276
FXUS66 KPQR 061058
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
357 AM PDT Mon May 6 2024

.SYNOPSIS...Cool onshore flow will maintain showers across SW
Washington and NW Oregon through Tuesday, with unseasonably low snow
levels in the Cascades. Isolated thunderstorms or heavy showers will
have the potential to produce small hail today. A dramatic shift
toward warmer and drier weather comes mid to late week as low-level
flow turns offshore and strong high pressure builds aloft. Once
showers come to an end Tuesday evening, dry weather is expected
through the weekend.

&&

.SHORT TERM...Now through Wednesday...Latest infrared and water vapor
imagery from GOES-West reveals plenty of low-level moisture being
driven onshore across SW Washington and NW Oregon. This is leading to
widespread and fast-moving showers moving onshore and across the
region early this morning, but the moisture is lacking a cohesive
forcing mechanism to organize the showers, so amounts have generally
been light. The frontal system approaching the Pac NW coast appears
to have occluded down to around 44N/126W, so any isentropic lift is
very weak and will continue to be so as the frontal zone progresses
into the Cascades this morning.

Behind the front, the 00z GFS shows 500 mb temps falling to -25 to
-28 deg C this afternoon, steepening lapse rates and thus enhancing
instability enough to warrant a chance of thunderstorms and small
hail. The coldest air aloft will swing across the northern portion of
the CWA, with instability looking a little more marginal over Lane
County. Northern portions of the CWA will also see the front move
through early in the morning, possibly yielding a few sun breaks
later in the day. If sunbreaks are sufficient to push sfc temps into
the mid-upper 50s, both NAM/GFS deterministic soundings suggest
equilibrium levels could reach 15-18 kft with momentum possibly
allowing cloud tops to reach 20 kft. This would be deep enough
convection to produce a few lightning strikes today. With limited
instability and SBCAPE values struggling to reach 500 J/kg, these
cells won`t be particularly strong nor organized. Brief heavy
downpours can be expected from these showers, but steering flow will
be W-SW at about 20 kt so the downpours shouldn`t last particularly
long unless training occurs.

A trailing shortwave will keep flow onshore and lapse rates steep
enough to maintain showers tonight into Tuesday morning. After that,
strong high pressure will build in from the Pacific while temps warm
aloft. This will bring a swift end to the showers after sunset
Tuesday, as a subsidence inversion increasingly pushes a capping
inversion down below 10 kft and the air mass stabilizes with the loss
of solar heating.

Showers will continue to push into the Cascades through Tuesday
morning, then they should become less numerous throughout the day
Tuesday. Snow levels remain unseasonably low, but marginal
temperatures and strong May solar heating (even behind the clouds)
will likely lead to just wet roads for the midday and afternoon
hours. As has been the case the last few nights, conditions
deteriorate for the Cascade passes during the cooler night and
morning hours, when the lack of solar energy makes it easier for snow
to accumulate on paved surfaces. With this in mind, will extend the
Winter Weather Advisory through 8 AM Tuesday morning, after which
conditions should improve considerably with solar heating and
decreasing showers. By the end of Tuesday, another 3-8 inches of snow
are expected for the Cascade passes and above.

Tuesday night will need to be watched for the potential of frost in
the lowlands, as residual cool air and good radiational cooling
conditions are expected. The air mass may warm up sufficiently to
keep the frost/freeze threat limited to our coldest and most outlying
valleys such as the upper Nehalem and Hood River Valleys. Will need
to assess how quickly skies will clear Tuesday afternoon and evening
to determine whether or not frost/freeze products are warranted.

Wednesday will be transition day as strong high pressure asserts its
dominance over the Pac NW weather pattern. Any morning clouds should
clear quickly for plenty of afternoon sunshine. With the chilly start
and flow still not totally offshore, Wednesday`s high temperatures
will probably remain in 60s for the lowlands despite the rapidly
warming air mass aloft. That said, NBM probabilistic guidance
suggests a 30-50% chance of reaching 70 degrees Wednesday afternoon
across the PDX metro, so it is not out of the question the inland
valleys reach the lower 70s as early as Wednesday.  Weagle

.LONG TERM...Thursday through Sunday...If you`re ready for a
break in the rain, the long term forecast is for you. If you`re
looking for temperatures in the 70s, and 80s, then mid and late
this week are definitely for you! A large area of high pressure
builds over the Pacific starting on Wednesday. Clusters are in
very good agreement of this ridging, and how amplified it is
stretching from off the coast of California up through southern
Canada. The extent of this high will increase easterly flow as
thermal troughs form along the coast. There will be a slight gap
winds through the Columbia River Gorge and downsloping off of
the Coast Range. There is still a lot of room for conditions to
change, but there are very high probabilities (90% or higher) of
temperatures rising into the 80s on Friday for all lowlands that are
more than a couple miles from the coast, with similar high
probabilities continuing into Saturday for lowlands east of the Coast
Range. NBM chances of reaching 90 degrees have now increased to
40-60% for the PDX metro Friday and 30-50% for Saturday. Elsewhere in
the interior valleys, the chance of reaching 90 deg F each day is
generally less than 20% but not zero. Regardless of whether or not
anyone actually reaches 90 degrees Friday or Saturday, confidence is
high that these will be the warmest two days so far this year for
much of the forecast area. Onshore flow likely returns for some
cooling and coastal low clouds Sunday.

The main concern later this week is not so much the rising
temperatures themselves, but the combination of the warm weather with
rivers that are still very cold. Most rivers draining the Cascades
will be loaded with fresh snowmelt, running fast and cold. With river
temperatures in the 40s for many drainages, cold water shock is a
very real possibility. So - while it may be tempting to jump into a
refreshingly cool river or lake as temperatures warm up - doing so
would be quite dangerous without proper equipment. Same goes for the
ocean - sea surface temperatures are mostly in the lower 50s along
the coast.  Weagle/Muessle

&&

.AVIATION...The next front is currently in the process of moving
through the area, with intermittent MVFR conditions at most
terminals. KONP sees the strongest rain, and will likely remain at
LIFR visibility and ceiling thresholds until the heaviest of the
rain passes at 17-18z Mon. By 21z, all terminals will see at least
a 75% chance of VFR conditions returning. Weak showers continue
throughout the rest of the TAF period, with generally VFR
conditions expected.

From late Monday morning onwards (18z), there is a chance
(15-20%) for some slight thunderstorm activity throughout the
region. Winds will generally shift west and become weak over the
course of daytime hours Monday.

PDX AND APPROACHES...A mix of VFR and MVFR conditions continue,
with passing showers bringing brief reductions to MVFR ceilings at
times for the next few hours until 15-17z Mon. Afterwards, less
than a 10% chance of MVFR ceilings for the rest of the TAF period.
From 18z onwards, there is around a 15% chance of weak isolated
thunderstorms throughout the rest of daytime hours and evening
hours, through 07Z Tue. /JLiu

&&

.MARINE...The active weather pattern decreases into Monday with a
small break of milder weather, followed by NW winds picking up
again Monday afternoon to just barely above 21 kts. Afterwards,
seas rise to just about 10-11 ft over Monday night, and the Small
Craft Advisory has been extended through Tuesday morning for the
north and central waters. Afterwards another break in significant
weather is expected during daytime hours Wednesday, but winds look
to pick up again Wednesday evening, bringing Small Craft Advisory
conditions back to the area. North winds look to peak right around
30 kts Wednesday night and Thursday morning. Chance of gales
looks to currently be 10-20% so holding off on a Gale Warning for
now. /JLiu

&&

.PQR WATCHES/WARNINGS/ADVISORIES...

OR...Winter Weather Advisory until 11 PM PDT this evening for
     ORZ126>128.

WA...None.
PZ...Small Craft Advisory until 8 AM PDT Tuesday for PZZ210.

     Small Craft Advisory until 11 PM PDT Tuesday for PZZ251-252-271-
     272.

     Small Craft Advisory until 8 AM PDT this morning for PZZ253-273.
&&

$$

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