Area Forecast Discussion
Issued by NWS Pendleton, OR

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FXUS66 KPDT 220430
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
830 PM PST Sat Jan 21 2017

.SHORT TERM...Tonight through Monday night...Have increased POPs
and snowfall in western Deschutes and Jefferson counties from 10
pm to 4 am. Next weather disturbance rotating around eastern
Pacific low pressure area has entered southwest Oregon and will
move northeast across the central Oregon after midnight. The
most snow will be near the Cascades crest and may approach
advisory criterion by late Sunday morning in Western Wasco,
Klickitat, and Yakima counties. Current forecast has 2-3 inches
which seems reasonable. Only light rain and snow is expected
later tonight and Sunday across the rest of the area. On Monday,
the offshore low pressure area will move south into California and
a drier and colder northerly flow will return.

&&

.AVIATION...06z TAFS...IFR conditions will occur in KBDN, KRDM,
KDLS, and KYKM as a disturbance moves north and brings mixed rain
and snow. The lower cigs/vsby will last around 6-9 hours and will
start around 10z at KRDM/KBDN, 13z at KDLS, and 15z at KYKM.
Generally MVFR/VFR will prevail at other TAF sites.



&&

.PREV DISCUSSION... /issued 213 PM PST Sat Jan 21 2017/

SHORT TERM...Tonight through Monday night...A series of weak
weather disturbances will be moving across the forecast area during
the short term period. A weather system is currently moving into
western Oregon and Washington at this time and spreading showers
east of the Cascades mainly in northern Oregon and southwest
Washington. These showers will affect the forecast area mainly late
this afternoon and this evening with a chance of lower elevation
rain and mountain snow. Precipitation amounts will be light and not
enough for winter weather highlights. There will then be a break
Sunday morning before the next system moves in the afternoon and
evening. This next system will bring a better chance of lower
elevation rain or snow showers and mountain snow showers. The lower
elevations may see snow only at night when snow levels are
lower...otherwise they will be around between 1700-3000 feet. There
will be some light accumulations above these elevations. More snow
will fall in the mountains, but not enough for winter weather
highlights again. A third system will then brush mainly the southern
portions of the CWA as the low pressure system passes farther to the
south. Again snow amounts look to be light but the atmosphere will
be colder and if there are any showers in the lower elevations it
should be all snow. After that high pressure aloft will move in
aloft over the region going into the extended period.

LONG TERM...Tuesday through Saturday...On Tuesday and Wednesday the
forecast area may see some scattered showers (mainly in the Mtns)
under a north to northwest flow. A rather weak upper level ridge
builds just offshore on Tuesday and starts to move east...overhead
by Wednesday. The inversion does not look particularly strong during
this time as weak impulses will be moving through the flow. Decided
to leave the patchy freezing fog in the forecast for early Tuesday
AM and again Tuesday night/Weds morning. A gradual cooling trend is
expected early to midweek as colder air filters into the CWA in the
northerly flow regime. There were some differences in the model
guidance temperatures with  the Euro being generally colder than the
GFS based guidance. For now took a middle of the road, blended
approach, as these temperature details will depend on fog formation
and inversion strength. Highs Tuesday and Wednesday are expected to
be in the mid 20s to mid 30s...with lows from 15 to 25...except some
single digits possible in the colder mtn valleys. A weather system
runs into the ridge Wednesday night and Thursday. After seeing
rising heights on Wednesday, heights remain nearly unchanged Weds
night/Thursday...indicating that this system weakens considerable
moving east. However...latest model guidance was hinting that this
system could still be enough to initiate some very light precip over
the area. Snow levels will be low...so the forecast now includes a
slight chance of snow showers during this time. After this weak
system moves through all current indications are that a
large...strong...upper level ridge of high pressure will build over
the Pacific NW from Friday through at least the end of the long term
period. Low level inversions will gradually strengthen heading into
next weekend. A strong surface high pressure system also
builds...with surface pressure perhaps reaching 1050mb over the
Treasure/Baker valleys. This will increase the pressure gradient
over the lower Grande Ronde Valley and southerly winds will begin to
increase Friday into Saturday. At this time, winds are not looking
particularly strong...mainly 15 to 25 MPH. This will also mean
increasing coverage of fog/freezing fog for locations below about
2500 MSL. Diurnal temperature ranges should begin to decrease in the
areas affected by fog...started to show this trend in the forecast
temperatures. High temperatures for Friday and Saturday are forecast
to be in the mid 20s to mid 30s areawide...with overnight lows
mainly 15 to 25 degrees. 77

AVIATION...00z TAFs...Generally VFR or MVFR conditions are expected
to continue over the next 24 hours. Periods of mixed rain...snow and
sleet showers will continue to move through the area late this
afternoon and into the early evening hours. There should be a break
in the activity mid to late evening. Then, the next system begins to
spread light snow into KBDN...KRDM and KDLS Between about 22/10-13z.
A light mix of precipitation then reaches KYKM and KPSC by 22/15z.
This snow/mixed precip could create several hours of IFR CIGS/VIS at
these sites Sunday morning. Some light rain showers will also be
possible at KPDT and KALW after about 22/15z...however strong south
winds above the surface will create a rain shadow and warmer
temperatures along the foothills of the Blue Mtns. Conditions should
remain MVFR or VFR here much of the time. Expect gradual improvement
Sunday afternoon as CIGs rise and scatter out. Surface winds will
remain under 15 kts through the period for most locations. However
will need to watch for some gusting south winds at KPDT...KALW
..KRDM and KBDN as winds are forecast to be around 30 kts at 1000
feet AGL Sunday morning. 77

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  32  40  26  31 /  40  60  10  10
ALW  33  42  28  32 /  30  70  10  10
PSC  28  37  25  32 /  30  60  10  10
YKM  25  35  21  32 /  60  90  10  10
HRI  28  36  26  31 /  30  60  10  10
ELN  26  34  20  30 /  70  90  10  10
RDM  26  38  21  31 /  50  70  40  40
LGD  31  35  21  28 /  40  70  20  20
GCD  29  38  21  31 /  30  70  40  40
DLS  29  37  28  33 /  40  80  30  30

&&

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

$$

79/77/79



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