968
FXUS66 KPQR 062220
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
320 PM PDT Mon May 6 2024

.SYNOPSIS...Active weather Monday and Tuesday will give way to
high pressure through the remainder of the week. Thunderstorms
are the threat this afternoon, then skies will partially clear
and temperatures will drop. Cannot rule out frost development in
the valleys tonight and again early Wednesday morning.
Temperatures will rise considerably.

&&

.SHORT TERM...Now through Wednesday Night...It`s an active
afternoon with widespread showers popping up throughout the
area. Radar is showing areas of small hail (around 50+ dBz) in
some showers, especially east of I-5. Rain accumulations with
these showers is around 0.1 inch in 10-15 minutes which is quite
heavy. However, they are fairly fast moving so the length of
time for the downpours are not necessarily long enough for urban
flooding to be an issue. However, if we continue to get heavier
downpours in cities or areas with poor drainage, cannot rule out
localized ponding of water. Thus far there have only been a
few strikes detected in eastern Clark County (Washington),
though with there being moisture in the charge separation zone
of the atmosphere (around -15 deg Celsius), there remains
around a 20% chance for thunderstorms to develop.

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 3-10 inches of snow are expected for
the Cascade passes and above. Will note though that the presence
of gusty winds may impact the ability for snow to properly
accumulate. Have decreased the snow rations to reflect this
possibility.

Front continues to be a challenge Tuesday night into Wednesday
morning. The National Blended Models suggests around a 20%
chance for at or below freezing overnight temperatures from
McMinnville south with higher probabilities along the Coast
Range and far southern Willamette Valley. The challenge will be
how dry conditions will be, if skies will clear, and whether
the east winds will be at the "right speed" for that frost to
form. Agricultural communities within the Willamette Valley,
Hood River Valley and valleys through the Coast Range.

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. It is not out of the question the
inland valleys reach the lower 70s as early as Wednesday.
-Muessle/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


.LONG TERM...Thursday through Monday...Fairly quiet long term
forecast as high pressure develops over the region. This pattern
is lovingly referred to as "big bubble no trouble" as skies will
be clear, conditions will be dry, and winds will be easterly.
Guidance ensemble model guidance is in good agreement that this
ridge will develop, it`s just the intensity and where the axis
peaks. Based on the surface level pressure gradient, expecting
that ridge to be vertically aligned with the west coast on
Wednesday afternoon which will develop a thermal trough. This
thermal trough (low pressure induced by increasing
temperatures). This thermal trough will cause easterly winds to
increase ushering in dry and warmer air from east of the
Cascades. Relative humidity will tank, and conditions will
continue to warm. Models are suggesting that Saturday will be
the warmest of the weekend, but, with the ridge beginning to
weaken that day, we will see a transition to onshore flow,
temperature rises may be stifled. Either way, looking at some
of the warmest temperatures thus far this year. With it being a
celebratory weekend AND having warm weather, remember the
hazards that are present. The water is very cold (around 40-45
degrees) and thus cold water drownings are possible. Heat
related illnesses are also possible to be sure to keep yourself
cool if you`re susceptible to heat. -Muessle


&&

.AVIATION...Northwesterly flow aloft and moist onshore lower level
flow will maintain post-frontal showers across the area today.
Predominately VFR expected (80-90% chance) 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. Westerly
winds with gusts to around 20 kt expected through 04Z Tuesday.
-DH

&&

.MARINE...Breezy west to northwest winds and postfrontal showers
continue through Tuesday morning. Generally expect winds around 10
to 20 kt across the waters, with stronger convective showers
producing gusts to 25 kt. Then high pressure begins to build
across the waters on Tuesday, bringing more tranquil weather. A
increasing northwesterly swell will cause seas around 7 to 8 ft
to build to around 9 to 10 ft through Tuesday. Will maintain the
Small Craft Advisory in place through Tuesday evening.

Then, winds turn northerly as high pressure persists offshore and
the thermal trough strengthens along the CA/OR coast. This summer
like pattern will bring breezy north winds with gusts to 30 kt on
Wednesday.-DH

&&

.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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