Area Forecast Discussion
Issued by NWS Great Falls, MT

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992
FXUS65 KTFX 220238
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
838 PM MDT Sun May 21 2017

.UPDATE...

A somewhat moist and unstable NW flow continues at this time across
the region. Of note, while the latest 00z TFX sounding and other
LAPS soundings show a fairly dry profile at this time, models and
satellite imagery show increasing mid level moisture dropping SE
through the area overnight as a s/w over SK also drops south through
the area. Ongoing shower activity east of I-15 will continue SE over
the next few hours. Despite some weak instability present, a capping
inversion around 500mb appears to be preventing much in the way of
lightning at this time. It`s unlikely that mid level temps will
cool much, so I don`t anticipate much in the way of lightning
through tonight, although a brief t-storm will still remain
possible (mainly from Havre to Lewistown). The increasing mid
level moisture mentioned above combined with weak convergence/lift
in the low levels may be enough to keep showers going overnight,
mainly from Cut Bank through Great Falls. For the update, I
adjusted pops some to reflect this thinking. Of note, though,
given the weaker lift/convergence and drier low levels, shower
activity may tend to be lower than currently forecast, so I
basically went in the middle with guidance and capped pops at
isolated. Martin

&&

.SYNOPSIS...

A northerly flow aloft will reside over the region through Monday.
As a result, a few passing light showers will affect the region
tonight and through the day on Monday, but any shower activity will
be short lived and should not produce any significant amounts of
precipitation. Drier and warmer conditions move into the region on
Tuesday, however strong winds, along with cooler temperatures move
back into the region for Wednesday. Looking ahead to Memorial Day
weekend expect seasonable temperatures along with scattered showers.

&&

.AVIATION...
Updated 2350z.

An upper level disturbance will move southeastward through the
region tonight through Monday. Prefer the WRF solution, thus a few
showers tonight in the Havre/Lewistown areas, then some showers in
the Helena/Bozeman area Monday morning/afternoon. Overall VFR
conditions will prevail, but some gusty winds to near 40kts are
possible from the showers this evening. Brusda

&&

.PREV DISCUSSION...
/ISSUED 550 PM MDT Sun May 21 2017/

Rest of this afternoon through Tuesday...Montana will remain
beneath a steady north to northwest flow aloft through Monday
night as a closed upper low remains anchored over the Great Lakes
region. To the west, a weak upper level ridge will remain along
the West Coast through tonight before becoming positively-tilted
and nosing inland over the Pacific Northwest Monday through
Tuesday. By Tuesday, the entire upper level pattern will undergo a
significant transition as a Pacific weather system moves
southeastward from the Gulf of Alaska and flattens the upper
ridge.

For the remainder of this afternoon through Monday, temperatures
will remain near seasonal averages with isolated/scattered showers
and a few thunderstorms possible. This activity will be
relatively weak as limited instability and weak disturbances aloft
interact in the northerly flow. Surface winds will remain
generally light to breezy and northerly in direction. By Monday
night, surface winds begin to veer clockwise in response to the
approaching Pacific weather system and surface pressure falls in
southern British Columbia and Alberta. Tuesday will remain dry but
temperatures will climb above seasonal averages as westerly winds
aloft and at the surface fuel increasingly strong downslope winds
across the region. mpj/cc

Tuesday night through Wednesday...Main forecast concern will be
the potential for high winds across portions of North Central
Montana and the Rocky Mountain Front. Strong cross barrier flow
will develop across the Rocky Mountain Front by 06z Wednesday, as
a strong upper level disturbance moves across the Southern
Canadian Provinces of British Columbia, Alberta, and Saskatchewan.
At the same time, an area of low pressure in lee of the Rockies
in Alberta will rapidly strengthen as it moves east towards
Saskatchewan, all the while dragging a cold front across the CWA
through the morning and afternoon hours on Wednesday. By 06z
Wednesday, H700 winds 50 to 80 knots will begin to overspread the
Rocky Mountain Front and there adjacent plains. BUFKIT soundings
depict a very shallow nocturnal inversion throughout this time
frame, thus any mixing during the overnight hours, especially
along the Rocky Mountain Front would result in high winds being
realized at the surface. By 15z Wednesday, BUFKIT sounding depict
strong momentum transfer across all of North Central Montana and
along the Rocky Mountain Front, with dry adiabatic lapse rates up
through H700. - Moldan

Wednesday evening through Sunday...An unsettled weather pattern
looks to begin across North Central and Southwest Montana, as an
upper-level trough settles southeastward over our area and
multiple disturbances pivot through the trough. Scattered showers
are forecast Wednesday night through the weekend, with the
potential for isolated afternoon/early evening thunderstorms
Thursday through Sunday. Temperatures on Wednesday may be near or
slightly above- normal, prior to the cold frontal passage.
Temperatures should then trend below-normal Thursday through the
weekend. Jaszka/Moldan

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  45  66  47  77 /  40  30  10  10
CTB  45  69  46  77 /  30  20  10  10
HLN  47  67  48  79 /  20  20  10  10
BZN  41  64  40  75 /  20  30  10  10
WEY  32  54  31  64 /  10  20  10  10
DLN  40  64  43  75 /  10  20  10  10
HVR  46  72  43  78 /  40  10   0   0
LWT  44  63  41  71 /  40  30   0   0

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
High Wind Watch from late Tuesday night through Wednesday
afternoon Chouteau...Eastern Glacier...Eastern Pondera...Eastern
Teton...Hill...Liberty...Northern Rocky Mountain Front...
Southern Rocky Mountain Front...Toole.

&&

$$

weather.gov/greatfalls



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