Area Forecast Discussion
Issued by NWS Pendleton, OR

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FXUS66 KPDT 211546

Area Forecast Discussion
National Weather Service Pendleton OR
845 AM PDT Thu Oct 21 2021

.SHORT TERM...Today through Friday...The transition to a wet
weather pattern continues today as the ridge of high pressure over
the region is retreating to the east. This is in response to an
upper level trough off the coast deepening and slowly moving
eastward. Currently there is a cold front approaching the coast that
will slowly move eastward over the Cascades this evening and then
across the forecast area on Friday. New model runs are showing a
slightly slower eastward progression of the front overnight and
Friday so updated the morning forecast to reflect this. This initial
front looks like it will produce a quarter to a third of an inch of
rain in the lower elevations and half an inch to around an inch in
the mountains.


.PREV DISCUSSION... /issued 458 AM PDT Thu Oct 21 2021/

SHORT TERM...Today through Saturday night...Impressive upper low
off the BC coast will lift newd while a shortwave trough digs sewd
and moves inland Friday across northern California and Oregon.
Primary uncertainty today is the amount of QPF that will fall
especially with eastward and northward extent with this upper trough
with rain spreading east of the Cascades this evening.
The atmospheric river associated with this storm system is projected
to be less pronounced than earlier anticipated and it may be
weakening as it moves across eastern Oregon. This may cut down on
rain amounts over the eastern part of the forecast area on Friday.
There are conflicting signals in the available data with the ECMWF
Extreme Forecast Index (EFI) highlighting a climatologically unusual
precipitation event for this time of year over central and northeast
Oregon but successive runs of the deterministic NAM have
significantly weakened the band of precipitation as it moves from
central into eastern Oregon. For now have used the WPC QPF which
keeps .25 to .60 inch amounts in the lower elevations and .50 to
1.00 inch amounts in the mountains. This is similar to the
deterministic NBM QPF. The best bet to reach these totals may end
up being in central Oregon where the IVT is greatest. Rain will
generally taper off Friday night but another impulse is forecast to
move across the region on Saturday and Saturday night bringing
mostly light rain. It will be a warm day today with highs mostly in
the 70s in the lower elevations and 60s in the mountains...but
turning cooler Friday and Saturday with highs in the mid 50s to
lower 60s except mid 40s to lower 50s mountains. 78

Sunday through Wednesday.  A challenge will be with the timing of a
series of weather systems that is expected to effect the two state
area through the extended.

A stationary and intense low pressure system over the ne pac should
usher abundant moisture into the two state area for the latter part
of the weekend. In addition pressure gradients tighten and breezy
winds are possible Sunday and Monday. The bulk of the pcpn drifts
away later Sunday night but showers persist into Monday. The low
pressure system over the ne pac moves inland Monday night keeping
the clouds and showers in the forecast. As the low pressure moves
away from the region Tuesday morning another weather
system/disturbance quickly follows Tuesday afternoon and into the
night. The confidence about the timing and strength of this
disturbance begins to lessen as the EOF/Variance begins to increase.
As this system rapidly moves east...drier conditions are likely
along with breezy to windy conditions possible Tuesday night and
Wednesday. Snow level remain high and above 6k feet while
temperatures remain around normal through the period.

AVIATION DISCUSSION...mainly vfr conditions are expected to persist
for the next 24 hours. Meanwhile a weather system is forecast to
move into the region tonight and effect all taf sites with lowering
ceilings. Showers are forecast to develop at first at taf sites krdm
and kbdn and kykm after 22z then the remainder of the taf sites
after 04z. Lcl mvfr conditions could be approached in heavier showers.
In addition brief gusty winds between 20 to 25 kts is possible at
taf sites krdm and kbdn after 18z.


PDT  73  49  59  40 /   0  40  90  20
ALW  73  52  59  42 /   0  30  90  20
PSC  69  54  60  45 /   0  50  90  10
YKM  63  47  61  37 /  10  90  70  10
HRI  71  51  62  42 /   0  50  90  10
ELN  60  45  57  35 /  10  90  70  10
RDM  70  44  57  36 /  10 100  70  20
LGD  68  50  58  40 /  10  20 100  40
GCD  71  49  61  38 /  10  30  90  20
DLS  68  52  61  45 /  10 100  70  40




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