Area Forecast Discussion
Issued by NWS Great Falls, MT

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FXUS65 KTFX 240513

Area Forecast Discussion
National Weather Service Great Falls MT
1113 PM MDT Sat Sep 23 2017

...Update to Aviation...


Most of tonight`s forecast appears valid. Only change was the
addition of patchy fog across the Hi-Line per latest forecast
guidance. However, patchy fog cannot be completely ruled-out
elsewhere. Still expect widespread frost to develop tonight as low
temperatures dip into the 20`s or lower 30`s in most places around
daybreak Sunday.


An upper level low pressure system will slowly move through the
central Rocky Mountains through Sunday, leaving southwest and
central Montana in a cooler and drier part of the system. The
result will be light winds, cool temperatures and clearing skies.
Given the recent precipitation, low level moisture combined with
clear skies may result in areas of fog tonight. Widespread frost
will also develop overnight as temperatures drop to below freezing
in many areas.



Tonight through Monday...A slow moving upper low will weaken as it
moves from the Great Basin tonight into northwest Wyoming on Monday.
Although this pattern brings little in the way of wind, dynamics, or
moisture to southwest and Central Montana, enough low-level moisture
lingers to produce isolated snow showers over the mountains tonight
and early Sunday. Likewise, a weak surface pressure gradient will
result in light winds. Tonight temperatures will drop to below
freezing in many areas so expect widespread frost to develop.
Temperatures will remain below normal through Monday. Britton

Monday night through Saturday...By the beginning of next week,
models are in good agreement with the development of broad ridging
over the Pac NW, with the slow moving trough over the Northern
Rockies finally deamplifying and moving off to the NE. This will
place us in a NW flow aloft between the exiting trough and
amplifying ridge to the west. As far as day-to-day details, models
slightly differ on the idea of small embedded shortwaves swinging
across the state along the NW flow. Moisture will be modest, given
PWAT anomalies above normal areawide which may be enough to squeeze
out a few isolated showers Tuesday and Wednesday afternoon.
Regardless, the bigger story will be moderating temperatures. As we
get into Thursday, the aforementioned ridge across the Pac NW will
build eastward across a large majority of Northwest US. This will
send temperatures back to near climatological normal, if not
slightly above normal with widespread temperatures in the 60`s and
70`s by late week for many, followed by dry conditions. Models begin
to display noticeable discrepancies as we get into next weekend,
with the general idea of the ridge slowly breaking down. EPS and
GEFS guidance suggests continued above normal height anomalies which
would side against any cold/winter weather issues. However due to
low confidence in this time frame, I`ve decided to keep temperatures
near normal next weekend with only slight rain chances. But this
will likely change as models become in better agreement in the
upcoming few days. KLG


Updated 0513z.

Latest infrared satellite imagery continues to show areas of low
clouds across spotty areas late tonight, with cloud ceilings ranging
5 to 8 kft at times, especially for EKS/BZN/LWT. However, the trend
is for clouds to slowly diminish overnight leaving behind mostly
clear skies to begin the day on Sunday with widespread VFR likely.
Some valleys may see some lingering/developing fog at times which
may cause temporary obscurations through sunrise. But should not be
widespread and will likely diminish by the morning hours leaving
behind clear skies and calm winds areawide. KLG


GTF  31  56  35  61 /   0   0   0   0
CTB  30  58  33  62 /  10   0   0  10
HLN  34  56  34  60 /  10  10   0   0
BZN  30  51  32  56 /  10  20  10  10
WEY  26  45  26  48 /  10  20  20  20
DLN  28  48  28  53 /  10  10  10  10
HVR  34  59  35  64 /   0   0   0   0
LWT  32  51  34  57 /  10  10  10  10



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