138
FXUS66 KPQR 011040
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
340 AM PDT Sat Jun 1 2024

.SYNOPSIS...An unseasonably strong Pacific jet stream is taking aim
at the Pacific Northwest, and will lead to increasingly active
weather this weekend. Today will be a bit of a transition day as high
pressure aloft weakens enough to allow a weak frontal system to brush
the forecast area. A stronger and much wetter system will follow for
Sunday, with locally heavy rain possible along the coast and in the
higher terrain. Rain will taper to showers later Sunday night,
persisting through Monday and likely into Tuesday. A turn toward
warmer and drier weather appears likely by the end of the week.

&&

.SHORT TERM...Today through Monday...Latest GOES-West infrared and
water vapor imagery reveals the first in a series of disturbances
presently approaching the Pac NW as high pressure weakens over the
region. Temperatures Friday afternoon climbed well into the 70s for
the inland valleys, with a handful of locations reaching 80 degrees.
The warm air mass and subsidence associated with high pressure aloft
dried out the low to mid levels considerably, so this first
disturbance will struggle to bring much more than a few sprinkles
east of the Coast Range today/tonight. However the onshore flow and
increased cloud cover should prevent anyone in our CWA from reaching
80 degrees today. For those who enjoyed the warm day Friday, at least
today won`t be a total bust - NBM probabilistic guidance suggests a
better than 75% chance our interior lowlands will climb into the 70s
this afternoon. Overall - today will see near-normal temps and a mix
of sun and clouds for most of the CWA, with the best chances of
precipitation likely limited to the coast.

For as normal as today is expected to be for the first day of June,
Sunday will be anything but normal as a potent late-season low
pressure system couples with a moderate to strong atmospheric river
to bring a very wet day to SW Washington and NW Oregon. The 00z ECMWF
Extreme Forecast Index (EFI) continues to suggest a high-end event
climatologically, with EFI values in excess of 0.9 for most of our
Cascades and a Shift-of-Tails (SOT) in the 1 to 2 range. The SOT
values in excess of 1 indicate the potential for rainfall to approach
record levels for this time of year. In other words, Sunday`s system
will behave much more like one of our moisture-laden mid- to
late-autumn frontal systems than the typical weaker systems we would
come to expect in late spring. If anything, the 00z GEFS has more
moisture involved with Sunday`s system, with NAEFS means solidly in
the +4 to +6 standardized anomaly range for moisture transport and a
return interval (in late May/early June) on the order of 10-30 years.
All this points to a high-end rainfall event for early June, and one
that will qualify as a significant rain event for any time of the
year across our region.

Perhaps the most concerning aspect of this system is the combination
of unusually deep moisture with an unusually strong early June
system, along with strong orographic enhancement for the Coast Range
and Cascades. Fortunately, most of our rivers are starting off low or
near summertime base flows, which should mitigate most hydro concerns
for this event. However, the potential for extreme rainfall rates is
there, especially for the higher terrain where HREF 90th percentile
rain rates suggest there could be a solid 6-12 hour period with
rainfall rates exceeding 0.25-0.40"/hr. This could be enough to cause
some minor debris flows, especially in recently burned areas. Such
rainfall rates could also cause some of our flashier creeks and
rivers to jump their banks - the Grays River in particular comes to
mind, but much will depend on the alignment of the atmospheric river
during the peak of our event.

Overall, based on NBM deterministic guidance, inland valleys can
expect 1-2 inches of rain from this event through Monday night,
though downsloping may hold Hood River short of 1 inch. The coast can
expect to see 1.50-3.00 inches, with 2-4 inches likely for the Coast
Range and Cascades. Looking at 75th percentile NBM guidance, there is
the possibility for some Coast Range/Cascade locations to exceed 4
inches. This potential rings especially true after looking at the 00z
HREF mean QPF, which only runs through 5 PM Sunday but already shows
multiple areas in the Coast Range/Cascades exceeding 2 inches by
then... with 6-12 hours still to go before the steady, locally heavy
rain comes to an end. WPC suggested a marginal risk of issues due to
excessive rainfall in their Day 3 outlook for 12z Sun to 12z Mon -
which certainly seems warranted considering all these factors.

Latest deterministic guidance brings an end to the steadier rain
Monday morning as a fast-moving cold front sweeps across the Pac NW.
Cool air aloft behind this system will result in plenty of showers
Monday and, depending on how many sunbreaks there are, potentially
even a couple thunderstorms. Sunday through Monday will see
occasionally blustery S-SW winds as mixing increases and 850 mb winds
occasionally push into the 40-50 kt range, though the strongest winds
appear to be focusing on Sunday night based on the latest guidance.
Given the potential for mixing down of the stronger winds aloft is
higher than it usually is during our winter events due to the
stronger sun this time of year, just about anywhere in our CWA could
see 30-40 mph gusts at times Sunday through Monday. Exposed, higher
elevations in the Cascades could easily see periods with W-SW wind
gusts exceeding 50 mph Sunday through Monday. All in all, the weather
Sunday and Monday is likely to bring the all-too- familiar
"June-uary" back into our lexicon here in the Pac NW. For those who
are craving more typical summer weather, you won`t have to wait long
- see the long term discussion below for details on a potential heat
wave by next weekend.  Weagle

.LONG TERM...Tuesday through Friday...Subtropical moisture remains
entrained in the broader baroclinic zone stalled across the NE
Pacific and Pac NW Monday night. Meanwhile, most deterministic
guidance continues to depict a fast-moving shortwave being pushed
toward the Pac NW on the unseasonably strong jet stream. Some models
guide this system closer to the Canadian border, while others such as
the 06z NAM hint at a track far enough south to bring another round
of significant rainfall to at least our northern zones. Given the
moisture rich environment and the recent rainfall from Sunday, this
system will need to be watched closely as another shot of heavy rain
Tuesday could push some drainages out of their banks. NBM median QPF
for Tuesday`s system is less than 1 inch throughout the CWA, but
looking upwards to the NBM 90th percentile QPF, one can see that
there is some potential for higher 1-3 inch amounts for the Coast
Range and Cascades north of about a Tillamook-Detroit line.
Conversely, NBM 10th percentile QPF shows less than 0.25" anywhere in
our CWA, so considerable variability remains.

Starting midweek, models are suggesting a high likelihood of a strong
upper level ridge of high pressure developing somewhere over the
Western United States. Looking at the 00z WPC cluster analyses, all
clusters are now indicating positive 500 mb height anomalies over
WA/OR (i.e. above-normal temperatures in most cases) by Thursday.
Therefore confidence in above-normal temperatures by the end of the
week is very high - the challenge is in determining just how much
above normal temps will be.

Most 00z WPC clusters suggest the upper ridge axis will be far enough
inland to maintain at least some vestige of onshore flow in the
low-levels, so this does not appear to be the type of warm spell that
reaches the coast. The WPC cluster most favored by ensemble members
is also the hottest one, with the most significant 500 mb height
anomalies. However this solution is still only shown by 40-50% of
members - mostly from the EC ensemble suite - so things can still
change. Unsurprisingly though, NBM probs of reaching 90 degrees for
the inland valleys have increased a bit, reflecting the greater
number of members showing the hot solution. As of the 07z run of the
NBM, the probabilities of reaching 90 degrees in the interior
lowlands were generally 30-60% for Friday and Saturday, but the
probabilities of reaching 100 degrees remain below 15%. Pattern
recognition-wise, this has the look of a prolonged but low-intensity
heat event with multiple days in the upper 80s to mid 90s for the
inland valleys. This type of heat event, while not necessarily record
breaking, can still be problematic for those who are sensitive to
heat. With all this in mind, those sensitive to heat should pay close
attention to the forecast the next few days, and perhaps consider
where they may go to stay cool late next week should the hotter
forecast guidance prevail.  Weagle

&&

.AVIATION...VFR conditions prevailing, but with marine stratus
along the coast for low MVFR to IFR conditions. A weak front
associated with an upper trough to bring some light showers to
the coast today. The marine stratus along the coast is expected
to deepen later this morning and afternoon with MVFR expected to
prevail. Inland areas will see variable mid and high clouds with
cigs above 5000 ft.

Potent for June storm approaches overnight with rain bringing
widespread MVFR to IFR conditions late tonight to the coast.
Along with lower cigs, visibilities likely reduced due to the
rain. Inland areas likely to remain VFR but will see clouds
filling in over the mountains. HREF shows about a 20-40% chance
for MVFR cigs over the valley after 08Z Sun.

PDX AND APPROACHES...VFR conditions expected through the entire
TAF period with cigs above 5000 ft and a few sprinkles this
afternoon. As the next front approaches, HREF shows a about a 20%
chance for cigs near 3000 feet. Better chances for rain develop
closer to 12Z Sun. Light winds in the morning becoming westerly
5-10 knots after 20Z Sat. /mh

&&

.MARINE...A weakening Pacific front moves to the waters today and
stall over the waters. A more organized and stronger storm
system will move into the northeast Pacific Sunday shifting winds
southerly. Small Craft Wind gusts are likely with 90+% chance of
gusts exceeding 21 kts. There is a 10-20% chance for Gale Force
winds of 35 kts per the NBM. But deterministic and HREF guidance
have trended stronger with a 50-80% chance for gale force gusts
on Sunday. At this point, gale are more likely over the outer
zones (10 to 60 NM) during the day Sunday, so have issued a Gale
Watch. Also issued a Small Craft Advisory for the inner zones for
Sunday into Sunday evening. Will have to watch for a coastal jet
to develop just as the front nears the coastline Sunday afternoon.
This could result in a short period of gales to the coastline.

Seas will build to 8-10 feet at 10-12 seconds Sunday afternoon,
persisting into Monday. Another westerly swell will enter the
waters toward the middle of next week with seas of 10-12 feet at
14-16 seconds.
/mh -Batz

&&

.PQR WATCHES/WARNINGS/ADVISORIES...

OR...None.
WA...None.
PZ...Small Craft Advisory from 5 AM to 11 PM PDT Sunday for
     PZZ251>253.

     Gale Watch from late tonight through Sunday afternoon for
     PZZ271>273.
&&

$$

www.weather.gov/portland

Interact with us via social media:
www.facebook.com/NWSPortland
www.twitter.com/NWSPortland