986
FXUS66 KPQR 021143
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
441 AM PDT Sun Jun 2 2024

.SYNOPSIS...Today`s weather will be more like early November than
early June as an unseasonably strong and wet low pressure system
moves into the Pacific Northwest. Deep moisture with tropical origins
will lead to locally heavy rain, especially this afternoon and
evening in the higher terrain. Rainfall along the coast, Coast Range,
and Cascades may be sufficient to result in minor flooding of small
creeks and streams, as well as minor debris flows in recently burned
areas. Southwest winds will be blustery at times through Monday,
especially along exposed portions of the Cascades. Unsettled and cool
weather will persist through Tuesday, then a sharp trend toward
warmer and drier weather will develop mid to late week.

&&

.SHORT TERM...Today through Tuesday...As of 3 AM, SW Washington and
NW Oregon remain dry, but that will change in dramatic fashion as
rain spreads across the region this morning and intensifies this
afternoon and evening. Infrared and water vapor satellite imagery
across the Pacific is more reminiscent of late autumn than late
spring, as a strong and consolidated jet stream carries a continuous
stream of moisture from Taiwan all the way to 130W, knocking on the
door of the Pacific Northwest. Total precipitable water (TPW) usually
not seen in the Pac NW are also approaching, with TPW values of 1.3
to 1.6" beginning to cross 130W. The moisture has been enhanced by
the remnants of earlier tropical activity in the West Pacific.

With a moderate to strong atmospheric river containing a deep layer
of moisture, moderate to strong synoptic lift, and surges of
orographic enhancement powered by surges of 35-45 kt SW flow at 850
mb, the ingredients are there for very heavy rainfall rates in the
Coast Range and Cascades. Between roughly 9 AM this morning and 6 AM
Monday morning, 00z HREF 90th percentile rainfall rates of
0.3-0.5"/hr are common in the higher terrain, with HREF means
generally around 0.25"/hr. This will add up quickly, with HREF means
suggesting widespread rainfall totals of 3 to 5 inches in the higher
terrain by Monday morning. The HREF 90th percentile QPF is closer to
4 to 8 inches. Fortunately, most of our mainstem rivers have come
down close to their summer base flow levels, so we are not
anticipating mainstem river flooding. Probabilistic river guidance
shows less than a 5 percent chance of any of our mainstem rivers
flooding due to this event. There will be significant rises on some,
but flooding is not anticipated. While mainstem rivers are not
expected to flood, smaller creeks and streams may be a different
story, especially in recently burned areas. There may also be minor
debris flows in recently burned areas due to the heavy rain. Due to
concerns about these issues, WPC has also issued a Slight Risk of
excessive rainfall for these areas of heavier rain in their Day 1
outlook for 12z Sunday through 12z Monday.

For the inland valleys, this will also be an impressive rain event by
June standards, but the rainfall rates should remain less problematic
than those in the higher terrain. High-res guidance shows significant
rain-shadowing of the Willamette Valley as strong SW flow downslopes
off of the Coast Range. Despite the heavy hourly rainfall rates
mentioned above for the Coast Range, HREF means are generally less
than 0.1"/hr for the Willamette Valley with 90th percentile values
rarely more than 0.2"/hr. This suggests a steady, light to
occasionally moderate rain today and tonight for the inland valleys.
A cold front will bring an end to the steady rain early Monday
morning, with more showery precipitation expected for Monday.
Overall, HREF means suggest 0.75-1.50" for the inland valleys today
through Monday, with the lighter amounts west of I-5 due to
above-mentioned rain-shadowing. HREF 90th percentile QPF is on the
order of 1-2 inches with a similar distribution.

Not only will today be unseasonably wet, but also blustery as
southerly MSLP gradients ramp up throughout the day. Latest 06z NAM
guidance shows 45-55 kt SW flow as low as 900 mb over much of NW
Oregon this afternoon. While the rain will do a lot to stabilize the
air mass, there will probably be enough mixing to bring some of that
momentum down to the surface. Therefore any location in our CWA could
easily see gusts 30-40 mph this afternoon, and possibly again late
tonight/early Monday morning as the cold front moves through.
Exposed, elevated portions of the Coast Range and Cascades will
likely see gusts exceeding 50 mph at times through Monday morning.

Snow levels will dip behind the cold front Monday, perhaps as low as
5000 ft north of Santiam Pass. This dip in the snow levels should be
brief and non-impactful. We do not issue winter highlights for
elevations above 6000 ft, but we are aware that May and June are peak
timing for those planning to climb the Cascade volcano peaks.  Anyone
with plans to climb these peaks should refrain from attempting to do
so...the peaks will see occasional blizzard conditions today through
Monday morning. Heavy snow, poor visibility, and strong winds will
make it hazardous for anyone exposed to the conditions up at summit
level.

The unseasonably strong/zonal Pacific jet stream will send another
system toward the Pac NW Monday night into Tuesday, but the strongest
forcing and deepest moisture for this system appear likely to aim
north of our forecast area. NBM 75th percentile QPF still bring more
than 1 inch of rain to the N OR Coast Range, Willapa Hills, and S WA
Cascades, with 0.25-0.50" for the Portland metro area. That said,
10th percentile NBM QPF shows less than 0.25" throughout our CWA with
no rain in the Portland metro southward, so there remains a wide
variety of model solutions for how Tuesday will play out. At this
point, it appears the best forecast is for some rain and slightly
below normal temps in our northern zones, with a drier forecast and
temps returning closer to seasonal norms (lower 70s in the interior
lowlands) for our southern zones.  Weagle

.LONG TERM...Wednesday through Saturday...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 remains 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. NBM probs of reaching 90 degrees for the inland valleys have
remained steady at 30-60%, highest near the Portland metro Friday and
Saturday. The hottest members of the ensemble suite appear to have
backed off a bit, as NBM probs of reaching 100 degrees are now
generally below 10%. Pattern recognition-wise, this has the look of a
prolonged but low-intensity heat event with multiple days in the mid
80s to lower 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...In advance of a soaking rain later today IFR/LIFR
stratus has filled in at the coast. Inland areas generally VFR
with areas of MVFR have develop mainly at locations near Gorge.
HREF showing widespread MVFR developing (60-80% chance) inland
after about 16Z Sun and continuing overnight while IFR conditions
continue across the coastal areas. As the rain increases will also
see visibility reduced. In additions, mountains will become
obscured in clouds and pcpn.

Winds start out generally be under 10 kt then increase out of the
south this morning. Southerly winds are forecast to gust up to 35
kt along the coast and up to 25 kt inland this afternoon and
evening.

PDX AND APPROACHES...VFR in the area as of 09Z Sun, but had a few
hours of MVFR at PDX, TTD. HREF captured this period of MVFR well
with MVFR chances rising to around 40% at 08Z Sun, then tapering
off to 10% by 11Z. So far, so good. HREF then shows MVFR
returning with 40% chance by 16Z Sun, and 80% chance by 18Z Sun.
MVFR chances decrease to around 50% after 00Z Mon as pcpn
decreases. Locally heavy rain could bring visibility down to
MVFR thresholds. Expect southerly gusts around 20 kt, mainly
after 18z Sun through about 05Z Mon.
/mh

&&

.MARINE...A strong storm system, more typical in winter, will
move into the northeast Pacific Sunday lifting a warm front
across the waters this morning. NBM probabilities for Gale Force
winds are 20-40% (about 10% higher than 24 hours ago) but the
deterministic and HREF guidance are higher with a 70-90+% chance
for gale force gusts, mainly 15-21Z on Sunday. Given the upward
trend in guidance and better potential for a brief coastal jet,
the Gale warning looks reasonable. Small Craft Advisory continues
into Monday for winds and/or seas.  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 this afternoon,
persisting into Monday. Another westerly swell will enter the
waters toward the middle of next week with seas of 10-12 feet at
12-15 seconds. /mh -Batz

&&

.PQR WATCHES/WARNINGS/ADVISORIES...

OR...None.
WA...None.
PZ...Small Craft Advisory from 11 AM this morning to 5 AM PDT Monday
     for PZZ210.

     Gale Warning from 8 AM this morning to 2 PM PDT this afternoon
     for PZZ251>253-271>273.

     Small Craft Advisory from 2 PM this afternoon to 5 PM PDT
     Monday for PZZ251>253-271>273.

&&

$$

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