Area Forecast Discussion
Issued by NWS Great Falls, MT

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FXUS65 KTFX 241136

Area Forecast Discussion
National Weather Service Great Falls MT
535 AM MDT WED AUG 24 2016

...Update to Aviation...


Today through Friday...Large upper low that has brought a weather
pattern change is now located in central Canada. A weak trough is
extending from that feature and into MT this morning...streaming
in wrap around moisture. Meanwhile a secondary circulation has
developed in Missoula`s CWA as evident on WV imagery. These two
features will set up a cool and unsettled day across the Treasure
State. The wrap around moisture and weak cold front from the
upper low in Canada is starting to bring isolated to scattered
showers...and should continue to do so through the morning and
into the afternoon hours across northern and central MT. Meanwhile
across the southwest...the secondary low will push
southeastward...and could fire off a few thunderstorms this
afternoon across sw MT. QPF from today`s precipitation is
generally expected to remain on the low side. WPC does have an
increase amount of qpf around 0.10 inches in their day 1 outlook
almost near Bozeman. There is a chance convection combined with
some upslope flow could bring some higher rain amounts in southern
MT...and especially across the higher terrain. Any thunderstorms
that can form in these areas will likely be slow movers with the
weakening flow aloft. Gave a slight nudge to WPC Guidance in the
QPF grids...but over rainfall amounts will be tame today except
under any mentioned slow moving thunderstorms. Flow then shifts to
a more dry and northerly flow tonight...effectively ending chances
for showers...except for a few isolated ones near the mountains.
Thursday will be mainly dry...with only a few isolated mountain
thunderstorms. Early Thursday evening through Thursday
night...another cold front slides out of Canada bringing our next
chances for showers and a few thunderstorms. This front and
associated disturbance could linger chances for showers and a few
thunderstorms into Friday...however...this disturbance may loose a
bit of its energy...making precipitation isolated to widely
scattered in coverage. Strong thunderstorms are generally not
expected for today and Thursday as both instability and shear are
lacking. Friday will see a slight increase in shear and
instability as a favorable jet comes overhead. Perhaps an isolated
stronger storm is possible...however...only small hail and some
gusty winds will be the main threat. Breezy winds may also
continue for today...and become northerly tonight. Winds should
then display a diminishing trend for the remainder of the short
term...with only a slight increase possible for Friday.

Today will likely be the coolest day of the week as the secondary
cold front comes through. Highs in the 60s are generally expected.
Slight warmer but still cool temps are then expected for Thursday
and again Friday...with 70s predicted for Friday. Although winds
will stay somewhat breezy today...with increasing moisture and
cool temps...little fire danger is expected. Fire danger will
remain low through Friday...although some low RH near 20 will be
possible across the far SW. Anglin

Friday night through Wednesday...Medium range models continue to
show an upper level positive-tilt trough over Montana moving
eastward into the Dakotas by Saturday morning. Montana will be left
beneath westerly flow aloft that backs to the southwest by the end
of the weekend. GFS and ECMWF each depict some degree of upper level
troughing along the British Columbia coast by Sunday night. However,
the GFS keeps energy associated with this feature as a closed upper
low over Southeast Alaska, while the ECMWF develops a narrow, weaker
trough that extends southward into Northern California. Model
solutions diverge significantly after Monday morning with the closed
low and associated trough of the GFS remaining along the West Coast
for persistent anti-cyclonically curved southwest flow aloft over
Montana. The ECMWF gradually broadens the upper trough across
western Canada as well as the Pacific Northwest and the Northern
Rocky Mountain region for cyclonically curved south to southwest
flow over the state.

These model trends continue to support temperatures warming back up
to near or above seasonal averages for the weekend with mostly dry
conditions prevailing through Sunday morning. Isolated storms appear
probably for Sunday afternoon and evening. Unfortunately, the
forecast for next week remains low confidence at this time but model
consensus tools support temperatures remaining near seasonal
averages with isolated showers and possibly thunderstorms each day.
One caveat being that if the ECMWF solution proves to be closer to
reality, the chance for showers could increase significantly by
Wednesday. mpj



Low and mid level clouds will continue to move southward through the
forecast area this morning. Isolated light rain showers are possible
across much of north central Montana though 00Z Thursday with areas
IFR/MVFR ceilings at times. South of a line from KHLN TO KHWQ
(Harlowton) will see prevailing VFR conditions with increasing mid
and high level clouds this afternoon along with a few isolated
thunderstorms. Conditions improve to widespread VFR after 00Z
Thursday with some breaks in mid-level clouds. mpj


GTF  63  41  69  45 /  40  30  10  20
CTB  64  42  67  45 /  30  20  20  30
HLN  65  44  70  47 /  30  30  10  10
BZN  65  41  71  42 /  20  20  10  10
WEY  63  34  63  34 /  20  20  20  20
DLN  66  40  69  42 /  10  20  10  10
HVR  67  46  72  49 /  20  20  20  20
LWT  59  43  67  45 /  40  30  10  20



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