NWS Portland, OR wrote its last area forecast discussion before the first event at May 28, 10:09 AM PST, 3 hours 51 minutes earlier.
Area forecast discussion, May 28, 10:09 AM PST, 3 hours 51 minutes before
Area Forecast Discussion
National Weather Service Portland OR
1109 AM PDT Sat May 28 2022
.SYNOPSIS...A particularly cool and wet storm system for late May
will persist through Sunday before conditions begin to dry out and
slowly warm early next week.
&&
.Discussion...Today through Memorial Day...Visible satellite imagery
this morning reveals an usually strong surface low pressure for late
May spinning just off the Oregon coast. Models have come into
agreement the surface low pressure will likely drop
east-southeastward across the Oregon coast somewhere between Waldport
and Lincoln City before pushing eastward into the southern Willamette
Valley later this afternoon or evening. The greatest 1 and 3 hour
surface pressure falls remain in the KSLE to KRBG corridor, which
lends further support to the aforementioned model(s) track. Doppler
radar and satellite imagery show a band of deeper clouds and moderate
rain with embedded heavier showers spreading northeastward across
Lane, Lincoln, Benton and Tillamook Counties at the moment. As the
surface low pressure moves closer to the coast, this band of rain
will pivot northeastward across the rest of the CWA this afternoon
and result in a rather soggy several hour stretch for places that
have temporarily dried out like Aurora, Portland, Vancouver, Hood
River and Kelso/Longview. It appears the band of rain should push
into the Portland metro for example between Noon and 130pm.
Breezy south winds will develop briefly along the coast today before
relaxing as the surface low pressure moves overhead. However, the
west to northwest winds behind this surface low pressure are
particularly strong and will likely slam rather abruptly onto the
central coast of Oregon (particularly across Lane County) late this
afternoon and evening per HREF and other model guidance. Individual
cloud elements per GOES17 imagery are moving at ~50 kt at the moment
on the backside of the low pressure so the strong wind field modeled
has observational support. Nonetheless, current thinking is that wind
gusts will generally top out in the 40-55 mph range along the central
coast of Oregon, particularly across Lane County where the strongest
winds appear most likely to materialize. Cannot rule out gusts to 60
mph, but think these would be extremely isolated and limited to the
most exposed headlands. As a result, issued a Special Weather
Statement to cover the wind hazard later this afternoon and evening.
Given winds this strong out of the west to northwest are fairly rare
and with deciduous trees being fully leafed out, we could see isolated
down trees and power outages, but do not think winds will be strong
enough to support anything more than that. Given the holiday weekend
and the fact there will be a lull in the winds ahead of the strong
burst of wind, am most worried that unsuspecting boaters along the
lower reaches of the Siuslaw River could get into trouble when the
winds pick up suddenly.
Modest low to mid level frontogenesis and strong low level westerly
flow should aid in the highest precipitation amounts being focused
across Cascades south of Santiam Pass later this afternoon and
evening. Do not foresee any hydro issues. Nonetheless, the area
around the Three Sisters will likely experience well over a foot of
snow when all is said and done above 6000-7000 feet. Snow levels will
be falling closer to 4000-4500 feet by late tonight into Sunday.
Accumulations may struggle to reach strict advisory criteria at that
point across Santiam and Willamette Passes, but given the holiday
weekend and the large numbers of people spending time outside and
traveling across the Cascade passes, will keep the Winter Wx Advisory
going to highlight it will still feel and look like winter in the
Cascades tonight into Sunday.
Shallow conditional instability and low to mid level west to
northwesterly flow will keep showers going along the coast and across
the higher terrain for much of Sunday. As inland valleys heat up
towards midday Sunday, expect showers to intensify and become more
widespread across the eastern half of the Willamette Valley and
Cascades. Even after modifying model soundings to reflect forecasted
surface high temperatures around 60F, instability appears likely to
remain shallow enough to prevent a mentionable risk of thunderstorms
due to a relatively strong subsidence inversion near 650-700mb. This
is supported by no contoured risk (i.e. <5% and <10%, respectively)
in the 12-hour SREF thunderstorm probabilities and the 4-hour HREF
thunderstorm probabilities. However, NBM 12-hour thunder probs climb
to 13-14% in the Willamette Valley Sunday afternoon/evening. Given
the bulk of the 300-500 J/kg of SBCAPE and 100 J/kg of MLCAPE will be
centered in the lowest 10kft of the atmosphere, any vorticity that
gets sucked into a stronger updraft, tilts more vertical and
stretches could produce a brief cold core funnel cloud or brief
landspout. As is often the case, the probability of this happening at
any given location is super low, but I will certainly not be
surprised if we get a report of one somewhere in the lower Columbia
or Willamette River valleys on Sunday.
Showers should then become increasingly confined to the coast and
higher terrain Sunday night into Monday morning as inland valleys
cool and stabilize. There may be just enough instability remaining
below 700mb to allow a few showers to redevelop Monday afternoon
across the inland valleys as daytime heating kicks in towards the
midday hours, but these showers should be lighter and much more
widely scattered than Sunday's showers. /Neuman
.LONG TERM...Monday night through Friday...No changes...previous
discussion follows...A warming trend will begin early next week as a
shortwave ridge over the eastern Pacific slowly moves eastward over
the Pacific Northwest. High pressure should be strong enough on
Tuesday and Wednesday for dry weather across the entire forecast
area. This is also when temperatures will become much warmer. Inland
high temperatures likely in lower to mid 70s for Tue, and closer to
80 for Wednesday. /Rockey
&&
.AVIATION...18z TAFs: Mostly just minor adjustments to the 12z
package to account for timing of the front. Recent obs have lagged
behind guidance a bit with timing of lower ceilings and onset of
steadier rain, but still anticipating cigs lowering to MVFR KEUG
and KSLE between 19-20z and then spreading to the Portland area
terminals between 20-22z. Coastal sites will deteriorate to low
MVFR/IFR through this afternoon as heavier rain moves onto the
coast in advance of the low. Expect a rapid wind shift from
southerly to northwesterly as at KONP as the low moves onshore
between 00-03z Sun. Expect partial clearing from southwest to
northeast as possibly a return to VFR by Sunday morning as rain
transitions to showers after 10-12z Sun. Backbuilding off of the
Cascade foothills may maintain MVFR or even IFR cigs a bit longer
in the eastern Willamette Valley and possibly some of the Portland
area terminals.
For detailed regional Pac NW aviation weather information, go
online to: https://weather.gov/zse
KPDX AND APPROACHES...Light rain now spreading into the area from
the southwest ahead of the approaching front should eventually
result in MVFR conditions as cigs lower to around 025. Expect rain
and MVFR to persist well into this evening before winds shift
around to WNW and rain transitions to showers after 09-12z Sun.
Expect VFR to return Sun morning, but HREF probs maintain a
roughly 20-30% chance for MVFR cigs through 18z Sun due to
backbuilding cloud cover off the Cascade foothills. /CB
&&
.MARINE...At 2 AM buoy observations across the board show seas
hanging around 6 ft at 9 seconds, with winds generally staying
below 15 knots. This will change quickly throughout the next
several hours as a compact low approaches the waters from the
west.
Unfortunately, model guidance is still not in great agreement on
the exact track of the low, which means confidence in the timing,
direction, and strength of the most intense winds across any point
within the waters remains low. This is particularly true of zones
PZZ275 and PZZ255 and the far southern part of PZZ250 and PZZ270,
as the heart of the low is forecast to pass directly through this
area. Calm winds will occur at the low's center, with the most
intense westerlies passing immediately south of it. Once the low
moves through, a tight pressure gradient will develop in its wake
and Gale Force northerly winds will be possible for a period of
several hours across all of the waters. The northern inner waters,
zone PZZ250, continues to have a high-end Small Craft Advisory, as
confidence in Gale Force northerly winds there remains lower than
in the other waters. However, a brief period of gusts to 35 knots
will be possible Saturday evening - mainly in the southern half
of that zone. We will continue to monitor trends in the track and
intensity of the low to determine whether an upgrade to a Gale
Warning is necessary for that zone.
A composite blend of models now indicates around an 80 percent or
greater chance of Gale Force gusts south of Cape Meares in the
24-hr period ending at 11 PM Saturday. This probability falls off
to less than 10 percent at the mouth of the Columbia River.
Likewise, a 50 percent or greater chance for wind gusts over 40
knots is forecast by the same model blend in the far southwestern
portion of zone PZZ275. As the low tracks over the waters strong
southerly winds are expected through Saturday afternoon south of
Cape Lookout. For areas north of Cape Lookout, offshore winds are
likely as the low moves closer to shore which will produce choppy
conditions. A bent back occlusion will produce strong
northwesterly winds into Saturday evening which will keep the Gale
Warning in place until 11 PM Saturday. Elevated seas into the mid
teens are possible within the Gale Warning zones.
Winds will gradually ease, but seas will remain elevated and
choppy to perpetuate marginal Small Craft conditions across the
waters on Sunday. Mariners are highly encouraged to keep an eye on
the forecast throughout the holiday weekend. Winds and seas will
diminish below advisory criteria late Sunday as high pressure
builds over the NE Pacific.
The next opportunity for Small Craft conditions is expected on
Wednesday, mainly across zones PZZ275 and PZZ255 where a thermal
low will extend north from northern CA to foster brisk northerly
winds gusting occasionally over 20 knots.
-Bumgardner/Weishaar/BrianaMuhlestein
&&
.PQR WATCHES/WARNINGS/ADVISORIES...
OR...Winter Weather Advisory from 8 PM this evening to 11 AM PDT
Sunday for Cascades in Lane County-Northern Oregon Cascades.
WA...Winter Weather Advisory from 8 PM this evening to 11 AM PDT
Sunday for South Washington Cascades.
PZ...Small Craft Advisory until 8 AM PDT Sunday for Coastal waters
from Cape Shoalwater WA to Cascade Head OR out 10 nm.
Gale Warning until 11 PM PDT this evening for Coastal waters
from Cascade Head to Florence OR out 10 nm-coastal waters
from Cape Shoalwater WA to Florence OR from 10 to 60 nm.
&&
www.weather.gov/portland
Interact with us via social media:
www.facebook.com/NWSPortland
www.twitter.com/NWSPortland