Area Forecast Discussion
Issued by NWS Great Falls, MT

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FXUS65 KTFX 291116

Area Forecast Discussion
National Weather Service Great Falls MT
516 AM MDT Thu Jun 29 2017

...Update to Aviation...


An upper level low pressure system will place much of North
Central and Southwest Montana beneath a cool, moist and somewhat
unstable north to northwest flow aloft on Thursday. This pattern
will favor a continuation of scattered showers and thunderstorms
this afternoon and evening. Shortwave ridging aloft builds into
the region Thursday night through Saturday for mostly dry
conditions and a significant warming trend through the weekend.



Today through Saturday...Upper level trough has brought a moist
and slightly unstable north to northwest flow aloft over Montana
this morning. Short term models are in good agreement bringing
scattered to numerous showers and a few thunderstorms to the
western mountains today, while lower elevations should see only
isolated to scattered activity. Latest model runs are also in good
agreement that CAPE and bulk shear should remain too weak to fuel
severe storms. However, I would not rule out the possibility for
one or two storms to generate borderline strong outflow winds,
especially in Southwest Montana this afternoon and early evening.
Additionally, high temperatures today are expected to be cooler
than those of the last few days as Canadian air slips into the
region. Friday, Friday night and Saturday morning will see mostly
dry conditions with a significant warming trend as the upper
trough moves eastward and short wave ridging builds over the
state. The axis of this upper ridge will move far enough east to
allow southwest flow aloft to return Saturday afternoon with
isolated showers and thunderstorms developing over the higher
terrain of western and southwest Montana. mpj

Saturday night through Wednesday night...Main forecast concerns
is rising temperatures and increasing fire weather concerns. A
complex upper level pattern highlights the first period of the
long term, as a H500 shortwave crosses the Northern Rockies/High
Plains Saturday evening and into the day on Sunday, all the while
H500 ridge amplifies to the north over Canada. This should
translate to scattered showers and thunderstorms during the
evening hours on Saturday across Southwest Montana, transitioning
to widely scattered storms over the mountainous terrain for the
day on Sunday (due to diurnal heating and orographic effects).
H500 ridging then briefly settles in Sunday evening across the
region, before breaking down during the day on Monday. As the
ridge breaks down and flattens out on Monday, high temperatures
are expected to rise into the mid-80s to mid-90s, with increasing
westerly winds and decreasing relative humidity values. This setup
would lead to elevated fire weather concerns, which is discussed
in more length within the Fire Weather Section. Following Monday,
flow becomes near zonal through much of the day on Tuesday. Models
then begin to differ with the exact placement of H500 features
beyond Tuesday, however, some indications are that a significant
ridge could try to build in across most of the Western CONUS for
the remainder of the work week. Biggest thing of note in the
second half of the long term, is the potential for a period of
temperature well above climatological norms. - Moldan


Updated 1116Z.

Upper level trough will remain over the region for a cool, moist and
somewhat unsettled northerly flow aloft across North Central and
Southwest Montana today through this evening. Scattered showers and
thunderstorms can be expected across the forecast area, especially
south of a KHLN-KGTF-KLWT line. High pressure ridging aloft builds
over the region after 03Z Friday with skies gradually clearing from
northwest to southeast overnight. VFR conditions expected to prevail
through the period except for brief MVFR conditons in and around
heavier showers. mpj


Fire weather concerns will begin to increase during the day on
Monday, as H500 ridging over the Northern Rockies/High Plains
undergoes somewhat of breakdown, and flow turns from southwest to
westerly. Anomalously warm H700 temperatures of +8C to +14C and
H850 temperatures of +24C to near +30C, should translate to
surface temperatures into the mid-80s to mid-90s Fahrenheit across
North Central and Southwest Montana. As the ridge undergoes its
breakdown, H700 cross barrier flow across the Continental Divide
will strengthen throughout the day to around 20-40kts by the late
evening hours. As this flow descends and subsequently dries on the
lee side of the Rockies, boundary layer moisture will quickly
become limited, especially across North Central Montana. This
should result in Monday afternoon relative humidity values falling
to near or below 20 percent across North Central Montana. In
addition, surface winds will become sustained at 10 to 20 mph,
with gusts approaching 30 mph. Finally, little to no fuel shading
is expected during the day on Monday, as skies are expected to
range from sunny to mostly sunny. The combination of unseasonably
warm temperatures, breezy west to southwest winds, low afternoon
relative humidity values, and antecedent dry fuels (especially
across Fire Weather Districts 113 and 115) will lead to elevated
fire weather concerns for the day on Monday across North Central

After the brief ridge breakdown during the day on Monday, there
are indications that significant H500 ridging could begin to
develop across the Northern Rockies/High Plains for the remainder
of the work week (Independence Day through Friday). While winds
beneath this ridge would be lighter compared to that on Monday,
surface temperatures in the mid-80s to 90s would likely persist.
In addition to the unseasonably warm temperatures, low afternoon
relative humidity values could also occur, which would further
exacerbate the curing of local fuels. - Moldan


GTF  69  48  80  54 /  30  10  10   0
CTB  68  47  79  52 /  20   0  10  10
HLN  71  50  81  54 /  30  10  10  10
BZN  69  44  78  48 /  20  10   0   0
WEY  62  36  70  38 /  40  20   0   0
DLN  67  44  76  48 /  30  10   0   0
HVR  71  48  83  53 /  20   0  10   0
LWT  67  46  76  51 /  20  10   0  10



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