Area Forecast Discussion
Issued by NWS Pendleton, OR

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FXUS66 KPDT 280230
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
730 PM PDT Mon Mar 27 2017

.SHORT TERM...Daytime instability has diminished and radar
indicates showers are quickly diminishing. Westerly flow aloft
combined with offshore warm front will result in plenty of mid and
high level clouds moving across the area tonight into Tuesday.
An approaching upper low will push the warm front through the
region Wednesday followed by a strong cold front Wednesday night.
This will bring additional rain to the forecast area with the
potential for some heavier rain over the Cascades and northeast
Oregon mountains. The steady precipitation will end Thursday
morning becoming showers during the day as the center of the upper
low moves overhead. 94

&&

.PREV DISCUSSION... /issued 300 PM PDT Mon Mar 27 2017/

SHORT TERM...Tonight through Thursday afternoon...Upper ridge moves
over for drying atmosphere and a dry forecast through tonight, except
for scattered showers over eastern mountains of Oregon into this
evening before tapering off and ending around sunset. Tuesday upper
ridge remains over forecast area for dry conditions through Tuesday
evening. Warm front moves into interior Pacific Northwest overnight
with chance PoPs from midnight to daybreak followed by rain likely
through the day on Wednesday along with -.50 to 1.00 inch of rain
in the Blue Mountains resulting in streams, creeks and rivers rising
through Wednesday and Wednesday night. Warm front on Wednesday
produce significant rainfall amounts in the eastern mountains. Snow
levels rise to around 5000-7000 feet during the day on Wednesday for
additional runoff due to snowmelt in the eastern mountains. Potential
for the John Day river to rise above flood stage late Wednesday into
Wednesday night. Cold front moves across region Wednesday night
followed by snow levels lowering through Thursday afternoon.
decreasing PoPs during the day on Thursday as the upper trough moves
overhead with sinking motion entering western sections of forecast
Thursday afternoon.  Polan

LONG TERM...Thursday night through Monday...An upper level trough
and associated low will be lingering over the eastern portion of the
forecast area for at least the first portion of Thursday night. This
will keep a chance of rain and higher mountain snow showers in the
forecast for the eastern mountains. As the aforementioned
trough/upper low digs into the Southwest US our area will be left
under a drying northerly flow by Friday morning. An upper level
ridge is forecast to build over the CWA during the day on Friday,
which will bring mostly sunny skies and seasonable temperatures to
the region. Skies remain mostly clear or partly cloudy Friday night,
with just some high clouds building over the top of the ridge. This
will allow for rather chilly temperatures, dipping into the 30s for
the Columbia Basin and surrounding locations, and into the 20s for
the mountains and high valleys. The latest model guidance then
diverges quite significantly on the timing and strength of potential
weather systems heading into the weekend and early next week. This
is leading to a lower than normal confidence forecast. The latest
GFS model would leave the forecast area under an upper ridge with
dry weather continuing into Saturday. The GFS then brings a potent
trough into the area for Sunday into Sunday night, with periods of
rain and high elevation snow moving through. Meanwhile the 12z ECMWF
model brings a fast moving weak disturbance through the CWA Saturday
and Saturday night. This would have the potential to bring ample
cloud cover and a period of light precipitation. The ECMWF would
then keep the forecast area under a northwest flow pattern, with
several weak disturbances moving through for Sunday and next Monday.
With such significant differences in the timing of systems in the
models, decided to not make many large changes to the official
forecast and mostly go with a model blend. Did lean slightly toward
the latest GFS solution. This gives a slight chance to low end
chance of rain and higher mountain snow Sunday, Sunday night and
lingering into next Monday. Snow levels will mostly be between 4500
to 5500 feet during this time. High temperatures are expected to be
in the mid 50s to mid 60s over the weekend, except some 40s and
lower 50s in the mountains. 77

AVIATION...00z TAFs...VFR conditions are expected to prevail over
the next 24 hours for most locations. Scattered rain showers have
developed this afternoon, mainly in the mountains. An isolated
shower cannot be ruled out at just about any terminal, mainly
through 28/03z...then the showers dissipate. Sct to bkn clouds
between about 4k to 10k ft AGL are also over the area this evening.
Expect high clouds, mainly between 15-25k ft AGL to build over the
area tonight. This will likely tend to inhibit any fog development
tonight. Then more bkn to ovc mid level clouds will develop through
the day on Tuesday. No precipitation is expected at the TAF sites
during the day on Tuesday. Winds remain breezy this evening, out of
the west-northwest between 10-20 kts...with a few gusts close to 25
kts. These winds will quickly decrease after sunset, and remain AOB
12 kts overnight and through much of the day Tuesday. 77

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  38  56  42  59 /  20  10  30  70
ALW  41  57  46  59 /  20  10  30  80
PSC  40  59  44  61 /  20   0  20  60
YKM  37  56  42  60 /  20  10  40  60
HRI  40  58  43  60 /  20   0  20  70
ELN  35  51  38  53 /  20  10  50  70
RDM  30  56  40  60 /  20   0  20  50
LGD  34  53  38  55 /  30  10  20  80
GCD  30  53  36  60 /  20  10  20  70
DLS  41  58  45  58 /  20  10  50  80

&&

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

$$

94


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