Severe Thunderstorm Warning 13, NWS Pendleton, OR

Area Forecast Discussion
National Weather Service Pendleton OR
421 PM PDT Sat May 20 2023

..Updated Aviation Discussion...

.AVIATION...00z TAFs...Thunderstorms have started to form across
parts of our forecast area, and are expected to spread northward as
we approach nightfall. The isolated nature of these storm cells
hinder confidence as to when exactly each site will be impacted, but
currently gauging about a 50% chance that BDN/RDM sees tsra, 40% for
PDT/ALW, 30% for YKM/PSC, and marginal for DLS. Storm threat will
wane overnight, likely by 10z, leaving VFR conditions heading into
Sunday. Winds will then become a concern, with W/NW winds gusting up
to 35 mph possible at some sites Sunday afternoon. Evans/74

&&

.PREV DISCUSSION... /issued 223 PM PDT Sat May 20 2023/

SHORT TERM...Tonight through Monday night...Convection is
increasing as of 1 PM PDT this afternoon, with a weak thunderstorm
over southern Wallowa County, southern Grant County and along the
WA Cascade east slopes. Convection has begun a bit later than
expected. However, conditions are still favorable for
thunderstorms, and SPC has a MARGINAL RISK for severe
thunderstorms over most of the forecast area today. On Sunday
(day 2), SPC now also has a SLIGHT RISK of severe thunderstorms
over the eastern two thirds of Wallowa County, far southeast Union
County, and far southeast Grant County. This is an increase from
the previous day 2 Convective Outlook by the SPC. The rest of the
eastern two thirds of the forecast area on Sunday has a MARGINAL
RISK of severe thunderstorms. Thunderstorm chances on Sunday will
be further east than today due to an expected upper trough and
cold front that is expected to move into the region from west to
east with drier and more stable air.

This afternoon, stability parameters are still favorable for
thunderstorms with MUCAPE values as high as 2000-3000 J/kg over
some areas, LI values as low as -6 degrees C, Precipitable Water
(PWAT) values around an inch in most places (some locations a bit
more than an inch), and increasing shear to 30-40 kts (especially
along the Cascades). There is high buoyancy in the hail growth
layer on some of the NAM12 forecast soundings. The latest HREF
Paintball tool is still showing the greatest concentration of
storms to be over central OR, but it has increased over the NE
mountains with the latest run of the HREF model. The NBM 4.1 is
showing thunderstorm probabilities between 30-40 percent over
central OR to the Ochoco-John Day Highlands, and another bull's
eye of higher probabilities (20-30 percent) over Yakima and
Kittitas Counties east of the WA Cascades. These probabilities
seem to be too low, and believe the chances will be higher than
what the NBM 4.1 is showing for this afternoon and evening. These
probabilities are also lower than what the previous run of the NBM
4.1 was showing this morning (50-60 percent). However, the
locations with the highest probabilities for thunderstorms are
close to where the HREF Paintball tool is showing. Any severe
thunderstorms could produce large hail of an inch or more in
diameter and winds greater than 58 mph. However, winds could be
higher than that. The best time period of thunderstorm development
this afternoon and evening will be from 00Z to 06Z (5PM to 11 PM
PDT) this evening.

Today will be the hottest day of this stretch of heat, with high
temperatures expected to be in the mid 80s to mid 90s, with a low
to moderate chance (30-50 percent) of 100+ temperatures in the
Tri-Cities and Hanford areas. The smoke that was over the forecast
area has decreased today from the last few days. As such solar
heating will be higher than if there was still a higher
concentration of smoke over the CWA. High temperatures in the
mountains will be from the mid 70s to mid lower 80s. However, some
of the valleys in the mountainous terrain could have temperatures
as high as the lower to mid 90s (such as the John Day River
basin).

On Sunday, the aforementioned upper trough with a cold front will
move into the forecast area beginning early in the morning from
west to east. The greatest thunderstorm probabilities will be
shifted eastward as a result, and will be mainly from the Blue
Mountains/Ochoco-John Day Highlands eastward and southward. Over
western areas of the CWA, drier and more stable air will be
filtering through the Cascade gaps and the eastern Columbia River
Gorge. Winds are expected to increase due to west to east
pressure gradients increasing beginning this evening. Wind speeds
may approach advisory criteria speeds in localized areas on Sunday
and Monday (mainly locally over the ridge tops of the lower
elevations. Wind speeds are expected to increase to 20 to 30 mph
with possible gusts of 35 to 45 mph. The NBM 4.1 does not show
winds exceeding advisory criteria. These winds will persist
through Monday, and then will decrease Monday night. Conditions
on Monday will be dry and stable with much cooler temperatures,
with only a few showers possible over the WA Cascades.

High temperatures on Sunday will be about 10-15 degrees cooler
than today, with highs in the mid 70s to mid 80s over the lower
elevations, and mid 60s to lower 70s over the mountains.
Temperatures will then cool about another 5-10 degrees on Monday,
with highs to be in the upper 60s to mid 70s in the lower
elevations, and mostly mid 50s to mid 60s in the mountains.
Overnight lows tonight will be quite mild...in the 60s in the
lower elevations, and 50s in the mountains. Low temperatures
tonight in the lower elevations will not provide much relief from
today's heat. Overnight lows Sunday night will be about 10-15
degrees cooler than tonight. 88

LONG TERM...Tuesday through Saturday...Models have grounded
themselves in a bit more consistency for the coming work week,
however discrepancies continue to arise toward the latter half of
the period. Ensembles and deterministic guidance are largely in
accord through around Thursday depicting a low overhead for the
midweek. The NW origins of this system from the Gulf of Alaska will
help drive a more cool and dry NW flow that may spawn marginal
chances for orographic showers across our mountain crests, however
guidance seems very lackluster on really any kind of QPF until the
later half of the period. We should see largely seasonable
temperatures, maybe slightly below in some areas, with this cooler
and drier air mass overhead, recovering from the unusually hot
temperatures we've experienced so far through mid-May.

Overall forecast confidence is on the lower end (<40%) Thursday and
beyond due to the arising discrepancies across the ensembles.
General consensus seems to lean towards a continued trough-y
pattern, which would suggest chances for showers and thunderstorms,
especially for our eastern mountains, but models don't really seem
to tap into any kind of precip until next weekend, towards the days
7 and 8 periods where forecast confidence is even more lacking. Both
the deterministic GFS and ECMWF hint at another organized low moving
toward the Pacific coast by around Friday, with sufficient SW flow
ahead of it to drive shower and thunderstorm activity, however
ensemble clustering suggests that this feature is not so apparent
across guidance, especially within the EPS and Canadian. NBM broadly
advertises a slight chance of showers/storms for the eastern
mountains during the afternoon hours of Thursday onward, increasing
heading into the weekend, so will leave that in the forecast package
for now. Will continue, however, to note that forecast confidence is
low given the discrepancies within ensemble guidance.

The overall trough-y pattern should keep temperatures roughly
seasonal throughout the duration of the period. Higher confidence in
this (60-70%) as the vast majority of ensemble members are free of
any synoptic feature that would allow for extensive warming for the
Interior Northwest. Thus far looks to be relatively quiet for the
midweek, with perhaps more active weather slated for the weekend
depending on how models evolve over the coming days. Evans/74

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  61  79  47  68 /  30  20  10   0
ALW  64  81  50  72 /  30  20  10   0
PSC  65  86  52  76 /  20  20  10   0
YKM  59  83  46  72 /  40  10   0   0
HRI  63  83  49  72 /  30  20  10   0
ELN  58  75  47  66 /  30   0   0   0
RDM  54  73  39  67 /  30  10   0   0
LGD  58  78  44  65 /  30  50  30   0
GCD  56  78  42  67 /  40  40  20   0
DLS  58  75  50  68 /  20   0   0   0

&&

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

SHORT TERM...88
LONG TERM....74
AVIATION...74

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