Area Forecast Discussion Issued by NWS Portland, OR
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000
FXUS66 KPQR 281035
AFDPQR
Area Forecast Discussion
National Weather Service Portland OR
330 AM PDT Thu Mar 28 2024
.SYNOPSIS...
Post-frontal showers with a slight chance of thunderstorms
continuing through Thursday evening as a low pressure system spins
offshore. Conditions begin to improve on Friday, with high
pressure bringing another period of warm and dry weather this
weekend into early next week.
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.SHORT TERM...-- Changed Discussion --
Thursday morning through Saturday night...Between
midnight-2am Thursday, radar and satellite observations depicted a
line of thunderstorms training over the Newport area, with a few
thunderstorms also drifting over Depoe Bay, Lincoln City and Siletz.
Based on spotter reports, some of these thunderstorms were producing
brief wind gusts of 20-30 mph as well as small pea size hail.
Accumulating hail was reported in Newport around 12:15am, with hail
covering sidewalks and roads. Anyone experiencing accumulating hail
during/following heavier showers or thunderstorms should exercise
caution while driving as roads will be slippery until the hail melts
off. Farther inland, a line of showers was beginning to move back
into the Portland/Vancouver metro, but with no observed lightning.
The ongoing convective activity is occurring in the wake of
yesterday`s cold frontal passage while a closed low pressure system
spins around 400 miles offshore the WA coast (~992mb surface low).
With a relatively cold airmass aloft/instability in place, SBCAPE
values are expected to hover around 250 J/kg through Friday morning
along and near the coast. This will help maintain thunderstorm
chances, and the latest NBM backs this up well with a 20-30% chance
of thunderstorms at the coast and a 15-25% chance in the Coast Range
and Willapa Hills. Thunderstorm chances do spread into the Willamette
Valley and Cascade foothills Thursday afternoon (20% chance), but any
thunderstorms that do form inland should be isolated in coverage due
to excess cloud cover limiting instability. If more sun breaks occur
than expected, then thunder chances will increase a bit. For now, the
latest HREF ensemble mean for cloud cover and current satellite
imagery/trends suggest cloud cover will be stubborn to clear out for
extended periods of time due to frequent rain showers and cloud cover
associated with convective debris. In addition to showers and
thunderstorms, Thursday afternoon will also feature breezy south
winds with gusts up to 25-35 mph, strongest across the central
Willamette Valley. Locally stronger gusts up to 45 mph will be
possible for brief periods of time near the strongest showers or
thunderstorms. Wind speeds of this magnitude are strong enough to
down some tree branches here and there, but likely not strong enough
to down entire trees (unless it`s a weak or dying tree that`s ready
to fall). Would also like to mention that precipitation is falling in
the form of snow in the Cascades with several inches of new snow
expected at pass level. While snow amounts are not high enough to
warrant headlines, anyone with travel plans over the passes should
still be prepared for winter driving conditions as off-and-on snow
showers linger over the area through Friday morning.
Showers should finally decrease Friday afternoon or evening as
offshore flow develops over the area. Models suggest light offshore
flow will then continue through Saturday, bringing a period of dry
weather with milder temperatures. This will be especially true on
Saturday as the entire day will be dry with minimal cloud cover,
allowing the strong late March sun to warm temps into the mid 60s.
There is a 30-50% chance for high temps above 65F Saturday afternoon
across the central/northern Willamette Valley from Salem to the
Portland/Vancouver metro, but upper 60s seems to be the upper limit
as the probability for highs at or above 70F is around 1-5%.
Conditions look to remain dry Saturday night as low level onshore
flow redevelops, albeit weak. It`s also worth mentioning that frost
is likely in the Upper Hood River Valley, Coast Range, Willapa Hills
and Cascade foothills Friday night and Saturday night as skies will
be mostly clear, winds will be light, and temps will likely drop to
around 32-35F. This setup is common with northerly flow aloft this
time of year, which models and their ensembles are indeed showing.
Anyone living in these zones with sensitive outdoor vegetation should
consider bringing plants inside if possible, or taking other measures
to protect your vegetation from frost. If you have a morning commute,
it may be a good idea to give yourself a few extra minutes in case
you need to scrape frost off your windshield before hitting the road.
If you do see frost, be aware there is the potential for a few slick
spots on sidewalks and/or roads. Will hold off on issuing Frost
Advisories as we are not in the heart of growing season quite yet; we
only issue Frost/freeze headlines after the start of April when
growing season quickly picks up and impacts start to become more
noticeable for the agricultural community. -TK
.LONG TERM...Sunday through Wednesday night...The long term forecast
is highlighted by mainly dry weather with mild temperatures. The
EPS/GEFS/CMC ensemble mean all show northerly flow at 500mb
transitioning to northeasterly flow Sunday into Monday ahead of a
southwest-northeast oriented ridge axis over the coastal waters. The
ridge axis is currently set to move inland Monday/Monday night before
shifting east of the Cascades Tuesday.
This ridge will bring a period of dry and warmer weather to the area,
however there is some uncertainty regarding exactly how warm it will
be, especially on Tuesday. This is when model spread ranges from the
upper 50s to mid 70s for high temps over the Willamette Valley and
interior lowlands of southwest WA. The most likely outcome will be
highs somewhere in the 60s Sunday through Tuesday, except upper 50s
to around 60F at the coast. Monday and/or Tuesday should be the
warmest days of the week, and this is when the NBM shows a 20-50%
chance for high temps of 70F or warmer (highest probabilities on
Tuesday). The more noticeable model spread for high temps on Tuesday
makes sense given the scenario at hand, as the next Pacific frontal
system will be approaching the region at that time. There is
uncertainty regarding the exact timing of this system and the cloud
cover that will come with it. If model solutions showing a faster
progression of this frontal system verify, then cloud cover would
begin to increase through the day on Tuesday and limit high temps
quite a bit. If models showing a slower progression verify, then most
if not all of the day will be sunny and highs would likely rise above
70F over the interior lowlands given the relatively mild temps to
start the day. By Wednesday, it appears very likely we will cool down
as light rain returns with the aforementioned frontal system. -TK-- End Changed Discussion --
&&
.AVIATION...-- Changed Discussion --
The majority of terminals currently are experiencing
VFR conditions early this morning although conditions will likely
fluctuate between MVFR and VFR cigs through the day. These
fluctuations will be tied to the prevalence of shower activity
which is expected to increase late this morning into the
afternoon(after ~17-19z) at all inland sites, thus increasing the
likelihood for sub-3kft CIGS. While chances(20-25%) for isolated
thunderstorms remain pinned to the coast this morning, they`ll
again bleed inland this afternoon - something to keep in mind.
Confidence in thunderstorm timing is too low to warrant adding it
to the latest TAFs. Winds will also become gustier later this
morning into the afternoon with intermittent gusts of to 20-30
knots possible at all terminals.
* Note: The observations out of KONP are generally limited to wind
and pressure at this time. Periodically cloud cover, vis, and
present wx are reported but are unreliable.
PDX AND APPROACHES...Mainly VFR conditions continue through the
period although waves of showers will briefly drop ceilings to
MVFR criteria. Chances(40-60%) for reaching MVFR criteria peak
late this morning and this afternoon. Intermittent gusts continue
around 22-28 knots as well. /Schuldt-- End Changed Discussion --
&&
.MARINE...-- Changed Discussion --
Expect active and unsettled conditions the next couple
of days before a more summer-like pattern shifts overhead this
weekend into early next week leading to a return of calmer winds
and seas. Current satellite observations early this morning depict
the center of a broad area of low pressure approximately 300
nautical miles off the south Washington coast and as the day
progresses this feature will slowly meandering eastward before
stalling offshore. The movement of the aforementioned low pressure
system likely leads to an uptick in southwesterly winds and
convective showers with guidance heavily favoring(70-90% chance)
of Gale Force wind gusts (33-40 kt) mixing down to the surface
through this evening. We’ll also see a large west-southwesterly
swell push into the coastal waters today as well with seas
increasing into the 15-20ft range - highest Cape Falcon
southward. Seas will most likely peak sometime during the late
afternoon and early evening hours before falling back down into
8-12ft range on Friday. From this point late Friday through the
weekend model guidance is in agreement on a positively tilted
ridge of high pressure moving across the Pacific Northwest, thus
facilitating a more summer-like northerly wind pattern across the
coastal waters. Looking ahead, model hint at the return of a more
active weather pattern around the middle of next week, although
confidence is low on the exact timing and marine impacts at this
time. /Schuldt-- End Changed Discussion --
&&
.PQR WATCHES/WARNINGS/ADVISORIES...
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OR...None.
WA...None.
PZ...Hazardous Seas Warning until 5 AM PDT Friday for PZZ210.
Gale Warning until 8 PM PDT this evening for PZZ251>253-271>273.
Hazardous Seas Warning from 8 PM this evening to 11 AM PDT
Friday for PZZ251>253-271>273.-- End Changed Discussion --
&&
$$
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