Area Forecast Discussion
Issued by NWS Pendleton, OR

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FXUS66 KPDT 260517 AAA
AFDPDT

Area Forecast Discussion...Updated Aviation Discussion
National Weather Service Pendleton OR
1020 PM PDT Sat Mar 25 2017

.UPDATE...Showers have ended across the area and upper level
trough continues to move east into Idaho. High clouds are already
increasing along the west coast ahead of next system that will
bring a warm front east of the Cascades through the day on Sunday.
Only updates to some clouds and overnight temperatures.  93

AVIATION...06z TAFS...VFR conditions are forecast to continue
through tonight.  The high clouds will lower and thicken toward
daybreak Sunday. Light rain will be possible over the western TAF
sites (KBDN, KRDM and KDLS) prior to 26/18z. Expect low end VFR or
Tempo MVFR cigs/vis in the steadier rain by late Sunday morning over
the western portion of the area...spreading eastward by Sunday
afternoon. The light rain should reach KPSC, KPDT and KALW between
26/18-21z and continue through 03z.  Winds will be generally under
10kts.  93

&&

.PREV DISCUSSION... /issued 425 PM PDT Sat Mar 25 2017/

SHORT TERM...Tonight through Tuesday...A weak upper level trough
exiting the region this evening will bring an end to the showers.
The next weather system is already approaching the West Coast with
increasing clouds. Warm frontal precipitation will reach the coast
late tonight then spread over the Cascades Sunday morning then over
the rest of the forecast area by afternoon. Then the associated cold
front moves across the forecast area early Sunday night and exits by
Monday morning with precipitation decreasing to showers or ending
behind the front. The upper level trough behind the front will pass
over the area on Monday which will re-energize the instability
leading to more showers during the afternoon and evening. A flat
ridge will build back over the region Monday night and Tuesday. Some
moisture in a westerly flow will still bring a low chance of
precipitation to the Cascades and the northeast mountains.

LONG TERM...Tuesday night through Saturday...The extended period
will start off unsettled, with several systems moving through the
area. Conditions should become drier with continued seasonable
temperatures for the end of the work-week and into next weekend as
an upper level ridge builds along the West Coast.

For Tuesday night and Wednesday, a weaker shortwave disturbance will
ride over the top of the departing, flat upper level ridge. There
remain some model differences in the timing and strength of this
system. The latest 12z ECMWF would suggest periods of rain for just
about the entire CWA from late Tuesday night through much of
Wednesday. Meanwhile, the 12z GFS has the disturbance weaker and
further north. Under the GFS solution much of the forecast area
would remain mostly cloudy with just some scattered showers
especially north and in the Mountains. How much precipitation the
area ultimately sees will also impact temperatures, especially
Wednesday afternoon. For now decided to take a blend of the
available guidance, which gives a slight chance to low chance of
rain and high mtn snow late Tuesday night, then a high chance to
likely PoPs during the day on Wednesday, as the system moves in.
Snow levels will remain near 5000 to 6000 feet during this time
period. Heading into Wednesday night and Thursday the model guidance
comes back into better agreement that a vigorous upper level
trough/low will develop along the Oregon Coast in response to a
strengthening up stream ridge near 140-145 degrees west. This system
is supported by a 140kt jet at the 300mb level, and is progged to
bring ample Pacific moisture into the region, with PWATS approaching
1.00 inch. Out ahead of the trough axis periods of stratiform
precipitation are a good bet for much of the area Wednesday night.
The latest model guidance is in remarkably good agreement that the
trough axis will be along the Cascade Crest by late Thursday
morning, with the strengthening mid and upper level low then
dropping into SE Oregon by Thursday afternoon. This system will also
bring a large pocket of cold air aloft over the region (especially
in Oregon). This will help to destabilize the atmosphere Thursday
afternoon...in fact the 12z GFS does have some surface based CAPE
and negative LIs over parts of the region Thursday afternoon. Did
not yet have enough confidence to add thunder into the forecast, but
this will need to be monitored closely in future forecast updates.
Either way, showers are in the forecast during the day on Thursday,
with likely PoPs in the Oregon mountains (which will be closer to
the track of the upper low), and slight chance to chance for the
lower elevations and further north. The Yakima and Kittitas Valleys
may be far enough north to have a drying NW flow on Thursday. Snow
levels will be 5000-6000 feet Wednesday night, falling to 4000-4500
feet Thursday...then 3000-4000 feet Thursday night. The
aforementioned system will quickly exit the area to the south and
east Thursday night, leaving behind gradually clearing skies. Friday
will remain dry and partly cloudy as an upper level ridge builds
just to the west of the forecast area. Overnight lows will be chilly
Friday and Saturday morning, in the lower to mid 30s for the
Columbia Basin and surrounding areas...with 20s in the mountains and
higher mountain valleys of Central Oregon. Daytime highs will be
seasonably mild Friday and Saturday, with lower to mid 60s in the
Valleys and 50s in the mountains. Conditions likely remain dry under
partly cloudy skies for next Saturday, as the upper level ridge
remains in place. 77



&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  37  53  40  55 /  10  60  80  40
ALW  41  55  43  56 /  10  60  70  40
PSC  37  53  43  60 /   0  60  60  20
YKM  35  52  39  61 /   0  60  60  20
HRI  37  52  40  57 /   0  60  60  20
ELN  31  47  34  53 /  10  70  60  20
RDM  32  54  34  54 /  10  70  70  20
LGD  34  53  39  53 /  20  50  80  60
GCD  35  54  37  53 /  10  60  90  50
DLS  39  52  41  57 /  10  70  70  20

&&

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

$$

93/99/93



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