785
FXUS66 KPDT 061713
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
1013 AM PDT Thu Jun 6 2024

Updated Aviation Discussion

.AVIATION...18Z TAFS...VFR conditions prevail with current clear
skies for this TAF period. Few high clouds will pass by tonight for
KDLS/KALW and then Friday morning for KPSC. KDLS/KRDM/KBDN will
experience gusty winds at 20-25kts with sustained winds at 10-15kts
this afternoon before decreasing tonight (>50% confidence).
Feaster/97

&&

.SHORT TERM UPDATE...Minor update made to the forecast to add a
slight chance of showers and thunderstorms in our far southeast
CWA this afternoon and evening, mainly covering southern Crook
and Grant Counties. There have been isolated thunderstorms over
Malheur County this morning, and CAMS are indicating more
activity there this afternoon. Visible satellite shows some
cumulus already developing in our SE zones. The instability is
present with mid- level lapse rates around 7 C/km. SPC Mesoscale
Analysis has yet to show any signs of CAPE at this time, but CAMs
have CAPES as high as 500 J/kg. Despite the limited moisture and
instability, what is lacking is anything to trigger storms. Weak
vorticity will keep the isolated storms over southeast Oregon,
therefore any probability for convection in our southeast zones is
15% or less. Wister/85

&&

.PREV DISCUSSION... /issued 423 AM PDT Thu Jun 6 2024/

SHORT TERM...Today through Saturday...High pressure with weak SW
flow aloft continues to drive our weather, with clear skies and
warming temps expected through the upcoming weekend. As more
prominent ridging takes place Friday into Saturday, breezy east to
northeast winds will pick up across the forecast area, impacting
primarily the Basin down through central Oregon. Not expecting
anything tremendously breezy (think 20 mph gusts at most), but
enough to prevent the atmosphere from becoming too stagnant, thus
putting a bit of a cap on warming. Still, highs are expected to
climb into the 90s across our population centers both Friday and
Saturday.

An oncoming Pacific low will help work to amplify the SW flow aloft
a bit more heading into the weekend, leading to a chance of showers
and thunderstorms for primarily the eastern mountains. Models
continue to tap into some mid-level moisture on Friday with some
decent MUCAPEs in the 400-800 J/Kg range, however with the ridge
axis overhead, am not too bullish on storm chances. That being said,
the tail end of CAM runs, such as the 00z HREF, do put some weak
reflectivity across the far southern portion of our forecast area, so
did leave in a slight chance for isolated t-storms for Friday
afternoon and evening across extreme southern Deschutes, Crook, and
Grant Counties. Current thinking is that better chances for storms
exist on Saturday, when a shortwave associated with the Pacific low
begins to ease into the forecast area, allowing for more available
moisture for showers and storms to fire off during the afternoon and
evening hours. Evans/74

LONG TERM...Sunday through Thursday...

Key Messages:

1. Thunderstorm potential Sunday afternoon and evening.

2. Warm, above normal temperatures through much of the week.

The extended period is characterized by an initial upper level
trough on Sunday, followed by several systems that keeps afternoon
winds breezy and temperatures warm through much of the period due to
persistent southwest flow aloft. However, the main weather concern
resides with thunderstorm chances (15-30%) over Central and Eastern
Oregon Sunday afternoon and evening as an upper level trough and
associated cold front passes through the region from the southwest.
This will effectively erode the incumbent upper level ridge that
built over the region Friday and Saturday, allowing enhanced
instability to be present along the boundary of the incoming
airmass. The ECMWF EFI highlights 70-90% of ensembles showing above
normal CAPE across Central Oregon, dropping to 50-70% of ensembles
further northeast into the northern Blue Mountains and Wallowa
County. The GFS suggests Most Unstable CAPE (MUCAPE) of 250-450
J/kg, 20-30 knots of low level shear, and low levels lapse rates of
6.5-7.5 C/km. These parameters are moderate for thunderstorm
development via the GFS, but the ECWMF only shows isolated areas
over Crook, Grant, and Wheeler counties exhibiting 100-150 J/kg of
CAPE - which does coincide with locations of higher probability for
thunderstorms (20-30%). These discrepancies are associated with the
GFS anticipating a stronger incoming trough and weaker eroding ridge
that the ECMWF, with the stronger trough solution currently
represented by 60% of ensemble clusters. Along with the primary
hazards of frequent lightning, hail, and gusty winds in the
proximity of any developing storm cells, moderate to heavy rain may
also be an issue as precipitable water (PWAT) amounts of 0.8-0.9"
(130-170% above normal) are advertised by both the ECMWF and GFS.
These could lead to a quick 0.05-0.15 of an inch of rain falling
near a developed thunderstorm. Elsewhere, expected rain amounts are
minimal as dry air (50-70% below normal PWATs) will follow closely
behind the frontal boundary.

Subsequent systems will continue to travel across the Pacific
Northwest Tuesday and again on Thursday. As a result, a slight
cooling trend will ensue, but it will be gradual as above normal
high temperatures are expected to persist until late in the week.
The current NBM suggests the area will approach normal high
temperatures on Friday, with values dipping into the upper 70s to
low 80s across lower elevations of the Basin and foothills. The slow
trend will commence on Sunday, as highs drop 3 to 7 degrees from
Saturday. These values drop another 2 to 3 degrees on Monday, 1 to 2
degrees on Tuesday, 2 to 4 degrees on Wednesday, and another 2 to 5
degrees on Thursday. These passing shortwaves are rather weak, as
indicated by the slow and gradual decline in temperatures, which
will keep dry conditions across the area as the bulk of each system
passes to our north. Another consistency during this timeframe will
be breezy afternoon winds as the passing shortwaves tighten isobars.
This will lead to winds of 10-20 mph and gusts of 25-35 mph across
the Basin, Gorge, Simcoe Highlands, and the Kittitas Valley each
day, with directions primarily out of the west-northwest. 75

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  84  54  88  59 /   0   0   0   0
ALW  85  57  90  60 /   0   0   0   0
PSC  88  59  93  62 /   0   0   0   0
YKM  88  54  91  58 /   0   0   0   0
HRI  88  56  92  61 /   0   0   0   0
ELN  84  53  89  58 /   0   0   0   0
RDM  85  51  89  57 /   0   0   0   0
LGD  83  51  86  57 /   0   0   0   0
GCD  89  54  91  59 /  10  10  10  20
DLS  89  58  93  64 /   0   0   0   0

&&

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

$$

SHORT TERM...74/85
LONG TERM....75
AVIATION...97