267
FXUS66 KPDT 091705
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
1005 AM PDT Tue Jul 9 2024

Updated Aviation Discussion

.AVIATION...18Z TAFS...VFR conditions expected for this period.
KRDM/KBDN will have few high-level clouds passing by with afternoon
gusty winds up to 21 kts. Otherwise, mainly clear skies with terrain-
driven winds less than 10 kts. Feaster/97

&&


PREV DISCUSSION... /issued 319 AM PDT Mon Jul 9 2024/

.SHORT TERM...Today through Thursday...Upper level ridging
will continue to dominate the region today through tomorrow, and
with it, heat and fire weather concerns will continue. The upper
ridge axis will continue east today, with the ridge axis moving
directly over eastern WA/OR through the afternoon. At the
surface, an inverted thermal trough centered along the Cascade
crest will gradually move east with the upper ridge passage. This
will result in the peak of the heatwave across the intermountain
PacNW with most, if not all, lower elevation locations in the
triple digits. In fact, NBM probabilities show a 60-95% chance of
areas outside of the mountain zones meeting or exceeding 105
degrees this afternoon, with a 40-65% of 110 degrees in areas of
the Columbia Basin and the eastern Gorge. Increasing instability
across the Cascade crest as well as central and eastern OR will
also be a concern today, owing to the surface thermal trough
over the region. This is congruent with 00Z HREF members
continuing to show weak reflectivity signals developing, forecast
soundings showing fairly impressive high based inverted V, and
even deterministic guidance showing increased relative humidity
values at 500mb. As a result, Red Flag Warnings have been issued
along the Cascade crest and central OR through this evening, with
Red Flag Warnings in effect across the eastern mountains today
through Wednesday evening. While instability will be present,
confidence in any isolated thunderstorms is very low(10-15%),
owing to the deep, dry airmass that has settled over the region.

By tomorrow, the upper ridge axis will be centered over eastern
ID with the thermal trough centered near the ID/OR border. There
will be two features to look at here for Wednesday, the first
being an upper trough passage into BC that will flatten the ridge
by Wednesday evening. This will support a cooling of 5 to 7
degrees across those areas on the periphery of the Cascade east
slopes, as well as across central OR and through the eastern Gorge
tomorrow afternoon. In these areas, chances of meeting or
exceeding 100 degrees are only 30-50%. The trough passage to our
north will also tighten pressure gradients across the Cascades,
resulting in breezy winds through the Cascade Gaps and into
portions of the Columbia Basin. Sustained winds 15-25mph and
gusts up to 40mph will develop in these areas, and will coincide
with relative humidities still in the teens. While confidence is
mod- high(60-80%) in red flag conditions, will keep the Fire
Weather Watch in place across the aforementioned areas to allow
for another run of short- term guidance before upgrading. The
second feature, will be a weak shortwave riding up the backside of
the ridge that will clip the eastern mountains. This shortwave
will provide enough synoptic lift to support unstable conditions
from the thermal trough across the eastern mountains, hence why
the Red Flag Warning continues through Wednesday evening in this
area. As for the Columbia Basin, Yakima valley, and the eastern
mountains/valleys, the lingering upper ridge influence will be
strong enough to produce temperatures near or 2 to 3 degrees below
this afternoon, with chances of exceeding 105 degrees 70-90% in
the Columbia Basin and Yakima valley.

By Thursday, zonal flow aloft will setup over the forecast area
as the upper ridge continue to flatten to our south. That
said, marginally hot conditions in the mid 90s to lower 100s will
continue over the forecast area, with chances of exceeding 100
degrees in portions of the Columbia Basin between 30-50%. The
zonal flow aloft will continue to promote a tightened cross
Cascade pressure gradient, resulting in breezy winds continuing
through the Kittitas valley and eastern Gorge. While fire weather
concerns will remain elevated with the continued winds, the cooler
temperatures and very weak marine push will keep relative
humidities above critical thresholds. Lawhorn/82


.LONG TERM...Friday through Tuesday...Don`t expect these hot and dry
conditions to go anywhere anytime soon, because even as the high
pressure driving this ongoing heat wave breaks down, ensemble
guidance remains in good agreement on a dry pattern through the
beginning of next week. Temps will certainly cool without the
presence of relentless high pressure, but the pattern replacing it
will keep hot temps in place, with highs across the lower elevations
and valleys well into the 90s, and potentially hovering around 100
for the lower Basin.

The synoptic pattern will be mostly zonal with a slight SW
orientation in the winds aloft, however guidance is pretty gun-shy
on moisture thus far, so not really anticipating widespread precip
threats until a more organized SW flow arrives in the region. That
being said, the deterministic GFS does have a shortwave skirting us
to our northwest on Friday, and both the GFS and ECWMF try to bring
in a trough around the latter half of the upcoming weekend. Neither
feature shows up particularly well in ensemble clustering, so
initial thoughts are that impacts will be on the lighter end precip-
wise, however these kind of systems trekking through our forecast
area do tend to lead to an uptick in winds, especially through the
Cascade Gaps. And with how dry we`ll likely be, this could
potentially lead to critical fire weather conditions. Confidence is
only moderate at this time (40-50%), as the overall synoptic pattern
still looks fairly benign, but models are in good agreement in RHs
remaining in the teens across the entire forecast area, so even a
weaker system could bring in breezy conditions that end up
triggering headlines.

The overall pattern through the tail end of the period is marked by
weak SW flow aloft, but should note that ensembles do hint at more
amplified SW flow around the middle of next week. Confidence is low,
however, as to be expected with a forecast more than a week out.
Evans/74

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT 106  68 103  61 /   0   0   0   0
ALW 108  70 107  65 /   0   0   0   0
PSC 109  71 109  65 /   0   0   0   0
YKM 108  70 105  64 /   0   0   0   0
HRI 110  70 108  65 /   0   0   0   0
ELN 107  68 101  62 /   0   0   0   0
RDM 105  61  98  53 /   0   0   0   0
LGD 101  65 100  58 /   0   0   0   0
GCD 104  64 102  59 /   0   0   0   0
DLS 110  71 100  63 /   0   0   0   0

&&

.PDT WATCHES/WARNINGS/ADVISORIES...
OR...Excessive Heat Warning until 10 PM PDT Wednesday for ORZ041-044-
     505-507-508-510-511.

     Red Flag Warning until 11 PM PDT this evening for ORZ610-611-639-
     640.

     Fire Weather Watch from Wednesday afternoon through Wednesday
     evening for ORZ641.

     Heat Advisory until 10 PM PDT Wednesday for ORZ049-050-502-503.

     Red Flag Warning from noon today to 11 PM PDT Wednesday for
     ORZ642>645.

     Heat Advisory until 10 PM PDT this evening for ORZ509.

WA...Excessive Heat Warning until 10 PM PDT Wednesday for WAZ024-
     026>029-521-523.

     Red Flag Warning until 11 PM PDT this evening for WAZ694-695.

     Fire Weather Watch from Wednesday afternoon through Wednesday
     evening for WAZ690-691.

     Heat Advisory until 10 PM PDT Wednesday for WAZ030.

     Red Flag Warning from noon today to 11 PM PDT Wednesday for
     WAZ692-693.

     Heat Advisory until 10 PM PDT this evening for WAZ522.

&&

$$

SHORT TERM...82
LONG TERM....74
AVIATION...97