578
FXUS66 KPDT 201743
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
1043 AM PDT Thu Jun 20 2024

Updated Aviation Discussion

.AVIATION...18Z TAFS...VFR conditions remain during this period with
current light winds. Mid to high clouds will prevail during the next
24 hours. KDLS will remain clear. Feaster/97

&&

.MORNING UPDATE...Updates to the forecast this morning were small.
The only changes to messaging include an increase in probability
of precipitation (PoPs) for the Washington Cascade crest and
adjacent east slopes as well as the Blue Mountains of OR and far
SE WA. Regarding current observations, altocumulus is present over
portions of the Blue Mountains region, though returns on radar are
weak.

Forecast surface-based CAPE of 250-750 J/kg, locally higher, is
advertised by 12Z HREF members for portions of the Blues and WA
Cascade crest. Coupled with steep mid-level lapse rates of 7-8.5
C/km and modest synoptic support, the environment appears capable
of supporting isolated showers and thunderstorms. Effective shear
is modest -- 30 kts or less for the Blues, and 20 kts or less for
the WA Cascades, so any cells that develop are not anticipated to
be severe. Plunkett/86

&&

.PREV DISCUSSION... /issued 218 AM PDT Thu Jun 20 2024/

SHORT TERM...Today through Friday...

 Bottom Line Up Front

1. Dry conditions with isolated dry thunderstorms along the
eastern mountains.

2. Hottest day will be Saturday with moderate heat risk.

With the northerly flow in place and the daily run of a surface
thermal trough, dry conditions will prevail through the period.
However, looking at some of the CAMs models, there will be some
instability moving across the eastern mountains. Model derived
sounding are showing MUCAPE values well above 400 J/kg with lifted
indexes of -1 to -4, lapse rates of 8.5 C/kg and bulk shear
nearing 40 kts. PWATs are moderate with 0.50-0.65 inches
available. Given the ingredients as well as daytime heating and
orographic lift, one can not rule out the possibilities of some
isolated thunderstorms over the eastern mountains tomorrow
afternoon (less than 25% probabilities). Not only are the
ingredients available along the eastern mountains, but CAMs models
show there are decent amounts of MUCAPE (250 J/kg) along the WA
Cascades. However, probabilities of isolated thunderstorms along
the eastern slopes are less than 10%.

Models are in firm agreement with the upper level flow being from
the northwest with a strong surface thermal trough making its way
into the region. Models show Saturday to be the day with the
strongest trough set up which will bring temperatures above
Normal. EFI shows temperatures to be well above normal
temperatures with over 70% of the raw ensembles putting the
majority of the region in low to mid 90s with some isolated areas
in the Basin in the high 90s. NBM is showing a moderate heat risk
across the majority of the Basin as well as through the Gorge and
portions of the southern Blues with very isolated locations seeing
high risk. If you are out and about this weekend, please plan
appropriately. For more information go to www.weather.gov/heat.
Bennese/90

LONG TERM...Sunday through Thursday...A dry cold front will result
in gusty winds on Sunday. Models have consistently advertised this
for at least several days now, so the confidence is high that it
will be breezy to windy. The NBM probability of gusts >= 22 kts is
over 90% for most of the forecast area and the probability of
gusts >= 34 kts is 70-100% in the Lower Columbia Basin, eastern CR
Gorge, Simcoe Highlands, and the Kittitas Valley. For gusts >=41
kts, the probabilities are around 80% for the Kittitas Valley, 30%
for the eastern CR Gorge, and 15% or less elsewhere. Confidence
is also high that the relative humidity will be in the teens for
areas in and around the Lower Columbia Basin and across south
central and southeast Oregon. All of the mean ensembles are
showing dry air aloft behind the front...less than 10% at 700mb.
The primary weather concerns for Sunday will be fire weather, and
highlights may be needed as we approach the weekend.

The westerly flow aloft across the eastern Pacific and the PacNW
will begin to buckle in response to a deep closed low developing
off the Gulf of Alaska. By Tuesday, a large trough will develop
between 130-150W, and the westerly flow over WA/OR will back to
the southwest. Models are not hinting at any convection and all
show very weak or no surface-based instability. However, there is
some elevated instability looking at the GFS with mid-level
theta-e decreasing with height and DIV Q aloft. Usually, these are
signs of isolated high based storms. For now, will not mention
thunderstorms for Tuesday until confidence improves. At this time,
confidence is only 15%.

There are differences in the model progs for Wed-Thur, but nothing
significant. Main difference is the amplitude of the trough as it
moves inland, but less than 25% of the ensembles from the GEFS,
ECMWF, and GEPS advertise a flat trough. The rest are showing a
fairly organized trough over WA/OR Thursday. Therefore, Thursday
is forecast to be a little cooler with 20-35% chance of showers
along the Cascades and the eastern mountains. Wister/85

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  87  54  89  56 /   0   0   0   0
ALW  91  57  92  60 /   0   0   0   0
PSC  92  58  94  60 /   0   0   0   0
YKM  88  55  93  59 /   0   0   0   0
HRI  93  57  95  60 /   0   0   0   0
ELN  86  55  90  59 /   0   0   0   0
RDM  83  50  88  53 /   0   0   0   0
LGD  83  53  85  54 /  20  20   0   0
GCD  85  51  89  55 /   0   0   0   0
DLS  92  62  94  63 /   0   0   0   0

&&

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

$$

MORNING UPDATE...86
SHORT TERM...90
LONG TERM....85
AVIATION...97