604
FXUS66 KPQR 061823 AAA
AFDPQR

Area Forecast Discussion...UPDATED
National Weather Service Portland OR
1123 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 Night...The front that has
brought widespread rain the last few days has moved eastward and
post frontal showers linger in it`s wake. 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...Northwesterly flow aloft and moist onshore lower level
flow will maintain post-frontal showers across the area today. A
mix of MVFR and VFR this morning is expected (80-90% chance) to
become predominately VFR by 21Z this afternoon as cigs continue
to lift. But, any stronger shower through this evening may briefly
reduce visibility or cigs at times. There is also a 15-25% chance
for thunderstorms this afternoon into early evening. Chances for
MVFR along the coast increases to around 30-50% overnight as high
pressure begins to build.

PDX AND APPROACHES...Predominately VFR expected with rain showers
through tonight. Heavier showers may bring brief reductions to
MVFR at times. Instability increases this afternoon bringing a
15-25% chance of thunderstorms through early evening. Southwest
winds around 10-12 kt expected to become more westerly by 21Z
Monday with gusts to around 20 kt. -DH

&&

.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 Advisory for
now. -JLiu

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...Winter Weather Advisory until 8 AM PDT Tuesday 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.
&&

$$

www.weather.gov/portland

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