Area Forecast Discussion
Issued by NWS Pendleton, OR

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000
FXUS66 KPDT 070533
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
1033 PM PDT Tue Jun 6 2023

Updated aviation discussion

.EVENING UPDATE...Lingering shower and T-storm activity has
almost entirely held to the south of our CWA this evening with
upper-level convective debris streaming across the region in the
form of cirrus caught up in the southerly flow aloft. Moving
through the rest of the night into Wednesday morning, the cut-off
low to our south will continue to swing mid-level moisture
northward but without diurnal heating it likely goes untapped.
That said, some CAM guidance like the HRRR does show some isolated
shower and/or weak T-storm activity overnight over portions of
eastern Deschutes County, Crook County, and Grant County. This
appears to line up with a swath of moisture currently associated
with T-storms to our south and some remnant 100-400j/kg of MUCAPE
aloft as indicated by the NAM and Canadian model. Low end
PoPs(~15%) were added to these aforementioned areas to account for
this potential scenario although confidence is rather low given
its nocturnal nature. We`ll certainly have to watch the eastern
Mountains overnight should this moisture and any isolated activity
push further north; confidence is high north-central Oregon
through the Columbia Basin and northward stays dry and quiet.

Otherwise, the forecast remains on track going forward with the
environment primed come Wednesday afternoon for scattered T-storms
across Central Oregon through the Eastern Mountains - isolated
severe activity possible. The previous discussion remains for
reference below. Schuldt/99

&&

.PREV DISCUSSION... /issued 440 PM PDT Tue Jun 6 2023/

.SHORT TERM...Tonight through Thursday night...So far this
afternoon there have been mainly just cumulus development over the
southern and eastern areas of the CWA. There have not been any
lightning strikes yet in the forecast area as of 2 PM PDT.
However, there still remains enough instability and moisture to
warrant a slight chance of showers and thunderstorms over southern
and eastern most areas of the CWA by late this afternoon and
evening. Thunderstorms today should just be the pulse type that
do not become organized supercell type storms with rotation.

The forecast area is in between a low pressure system to the south
and a weak upper trough to the north over southwest Canada and far
northern WA. There will be some moisture that will wrap around the
low to the south into the southern and eastern areas of the CWA
this evening that will produce some showers and possible
thunderstorms. SPC has a general risk of non-severe thunderstorms
over the southern and eastern areas of the CWA for this evening.

These will decrease overnight, and then there will be a better
chance, with increasing instability and moisture on Wednesday,
which will also push further to the north, reaching the Blue
Mountains and central OR. There will even be enough moisture and
instability for a marginal risk of severe thunderstorms on
Wednesday afternoon and evening for areas as far north as the Blue
Mountains. Currently, SPC has a MARGINAL risk of severe storms in
these areas.

The upper low over the Great Basin will then weaken, however a
southeast flow will remain, pushing moisture and instability into
the forecast area on Thursday and Thursday night. Precipitable
Water (PWAT) values will be higher, so thunderstorms will have a
better chance of producing more rain with them. However, due to
the increased cloud cover and expected cooler temperatures, it
will not be as unstable on Thursday, as it will be on Wednesday.
Therefore, only general thunderstorms will be expected for
Thursday and Thursday night. This is in agreement with both the
NBM 4.1, and also with the SPC convective outlook for Thursday.

Wednesday will be the hottest day, with high temperatures mostly
in the 90s, with a slight chance that some of the hotter areas of
the WA Lower Columbia Basin may reach 100 degrees. In the
mountains, highs will be mostly in the mid 70s to mid 80s. These
temperatures will be about 10-20 degrees above normal for early
June. Cooler temperatures are then expected for Thursday, by
about 4-6 degrees. Overnight lows will be in the 60s lower
elevations and mostly 40s to mid 50s in the mountains.

It will become locally breezy to windy during the afternoons and
evenings, mainly in central to north central OR, as well as the
eastern Columbia River Gorge. The strongest winds will be on
Thursday when a westerly pressure gradient tightens up across the
Cascades and causes winds to increase through the Cascade gaps
and the eastern Columbia River Gorge. 88

.LONG TERM...FRIDAY THROUGH TUESDAY...The extended period is
characterized by a passing open wave from our south, as another
upper level low pressure system drives into California before slowly
opening up and moving through the Pacific Northwest into the early
part of the workweek. This will provide thunderstorm chances (30-
50%) on Friday across Central Washington and along the Blue
Mountains and east, staying more confined to the eastern mountains
Saturday through Tuesday as chances wane between 20-30%. The ECMWF
EFI highlights Friday`s storm threat as 50-80% of ensembles show
unclimatologically high amounts of CAPE extending from Deschutes
County through the John-Day/Ochoco Highlands and into the Northern
Blues/Wallowas. Temperatures will also be warming through the
period, especially Sunday onward as an upper level ridge builds off
the Pacific coast. High temperatures will be increasing from the
upper 70s to low 80s for lower elevations of the Basin on Friday
into the upper 80s to low 90s for lower elevations of the Basin on
Monday, which is about 10 degrees above normal. Conditions will also
be drying through the weekend and into next week with max RH`s
dropping into the low to mid 60% range Tuesday morning with min RH`s
around 25% Tuesday afternoon.

An upper level wave, that was previously a closed low pressure
system over Southern California, will begin pushing in from the
south on Thursday and continue to slowly track through our area
Friday before exiting to our northeast on Saturday. This will allow
for shower and thunderstorm potential beginning Friday morning and
extending through the day before focusing along the Blue Mountains
and east into Saturday morning. Pockets of MUCAPE values between 250-
750J/kg  are present from Deschutes county through the John-
Day/Ochoco Highlands and into the eastern mountains with 0-6km shear
between 20-30kts. Heavy downpours will be possible with any
developing storm cells as precipitable water (PW) values are between
0.75-1.25", which is 150-200% above normal on Friday. These PW
values will be decreasing and closer to 100-120% above normal
starting Saturday as drier air moves into the region. Breezy
conditions will also ensue through the day on Friday, especially
across the Eastern Gorge, Lower Columbia Basin, and Kittitas Valley
as a pressure gradient of around 5mb will be present between
Portland and Spokane to allow for gusts of up to 35mph out of the
west-southwest, peaking Friday morning. Conditions will be drier on
Saturday as the wave continues to lift through the area and push
north late in the day into Sunday morning to allow for another
chance for showers and thunderstorms across the eastern mountains
(35-55%), John-Day/Ochoco Highlands (30-40%), and the Northern Blue
Mountain foothills (15-30%) before tapering off in the evening. An
upper level low pressure will be developing off the coast of
Southern California Saturday morning before moving inland Sunday
morning and stalling over Northern California on Monday. This will
allow for flow aloft to have more of a southeast component to advect
a drier and warmer air mass into the area to provide high
temperatures into the low 90s for areas of the Eastern Gorge and
Lower Columbia Basin on Sunday. An upper level ridge offshore of the
California closed-low feature builds on Sunday and infiltrates into
the Pacific Northwest on Monday, helping to further dry conditions
and bump temperatures up 3 to 5 degrees for lower elevations of the
Basin from Sunday to Monday. This continues into Tuesday until a
weak shortwave over the Yukon suppresses the ridge slightly to allow
for more northwest flow aloft and cooler conditions heading into the
midweek.

Guidance is in good agreement with the overall trend of an opening
wave passing through our area, a subsequent low pressure system to
our south, and an offshore upper level ridge, but differences arise
regarding the strength and timing of the low pressure system and
building upper level ridge into the early week. These discrepancies
are visualized via the 500mb EOF Patterns as a 40-50% ensemble
variance is present along with a dipole indicating the primary issue
is with the timing of the incoming low pressure system through the
weekend before turning to differences in ridge strength into the
early workweek. This would lead to a warmer and drier outcome with
the GFS versus the ECMWF, as the GFS pushes the upper level ridge
into our area much earlier on Monday then the ECMWF, which moves it
in Tuesday morning. The cluster phase space lends more confidence in
the ECMWF outcome of a later building ridge, which keeps the
potential for developing afternoon showers and thunderstorms across
the eastern mountains Sunday and Monday. This also provides
additional confidence in high temperatures breaking into the upper
80s to low 90s for lower elevations of the Basin over both days.
Cluster heights also lend additional confidence in the ECMWF
scenario with the majority of ensembles aligning better to a slower
building ridge. Thus, the NBM was utilized through the extended
period to provide an applicably weighted scenario of warming
temperatures through Monday and lingering storm chances into early
next week for the eastern mountains. 75

&&

.AVIATION...06Z TAFS...VFR conditions are expected to persist
through the period. Some convective debris clouds will linger over
portion of the forecast area overnight. Convection will fire off
again midday Wednesday over central Oregon and over the eastern the
mountains mainly impacting BDN and RDM with possible showers and or
thunderstorms. Winds are expected to remain less than 10 kts but
with possible gusty winds near convection on Wednesday.


&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  57  92  62  87 /   0   0  10  20
ALW  60  95  65  90 /   0   0   0  20
PSC  60  98  67  93 /   0   0   0  10
YKM  58  95  64  94 /   0   0   0  10
HRI  59  97  65  93 /   0   0   0  10
ELN  55  94  64  92 /   0   0   0  10
RDM  53  87  56  83 /   0  20  20  20
LGD  57  88  60  84 /   0  20  20  40
GCD  54  87  56  83 /  10  40  40  50
DLS  59  97  64  88 /   0   0   0   0

&&

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

$$
UPDATE...99
SHORT TERM...88
LONG TERM....75
AVIATION...99


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