with GFS operational run a
little faster then EC, but net effect is still for the front to move
in during the daytime. This system does not appear much different
from the Friday front, in that a narrow band of moisture associated
with the front moves through during the daytime. QPF is not likely
to be particularly high as the front moves through pretty quickly,
but dynamics and moisture suggest at least likely pops reasonable
for the entire forecast area.

.LONG TERM...Monday night through Friday...No changes. Previous
discussion follows. Active weather next week, as a large upper-level
trough sits off the coast of Washington and Oregon most of the week,
sending shortwaves into the Pacific Northwest. Used the National
Blend of Models primarily for precipitation chances through next
week because deterministic guidance doesn`t reflect the uncertainty
in the timing of these shortwaves that is reflected in more of a
probabilistic forecast produced by the NBM.

Monday night we will have lingering post-frontal showers from the
first of these shortwaves. Deterministic models (GFS and ECMWF) show
us drying out on Tuesday as we are between systems. The NBM,
however, keeps a chance of a few showers suggesting a the
possibility of weaker, unresolved shortwaves generating a few
showers on Tuesday. Have left a chance of showers in the forecast on
Tuesday. Wednesday, deterministic models show another shortwave
moving in, and this is supported by an increase in chance for rain
in the NBM. Therefore, have likely PoPs in the forecast for
Wednesday. Thursday, the trough that has been sitting offshore all
week finally starts to weaken and move onshore. This will keep
showers over our area through Thursday. There is greater uncertainty
on Friday, with some guidance showing the development of an upper-
level ridge starting Friday into next weekend, which would dry us
out, or the pattern turning to more zonal flow which could keep a
chance for showers on Friday. For that reason, the NBM keeps a
chance for showers on Friday. The NBM does show a trend toward lower
PoPs, however, suggesting most of the ensemble members are leaning
toward drier conditions Friday going into next weekend. -McCoy

&&

.AVIATION...Showers will continue overnight, with a slight chance
of thunder developing along the coast after 12Z Sunday and
shifting inland Saturday afternoon. Conditions will be a mix of
categories through Saturday morning with IFR fog developing in
areas which clear out and areas which keep clouds remaining VFR.
Many sites will be up and down overnight as showers traverse the
area. Expect conditions to trend to VFR by late Saturday morning,
but brief MVFR or lower CIG/vis will be possible with showers
and any storms. Bowen

KPDX AND APPROACHES...Expect mainly VFR, but shower chances will
continue through the overnight and any showers may bring the
terminal to MVFR. There is a chance for a thunderstorm to impact
the terminal sometime Saturday afternoon. Bowen

&&

.MARINE...As showers continue through the Friday night/Saturday
morning period, may see a thunderstorm or two develop. Seas will
remain around 13 ft through the day Saturday with west swell,
although winds have decreased significantly and will remain
benign. The only exception would be in the vicinity of any
thunderstorms through Saturday morning, where strong, gusty winds
are possible.

The next system sweeps a cold front through the waters Sunday
night/Monday with seas building back up to or above 10 ft Monday
briefly Monday and Monday night. May see some brief wind gusts
above 20 kt with this front. The waters remain under the
influence of a large trough, but with little additional impact
through midweek. Seas are forecast to drop back below 10 ft
Tuesday and remain around 5 to 7 ft with winds staying below 20
kt out of the south and southeast. Bowen

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...None.
WA...None.
PZ...Small Craft Advisory for hazardous seas until 11 AM PDT Sunday
     for Coastal Waters from Cape Shoalwater WA to Florence OR
     out 60 NM.

     Small Craft Advisory for Rough Columbia River Bar until 11 AM
     PDT Sunday.
&&

$$

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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 forecast area.


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