476
FXUS66 KPDT 081749
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
1049 AM PDT Mon Jul 8 2024

.SHORT TERM UPDATE...The forecast updates for this morning were
fire weather related--issuing Red Flag Warnings and Fire Weather
Watches for hot, dry and unstable conditions. The warnings cover
the Cascades and central Oregon starting today through Tuesday
evening, and the watches cover the eastern mountains and valleys
starting Tuesday afternoon through Wednesday evening. The upper
level ridge is shifting eastward, and the axis of the ridge is
along the Cascade range. The inverted surface thermal trough will
also be positioned along the Cascades. This is a pattern conducive
to enhanced fire behavior, and confidence is about 80% that any
fires will exhibit growth and instability. Watches were issued for
the Blues, Wallowas, and south to the John Day Basin for hot, dry
and unstable starting late Tuesday. Not only is there a moderate
degree of confidence in the instability and high confidence in
critically low RHs, there are hints of mid level moisture that
could contribute to isolated high based thunderstorms. Not all of
the HREF members advertise weak simulated reflectivity, so the
probability at this point is around 15%. Since this is a slight
chance, the afternoon package will show this in our southeast
zones. Wister/85

&&

.AVIATION...18Z TAFS...VFR conditions expected throughout
this period. Skies will be clear with terrain-driven winds less than
10 kts. Feaster/97


&&

.PREV DISCUSSION... /issued 335 AM PDT Mon Jul 8 2024/

SHORT TERM...Today through Wednesday...Water vapor imagery
tonight shows the axis of a strong upper level ridge has moved
inland, and is settled over western OR/WA at this time. Great
agreement amongst ensemble and deterministic guidance that the
upper ridge axis will transit the PacNW through Tuesday, where it
will exit into ID/MT where it will ultimately stall across
eastern ID and western MT Wednesday. Tuesday afternoon into
Wednesday morning, a weak vort and jet max in the mid levels will
lift across central and eastern OR, providing weak
instability/uplift to the region, though moisture support will be
weak with a fairly pronounced dry layer in the lower levels. All
said, confidence in instability leading to isolated dry
thunderstorm chances is very low(10%). However, there is a threat
for hot, dry, and unstable conditions to lead to enhanced fire
weather potential on new and existing fires across central and
eastern OR Tuesday(confidence 40-50%). At this time, confidence
not high enough to issue any fire weather highlights for the
unstable conditions. The upper ridge will also flatten Wednesday
as an upper trough in the Gulf of Alaska ejects into BC. This will
introduce a cooling trend into the region, though HeatRisk will
remain major across the Columbia Basin and adjacent valleys
Wednesday. Surface pressure gradients will also tighten in
response to this system, producing breezy winds through the
Cascade gaps and into portions of central OR and the Columbia
Basin. Little to no surface moisture will make it`s way into the
intermountain PacNW, resulting in widespread RHs still in the
teens. The combination of the winds and low RHs have warranted the
issuance of Fire Weather watch for the Columbia Basin and the
Kittitas valley Wednesday. More details will follow in the fire
weather section.

Excessive Heat warnings and Heat advisories will continue through
Wednesday for widespread HeatRisk values in the major category,
with localized areas of extreme categories in the Columbia Basin
Tuesday. The peak of this heatwave is expected Tuesday as the
upper ridge axis moves across the forecast area. In most lower
elevation zones, chances of exceeding 105 degrees are between
65-95% Tuesday, while chances of exceeding or meeting 110 in
portions of the Columbia Basin are 40-60%. On Wednesday, areas
closer to the Cascades are expected to cool by about 5 to 7
degrees into the upper 90s to around 102. However, the Columbia
Basin, Yakima valley, and the northern Blue mountain foothills
will continue to see the hotter temperatures near 105, with
chances of exceeding 105 between 60-90%. Lawhorn/82

LONG TERM...Thursday through Monday...While the long-lasting ridge
will lose its potency by Thursday and we`ll see a relief from
widespread triple digits temps, the synoptic pattern in the long
term still looks to support relatively hot conditions as well as
heightened fire weather concerns via breezy winds through the
Cascade Gaps, which could extend well into the Basin.

Ensembles are in good agreement that by Thursday, the ridge will
flatten out and a more zonal pattern will take its place, however
these types of patterns are typically supportive of gap flows due to
a favorable orientation of the winds aloft. NBM probabilistic
guidance already showing several consecutive days of >70% chance of
wind gusts exceeding 35 mph through our usual wind prone areas: the
Columbia River Gorge, Kittitas Valley, Simcoe Highlands, and
extending into the Oregon Basin. While this zonal pattern may
support advection of more mild air through the Cascade Gaps via west
winds, the dry synoptic pattern will allow high temps to remain well
in the 90s for most of our lower elevation and valley zones, so RHs
will likely (70-80% confidence) remain at critical levels well into
the teens even as temps begin to cool after the peak of this ongoing
heat wave.

Much of the period is dominated by this dry zonal pattern, however
should note that both the deterministic GFS and ECMWF try to bring
in a SW trough through the region around the beginning of next week.
Obviously a bit too far out to determine how much moisture would
potentially accompany this system, but the potential for mountain
showers and thunderstorms always exists with these SW lows. Given
how dry the boundary layer will be through the period, however, dry
thunderstorms could be a concern as well. Confidence in the forecast
beyond Sunday, however, is low (<20%).


&&

.PRELIMINARY POINT TEMPS/POPS...
PDT 102  64 107  69 /   0   0   0   0
ALW 104  69 108  72 /   0   0   0   0
PSC 106  69 110  72 /   0   0   0   0
YKM 105  68 108  70 /   0   0   0   0
HRI 107  66 111  70 /   0   0   0   0
ELN 103  67 106  68 /   0   0   0   0
RDM 102  61 105  61 /   0   0   0   0
LGD  99  62 102  66 /   0   0   0   0
GCD 101  62 105  64 /   0   0   0   0
DLS 110  71 112  72 /   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 Tuesday 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.

     Fire Weather Watch from Tuesday afternoon through Wednesday
     evening for ORZ642>645.

     Heat Advisory until 10 PM PDT Tuesday for ORZ509.

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

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

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

     Heat Advisory until 10 PM PDT Wednesday for WAZ030.

     Fire Weather Watch from Tuesday afternoon through Wednesday
     evening for WAZ692-693.

     Heat Advisory until 10 PM PDT Tuesday for WAZ522.

&&

$$

SHORT TERM...85/82
LONG TERM....74
AVIATION...97