Area Forecast Discussion
Issued by NWS Pendleton, OR

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FXUS66 KPDT 280341

Area Forecast Discussion
National Weather Service Pendleton OR
841 PM PDT Mon May 27 2024

.SHORT TERM UPDATE...Forecast for tonight`s thunderstorms is on
track and no updates planned. Isolated thunderstorms have
developed over southeast Oregon, including Crook and Grant
Counties. The weak storms are moving north-northeast, and most of
the 00Z CAMS have done a good job depicting the area of higher
reflectivity for this evening. The increasing southwest flow aloft
ahead of an upper level trough will push mid level moisture
where MUCAPES are as high as 1000 J/kg on SPC`s Mesoscale
Analysis. The forecast shows isolated thunderstorms over the Lower
Columbia Basin tonight which is in agreement with the HREF
ensembles and paintball probabilities of reflectivity greater than
40dBZ. Confidence is increasing that there will be a band of
light showers and a few lightning strikes in a line in the Lower
Columbia Basin of WA/OR late this evening and early Tuesday
morning. Otherwise, the forecast area will have a quiet weather
night with partly to mostly cloudy skies and winds 15 mph or less.


.PREV DISCUSSION... /issued 450 PM PDT Mon May 27 2024/

SHORT TERM...This afternoon through Wednesday night...Quiet
weather, albeit with FEW-SCT cirrus (area-wide) and fair-weather
cumulus (over the mountains) clouds have prevailed through mid-
afternoon. The axis of an upper-level ridge continues to amplify
as it tracks eastward over the northern Rockies. Offshore, an
upper-level trough is looming, a harbinger of weather to come.

Later this afternoon, surface-based showers and thunderstorms may
develop (10-20% chance) over the mountains of central and east-
central OR, though 12Z HREF members were not in good agreement on
coverage due to a wide spread in forecast boundary layer moisture
and surface-based CAPE. As is often the case, the HREF ensemble
mean dew point field has done a decent job depicting observed
surface moisture -- observations range from the mid-40s to lower
50s, except for lower 30s to lower 40s in central and east-central
OR. While confidence remains low in anything more than a cumulus
field this afternoon, the best chances for showers and isolated
thunderstorms will be over the high terrain, namely the Ochocos,
Elkhorns, and other sub-ranges of the central and east-central OR
Blue Mountains. East of the Cascade crest, the best chances will
be over the Newberry Crater area. Any showers that do develop may
produce gusty outflow winds due to ample DCAPE (500-800 J/kg)
which has resulted from a well-mixed boundary layer and steep low-
level lapse rates (9-10 C/km).

As we transition to the evening and overnight hours, CAMs are in
excellent agreement that surface-based instability from daytime
heating will wane. Any subsequent instability would be
facilitated by southwesterly flow aloft advecting moisture and
increasingly steep mid-level lapse rates (7-7.5 C/km) northward
across central/north-central OR and limited portions of south-
central WA and northeast OR. Should the moist, steeper-lapse-
rate solutions materialize, showers and a few isolated
thunderstorms may develop this evening and track northward
overnight (10-20% chance PoPs). Of note, global guidance is
highlighting areas of negative Theta-E lapse rates coincident
with 70-95% 700-500 mb RH, increasing confidence in at least some
altocumulus and showers, though synoptic-scale lift is not robust
until later Tuesday morning.

As the upper-level trough approaches Tuesday morning into Tuesday
afternoon, concerns shift to a low-end severe thunderstorm threat
for east-central and northeast OR, mainly eastern Grant, Union,
and Wallowa counties. While CAMs have backed off somewhat on
forecast mid-level lapse rates (now 7-8 C/km) compared to
yesterday`s runs, consensus is that surface dew points ranging
from the mid-40s to mid-50s will lead to surface-based CAPE of
250-750 J/kg across much of the Blue Mountains with values of
500-1500 J/kg (locally higher) over far northeast OR. Upper-level
support looks good as a mid- to upper-level jet swings into the
region. Effective bulk shear of 40-50 kts is evident in plan view
and sounding analysis of the aforementioned region. While some
members show effective SRH values of up to 150 m2/s2, a broad
range leads to low confidence in this field. That said, portions
of Wallowa and southeast Union counties are under a Marginal Risk
(5% chance of severe weather within 25 miles of a point) from the
Storm Prediction Center. 12Z HREF members do show a signal for
some organized convection with rotating updrafts, and the main
threat will be gusty outflow winds and small hail.

Breezy to windy westerly winds will accompany frontal passage
Tuesday and linger Wednesday as the upper-level trough axis swings
overhead. Confidence in widespread advisory-level winds was too
low to issue any wind highlights this afternoon.

Lingering showers across the forecast area on Wednesday will
taper off through Wednesday night. Plunkett/86

LONG TERM...Thursday through Monday...Models in good agreement
early, but begin to diverge significantly by Sunday. The EOFs show
discrepancies increasing Saturday and continuing into next week.
WPC cluster analysis shows 57% of members showing an upper level
ridge Saturday while 43% show a trough. By Sunday, it`s still
leaning slightly ridgy, but by Monday, it`s about a 50% chance of
either panning out. The ensembles are similar with GFS leaning
towards a ridgier pattern while the ECMWF leans troughy. Given
discrepancies, confidence is low (10-20%) on overall pattern so
will stick with NBM. If the GFS pans out (a more ridgy pattern),
PoPs will need to be lowered Sunday and Monday. If the ECMWF pans
out (slightly more likely given better track record), we`ll see
1-3 days of afternoon/evening thunderstorms over our CWA next week
under southwest flow aloft.

At this time, the best chance of seeing 0.10 inch or more of
precipitation will be Monday with the mountains approaching a
moderate chance (40-50%) while the Columbia Basin is low to
moderate  (20-30%).

Daytime highs are expected to be slightly below normal Thursday,
but aforementioned discrepancies decrease confidence (low-
moderate) highs will be near to slightly above normal. Earle/81

AVIATION...00Z TAFs...Various cumulus, altocumulus and cirrus
clouds will be observed over the next 24 hours.  Bases will range
from 6-10 kft and 15-25 kft.  There will be some towering cumulus
and a slight chance that a thunderstorm will occur along the Blue
Mtns and in the vicinity of PDT and ALW on Tuesday afternoon. West
winds will increase on Tuesday and will be sustained 10-20 kt
gusting to 25-30 kt by early afternoon.  Wister/85


PDT  53  73  45  64 /  20  20  10  10
ALW  56  78  49  67 /  20  20  20  10
PSC  58  81  50  71 /  20  20   0  10
YKM  50  76  40  67 /   0  10   0  10
HRI  56  78  48  70 /  20  20   0  10
ELN  52  70  42  62 /  10  10   0  10
RDM  46  69  40  62 /  10  10   0   0
LGD  50  73  45  60 /  10  50  50  10
GCD  49  77  43  62 /  20  60  40  10
DLS  55  70  48  66 /   0  10   0  10




SHORT TERM...86/85