Area Forecast Discussion
Issued by NWS Great Falls, MT

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FXUS65 KTFX 160518
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
1018 PM MST Sun Jan 15 2017

UPDATED AVIATION SECTION

.UPDATE...

Satellite imagery this evening shows the weak shortwave in NW flow
aloft has passed across most of the forecast area with skies
clearing in its wake as an upper level ridge moves into the
Northern Rockies. Similar temperature variations to previous
nights are occurring tonight due to low level temperature
inversions and radiative cooling with some locations in SW MT
valleys already near or below zero, with milder temperatures along
the east slopes of the Rockies, where somewhat breezy conditions
continue due to a lee-side trough of low pressure at the surface.
Have made some minor edits to minimum temps tonight based on
observed trends this evening, but overall forecast remains in good
shape. Hoenisch

&&

.AVIATION...
Updated 0518Z.

VFR conditions will generally continue over the forecast area under
a dry northwest flow aloft. However, haze and fog will continue
through the period in the Helena Valley, bringing MVFR visibility at
times to the KHLN terminal. Patchy fog may also redevelop in other
valleys in Southwest Montana, but it will likely not caused worsened
flight conditions at KBZN and KEKS. Breezy westerly winds will also
persist through the period over the plains of North Central Montana.
Coulston

&&

.PREV DISCUSSION...
/ISSUED 246 PM MST Sun Jan 15 2017/

Tonight through Tuesday...

Mid- to High-level clouds are spreading across N-central MT this
afternoon associated with a weak upper level disturbance. Radar
reflectivity is picking up weak returns aloft, but this shortwave is
not expected to produce any precipitation at the surface while
passing through this evening. A strong surface pressure gradient is
producing wind gusts up to 50 mph along the Rocky Mountain Front.
Winds are expected to slow somewhat overnight as the upper level
shortwave departs to the east and results in slightly higher surface
pressure.

Temperatures remain the most noteworthy impact, and challenge to
forecast through Monday night. Net radiational cooling under clear
skies, high pressure, and subsidence aloft are combining to maintain
valley temperature inversions in the valleys south and west of a
line from Helena to Bozeman. Reflection of sunlight during the day
off the snow covered ground inhibits surface heating, and therefore
prevents mixing as warm air over cold maintains the stable
inversion. Air Quality Advisories are in effect for some of these
areas due to high particulate concentrations under these temperature
inversions. The inversions are sufficiently strong and shallow that
all forecast guidance is not matching observed trends. Therefore,
persistence is being used to heavily weight and maintain the
temperature forecasts for these cold valleys far below guidance for
at least another day. Cold air also remains in place east of I15
along the HiLine. A rapid temperature rise from -1F to 36F occurred
in the Marias River valley near Shelby this morning, suggesting a
weakness in the temperature inversion. Clearing skies tonight will
enable rapid cooling once again, but with increasing winds later
Monday afternoon expect more widespread moderation of temperature
across the N-central plains.

Mostly sunny skies prevail through Monday afternoon as the upper
level ridge axis leans across the forecast area. A strong shortwave
in the westerly flow approaches Monday night. This will act to lower
surface pressure in the lee of the Rocky Mountains, thereby
increasing winds along and adjacent to the Rocky Mountain Front. A
High Wind Watch is in effect for this area with growing confidence
that winds speeds could reach near high wind criteria early Tuesday.
Precipitation advances up to the Continental Divide Monday night and
Tuesday, but westerly downslope flow will suppress precipitation
further east. PN

Tuesday Night through Sunday...

The period starts with an upper level ridge over much of the
western US with a broad upper level trough over the Pacific. By
Thursday, this trough will move into the western US as the ridge
shifts east. From late in the week through next weekend, the
models generally agree on a broad trough remaining across the
western US. Locally, this translates to generally dry and, at
times, windy conditions across the plains. For the SW valleys and
western/southwestern Mountains, it will likely be a bit more
active. Temp-wise, a general moderating trend is expected through
mid-week, followed by a gradual cooldown heading into the end of
the week. A few quick summary notes below:

*Wind: the strong winds developing in the short term period will
likely continue on through at least Wednesday morning across
central Montana. For now, the High Wind Watch will only go
through Tuesday, however based on the latest model guidance, it
is likely that this will need to be extended through at least
Tuesday night in later shifts. But, since Tuesday night/Wednesday
is still 3-4 days away, we`ll hold off on going this far with
the watch as the models may still come in lower in later runs.
That said, if the current models are correct, the potential
exists for 60-80+ mph wind gusts along the Rocky Mtn Front and
40-60 mph gusts across parts of the plains Tuesday
night/Wednesday.

*Precip: quite the surge of moisture will move in off the Pacific
through mid-week as a potential atmospheric river event sets up
briefly. This will favor the western and southwestern mountains
until the jet stream shifts south by Thursday. I raised pops some
in the mountains given the setup. For central MT, downsloping
still looks to limit precip, but given the seasonally high
moisture, I wouldn`t be surprised if a few showers make it out of
the mountains.

*Temps: 700mb temp anomalies of 2-3 std deviations plus a
downsloping W/SW wind should allow much, if not all, of central
Montana to see above normal highs for a change...a January thaw,
if you will. CIPS analog guidance even suggests a few locations
may approach or exceed 50 degrees by mid-week before cooler air
moves in late in the week. Temps are trickier across SW MT. The
current forecast assumes the current inversions will be able to
break with some wind and rising 1000-500mb thickness values, but
temps may end up cooler longer if the inversions remain stronger
longer. Martin

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  17  36  31  46 /   0   0  10  10
CTB  17  35  29  45 /   0   0  10  10
HLN  -5  17  10  37 /   0   0   0  10
BZN -10  20   8  37 /   0   0   0  10
WEY -19  16  -3  21 /   0   0   0  10
DLN   2  28  14  36 /   0   0   0  10
HVR  10  28  20  45 /   0   0  10  10
LWT  13  34  24  46 /   0   0   0  10

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
High Wind Watch from late Monday night through Tuesday afternoon
Eastern Glacier...Northern Rocky Mountain Front...Southern Rocky
Mountain Front.

&&

$$

weather.gov/greatfalls



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