526
FXUS66 KPDT 200535
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
1035 PM PDT Fri Jul 19 2024

.AVIATION...06z TAFs...VFR under dry conditions with smoke/haze
present at times. Visibility obscuration due to smoke are not
currently anticipated, though will remain monitored. Wind will
generally remain light at 12 kts or less through the TAF period.
Rising temperatures over the weekend could result in High Density
Altitude at terminals. Branham/76

&&

.PREV DISCUSSION... /issued 345 PM PDT Fri Jul 19 2024/

SHORT TERM...This Afternoon through Sunday night...A weak
shortwave passing over the forecast area this afternoon has led to
slightly suppressed afternoon temperatures relative to yesterday,
but temperatures remain 5-10 degrees above normal. Despite modest
instability (MUCAPE of up to 500-1000 J/kg) accompanied by some
weak synoptic forcing associated with the aforementioned
shortwave, convection over the eastern mountains has been limited
to a cumulus field, presumably due to very dry air aloft (visible
on low/mid-level water vapor imagery).

Tonight through Saturday, an anomalously strong mid- to upper-
level ridge of high pressure will continue to rebuild over the
Pacific Northwest with hot to very hot conditions Saturday
through Monday. Excessive Heat Warnings and Heat Advisories are in
effect.

Saturday night into Sunday, a weak vorticity max/lobe riding up
the coast coupled with increasing PWATs and mid-level moisture
will facilitate some high-based showers. Confidence in
thunderstorms for any given location is currently low, but
confidence is medium-high (50-80% chance) in at least infrequent
lightning for central OR and portions of eastern Oregon and
south-central Washington. Currently, highest confidence in
lightning is for central Oregon in Deschutes, Jefferson, and Crook
counties, as well as western Grant County in eastern Oregon.

Sunday evening, an approaching trough of low pressure will induce
strengthening onshore-directed surface pressure gradients and a
push of cooler marine air east of the Cascades, channeled through
the Cascade gaps. This will persist through Sunday night and
present elevated fire weather concerns for new and existing fires.
The moister air mass will yield higher dew points and RHs which
may limit widespread overlap of wind and RH criteria. Plunkett/86

LONG TERM...Monday through Friday...

Key Messages:

1. Pattern change expected early in the long term

2. Possible thunderstorms over the eastern mountains on Monday

3. Much cooler than in recent days

4. Breezy afternoon winds through the Cascade gaps each afternoon

Models are in good agreement in having a pattern change from the
persistent strong ridge of the last couple of weeks to having a
trough in control of the Pacific Northwest through the long term
period. This will give us cooler temperatures through the week
dropping to a few degrees below normal by Friday. The timing of
the arrival of the trough is still a bit up in the air on Monday
with the GFS bringing in cooler air earlier and the ECMWF giving
us even warmer high temperatures than on Sunday. Monday will be a
classic breaking down of the ridge situation and the GFS has ample
instability over the eastern mountains so have kept a slight
chance of thunderstorms with little if any rain Monday afternoon
and early evening. Foresee no rain or thunderstorms after Monday.
Pressure gradients will also tighten up with the trough and breezy
winds through the Cascade gaps are likely each afternoon.

Monday will have the most uncertainty as to the temperatures. The
cooler GFS and NBM have temperatures in the 90s to about 103 in
the lower elevations while the ECMWF has temperatures of 100 to
108 in the Columbia Basin and in the mid to upper 90s in the rest
of the lower elevations. Have gone with the NBM temperature which
are about 5 degrees cooler than Sunday with highs in the mid 90s
to lower 100s and mainly 90s in the mountains. As mentioned above,
have kept a slight chance of thunderstorms in the eastern
mountains as the instability is there though front timing may keep
it from happening. Afternoon winds will reach 15 to 25 mph in the
Kittitas Valley, Columbia Gorge and the western Columbia basin
and 10 to 20 mph elsewhere.

Tuesday and Wednesday, the trough will move close to the coast
while the ridge moves into the Rockies. Temperatures Tuesday
will cool to the lower to mid 90s and in the 80s in the mountains.
Wednesday will be a degree or two warmer. Afternoon winds will be
a few mph lighter than Monday.

Thursday will see the trough move ashore with a central low in
British Columbia. This will bring in cooler air and temperatures
will drop 5 or so degrees to the 80s and lower 90s. Friday will
see the the trough axis move overhead, cooling temperatures a
couple more degrees to the 80s with mid 70s to lower 80s in the
mountains. Breezy west to northwest winds of 15 to 25 mph are
expected each afternoon. Perry/83

FIRE WEATHER...Hot, dry and unstable conditions are anticipated
over the weekend as a robust mid- to upper-level ridge of high
pressure amplifies over the Pacific Northwest. Red Flag Warnings
are in effect from 11AM Saturday until 11PM Sunday for fire
weather zones OR642 and OR644. Additionally, there are concerns
for an elevated fire threat due to increasing winds late Sunday as
cooler air associated with an approaching trough pushes into the
region. This will produce sustained winds of 10-20 mph with
minimum RH values in the teens and 20s. Through the remainder of
the week, onshore gap flows are expected to redevelop each day.
Plunkett/86

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  96  59 102  64 /   0   0   0   0
ALW 100  64 105  67 /   0   0   0   0
PSC 101  64 105  66 /   0   0   0   0
YKM  99  60 102  65 /   0   0   0   0
HRI 101  63 105  65 /   0   0   0   0
ELN  96  62 102  64 /   0   0   0   0
RDM  93  53 100  60 /   0   0   0  10
LGD  94  59 100  62 /   0   0   0   0
GCD  97  58 103  65 /   0   0   0   0
DLS  96  64 106  71 /   0   0   0   0

&&

.PDT WATCHES/WARNINGS/ADVISORIES...
OR...Excessive Heat Warning from 11 AM Saturday to 10 PM PDT Monday
     for ORZ041-044-049-050-502-503-505-507-508.

     Red Flag Warning from 11 AM Saturday to 11 PM PDT Sunday for
     ORZ642-644.

     Heat Advisory from 11 AM Saturday to 8 PM PDT Monday for ORZ506-
     510-511.

WA...Excessive Heat Warning from 11 AM Saturday to 10 PM PDT Monday
     for WAZ024-026>030-521.

     Heat Advisory from 11 AM Saturday to 8 PM PDT Monday for WAZ523.

&&

$$

SHORT TERM...86
LONG TERM....83
AVIATION...76