Area Forecast Discussion
Issued by NWS Pendleton, OR

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000
FXUS66 KPDT 212101
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
201 PM PDT Sat May 21 2022

.SHORT TERM...Tonight through Monday night...Currently there is a
fair amount of convection taking place, however, the lack of
shear and a strong trigger is causing the convection to actually
decrease slightly at this time. There were some stronger cells
with a fair amount of lightning with them, mostly over the
northeast mountains and the WA Cascades, and the valleys to the
east of the WA Cascades. During the last hour or so the amount of
lightning has decreased, indicating that the development stage of
the convection has decreased and cells are now actually
weakening more than developing. However, there is still an upper
shortwave disturbance in the northwest flow aloft, along with a
cold pool of air aloft, which should keep convection going through
at least early evening over these areas. But they are unlikely to
become strong, and remain a pulse variety of thunderstorms,
rather than strong or severe organized convection, despite the
decent amount of available CAPE and negative LI`s this afternoon.
The best chance for thunderstorms late this afternoon and evening
continues to be over the eastern/northeast mountains, where the
coldest air aloft is, and thus the steepest temperature lapse
rates, resulting in the greatest CAPE/negative LI`s. The latest
HREF ensemble Paintball is still showing that the best chance of
thunderstorms will be over the northeast Mountains, with another,
but weaker, maximum over the WA Cascades, and to a lesser degree
over the northern OR Cascades. Central OR, north central OR and
south central WA looks to remain dry and to have mainly just some
possible heavy cumulus, but no thunderstorms after about 03Z this
evening. All convection will dissipate after 06Z this evening.
However, there will still be some possible lingering showers over
the extreme NE mountains overnight.

On Sunday, the flow aloft will become more northwest, with the
current parent upper trough having moved to the east of the CWA in
the morning. However, there will still be some limited
instability and moisture in the afternoon for more possible
isolated thunderstorms over the northern Blue Mountains and
Wallowa County. However, the 500 mb flow will take on a more
anticyclonic pattern as a weak shortwave upper ridge embedded in
the flow develops over the PacNW by late afternoon. Therefore, any
convection that gets going during the late morning and early
afternoon will quickly decrease in the late afternoon or evening,
with nothing expected after 00Z Monday (5 PM Sunday afternoon).
This is in good agreement with the ensembles mean 500 mb pattern
at that time. The latest 12Z run of the models are then indicating
another weak shortwave disturbance embedded in the northwest flow
to move across the northeast portions of the forecast area during
the late afternoon/evening. However, there will be a lack of
moisture and instability to support any new convection late Sunday
afternoon or evening after 00Z.

On Monday, another weak disturbance embedded in the northwest
flow will move southeast across the forecast area, and clip the
northeast portions of the forecast area. It will also have some
impact over the central WA Cascades. Therefore, there will be a
slight chance to chance of afternoon/evening showers over these
areas on Monday. The only locations that may have a rumble of
thunder or two will be over the Wallowa Mountains and the high
country of eastern Wallowa County on Monday. These will persist
into early Monday evening, and then dissipate.

Temperatures will be on a slight warming trend during the short
term period by a degree or two each day. Maximum temperatures are
expected to be in the lower to mid 70s both on Sunday and Monday.
A more pronounced warming trend is then expected going into the
extended period. Winds will be light through the short term up
until Monday morning, and then it will become breezy to windy
by Monday afternoon, especially through the Cascade gaps, the
eastern Columbia Gorge, north central OR, the Kittitas Valley, and
portions of central OR. 88

.LONG TERM...Tuesday through Saturday...At the start of the period,
a ridge of high pressure will be building over the Northwest.
This ridge will build into Wednesday before moving eastward and
allowing for a more southwest flow on Thursday. The ridge will
bring dry and increasingly warm conditions. Temperatures on
Wednesday and Thursday look to be the warmest of the year so far
in many areas (though that is a low bar to clear)!

Southwest flow increases on Thursday into Thursday night in advance
of the next upper low and trough that will move onshore in
Washington on Friday or Friday night.  There are still some timing
differences with the speed of this feature, which can be expected
this far out.  This low and trough will bring a return to unsettled
weather and cooler temperatures for the start of the Memorial Day
Weekend. Temperatures will be a bit below normal by Saturday.,
though rain chances will be decreasing during the day on Saturday as
well.

The ensemble cluster are in generally good agreement Tuesday through
Thursday, with the ridge over the west.  By Friday, there is more
uncertainty in the ensembles with regard to the speed and depth of
the trough moving onshore. Most of the ensemble clusters support a
deeper trough with slower timing, which would trend away from the
faster GFS timing, though not necessarily the GFS depth.  The
uncertainty continues on Saturday with the depth and placement of
the trough, with the GFS still being the fastest and deepest while
the other ensemble clusters are slower.  The depth varies depending
on the individual solutions and other variables which will come into
better agreement as time progresses.

High temperatures will be mainly in the 70s on Tuesday, then climb
into the upper 70s to lower 80s Wednesday and Thursday, with some
mid 80s possible in the Columbia Basin.  Friday, will see a drop of
about 10 to 15 degrees, with highs ranging from the mid 60s in
central Oregon, to the low to mid 70s in the Columbia Basin.
Saturday`s temperatures will be similar but possibly a degree or two
cooler.  Overnight lows will mainly be in the 40s and 50s until
Friday night, then 30s and 40s.

&&

.AVIATION...18Z TAFS...VFR conditions are expected through the TAF
period. Winds could gust to around 20 kts at BDN and RDM this
afternoon through early evening. Otherwise winds should be 10 kts
or less. Isolated SHRA/TSRA are possible mainly across far eastern
Oregon and Washington, and should generally remain over the
mountains and east of the TAF sites. However, current radar does
show some SHRA popping up across other locations as well.

Current expectation is that any impacts from SHRA/TSRA at any
individual TAF site, were that to occur would be brief, and
therefore are not currently included. If it becomes apparent that
SHRA/TSRA are heading towards a TAF location later this morning
or during the afternoon, will handle with amendments, once timing
is more apparent.

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  42  69  48  70 /  20  10   0  10
ALW  45  72  52  72 /  20  10   0  10
PSC  47  76  54  76 /  20   0   0  10
YKM  40  75  49  73 /  30   0   0  10
HRI  46  74  53  75 /  10   0   0  10
ELN  40  72  49  68 /  40   0   0  10
RDM  35  68  44  70 /  20   0   0   0
LGD  37  64  43  64 /  20  10   0  20
GCD  37  66  44  67 /  20  10   0  10
DLS  48  76  55  73 /  30   0   0  10

&&

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

&&

$$

SHORT TERM...88
LONG TERM....77
AVIATION...77


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