923
FXUS66 KPDT 132350
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
450 PM PDT Tue Aug 13 2024

Updated Aviation Discussion

.AVIATION...00Z TAFs...VFR conditions expected over the next 24
hours aside from BDN/RDM/YKM where local smoke from fires may be
possible. Confidence is too low (<40%) to list in TAFs. Breezy
flow expected to continue through the gaps for DLS, with diurnal
flow 10 knots or less otherwise all other sites. Goatley/87


&&

.PREV DISCUSSION... /issued 340 PM PDT Tue Aug 13 2024/

SHORT TERM...This afternoon through Thursday...A vorticity
maximum is slowly moving north-northeast across far NE OR, SE WA,
and NRN ID this afternoon. The showers and thunderstorms
associated with it have exited our forecast area, but a cumulus
field is building across portions of the northeast Blue Mountains
that cleared this morning. CAMs are advertising some isolated
convection over the Blue Mountains, primarily Wallowa County, this
afternoon. Any activity that develops will diminish this evening
with dry conditions forecast overnight into Wednesday.

Attention then turns to the next in a series of upper-level low
pressure systems, a developing low offshore in the Pacific.
Ensemble and deterministic guidance are in excellent agreement
that the closed low will track onshore by Wednesday afternoon and
evening, increasing large-scale forcing for ascent across central
Oregon. Wednesday night through Thursday morning, the combination
of the aforementioned forcing, ample mid-level moisture, and
modest elevated instability (MUCAPE of 200-500 J/kg for parcels
lifted from around 700 mb) should facilitate some high-based
showers and thunderstorms. Have issued fire weather watches for
OR611, OR640, and OR642 for abundant lightning potential given the
high-based showers will have a low chance (10-30%) of producing a
wetting rain.

Thursday afternoon, the upper-level low will swing over the PacNW
with another round of surface-based convection forecast for the
majority of the Blue Mountains and Cascades. While the main jet
will be across southeast OR and northern NV, hence bulk shear
values will be low (10-20 kts), the cold core aloft will
facilitate modestly steep lapse rates and MUCAPE of 500-1500 J/kg.
Given weak steering flow and ample PWATs (~150% of normal), these
storms will have a slow storm motion (5-15 mph) and a higher
chance of producing a wetting rain (30-60% chance), so no fire
weather watch has been issued for this activity. The low will
track north-northeast out of our forecast area overnight into
Friday morning with a slight chance (15-24%) of lingering shower
activity overnight. Plunkett/86

LONG TERM...Friday through Monday...Ensemble guidance is in
excellent agreement (>90% of members) that a deep, closed low
will develop off the PacNW Friday through the weekend. However,
analysis of ensemble clusters reveals a spread in solutions,
ranging from a retrograding low with warm, dry conditions for the
forecast area to a more progressive solution that tracks the low
onshore overhead. The latter solution would result in cooler, more
showery weather with potential for thunderstorms. Plunkett/86


&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  49  82  53  82 /   0   0   0  20
ALW  55  86  58  86 /   0   0   0  20
PSC  59  87  60  88 /   0   0   0  10
YKM  52  86  55  86 /   0   0   0  10
HRI  56  87  58  86 /   0   0   0  10
ELN  55  86  57  87 /   0   0   0  10
RDM  44  82  50  78 /   0   0  30  30
LGD  48  84  52  81 /   0   0  10  50
GCD  48  86  53  82 /   0   0  20  60
DLS  57  85  61  82 /   0   0   0  10

&&

.PDT WATCHES/WARNINGS/ADVISORIES...
OR...Fire Weather Watch from Wednesday evening through Thursday
     morning for ORZ611-640-642.

WA...None.
&&

$$

SHORT TERM...86
LONG TERM....86
AVIATION...87