638
FXUS66 KPQR 020456 AAA
AFDPQR

Area Forecast Discussion...UPDATED
National Weather Service Portland OR
955 PM PDT Sat Jun 1 2024

Updated aviation discussion...

.SYNOPSIS...Increasing clouds and little light rain around this
evening. But, rain will be increasing later tonight into Sunday
morning as rather strong weather system arrives. Rain, will be heavy
at times Sunday afternoon through Sunday night. Front will push
inland early Monday, with some showers around for the rest of the day
through Tuesday. Then, high pressure builds over the western USA,
with a return of more typical late June warmth.
&&

.SHORT TERM...(Sat night through Mon)
KEY NOTES:
  1)  Rain, heavy at times, for Sunday and Sunday night

  2)  Local ponding of water expected in low-lying areas Sunday
      through Sunday night, but river flooding is NOT expected.

  3)  Showers for Monday and Tuesday

  4)  Warmer for Thu into the weekend, with inland highs in 80s

Mild afternoon around the region, under quite a bit of mid and high
clouds.  Temperatures mostly in the upper 50s to lower 60s along the
coast, with upper 60s to lower 70s across the interior. Have had few
sprinkles around the area, and that will continue into this evening.

No big changes to the current forecasts, as everything seems be on
track. Will leave previous discussion intact, as most seems
appropriate to repeat:

Weather for 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. Recent model trends of 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 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. Will have to monitor this, and see
where rainfall starts setting up on Sunday.

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...A robust frontal system will approach the region late
tonight, bringing widespread rain and falling cigs. Expect
predominately IFR cigs or lower along the coast through the TAF
period. As of 0430z Sun, conditions are VFR inland, but expect cigs
to drop to MVFR across all inland terminals after 16-18z Sun as the
front moves further inland. Locally heavy rain is possible at any
given terminal tomorrow (Sunday), which could lead to visibility
dropping down to IFR/MVFR thresholds. Winds tonight will generally
be under 10 kt and turn more southerly. However, after 12z Sun,
winds will begin to strengthen and become gusty. Southerly winds
tomorrow are forecast to gust up to 35 kt along the coast and up to
25 kt inland.

PDX AND APPROACHES...Predominately VFR cigs through early Sunday
morning with southerly winds 8-12 kt. After 16-18z Sun, there is
high confidence that cigs will fall to MVFR as the frontal system
pushes through. Locally heavy rain could bring visibility down to
MVFR thresholds. Expect southerly gusts around 20-25 kt, mainly
after 18z Sun.     -Alviz

&&

.MARINE...A strong 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. NBM probabilities
for Gale Force winds remain around 10-30% but the deterministic and
HREF guidance have trended stronger with a 60-90+% chance for gale
force gusts, mainly 15-21Z on Sunday. Given the upward trend in
guidance and better potential for a brief coastal jet, have upgraded
the Gale Watch to a Gale Warning for all zones. A Small Craft
Advisory goes into affect at 12Z before the Gale Warning at 15Z and
will likely need another Small craft as the winds ramp down later
Sunday.

Small Craft criteria wind gusts are also expected for the Columbia
River Bar late Sunday morning into the afternoon.

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 12-16
seconds. -Batz

&&

.PQR WATCHES/WARNINGS/ADVISORIES...

OR...None.
WA...None.
PZ...Small Craft Advisory from 11 AM to 5 PM PDT Sunday for PZZ210.

     Small Craft Advisory from 5 AM to 8 AM PDT Sunday for
     PZZ251>253-271>273.

     Gale Warning from 8 AM to 2 PM PDT Sunday for PZZ251>253-
     271>273.
&&


$$

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