687
FXUS66 KPDT 171736
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
1036 AM PDT Wed Jul 17 2024

.AVIATION...18z TAFs...Mostly VFR with the exception for instances
of MVFR to IFR conditions through around 02z due to showers and
thunderstorms. KDLS, KYKM, and KALW are the favored terminals to
see this continued shower or storm development through the late
afternoon hours. Thunderstorm probabilities are low (10-20%) for
development across the terminals. Otherwise, VFR conditions will
be found. Surface wind across the western terminals (KDLS, KRDM,
KBDN) will be 10-20 kts after 20z today, with locally higher
gusts. The remainder of the area should see generally light wind
at less than 10 kts, with the exception of gusty and erratic wind
near any thunderstorm development. Branham/76

&&

.PREV DISCUSSION... /issued 934 AM PDT Wed Jul 17 2024/

MORNING UPDATE...Water vapor imagery reveals the center of a low
pressure system over western Oregon this morning, with a band of
broken shower and isolated thunderstorm activity leading the warm
front as it lifts through Oregon and into central Washington. This
system has produced thunderstorm activity across central Oregon,
and has been somewhat weakened this morning by a lack of
instability, but that is expected to change once the afternoon
rolls through and the low gets an opportunity to tap into midday
sunshine. 12z HREF suggests shower and storm activity will flare
up once again this afternoon and evening across primarily the
Oregon Basin northward, with some more isolated activity across
the eastern mountains. Red Flag Warning remains in effect for the
Oregon and Washington Basin zones, as well as the Washington
Cascades zones, as storms are still expected to remain dry in
nature.

The Red Flag Warning for the Oregon Cascades and central Oregon
zones were allowed to drop off as the low has moved north out of
the area. Minor touch-ups were made to PoPs based on the 12z HREF,
and temps were lowered a tad, otherwise changes to the forecast
were overall minor. Evans/74

PREV DISCUSSION... /issued 520 AM PDT Wed Jul 17 2024/

AVIATION...12Z TAFs...VFR conditions are forecast through the
period for all sites, though a low pressure system transiting the
region will facilitate widespread mid- and high-level clouds as
well as shower and thunderstorm chances across the region today.
Ongoing shower and thunderstorm activity will continue to lift
north-northeast across north-central Oregon into south-central
Washington through the morning. Confidence in lightning activity
at terminals is too low to include mention in TAFs at this time.
This afternoon, there is more uncertainty with regard to
convection across the Columbia Basin and foothills of the Blue
Mountains; current thinking places a 15-20% chance of isolated
thunderstorms in the vicinity of PDT/ALW/PSC/YKM, but confidence
in timing and location precludes mention in TAFs. Gusty outflow
winds of up to 45 mph have been observed with ongoing convection
and will be possible with any afternoon activity. Plunkett/86

PREV DISCUSSION... /issued 418 AM PDT Wed Jul 17 2024/

SHORT TERM...Today through Friday...Multiple weather concerns
over the next 12-24 hours revolving around isolated thunderstorm
potential and heat. Initial concerns revolve around isolated
thunderstorms capable of infrequent CG lightning and strong
outflows, the former promoting potential for new fire starts.
Isolated storms have been confined to central OR and west of the
Cascades overnight so far with predominantly very infrequent
lightning (5 or less CG strikes/5 mins) with the activity over the
last three hours. Some gusts this evening have been notable with
some sites seeing above 45 mph from thunderstorms. Otherwise, heat
products remain unchanged with existing heat advisory areas
outside the Lower Columbia Basin, Yakima Valley, and Kittitas
valley dropping off this evening. The aforementioned areas will
see an additional day of heat-impacts with the head advisory and
excessive heat warning ending tomorrow/Thursday evening.

Latest water vapor imagery shows dry slot moving across central OR
as a shortwave trough is lifting north northeast across western
OR. Isolated thunderstorms have been seen to be forward of the
tropopause anomaly where there is strong broad ascent. Very
limited instability but a primed environment with steep mid-level
lapse rates and decent mid-lower level moisture over 700-500 mb as
moisture continues to move north and east. This combined with
enhanced orographic lift will keep a risk for isolated
thunderstorms in place. Of which, the risk will be spreading and
shifting north and east through the morning becoming more confined
to south central WA today. That said, there is a low chance
(15-20%) for isolated storms across the Blues this afternoon as
well. A decent signal is seen across the WA Cascades with the
latest HREF when it comes to 4-hr max 1-km dBZ > 40 dBZ (which
exceeds 50%), alongside 4-hr lightning probs that exceed 60%. This
increases confidence in the RFW for abundant lightning with lower
chances then extending farther east into the Lower Basin of
Washington and Oregon. The threat remains sufficient across the
Lower Basin so no change to the Red Flag Warning.

Meantime, highs will be 2 to 4 degrees warmer outside the Cascades
with heat headlines continuing today owing to NWS HeatRisk of
high-end moderate to major. Greatest risk for heat impacts tied to
the Kittitas and Yakima Valleys driven by mild morning lows this
morning and overnight tomorrow. Little to no change in highs
tomorrow across the Lower Columbia Basin and the Yakima and
Kittitas valleys. As such, the headlines extend one day further
in those areas. Of which, highs around 15 to approaching 20
degrees above normal today and tomorrow.

Looking ahead, shortwave ridge builds in behind the exiting
disturbance Thursday with southwest flow in place. Highs and lows
still above average but are sufficient to lower the risk of
impacts from heat. The dominant upper ridge across the desert SW
shifts west by Friday with its ridge axis extending north across
the Interior NW promoting dry and fair weather. However, triple
digit temperatures more likely than not across the lower
elevations. 60-80% for exceeding 100 F on Friday across the Lower
Basin with lower chances/lower confidence elsewhere.

LONG TERM...Friday night through Wednesday...Confidence is high
(>95% chance) in continued above-normal temperatures Saturday
through Monday as an amplified mid- to upper-level ridge remains
over the West. While ensemble guidance does showcase some spread
in temperatures over the weekend, the current forecast is
supportive of heat highlights for most of our forecast zones.
Additionally, low- elevation population centers in the Columbia
Basin and adjoining foothills and valleys have a 20-60% chance of
exceeding 105 degrees on Saturday and Sunday.

More uncertainty surrounds chances of convection. Analysis of
ensemble clusters reveals over 60% of members track some form of
a vorticity maximum into the Pacific Northwest sometime between
late Saturday through Sunday. Timing of the shortwave currently
favors convection the most on Sunday.

Monday through Wednesday, ensembles suggest a potential ridge
breakdown as a low pressure system in the Pacific attempts to
push onshore. While significant variance among ensemble members
exists with regard to pattern details, a ridge breakdown would
facilitate shower and thunderstorm chances as well as cooler
temperatures and a return to breezy winds through the Cascade
gaps. Ensemble clusters are roughly split on whether the low will
be able to displace to ridge or if the ridge will divert the low
northwest into Canada. Plunkett/86

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  99  65  98  64 /  20  10   0   0
ALW 102  69 101  67 /  20  20   0   0
PSC 101  69 103  68 /  20  20   0   0
YKM  98  65 101  63 /  20  20   0   0
HRI 102  69 103  67 /  20  10   0   0
ELN  96  66  99  65 /  20  20   0   0
RDM  94  56  95  55 /  10   0   0   0
LGD  95  62  96  61 /  20  10   0   0
GCD  98  60  97  60 /  20   0   0   0
DLS  98  65  94  67 /  20   0   0   0

&&

.PDT WATCHES/WARNINGS/ADVISORIES...
OR...Red Flag Warning until 10 PM PDT this evening for ORZ641.

     Heat Advisory until 10 PM PDT this evening for ORZ041-505-507-
     508-510.

     Heat Advisory until 10 PM PDT Thursday for ORZ044.

WA...Red Flag Warning until 10 PM PDT this evening for WAZ690-691-694-
     695.

     Heat Advisory until 10 PM PDT this evening for WAZ024-029-521.

     Heat Advisory until 10 PM PDT Thursday for WAZ028.

     Excessive Heat Warning until 10 PM PDT Thursday for WAZ026-027.

&&

$$

SHORT TERM...80
LONG TERM....86
AVIATION...76