916
FXUS66 KPQR 131707
AFDPQR

Area Forecast Discussion...Aviation Update
National Weather Service Portland OR
1007 AM PDT Sat Jul 13 2024

.SYNOPSIS...Above normal temperatures will persist for inland
portions of SW Washington and NW Oregon through next weekend, though
onshore flow should keep triple-digit heat out of the forecast area.
Flow aloft will turn more southerly Tuesday, potentially carrying
moisture and instability northward from California. This will lead to
about a 5-10% chance of thunderstorms Tuesday afternoon into Tuesday
night, with the best chance being over the Cascades Tuesday afternoon
and evening.

&&

.SHORT TERM...Today through Monday...Strong upper level high pressure
has taken residence over the Four Corners region while a weak upper
level low persists off the California coast. Stable W-SW flow aloft
persists over the Pac NW with high 500 mb heights in the upper 580s
dam range, which remains about 5-10 dam above normal. Thermal low
pressure remains stronger east of the Cascades versus west, which
will allow onshore flow to persist through the weekend. The result is
a shallower-than-normal marine layer which will keep the coast and
coastal valleys cool, but not so much the interior. With 850 mb temps
in the +17 to +19 deg C range, this means inland valleys can expect
highs around 90 degrees today and Sunday. The air mass cools slightly
Monday, with the 00z EC ensembles showing 850 mb temps in the +15 to
+18 deg C range. The difference will probably be hardly noticeable,
but a few more valley locations may stay in the 80s Monday afternoon,
and a slightly deeper marine layer may lead to a few morning clouds
pressing inland into the Portland metro area Monday morning.

Although the air mass is presently too dry and stable for
thunderstorms, there are signs that may start to change Sunday night
into Monday. Various models open up and eject the upper low presently
off the California coast northward, potentially advecting elevated
moisture and instability northward by Tuesday. At this point the NBM
still suggests moisture and instability will be insufficient for
thunder, giving less than a 5% chance anytime in the forecast period.
Suspect chances are in fact a little higher than this, based on
deterministic 00z GFS/GEM - this may show up in future cycles of the
NBM. Given that this is still 3-4 days out, will allow the dry NBM
forecast to ride for the time being - but the evolution of that upper
low/trough will be important to watch over the coming days, as our
recent hot/dry weather has dried out vegetation across our area...
making it more susceptible to ignition by any lightning strikes that
would occur.  Weagle

.LONG TERM...Tuesday through Friday...As mentioned in the short term
discussion, all eyes will be on the upper low/trough lifting
northward from the California coastal waters Tuesday. Flow aloft
turns more southerly, adding moisture and instability aloft, but NBM
suggests this will be insufficient for thunderstorms in our CWA,
giving only about a 5% chance as the highest probability. In reality,
would put the probability higher than that based on pattern
recognition, with the best chance (10-15%) being Tue night/Wed
morning. Given the low confidence, we decided not to fight NBM and to
let its dry forecast to ride for the time being. If the upper
low/trough ejects north-northeastward Tue/Tue night as the 00z
deterministic GFS/GEM suggest, this would probably be followed by a
deeper marine layer and somewhat cooler temps inland Wednesday and/or
Thursday. Latest ensembles suggest a fairly quick recovery in 500 mb
heights behind the upper trough, so it still appears temperatures
will remain above mid-July climatological averages for the
foreseeable future.

Another note about Tuesday - this time regarding temperatures.
Tuesday has the potential to be the hottest day this week with 00z
GEFS/EC ensemble mean 850 mb temps generally in the +20 to +22 deg C
range. This could support mid 90s with full sunshine and no marine
influence, but that may not be the case on Tuesday due to the
shortwave energy advecting toward us from the south at that time. The
southerly flow aloft could easily advect debris clouds (or smoke)
from Monday`s convection and fires, which could keep temps a little
cooler. That said, any relief in temperatures would probably come
with an increase in humidity, so Tuesday has the potential to be an
uncomfortably very warm and humid day across the forecast area.
Weagle

&&


.AVIATION...Surface high continues to impact the region, with
marine stratus currently in the process of lifting. Mixed MVFR/IFR
conditions in place at coastal terminals, expected to lift by
early afternoon, 20z Sat at the latest. Coast terminals return to
IFR by 02z Sunday, with a 20% chance of LIFR during that time.
Inland terminals will just remain VFR throughout the TAF period.
Winds will also pick up in the afternoon, with winds around 10-15
kts, gusting to 20 kts.

Residual smoke and smog across the general area continue to
obscure distant topography, but visibilities should continue to
remain well above 6 SM.

PDX AND APPROACHES...VFR conditions through the TAF period.
Northwesterly gusts, with winds up to 10 kt at times during
Saturday afternoon. Generally few/no concerns expected. /JLiu

&&

.MARINE...High pressure will maintain northerly winds across the
waters for much of the next week. Winds will generally be
strongest off the central coast of Oregon and lightest off the
mouth of the Columbia River each day. Winds will typically peak
each day in the late afternoon and evening hours.

Probabilities of 20 kt winds or greater do decrease substantially
(to less than 20%) across the waters off even the central coast
of Oregon towards next Tuesday and Wednesday. In the meantime,
seas will primarily be a hybrid of fresh swell/wind-driven waves
through the weekend. /Neuman

&&

.PQR WATCHES/WARNINGS/ADVISORIES...

OR...None.
WA...None.
PZ...Small Craft Advisory until 5 PM PDT this afternoon for PZZ252-
     253.

     Small Craft Advisory until 5 AM PDT Sunday for PZZ271>273.
&&

$$

www.weather.gov/portland

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