Area Forecast Discussion
Issued by NWS Pendleton, OR

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FXUS66 KPDT 261557

Area Forecast Discussion
National Weather Service Pendleton OR
857 AM PDT Tue Sep 26 2017

.SHORT TERM...Today through Thursday...A mid/upper level high
pressure ridge just offshore puts the forecast area on the eastern
flank of the ridge today yielding a northerly flow aloft through
tonight. The ridge axis moves over the region on Wednesday
resulting in mostly clear skies during the day on Wednesday.
Sinking air associated with the ridge will produce a warming
trend, which begins today and lasts through Thursday. The ridge
axis moves east of OR/WA Thursday just ahead of an upstream cold
front nearing the coast Thursday evening, which brings increasing
clouds Thursday night. As such Thursday will be the warmest day
with lowlands highs in the upper 70s to lower 80s and highlands
max temperatures in lower 60s to mid 70s.

Forecast adjustments for today...I decreased sky cover in western
sections of the forecast area for this morning and increased the
maximum temperatures for today by a few degrees in the Lower
Columbia Basin, the Blue Mountain Foothills and in central Oregon.


.PREVIOUS DISCUSSION... /issued 410 AM PDT Tue Sep 26 2017/

SHORT TERM...Today through Thursday night...The main feature in the
short term will be an upper ridge that is currently building over
the northeast Pacific. It will slowly shift eastward and move into
the region this week with a warming trend and dry stable conditions.
However the warming trend will not be too strong due to the flow
being mostly out of the north to begin the warming trend. By the end
of the short term period (Thursday) the flow will become southwest
as the ridge axis moves to the east. This will allow stronger
warming to take place on Thursday. Skies will be mostly cloudy to
start the short term period (today), but they will gradually lessen
with skies becoming mostly clear by Thursday. Winds will be light
throughout the short term period and under 15 kts. 88

LONG TERM...Friday through Tuesday...The latest model guidance has
trended toward a much cooler and wetter weather pattern through the
extended period. There is solid support from the operational 00z
GFS, CMC, ECMWF and 00z GFS ENS mean that a deep trough, supported
by a 130+ KT jet will approach and move through the forecast area
Friday through Saturday. Friday morning there will be slight chance
PoPs along the Cascades for showers...with increasing clouds
spreading east. By Friday afternoon chances for showers make it as
far east as Central Oregon, The Columbia Gorge and Yakima Valley.
Finally, Friday evening and night the precipitation is now expected
to spread east, overtaking the entire CWA. Kept PoPs at slight
chance for the Lower Columbia Basin, with high chance and some
likely PoPs in the mtns. On Saturday, this deep upper level trough
remains in place over the forecast area with a good chance for
additional showers. Temperatures and snow levels were also lowered
quite significantly from previous forecasts...with highs now
expected to only be in the 60s...except upper 40s and 50s mountains.
Snow levels will be around 5000-6000 feet on Saturday as 1000-500mb
thickness drop to around 545dm. With the upper level low moving
overhead there could very well be enough instability due to the cold
temps aloft for isolated afternoon t`storms...but confidence was not
quite high enough to add these into the official forecast just yet.
The trough axis moves overhead Saturday afternoon, and just behind
the trough/front expect westerly winds to become rather breezy
..likely between 15-25 mph. On Sunday, the aforementioned system is
forecast to move east, and gradually out of the area. However,
lingering showers are a good bet, especially over the eastern
mountains in the morning. The Lower Columbia Basin and surrounding
areas should be dry, under partly cloudy skies on Sunday.
Temperatures remain cool, with highs in the 60s, except upper 40s
and 50s mountains. Snow levels remain near 5500-6500 feet. Heading
into early next week there is significant disagreement among the
various model guidance. The 00z GFs seems to be somewhat of an
outlier, and would keep the area under a northerly flow pattern on
the front side of a high amplitude offshore ridge. Meanwhile the
ECMWF, CMC and ECMWF ENS mean suggest yet another upper level low
would drop into the Pacific Northwest Monday and Tuesday. With such
significant differences in the available guidance decided to take a
blend (mainly the Superblend and WPC guidance) for this portion of
the forecast. This allowed for slight chances or chances for
additional showers over the forecast area Monday afternoon into next
Tuesday. Temperatures were also based off a blend of available
guidance and were again mainly in the 60s, except upper 40s and 50s
in the mountains for daytime highs. Overnight lows through the
extended period look to be in the 40s to lower 50s....except 30s in
the mountains. These temperatures are near to slightly below normal
for this time of year. 77

AVIATION...12Z TAFs...VFR conditions will prevail through the
period. Some mid level strato-cumulus clouds are present this
morning, mainly affecting KPDT, KPSC and KALW with a bkn to ovc deck
between 8-12K FT AGL. Other taf sites are mainly clear this morning.
Any lingering clouds are expected to dissipate, giving way to SKC
conditions this afternoon, evening and tonight as an upper ridge
builds overhead. Winds will remain light, AOB 10 kts and diurnally
terrain driven. 77


PDT  73  49  76  50 /   0   0   0   0
ALW  75  54  77  53 /   0   0   0   0
PSC  77  48  80  46 /   0   0   0   0
YKM  76  47  81  45 /   0   0   0   0
HRI  76  48  81  45 /   0   0   0   0
ELN  76  47  81  45 /   0   0   0   0
RDM  72  40  77  39 /   0   0   0   0
LGD  69  40  75  39 /   0   0   0   0
GCD  71  45  77  44 /   0   0   0   0
DLS  80  51  82  49 /   0   0   0   0




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