289
FXUS66 KPDT 171727
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
1027 AM PDT Mon Jun 17 2024

Updated aviation discussion

.MORNING UPDATE...Current Satellite and radar imagery show
isolated showers with low stratus across portions of the Cascades
and eastern mountains, while mid to upper level cloud cover is
pushing in from the north across the lower elevations. This
activity and the well below normal temperatures this morning is
associated with an upper trough that is currently sitting over
eastern WA/OR. This trough will continue to bring isolated to
scattered showers across the mountains today, where higher areas
along the Wallowa and Strawberry mountains will see snow showers
through the afternoon. There is also increasing chances (15-30%)
of isolated thunderstorms across the eastern mountains this
afternoon, thanks to modest mid-level lapse rates between 7-8 C/km
and increasing surface CAPE values of 100-300 J/kg. Otherwise, an
increasing cross Cascade pressure gradient will result in breezy
to locally windy conditions through the Cascade gaps and the Lower
Columbia Basin, with the strongest winds along the Simcoe
Highlands, the eastern Columbia Gorge, and the Kittitas valley.
Lawhorn/82

&&

.PREV DISCUSSION... /issued 405 AM PDT Mon Jun 17 2024/

SHORT TERM...Today through Wednesday...Bottom Line Up Front:

1. Mountain snow/rain showers over higher terrain with slight
chance thunderstorms along the eastern mountains.

2. Breezy conditions Monday through the Cascade Gaps.

3. Below normal temperatures through today then a slight warm up.

Short term models are good agreement with the upper level low moving
over the area bringing with it precipitation. Along with the
precipitation, the upper level low is bringing in a rush of cold air
which will bring the snow levels down to near 4500-5500 feet.
Therefore some of the precipitation falling, especially above
4500 feet, will be snow with 80-100% probabilities of more than a
trace along the higher peaks of the WA & OR Cascades, Blues, and
along the Ochoco-John Day Highlands. With the influx of
instability associated with the system, model derived soundings
are showing an increase in MUCAPEs of 150-300 J/kg and lapse rates
of 8.4 C/km and higher. However, the remaining ingredients needed
for better lift and mid level moisture is lacking. PWATs are low
at only 0.4-0.5 inches, bulk shear is below 15 kts, and LIs are
only -1. However, cannot rule out a few lighting strikes. With the
incoming cold air and the amount of cloud cover that is over the
area, confidence in thunderstorms is low (less than 25%) along
the aforementioned areas.

Another round of breezy conditions will swing through as the upper
level low continues to push across the region. Looking at surface
model data, there is a clear tightening of the isobars as the low
pushes towards and over the Cascades. Looking at the pressure
gradient tool using the NAM40, GEFS, SREF and RAP40, it clearly
shows a strong gradient between PDX and GEG. The NAM is the most
bullish with an 11 to 12 mb change while the others are 9 to 10
mb. This leads to the conclusion that we will see yet another
round of breezy conditions along the Gorge, Kittitas and Yakima
Valleys as well as along the Southern Blues. 40-60% of the raw
ensembles show probabilities of these areas seeing 25 mph or
greater sustained winds with gusts to 50-70% probabilities of
seeing gusts to 40 mph. After today, winds will settle and return
to normal.

Lastly, EFI show temperatures to take a bit of a dive with this
upper level low. Snow levels drop, as do the surface
temperatures. Equal amounts of the raw ensembles have temperatures
in the upper 50s to mid 60s for today across the PDT area, 56%
show the Basin in the mid 60s to low 70s, 67% show the Gorge and
surrounding areas in the low to mid 60s and lastly the higher
terrains in the low 60s. Today will be the coolest day of the
forecast period through the weekend and next Monday. Models show
the upper level low to push the moisture and cooler air off to the
east by Monday night. Models show a broad trough move over with
northwest to westerly flow. This will warm the region up a bit
bringing temperatures back to near normal with the EFI in
agreement. NBM and 77% of the raw ensembles have the PDT area and
central OR in the low to mid 70s, the Basin, adjacent valleys and
the Gorge in the mid to upper 70s. Temperatures are expected to
slowly increase through the remainder of the period. Bennese/90

LONG TERM...Thursday through Monday...The main sensible weather
highlights include: a warming trend with departures peaking 10-15
degrees above average on Saturday, a cold front and windy
conditions Sunday, low-end precip chances returning to the
mountains late Sunday-Monday, and elevated fire weather potential
Sunday revolving around winds and low RH.

The large scale pattern early Thursday will be characterized by
weak mid-level cyclonic flow and a weak upper-level trough over
the west coast. Elsewhere, a modest upper-level ridge is forecast
in the eastern North Pacific with the high centered north of
Hawaii. Ensemble clustering and guidance are in good agreement
through Saturday. A 500 mb shortwave ridge is anticipated to build
and move east to be overheard 48-hrs later/by 12Z Saturday while
a closed upper-low develops and moves east across the far northern
east North Pacific/Gulf of Alaska. This will continue to support a
warming trend Thursday through Saturday. The greatest departures
from average are expected to be seen on Saturday at mainly 10-15
degrees area-wide. We are seeing widespread moderate HeatRisk
(category 2) across much of the Lower Basin into the Gorge Friday
and Saturday driven largely by daytime temperatures, though mild
lows are forecast around 5-10 degrees above average. While highs
are above seasonal averages, they are not that unusual and heat
will affect mainly those sensitive to heat and those without
cooling/hydration. High confidence (>80%) in 90 F or higher highs
across the lower Basin with moderate-high confidence (50-80%)
across the foothills Friday and Saturday. Low confidence (< 15%)
in exceeding 100 F on the warmest day (Saturday).

The spread in gudience increases by Sunday with the differences
centered on the strength and timing of a shortwave trough and its
attendant front associated with the evolving upper-level trough in
the eastern North Pacific. While some differences exist with the
timing of the front/marine push, fairly confident in Sunday being
cooler (60-70%) with highs trending to drop 5-10 degrees from
Saturday. Limited moisture is anticipated on top of dry
conditions with a marked increase in winds expected Sunday.
Chances for 24-hr daily highest wind gust exceeding 34 kts/39 mph
exceeds 60-80% across much of the eastern Gorge spilling into the
Lower OR Basin with Kittitas Valley and Simcoe Highlands exceeding
90%. Meantime, reasonable high-end gusts, the 75th percentile
wind gust forecast (1 in 4 chance of higher gusts), are seen to
exceed 40 mph at spots in the afternoon Sunday such as KPDT,
KDLS, and KELN with the latter seeing gusts reach 50 mph. Even
so, most likely gusts of around 30-45 mph will combine with low
afternoon humidity across the Lower Basin and north central OR
that will promote potential for elevated fire weather concerns.
Cooler air will then continue to move into the region on Monday.
Finally, there will be low-end chances for precip (~30% and less)
across the upper slopes of the eastern WA Cascades Sunday- Monday.
Of note, there could be conditional potential for
showers/isolated thunderstorms across the far eastern mountains
but confidence is very low (less than 15%).

AVIATION...18z TAFs...Main aviation concerns with be the breezy to
windy conditions today followed by some shower this afternoon
through evening but overall expecting VFR conditions. Will see
sustained winds 10-20kts with gusts 30-35kts this afternoon through
evening before decreasing overnight becoming less than 10kts.
Showers will produce some local CIGS 040-080 into this evening
before showers dissipate and clouds decrease.

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  67  43  73  45 /  30  30   0   0
ALW  70  47  76  48 /  40  40   0   0
PSC  74  49  79  49 /  20  30   0   0
YKM  72  43  77  46 /  10  10   0   0
HRI  73  47  79  48 /  20  20   0   0
ELN  67  45  75  46 /  10  10  10   0
RDM  61  33  72  39 /  10   0   0   0
LGD  60  39  69  41 /  50  40  20   0
GCD  59  36  71  41 /  50  20  10   0
DLS  68  48  77  51 /  10   0   0   0

&&

.PDT WATCHES/WARNINGS/ADVISORIES...
OR...None.
WA...None.
&&

$$

MORNING UPDATE...82
SHORT TERM...90
LONG TERM....80
AVIATION...80