FXUS66 KPDT 210157

Area Forecast Discussion
National Weather Service Pendleton OR
657 PM PDT Tue Mar 20 2018

.UPDATE...A SW flow of increasing moisture will begin to
overspread the region beginning tonight as an upper low
consolidates over the Gulf of Alaska. A few light rain showers are
expected tonight over Oregon with snow levels around 5000 feet.
Added some slight chance POPS for the Blues this evening as radar
and the ASOS at Meacham are indicating some -SHRA. Otherwise the
current forecast is largely on track with just some small changes
to min temperatures and sky cover for the overnight period
tonight. The heaviest rains still look like they will occur on
Thursday. 78


.PREV DISCUSSION... /issued 459 PM PDT Tue Mar 20 2018/

SHORT TERM...Tonight through Friday...Two weather systems will
impact the forecast area over the next three days. An upper level
low dropping into the Gulf of Alaska will begin to turn the flow over
the region more southwesterly. This in turn will pick up moisture
from another low pressure system off the California coast and advect
it into the region starting tonight and continue through Thursday.
This moisture is part of an atmospheric river that will be directed
into California from the sub tropic latitudes. Initial precipitation
will be light and mainly over Oregon tonight but will see a gradual
increase in precipitation Wednesday through Thursday. Snow levels
will rise to 5000 to 7000 feet so precipitation will primarily be
rain Wednesday and into Thursday. The the upper level low in the
Gulf of Alaska will send a fast moving cold front through the
forecast area Thursday afternoon. The atmospheric forcing of the
front combined with some instability over the eastern portion of the
forecast area could result in some thunderstorms Thursday afternoon.
Will also see increasing southerly winds Wednesday and Thursday
ahead of the front and then quick switch to westerly behind the
front Thursday. Snow levels will also drop rapidly behind the front
but precipitation also dwindles quickly so not expecting any
significant snow accumulation in the mountains. The upper level low
will remain off the coast Friday but will still spread enough cold
air aloft over the forecast area Friday for it to be a convective
unstable day producing showers with snow levels down to 1000 to 2000

LONG TERM...Friday night through Tuesday...A large mid/upper level
low pressure system just offshore Friday night will allow a couple
of mid/upper level shortwaves to move across the forecast area
Friday night through Saturday night. The first disturbance moves
through the region Friday night and Saturday morning. As a result
snow showers will be likely in central and east central Oregon and
the Blue Mountains through Saturday morning. Snow levels will be
1000 to 2000 feet Friday night through Saturday afternoon. The
second strong disturbance moves through the region Saturday night,
but this system will have significant westerly flow aloft thus
yielding sinking motion east of the Cascades for mainly dry
conditions Saturday night at lower elevations and a chance of snow
showers in the Ochoco Mountains and the northern Blue Mountains.
Sunday a moist northwest flow aloft will keep a chance of snow
showers along the east slopes of the Cascades, the Blue Mountains,
the Grande Ronde Valley and Wallowa County. Sunday night through
Monday night a somewhat drier northwest flow aloft brings mainly dry
conditions to the forecast area due to a ridge of high pressure in
the NE Pacific. Models suggest a weak system moving over the ridge
and east of the Cascades on Tuesday so there is a slight chance of
snow showers in the Blue Mountains and the Kittitas and Yakima
Valleys during the day on Tuesday as well as a chance of snow
showers along the east slopes of the Cascades Tuesday. Polan

AVIATION...00Z TAFs...VFR conditions for the next 24 hours. Mostly
clear into this evening for KYKM, KPSC, KPDT and KALW. Winds tonight
5-10kt. Increasing clouds this evening at KRDM and KBDN spreading
over these TAF sites from the southwest with ceilings becoming 050-
100. Lowering cloud bases spread northeast to the other TAF sites
overnight. Light rain expected at KRDM, KBDN, and KDLS beginning
around 12Z. Rain spreading to other TAF sites Wednesday morning
through afternoon. Winds overnight 5-10kt becoming 5-15kt by late
morning. Polan


PDT 39 62 46 61 / 20 20 40 80
ALW 40 62 48 62 / 10 20 30 80
PSC 36 62 46 60 / 0 20 40 70
YKM 35 60 43 56 / 10 20 70 60
HRI 38 63 46 61 / 20 20 40 80
ELN 32 55 39 53 / 0 20 70 70
RDM 34 58 39 53 / 30 60 70 70
LGD 37 56 44 57 / 30 30 40 90
GCD 38 57 45 57 / 20 40 40 90
DLS 40 61 44 53 / 20 50 80 70




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