Area Forecast Discussion
Issued by NWS Great Falls, MT

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FXUS65 KTFX 240545

Area Forecast Discussion
National Weather Service Great Falls MT
1145 PM MDT Thu May 23 2019

...Updated Aviation Discussion...


After a mostly dry and sunny day today for most of the region, a
series of disturbances will bring more unsettled weather with
slightly cooler than normal temperatures through much of the holiday
weekend. The first one will spread scattered showers northeast over
the part of the region along and east of a Dillon to Havre line this
tonight, which will linger into Friday. Disturbances will then
move through the area Friday through Sunday, bringing a good
chance of showers and thunderstorms each day, with the strongest
thunderstorms likely occurring on Saturday. Weak high pressure may
bring drier conditions for Monday.



Tonight through Saturday Night...

Upper level low centered over the Great Basin this afternoon, within
a larger scale trough, will lift north tonight through Friday,
reaching the MT/ND border region by Friday afternoon. Clouds and
moisture to the north of this low have already spread north into SW
MT today with cloud-cover spreading north through the remainder of
the area this evening. Showers and a few weak thunderstorms will
develop late this afternoon and evening over SW MT with showers
spreading north across eastern portions of central MT overnight. The
bulk of the precipitation through Friday should be east of a line
from Dillon to Havre on the western periphery of the low tracking
north, but a few showers cannot be ruled out across areas west of
this line as well. Meanwhile, another upper level system drops south
on the backside of the larger scale trough into WA/OR by Friday
evening, bringing an increase in showers and possibly a few
thunderstorms to western portions of the State Friday afternoon
which could creep into western portions of central and SW MT late
Friday afternoon and evening. Yet another upper level low dropping
down the BC coast Friday will absorb the Pac NW low and continue
south along the US west coast Saturday and Saturday night. This
brings an increasingly moist southerly flow aloft into the region
Saturday with showers and thunderstorms increasing late in the day
and overnight. Diffluent southerly flow aloft and a good tap of
moisture (PW`s near 0.75") could promote some stronger storm
development Saturday afternoon and evening, with heavy rain and
small hail being the primary concerns. Will need to watch late
Saturday trends with potential for slow moving storms and localized
heavy precipitation in this kind of a setup. Hoenisch

Sunday through Thursday...

Unsettled conditions should continue into Sunday, but there may be a
break in the weather for Memorial Day itself. The low pressure area
will dig south into the southwestern United States on Sunday. This
will help drag a disturbance and associated cold front south out of
Canada over the forecast area, bringing another good chance of
showers and thunderstorms, as well as temperatures of around 10
degrees below normal. The latest forecast model runs now move this
low pressure area more so to the east over the Four-Corners region
on Monday, rather than previously moving it northeast into the
central Great Plains. This will bring a better chance that weak high
pressure ridging will move into Montana from the northwest, so have
decreased the chance for additional showers and thunderstorms, as
temperatures remain up to 10 degrees below normal. There is still
some uncertainty as to whether the day will be completely dry, so
have kept at least some mention of precipitation in the forecast for
now. A drier northwesterly flow aloft for Tuesday through Thursday
will allow temperatures to warm back closer to normal, with the main
chance for additional showers occurring during the afternoon and
early evening due to weak instability caused by daytime heating.


Updated 1145 PM MDT Thu May 23 2019 (06Z TAFs)

Showers are expected to develop today across Southwest and Central
Montana. VFR conditions prevail, with periods of MVFR being possible
beneath the most persistent showers.


GTF  39  63  42  64 /  10  30  20  70
CTB  39  64  41  61 /   0  30  20  60
HLN  39  63  42  65 /  10  60  30  70
BZN  36  61  38  64 /  40  40  40  50
WEY  30  51  33  56 /  50  80  70  50
DLN  35  58  38  60 /  20  70  50  60
HVR  43  66  43  69 /  10  20  10  40
LWT  39  57  38  62 /  50  60  10  80



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