192
FXUS66 KPDT 150431
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
931 PM PDT Wed Aug 14 2024

.EVENING UPDATE...Water vapor imagery reveals a closed low
pressure system over southwest Oregon, moving slowly inland.
Southerly flow aloft is advecting moisture northward into central
Oregon, and radar shows a cluster of showers and thunderstorms
rumbling slowly northward across south-central Oregon,
predominantly in Klamath and Lake counties. While the 00Z MFR
sounding recorded a modest 207 J/kg of MUCAPE, these storms have
produced prolific lightning, including cloud-to-ground lightning,
with GLM flash density rates as high as 97 flashes/5min. Heavier
precipitation cores have yielded wetting rain in areas with
surface observations picking up over a half inch.

The question remains, how will this convection evolve as it
propagates into our CWA? RAP-based mesoanalysis suggests the best
700-500 mb (mid-level) layer-average omega is over southeast
Deschutes and Crook counties. Additionally, 00Z global guidance
indicates negative mid-level theta-e lapse rates in this region,
and 00Z HREF forecast soundings continue to advertise modest
MUCAPE of 50-200 J/kg through the night. This instability coupled
with ample moisture through the column and strong synoptic-scale
forcing from the vorticity maximum lifting northward should
support showers and thunderstorms into central and eastern Oregon.

As far as forecast updates, have adjusted PoPs and thunder
coverage based on satellite and radar trends and 00Z NWP guidance.
Forecast morning lows were also adjusted based on the assumption
that many areas will see precipitation and cool to near forecast
wet bulb temperatures.

Red Flag Warnings remain in effect tonight and Thursday due to
forecast abundant lightning. Plunkett/86

&&

.AVIATION...06Z TAFs...Cloud cover will increase across central
and eastern Oregon overnight due to ongoing convection in south-
central and central Oregon. BDN/RDM will likely (50-60% chance)
see rain showers between 07Z and 11Z Thursday with a chance
(25-30%) of thunder. Elsewhere, precipitation chances are low (30%
or less) through the period, highest at PDT between 15Z and 18Z,
but mid- and high-level clouds will increase through Thursday for
all sites.

Aside from the potential for gusty outflow winds with any showers
and thunderstorms, diurnally driven gap winds are forecast,
strongest at DLS Thursday afternoon and evening.

Any showers and thunderstorms that pass over terminals will be
capable of dropping CIGs/VSBYs temporarily to MVFR. Plunkett/86

&&

.PREV DISCUSSION... /issued 501 PM PDT Wed Aug 14 2024/

SHORT TERM...Today through Tomorrow Night...A transient ridge is
crossing the region today. Mostly clear skies and near normal high
temperatures today. However, few cumulus clouds are developing
within Wallowa county, the crest of WA Cascades including south
central OR, observed from satellite imagery. Confined smoke aloft
still remains in central OR and Ochoco-John Day Highlands from
local wildfires. Winds will be around at 20 mph or less across the
forecast area this afternoon.

Tonight through tomorrow afternoon, an upper level trough makes
its way through the PacNW which is now offshore the OR/CA
border. There are chances (40-60%) of thunderstorms starting late
tonight over central OR that then ramp up tomorrow afternoon and
evening in terms of both coverage and lightning potential. A bit
more spread/uncertainty in terms of elevated instability overnight
that spreads across south central OR northward. Best signal is
focused across northern Klamath-Lake. Thereafter, environment
becomes more conducive for storms tomorrow. Model soundings
indicated MUCAPEs between 500-1000 J/kg, Lifted indices (LI) of
-3, and lapse rates 6.5-8 C/km across central OR to eastern OR.
Thus a moderately unstable environment. Of note, HREF probability
of thunder is forecast around 30-60% overnight that then ramp to
greater than 80% across portions of eastern OR.

Of note, a mix of wet and dry storms are expected in this event
with storms likely to be more wet than not across the Southern
Blue and Strawberry Mountains north and east. Of which, HREF LPMM
highlights this nature. A reasonable high-end is expected to be
approaching one inch or more. HREF shows raw probs of exceeding
0.5" at around 40-50% in spots. Wetting rains are further
supported by NBM probs. Of which, a tenth or more exceed 60%
across a large area, roughly Crook county southward and northeast
into Wallowa through 06Z Friday. While storms are wet there, the
threat for new fires is sufficient between fuels and lightning
activity. Thus Red Flag Warnings were issued for ORZ644-645.
Elsewhere, the Fire Weather Watches were replaced with a warning
with these zones having two windows of lightning potential, albeit
storms more isolated-widely scattered in character compared to the
storms in the east.

Overall, models are in good agreement with trough returning over
the PacNW with a cyclonic low for tomorrow. Breezy winds will also
develop up to 20-30 mph tomorrow around the Cascade Gaps extending
to Columbia Basin (>50% confidence). This is due to increasing
surface pressure gradients up to 5-8 mb.

Tomorrow night, the worst will come to an end as thunderstorms
begin moving out of the area. Gusty winds will then return at
15-25 mph with the surface pressure gradient decreasing to 3-5 mb.
Clear skies will return as well as clouds decrease. Temperatures
will then steadily increase by a few degrees with relative
humidity above critical levels. Confidence enhances with the
Extreme Forecast Index (EFI) showing temperatures to be near-
normal through the forecast period. Feaster/97/80

LONG TERM...Friday through Wednesday...The previous area of low
pressure continues to lift northeast out of the forecast area on
Friday morning. Lingering moisture will provide a slight chance
(15-20%) for continued shower activity, mainly over Wallowa
County in Oregon and Columbia County in Washington. This system
should generally lift out of the forecast area by the late morning
hours, with another area of low pressure moving over the Pacific
Coast. Models continue to indicate instability, especially over
the Eastern Mountains of Oregon. With atmospheric moisture over
the area, showers and isolated thunderstorms (15-25% chance) will
be possible through Friday afternoon. The best opportunity for
thunderstorm development will be over eastern Union County, with
showers otherwise.

The previously mentioned area of low pressure off the Pacific
Coast will advance east with moisture moving over Washington and
Oregon between Saturday and Sunday. Ahead of the arrival on
Saturday, temperatures will warm around 5 degrees across the area,
with cooler conditions anticipated on Sunday. The best moisture
will be found west of, and including, the Cascades, though
moisture will spill over into the central portions as well (30-50%
for favored locations). Guidance indicates the afternoon and
evening hours to be the favored timeframe for shower activity .
Instability is generally faced in the western reaches of the
moisture with slight chance (10-20%) of development.

The area of low pressure will drift far enough northward to lose
influence over area weather by Sunday afternoon/evening. Drier
conditions expected to persist through the remainder of the
forecast period, though pockets of moisture will produce isolated
showers and thunderstorms over the Washington Cascades through
Wednesday. Temperatures through this period will remain near to
slightly above normal.

Overall, clusters are demonstrating fair agreement with the
progression of these systems. Intensity differences to exist, as
well as how far east the weekend system moves, but significant
differences are not currently present. Branham/76

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  54  80  54  81 /   0  30  20  10
ALW  57  84  59  85 /   0  20  30  10
PSC  59  87  62  86 /   0  10  20   0
YKM  55  85  54  84 /   0  20  20   0
HRI  59  85  59  86 /   0  10  20   0
ELN  57  85  56  83 /   0  10  20  10
RDM  52  75  47  80 /  60  40  20  10
LGD  52  80  50  83 /   0  50  60  20
GCD  54  80  50  83 /  40  70  40  20
DLS  61  80  59  83 /  10  20  10   0

&&

.PDT WATCHES/WARNINGS/ADVISORIES...
OR...Red Flag Warning from 11 AM to 11 PM PDT Thursday for ORZ644-645.

     Red Flag Warning until 11 PM PDT Thursday for ORZ611-640-642.

WA...None.
&&

$$

EVENING UPDATE...86
SHORT TERM...97/80
LONG TERM....76
AVIATION...86