Area Forecast Discussion
Issued by NWS Great Falls, MT

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901
FXUS65 KTFX 201007
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
407 AM MDT Thu Jul 20 2017

.SYNOPSIS...
A few showers or thunderstorms may linger along the Hi-Line
today, but the area will otherwise remain dry through the
weekend. A dry cold front will bring gusty west winds to the area
this afternoon through the day on Friday, with some decrease in
winds tonight. Ongoing very dry conditions will combine with this
wind for critical fire weather conditions. High pressure aloft
will then help decrease winds for Saturday into Sunday.
The cold front will cool temperatures from 10 degrees above
normal today closer to normal for Friday into Saturday, before
temperatures warm back up for Sunday.

&&

.DISCUSSION...

Today through Saturday...A weak disturbance moving through the
high pressure ridge will keep a chance of weak showers and
thunderstorms along the Hi-Line today. Otherwise, a shortwave
trough will move east along the Canadian border, breaking down
the ridge temporarily this afternoon through Friday. An associated
relatively dry cold front will also move through the area this
afternoon into this evening, and the strong pressure gradient in
the wake of it will bring gusty west winds. Winds will decrease
somewhat overnight as the pressure gradient weakens, but mid
level winds will increase to around 30 kt as the shortwave passes.
These winds will be brought down to the surface during the day on
Friday due its associated cold air advection. Despite temperatures
falling to near seasonal averages on Friday, the airmass will
remain quite dry. (Please see the Fire Weather section for details
on fire weather highlights in effect.) The high pressure ridge
will then rebuild into the area Friday night into Saturday,
which will help decrease the winds, keep the area dry, and warm
temperatures a few degrees in the process.
Coulston

Saturday Night Through Thursday...Dry weather prevails through the
weekend with temperatures climbing back into the 90s. Broad ridging
aloft amplifies through Sunday, but moves east of the forecast area
Sunday night. Increasing southwest flow aloft associated with a
closed low over western Canada will allow shortwave energy to move
into the region, raising chances for showers and thunderstorms.
Models are arriving at a better consensus for the large scale
pattern, but timing and coverage of thunderstorm activity has yet to
be resolved. Temperatures will remain near to above seasonal
averages into next week. PN

&&

.AVIATION...
Updated 0438Z.

VFR should prevail next 24-hours. High pressure aloft will persist
into Thursday, but a disturbance will ripple NE`ward through this
ridge tonight through ~18Z/Thur. Accordingly, isolated showers and
thunderstorms are possible over North-Central MT through Thur
morning, but confidence in this activity impacting any TAF site is
very low. Late Thur morning and afternoon, a weak cold front will
sweep E`ward through the area and be followed by breezy westerly
surface winds, especially over North-Central MT. After ~00Z/Fri, a
more pronounced weather disturbance will begin overspreading the
region from the Pac NW. Thus, some additional isolated showers and
thunderstorms may occur Thur afternoon and evening, mainly over SW
MT.
Jaszka

&&

.FIRE WEATHER...

The forecast models continue to move a relatively dry and weak
cold front through the area this afternoon. As a result,
sustained west winds of 10 to 20 mph will spread east across the
area with gusts as high as 35 mph at times. Relative humidity
will fall well into the 10 to 20 percent range once again which,
when combined with the gusty winds and high to very high fire
danger class, will make for critical fire weather conditions
this afternoon and evening. Breezy but less gusty west winds will
continue overnight with only moderate to poor humidity recovery.
Winds will then increase again to 15 to 25 mph with gusts to 40
mph on Friday due to mixing down of stronger mid level winds, as
humidity falls into the 10 to 20 percent range. Have therefore
upgraded the previous Fire Weather Watch to a Red Flag Warning,
expanded that Warning to include all of TFX`s fire weather zones,
and extended the valid time from this afternoon and evening to
this afternoon through Friday evening. NWS Missoula also has a
Red Flag Warning in effect for much of Southwest Montana this
afternoon and evening due to the dry conditions and passage of
the cold front.
Coulston

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  94  54  85  52 /   0  10   0   0
CTB  88  51  79  49 /  10   0   0   0
HLN  93  56  86  55 /   0   0   0   0
BZN  92  50  86  48 /   0   0   0   0
WEY  81  40  78  38 /  10  10   0   0
DLN  90  48  84  48 /   0   0   0   0
HVR  96  55  87  53 /  20  10   0   0
LWT  91  53  84  52 /   0  10   0   0

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
Red Flag Warning from noon today to 9 PM MDT Friday Central and
Eastern Lewis and Clark National Forest Areas...Chouteau and
Fergus Counties...Eastern Glacier/Toole/Central/Eastern
Pondera/Liberty...Helena and Townsend Ranger Districts of the
Helena National Forest...Hill and Blaine Counties...Lewis and
Clark National Forest Rocky Mountain District-Rocky Mountain
Front...Lincoln Ranger District of the Helena National Forest.

Red Flag Warning from noon today to 9 PM MDT this evening
Deerlodge/Western Beaverhead National Forest...Eastern
Beaverhead National Forest.

&&

$$

weather.gov/greatfalls



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