052
FXUS66 KPDT 161622
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
922 AM PDT Tue Jul 16 2024

.MORNING UPDATE...No real update to the forecast. However, a Red
Flag Warning has been issued for the southern Blues and Strawberry
Mountains for dry and unstable conditions. With the fire already
on the landscape, these conditions will cause the fire and any new
fires to exhibit extreme fire behavior. Bennese/90



&&

.PREV DISCUSSION... /issued 444 AM PDT Tue Jul 16 2024/

SHORT TERM...Today through Thursday night...A compact vorticity
maximum that is currently approaching northwest CA and southwest OR
will be the main focus for weather this afternoon through Wednesday
as it tracks north-northeastward across western Oregon this evening
and overnight before ultimately exiting the region via northeast
Washington on Wednesday night.

Confidence is very high (>80% chance) that some high-based
convection will develop this afternoon across central Oregon as a
result of synoptic forcing and moisture advection associated with
the incoming shortwave coupled with modest (<500 J/kg) MUCAPE
advertised by the 00Z HREF. A deep, well-mixed boundary layer will
increase the likelihood of dry lightning and gusty outflow winds.

This evening, as the wave tracks northeastward, mid- to upper-level
forcing coupled with increasing PWATs should facilitate at least
isolated to scattered high-based showers (>50% confidence), though
CAMs are still exhibiting some run-to-run and model-to-model
differences regarding how robust the convection will be. Current
thinking is lightning will be limited overnight, though any that
occurs will likely not (>90% chance) be accompanied by a wetting
rain.

Instability ramps up along the east slopes of the Washington
Cascades Wednesday morning and afternoon, and forecast soundings
show more robust MUCAPE of 500-1000 J/kg, locally higher. Confidence
in isolated to scattered thunderstorms is highest there. What
remains in question is how much rain will accompany convection.
Forecast PWATs are certainly high enough to support a wetting rain
with some cells. Further east across the Blue Mountains of northeast
Oregon and far southeast Washington, uncertainty in convective
potential is high due to a wide spread in the forecast surface
moisture field. HREF members suggest dew points ranging from roughly
35 to 55 degrees. Should the higher moisture materialize, convection
would initiate along the Blues Wednesday afternoon, subsequently
diminishing Wednesday evening. Have opted to nudge PoPs over the
NBM`s values, but have not included a mention of showers or
thunderstorms due to low confidence.

Continued heat has prompted additional heat highlights across many
of our low-elevation zones today through Wednesday or Thursday.
Cloud cover Wednesday night should limit radiational cooling such
that overnight lows remain warm enough to push HeatRisk values to
Major across the majority of the Yakima and Kittitas valleys. In
response, have opted to upgrade the existing Heat Advisories into an
Excessive Heat Warning, valid noon today through 10PM Thursday.
Elsewhere, have included the John Day Basin in a Heat Advisory from
noon today until 10PM Wednesday.

Thursday, locally breezy westerly winds will develop through the
Cascade gaps and into the Columbia Basin as another shortwave trough
approaches the PacNW. Confidence is currently low (<50% chance) in
any wind or fire weather highlights on Thursday. Plunkett/86

LONG TERM...Friday through Tuesday...The primary sensible weather
concerns revolve around heat concerns this weekend and elevated
fire weather conditions. The latter for: a) dry and windy
conditions Monday across Cascade gaps/east slopes, the eastern
Gorge spilling into the Lower Basin, and Kittitas Valley and b)
very low isolated thunderstorm potential mainly late Sunday and
Monday, albeit with appreciable spread in timing/details.

An amplified large scale pattern is forecast early Friday that
will be characterized by a passing embedded mid-level shortwave
trough lifting northeastward across the PacNW with a longwave
ridge anchored across the Desert Southwest extending into Nunavut
and Northwest Territories. Ensemble guidance is in fairly good
agreement with the synoptic evolution through Sunday. Of which,
the strong amplified mid-level ridge will evolve westward to be
move over NV extending north into the Northwest Territories by
Saturday and Sunday with an upper trough/closed low upstream in
the eastern North Pacific.

Impressive ECMWF mean 500 mb heights forecast with mean heights
forecast outside model climo across Canada with interior PacNW
heights exceeding the 90th percentile. After a brief decline in
highs on Friday owing to a weak shortwave trough and its
associated front that hangs in the western NW, oppressive heat is
anticipated to build back across the interior NW this weekend.
Increased confidence in building heat buttressed by increasing 850
mb temperatures, ample insolation, dry air mass, and deep mixing.
ECMWF mean 850 mb temps forecast to exceed the 97th percentile
Saturday and 99th percentile Sunday. Of which, warmest 850 mb
temperature anomalies are centered over eastern WA, northeast OR,
and northern ID extending into Canada. Confidence in heat further
supported by Extreme Forecast Index values of 0.6-0.9 indicative
of a large number of raw ensemble members forecasting warmer than
average highs with many extreme forecasts in excess of 110 F in
the lower elevations. Latest NBM guidance show odds for 100 F or
more in excess of 75% across large parts of south central WA into
the lower Basin, foothills of the Blues, and John Day. Odds for
110 F are limited Saturday with the greatest seen Sunday between
15-25% across parts of the lower elevations. NWS HeatRisk is
showing widespread Major (category 3) Saturday and Sunday, the
latter day appearing the most impactful driven by mild, warm lows
with upper 60s forecast. Of which, if underdone, lower 70s would
increase the risk to sensitive groups and at-risk populations.

Start to see increasing differences Monday onward with clustering
scenarios showing the main source of spread/uncertainty revolving
around the upstream trough, its eastward progression/shunting of
the upper ridge, and possible embedded open waves. An initial
shortwave trough embedded in the flow is possible to rotate late
Saturday or Sunday, however, significant differences exist with
gudience in terms of timing. Confidence in this open wave
disturbance is not well supported (~15% confidence) Saturday with
a little more support in its occurrence Sunday (~25% confidence).
That said, moisture will be a limiting factor but the combination
of dry fuels/antecedent conditions with isolated thunder risk
bears mentioning. The greatest risk area is across the upper
slopes of the WA Cascades and central OR Sunday. The low chances
precludes a mention in the forecast at this time.

Thereafter there is greater potential for a disturbance affecting
the area on Monday (~40% confidence) based on ensemble clustering
with greater support amongst ensemble members. This will promote
very low potential for thunderstorms as well, though with the
threat area spreading eastward across the eastern mountains as
well. Furthermore, increased potential for dry and windy
conditions are anticipated across the eastern Cascade gaps,
eastern Gorge spilling into the Lower Basin, and Kittitas Valley.
24-hr chances for peak daily gusts greater than 39 mph exceeds 50%
across the aforementioned areas, except near 80% across Kittitas
valley. While there is growing confidence in a reprieve in temps
from this weekend by Tuesday (60-70%), there is still potential
that the upper troughs progress will be slow that would help the
heat linger into early next week.

AVIATION...12Z TAFs...VFR conditions area-wide presently with
some passing mid-clouds across northeast OR and south central WA.
Smoke has been noted at KJSY and KALW overnight. Outside haze,
mostly clear skies across the area until this afternoon as a
disturbance promotes increasing high-end VFR, mid- and high-level
clouds across central OR spreading across the remainder of the
area overnight.

There is low potential for isolated thunderstorms this
afternoon and evening at central OR terminals. This threat then
spreads to KDLS and KYKM during the small hours-tomorrow, and more
so the KPDT and KALW tomorrow afternoon/evening. However, low
confidence precludes a mention in the TAFs at this time.
Otherwise, winds will be light (less than 12 kts) with a weak
surface pressure differences in place. Otherwise, surface smoke
may be seen across northeast OR but confidence in sub-VFR
visibility is low (less than 50%) based on latest HRRR forecasts.

FIRE WEATHER...Fire weather watches remain in effect for much of
central and north-central Oregon as well as south-central Washington
due to the potential for dry thunderstorms late this afternoon,
overnight, and Wednesday. While locally breezy conditions are
forecast, especially Wednesday and Thursday through the Cascade
gaps, we are not currently forecasting overlap of critical wind and
RH.

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  99  65 100  65 /   0   0  20   0
ALW 102  67 103  68 /   0   0  20   0
PSC 102  67 102  68 /   0   0  20   0
YKM 101  68 100  65 /   0  10  20   0
HRI 103  67 104  68 /   0  10  20   0
ELN 101  66 100  66 /   0   0  20  20
RDM  98  60  96  55 /  10  20   0   0
LGD  98  61  97  60 /   0   0  20  10
GCD  98  64  99  58 /   0  20  20   0
DLS 105  70  99  67 /   0  20  20   0

&&

.PDT WATCHES/WARNINGS/ADVISORIES...
OR...Heat Advisory until 10 PM PDT Wednesday for ORZ041-505-507-508-
     510.

     Fire Weather Watch from 5 PM PDT this afternoon through
     Wednesday afternoon for ORZ610-611-639-640.

     Heat Advisory until 10 PM PDT Thursday for ORZ044.

     Red Flag Warning from 2 PM this afternoon to 11 PM PDT this
     evening for ORZ642.

WA...Heat Advisory until 10 PM PDT Wednesday for WAZ024-029-521.

     Heat Advisory until 10 PM PDT Thursday for WAZ028.

     Fire Weather Watch from this evening through Wednesday evening
     for WAZ690-694-695.

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

&&

$$

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