716
FXUS66 KPDT 160316
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
816 PM PDT Thu Aug 15 2024

.Update...Convection is dwindling to showers and will continue to
move off to the north and east overnight as the weak shortwave
causing the showers exits into the Idaho Panhandle. Forecast has
been updated to end thunderstorms for tonight and transition the
showers out of the forecast area overnight.

.SHORT TERM...Today through Saturday

Bottom Line Up Front

1. Thunderstorms today and through this evening

2. Thunderstorms again Saturday

The upper level shortwave trough has been causing some wrap
around convection and showers this afternoon. Current radar shows
quite a few cells popping up over the eastern mountains and
putting out some lightning and rain showers. Ground observations
show that some isolated locations have seen near 0.04 inches of
rain with some of these cells as well as some lightning
accompanying them. These storms will continue through the
remainder of the afternoon and early evening bringing more rain
chances as well as thunderstorms. The thunderstorms will stretch
from central OR along the central and eastern mountains. A RED
FLAG WARNING remain in effect along the aforementioned areas for
abundant lightning through 11 PM tonight.

Friday the models show a brief break form all the action before
another round of thunderstorms roll through on Saturday and will
primarily be along the east slopes of the Cascades.

Models show another upper level low parking itself just off shore
while its leading edge shimmies up along the Cascade crests
Saturday. This will allow some added moisture to influx across
the Cascades as well as increased instability. Model derived
soundings from the CAMs shows relatively high MUCAPE values
beginning to crest into the 700-800 J/kg with a LI of -4 to -6,
mid level lapse rates over 8.5 C/km and effective bulk shear
nearing 45 kts. These ingredients coupled with the southwest flow
and orographic lift, thunderstorms will likely form along the
crests and portions of the eastern slopes on Saturday. Primary
concerns for thunderstorms on Saturday will be the amount of rain
associated with them. Model derived soundings show PWATs over 1
inch which could lead to heavy downpours with these storms
(50-70%). Raw ensembles show 30-50% chances for near 0.50 inches.
Models are also showing these storms could be slow movers so they
could train over the area and if near any recent burn scars, could
cause flash flooding. Due to this fact, a flash flood watch is in
effect for the eastern slopes of the Cascades Saturday afternoon
through late Saturday night primarily concerns being over the many
burn scars across the area (i.e. Retreat Fire, Williams Mine fire
as well as some of the larger burn scars from 2021 and 2022).
Bennese/90

.LONG TERM...Sunday through Thursday...The 500 mb pattern during
the extended period will be dominated by an upper low remaining more
or less stationary off the PACNW coast held in place by a full
latitude blocking ridge centered over the SWRN states. The GFS does
break down this ridge a bit Wednesday into Thursday by moving a
shortwave across the nrn tier of states including the PACNW but this
is not supported by the ensemble clusters which for the most part
keep the upper low intact offshore. This will set up a swly flow
aloft pattern across the region with relatively benign conditions.
It will be warm but not hot with probabilities of reaching 90
degrees for high temperatures peaking at 20-30% in the Columbia
Basin on Sunday then less than 10% areawide the remainder of the
forecast period. Aside form some low pops (30% or less) along the
Washington Cascade Crest, chances for rain will be less than 15%
through the period. There is always some concern that a shortwave
embedded in the flow could spark thunderstorms but the NBM
probabilities of thunder are 10% or less each day. Weak marine
pushes are also likely each day in this flow pattern but the latest
suite of model guidance indicates that the probability of needing
wind highlights is rather low (10-20%). 78


&&

.AVIATION...00Z TAFS...General VFR conditions will prevail through
the forecast period with some mid level CIGS at 6-12 kft AGL
through tonight for PDT and ALW along with a chance of showers
through this evening and a 10% chance of some lightning.
chance) at KBDN and KRDM tonight. Winds will generally be less
than 10 kts overnight except for DLS where some 10-20kts winds
will last into the evening.


&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  54  82  54  90 /  10   0   0  20
ALW  59  86  58  96 /  20   0   0  10
PSC  63  87  59  96 /  20   0   0  10
YKM  54  84  54  90 /  30   0   0  40
HRI  60  86  57  94 /  10   0   0  20
ELN  56  84  54  91 /  10  10   0  30
RDM  46  81  49  82 /  10  10  10  50
LGD  49  83  51  92 /  50  10  10  10
GCD  49  83  53  92 /  50  10  10  10
DLS  59  83  60  90 /  10   0  10  50

&&

.PDT WATCHES/WARNINGS/ADVISORIES...
OR...Red Flag Warning until 11 PM PDT this evening for ORZ611-640-642-
     644-645.

WA...Flash Flood Watch from Saturday afternoon through late Saturday
     night for WAZ522-523.

&&

$$

SHORT TERM...90
LONG TERM....79
AVIATION...79