Area Forecast Discussion
Issued by NWS Great Falls, MT

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FXUS65 KTFX 231145
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
545 AM MDT TUE AUG 23 2016

...Update to Aviation...

.DISCUSSION...

Today through Thursday...Large upper low showing up clearly on WV
imagery across Canada this morning. A clear slot from about
Hamilton to Great Falls to Havre also showing nicely where the mid
level frontal boundary is. Behind this front is some mid level
moisture currently producing a broken to overcast layer...with
high level clouds in front of the front. Smoke from the Copper King
Fire in western MT seems to be following behind this front...and
could linger across north central MT this morning. This front will
push eastward today...while the upper low also skirts eastward
across Canada just north of the MT border. Wrap around moisture
does look to enter into the region today...however...models keep
pulling back the amount of precipitation associated with this wrap
around feature. Perhaps the Rocky Mountain Front and adjacent high
plains can still see a few showers today...but other than across
the mountains...coverage will be isolated to widely scattered.
Cool air behind this mornings front will allow for some weak
instability...making a few isolated thunderstorms also possible
today. Shear will be weakening throughout the day...so stronger
storms are not expected. Breezy west to northwest winds will also
be possible today...however...winds will not be as high as Monday.
More moisture streams in the region tonight...with a secondary
trough and cold front extending from the upper low...which will
be in eastern Manitoba by this time. This could bring more
isolated to widely scattered showers tonight...with perhaps a few
mountain snow showers at the highest peaks above 8000 feet. More
north to northeasterly flow is then expected for Wednesday with
lingering moisture perhaps bringing a few isolated to scattered
showers. Models are now not hitting the upslope component as high
for Wednesday...thus continue only the slight to chance pops. We
will then be in between weather systems for Wednesday night
through the day Thursday. This limits precip chances to southern
CWA where moisture still lingers...with mainly dry and continued
cool conditions elsewhere. A weak front could then come out of
Canada late day Thursday...perhaps increasing chances for a few
showers and thunderstorms across the north. Temperatures will cool
into the 70s today...and even cooler tomorrow behind the secondary
front...with Wednesday`s highs expected to only reach in the 60s.
Slightly warmer...but still cool temps in the 60s and 70s are
expected for Thursday. Anglin

Thursday night through Tuesday...Tonight it appears that the medium
range models have been reset and have come into good agreement for
Thursday night through Sunday. Period will start off with an upper
level positive-tilt trough whose axis stretches from southwest
Montana northeastward into Saskatchewan. This feature digs southward
overnight then sweeps eastward across Montana and into the Dakotas
by Saturday morning. Montana will be left beneath an initially
northwesterly flow aloft that gradually backs to the southwest over
the weekend period. GFS and ECMWF each attempt to carve out a low
pressure trough along the British Columbia coast but, as usual,
disagree on strength and timing. As a result, each indicate
southwest flow aloft over the region Monday and Tuesday. However,
the flow aloft in the ECMWF indicates cyclonic flow, while the GFS
has an anti-cyclonic component.

Fortunately, the overall trend for sensible weather across Montana
remains relatively unchanged from that of previous forecast periods.
Thursday night and Friday should see variable cloud cover along with
isolated showers/thunderstorms beneath the influence of the upper
level trough. Temperatures warm back up above seasonal averages for
the weekend with mostly dry conditions and mostly clear to partly
cloudy skies prevailing. Forecast for early next week is somewhat
uncertain but I have kept temperatures relatively stable and
generally in agreement with those of the weekend with a slight
chance for showers/thunderstorms being introduced under the
assumption that southwest flow aloft over the region will be
slightly unstable. mpj


&&

.AVIATION...
UPDATED 1145Z.

Deep, vertically stacked low pressure system over southern
Alberta/Saskatchewan will move slowly eastward today through early
Wednesday morning. Flow aloft will veer from westerly early in the
forecast period to north/northwest by this afternoon. Surface cool
front currently pushing southward through north central Montana
continues as far south as Yellowstone Park with widespread
northerly/upslope winds prevailing for much of the forecast period.
Low and mid level cloud cover will increase behind the front with
isolated to scattered showers and possibly a few thunderstorms
developing this afternoon and areas of light rain persisting
overnight. VFR conditions will prevail but areas IFR/MVFR conditions
will develop overnight along with widespread mountain obscurations.
mpj

&&

.FIRE WEATHER...

RH recovery continues to struggle this morning. Moisture should
increase across the region today with wrap around moisture from
an upper low pushing just to the north of Montana. As a result
most areas should see min RH 20 to 40 percent...except across sw
MT where RH in the teens will be possible. A few isolated to
widely scattered showers and thunderstorms will also be possible
today...mainly across the Rocky Mountain Front and north-central
MT. The mentioned disturbance will also keep breezy west winds
today...although gusts will not be as high as Monday. A critical
fire weather day is not expected today...however...portions of the
sw could still see elevated fire danger with the continued low Rh
values. Cooler temperatures and increasing moisture will then be
found for Wednesday...keeping fire danger low...although far sw MT
will still see low afternoon RH values. A few isolated to widely
scattered showers and thunderstorms may also be found on
Wednesday. Anglin

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  71  46  65  42 /  20  30  20  10
CTB  65  46  64  41 /  20  20  10  10
HLN  74  50  66  45 /  10  20  20  10
BZN  75  43  68  41 /   0  20  20  20
WEY  70  35  63  33 /   0  10  10  20
DLN  73  42  67  41 /   0  10  10  10
HVR  67  50  68  46 /  20  20  20  10
LWT  70  47  62  43 /  10  30  20  20

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
None.
&&

$$

weather.gov/greatfalls



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