857
FXUS66 KPQR 180946
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
245 AM PDT Sat May 18 2024

.SYNOPSIS...An upper level trough will lead to cool weather this
weekend, with a few showers mainly in the north. Temperatures bounce
back to mid-May normals Monday as a transitory high pressure ridge
slides across the Pac NW. More widespread showers are possible
Tuesday into Wednesday as another upper trough swings through the
region. Unsettled and generally cool weather is expected to persist
through the end of the week.

&&

.SHORT TERM...Now through Monday...Latest GOES-West satellite derived
winds show a 90-100 kt jet stream over the NE Pacific, aiming toward
the Pac NW coast. This will carry a pair of weak shortwave upper
disturbances onshore into the Pac NW this weekend, leading to cool
and somewhat unsettled weather. That said, a strong capping inversion
around 700 mb will keep showers from becoming too boisterous or
widespread. In fact, many locations in the CWA may make it through
the entire weekend without a drop of rain. So the more noticeable
weather will be somewhat cool temperatures as 850 mb temps generally
range between -2 and +2 deg C throughout the weekend. West to
northwest winds will be breezy at times. Snow levels dip to around
3500 feet tonight into Sunday morning, so the passes may see a brief
dusting of snow. With marginal temperatures, impacts should be
minimal.

Low to mid level flow becomes anticyclonic Sunday night into Monday
as a shortwave ridge of high pressure slides SE across the forecast
area. This will lead to a dry day with plenty of sunshine. After a
chilly start, temps should warm to around 70 degrees for the inland
valleys Monday afternoon - which is pretty close to normal for the
time of year.

Will need to watch the Upper Hood River Valley for the potential of
frost each morning through Monday, given the chilly air mass. Latest
NBM probabilistic guidance suggests the best chance of temps reaching
35 deg F or cooler will be early Sunday morning, when there is
roughly a 60% chance of Parkdale being that cold. These probabilities
fall off dramatically north of Parkdale, with only a 10% chance shown
for Odell. For now this coverage/probability does not seem sufficient
to warrant a Frost Advisory.  Weagle

.LONG TERM...Tuesday through Friday...Unsettled weather looks to
return Tuesday as another upper trough dives S-SE from the BC coast.
The 00z WPC clusters show a variety of trajectories for the upper
trough, with members almost evenly split between an "inside slider"
solution with an inland trajectory versus a track closer to the
coast. The latter would pose a better chance for showers across the
forecast area, and it would be colder - potentially cold enough to
bring some snow to the Cascade passes. The latter solution would also
invite a second system to follow later in the week, prolonging the
cool/unsettled weather. Given the uncertainty in track and timing,
our forecast mostly uses the National Blend of Models deterministic
forecast onward from Tuesday. This shows the best shower chances on
Tuesday, with chances generally decreasing after that. Lowland high
temps are forecast to generally be in the upper 50s along the coast
and 60s inland, while the higher terrain sees 40s and 50s. Some of
our cooler valleys may not be totally out of the woods yet when it
comes to frost - NBM shows roughly a 10-20% chance of lows 35 deg F
or cooler for Parkdale each morning Wednesday through Friday. This
would require the upper trough to set up far enough west to bring a
chill to the air mass, but not too far west to where clouds and
precipitation would hold temps up during the overnight hours.  Weagle


&&

.AVIATION...VFR conditions will prevail across the region over the
next 24 hours as high pressure remains in place. An upper level
disturbance will bring increasing chances for light rain showers
after 14z Saturday along the north coast and after 20z inland.
That being said, significant drops in visibilities and/or cigs are
not expected with passing showers (very brief periods of high-end
MVFR cigs at best).

PDX AND APPROACHES...Expect VFR flight conditions through
Saturday night. Winds will generally remain out of the northwest
sustained between 5-10 kt, however gusts should temporarily
increase to around 20 kt between 00-03z Sunday. -TK

&&

.MARINE...Buoy observations from early Saturday morning showed the
continuation of northerly winds, however gusts had fallen to
around 20 kt. Seas were improving and becoming less steep as wave
heights were falling to around 6 ft with a dominant wave period of
9 seconds. As such, will allow the Small Craft Advisory to expire
at 6am Saturday.

High pressure over the upcoming weekend will result in a typical
summertime northerly flow regime with wind gusts peaking in
strength during the afternoon/evening hours each day. The
strongest winds will occur over the southern zones where marginal
small craft advisory level wind gusts to 25 kt are possible
(30-40% chance on Saturday increasing to a 40-70% chance on
Sunday and Monday). Waves will become steep and choppy again early
Sunday morning and then remain steep into Monday as a fresh
northwest swell around 7-8 ft with a wave period around 8 seconds
interacts with northerly wind waves.

The next frontal passage of any significance is set to arrive
late Monday night into Tuesday morning, bringing rain and a shift
to westerly winds. However, wind gusts look to generally stay
around or below 20 kt as the pressure gradient associated with
this frontal passage appears rather weak. The exception to that is
over the northern outer waters where isolated small craft
advisory level gusts to 25 kt are possible (20-50% chance). -TK

&&

.PQR WATCHES/WARNINGS/ADVISORIES...

OR...None.
WA...None.
PZ...Small Craft Advisory until 6 AM PDT early this morning for
     PZZ251>253-271>273.

     Small Craft Advisory from 11 AM Sunday to 5 AM PDT Monday for
     PZZ251>253.

     Small Craft Advisory from 5 PM this afternoon to 5 AM PDT
     Monday for PZZ271>273.

&&

$$

www.weather.gov/portland

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