Area Forecast Discussion
Issued by NWS Pendleton, OR

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FXUS66 KPDT 191724 AAA

Area Forecast Discussion...Update
National Weather Service Pendleton OR
1025 AM PDT Tue Sep 19 2017

Updated Aviation Discussion

.SHORT TERM...A large upper level trough is centered over British
Columbia and the Pacific Northwest this morning. Moisture
continues to stream into the area on a northwest flow and aided
by a 160 kt jet aloft, rain is expected to continue through the
next 24 hours. The air mass overhead is very cold (-26C) and this
is making the atmosphere unstable. Combined with the dynamic lift
from the jet aloft, would not be surprised to see a thunderstorm
or two pop up just about anywhere in the area, similar to
yesterday afternoon. Do not expect any thunderstorms that develop
to be strong. Have added a slight chance of thunderstorms across
the area this afternoon and early evening. Rain amounts will be up
to a tenth of an inch in the lower elevations today with similar
amounts tonight. The mountains will get a quarter to a third of an
inch today and up to a half inch tonight and heavier stratiform
rain arrives overnight. Snow levels are down to 4500-5000 feet
this morning, especially in Central Oregon and webcams show some
snow over the higher passes, which has been melting on the
roadways. Snow levels should rise to around 6000 feet tonight as
some warmer air filters in, especially in Central Oregon. Breezy
to locally windy conditions are expected today with generally
westerly 15 to 25 mph winds decreasing this evening. Temperatures
today will be unusually cool with highs in the mid 50s to lower
60s in the lower elevations and in the mid 40s to mid 50s in the
mountains. Forecast update already out. Perry

.AVIATION...18Z TAFS...A storm system with a strong upper level jet
is pushing through the forecast area today/tonight. This will keep
scattered showers in the forecast for the remainder of the day.
Could see an isolated t`storm over the Blue Mtns this
afternoon...much like yesterday. A period of steady rain arrives
after 20/06z over north-central Oregon and continues into the
morning hours on Wednesday. Expect bkn-ovc CIGS between 3.5
- 8K FT AGL through the period. There could be occasional MVFR CIGS
or VIS in heavier showers. Westerly winds will be quite breezy this
afternoon...between 15-30 kts most locations. Winds will gradually
decrease overnight and Wednesday morning...mainly AOB 15 kts. 77


.PREV DISCUSSION... /issued 259 AM PDT Tue Sep 19 2017/

SHORT TERM...Today through Wednesday night night...An upper
level jet stream and attendant moisture will yield an unsettled
atmosphere with below normal temperatures and showers through
Wednesday night. A mid/upper level disturbance in the Gulf of Alaska
this morning will develop into an amplifying trough through tonight
as it moves SE toward the WA/OR coast. In the meantime, the
aforementioned jet stream and moisture will remain over the forecast
area and yield mostly cloudy skies and showers at times in the
mountains with scattered showers at lower elevations through
tonight. The trough moves inland into the Pacific Northwest during
the day on Wednesday with dynamic lift associated with the trough
keeping periods of showers in the mountains on Wednesday. Wednesday
night the western flank of the upper trough enters the forecast area
with sinking motion providing subsidence and drying conditions at
lower elevations thus resulting in showers tapering off and ending
Wednesday evening in rain shadowed ares.  Polan

LONG TERM...Thursday through Tuesday...The upper level trough will
continue to impact the region Thursday and Friday with instability
showers primarily during the afternoon and evening hours. Snow
levels will remain around 5000 to 6000 feet so some light
accumulations possible in the higher mountain elevations. The trough
begins to exit the region late Friday and and into Saturday with a
noticeable decrease in precipitation threat Friday night and into
Saturday with just some lingering showers over the eastern
mountains. An upper level ridge of high pressure will build over the
region through the weekend and remain through most of the coming
week with a return of warmer weather.


PDT  60  43  56  42 /  40  70  70  30
ALW  59  47  56  44 /  50  70  70  40
PSC  62  45  59  44 /  30  60  50  10
YKM  64  43  59  39 /  30  50  40  10
HRI  62  44  58  44 /  40  70  60  20
ELN  61  41  59  39 /  30  40  40  10
RDM  57  42  55  33 /  40  70  50  20
LGD  54  38  54  37 /  60  80  70  50
GCD  54  48  55  38 /  70  70  70  40
DLS  64  54  61  48 /  40  80  60  20




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