Special Weather Statement, NWS Great Falls, MT

Area Forecast Discussion
National Weather Service Great Falls MT
210 PM MDT Thu Jul 22 2021

.SYNOPSIS...

Westerly winds will continue to increase this afternoon from west
to east as a Pacific front crosses the Northern Rockies.
Scattered shower and thunderstorm activity over the southwest and
eastern portions of Central and North-central is expected will
likely persist into the evening hours, with some strong to severe
storms possible. The dry weather and gusty westerly winds over the
north- central plains will expand to cover the entire region on
Friday as temperatures cool down slightly.

&&

.DISCUSSION...

Today through Friday...Primary concern throughout the period will
be winds, especially during the daylight hours of both days;
along with afternoon and evening thunderstorms over the
southeastern third of the forecast area.

Showers and thunderstorm development is already ongoing over
Southwest Montana early this afternoon, especially along a Dillon
to Bozeman line. This activity is expected to increase as the
afternoon progresses and will likely expand northeastward and
impact the eastern portions of Central/North-central MT later this
afternoon and evening. Lightning and strong gusty winds continue
to be the most likely hazards with these storms, with hail and brief
downpours being secondary. Hail may become more likely over
Fergus, Blaine, Judith Basin, and Meagher Counties, where surface
based CAPE of 1000 j/kg is supplemented with 40+ kts of bulk
shear. The SPC has highlighted this area with a slight severe
risk, again mostly for severe wind gusts. Thunderstorm activity
will likely persist over the aforementioned area through 9 pm,
before diminishing and ending thereafter. As earlier mentioned,
westerly winds are on on the increase this afternoon, mostly over
areas west of a Helena, to Great Falls, to Havre line. Sustained
afternoon winds of 20 to 30 mph can be expected across these
areas, with gusts potentially reaching or briefly exceed 40 mph.
Winds are then expected to decrease slightly during the overnight
hours tonight, but then increase for all areas during the day on
Friday as deep mixing ensues with the passage of the mid- and
upper level trough axis. Deep mixing and strong subsidence behind
the passage of the trough axis is expected to translate to
sustained wind speeds across the entire CWA in the 15 to 30 mph
range, with afternoon relative humidity values dropping into the
single digits in places. This has resulted in all of our fire
weather zones being included in a Red Flag Warning for Friday
afternoon and evening. High temperatures on Friday will be
slightly cooler than today in wake of the Pacific front, but still
remain near to above normal. -RCG/Moldan

Saturday through next Thursday...H500 ridge will slowly build
back in over the Western CONUS throughout the period, with several
weak embedded waves riding over the ridge axis (especially over
Southwest Montana) during the Tuesday through Thursday timeframe.
These weak waves will likely bring about some mountain showers and
thunderstorms, but confidence in coverage remains low at this
point. Temperatures are expected to slowly warm through the
weekend, peaking on Monday, before falling slightly for the
remainder of the work week. - Moldan

&&

.AVIATION...
1153 AM MDT Thu Jul 22 2021 (22/18Z TAF period)

VFR conditions generally prevail with scattered thunderstorms
developing initially across southwest MT after 22/19Z, before
tracking NE and expanding across eastern portions of central MT
later this afternoon into early this evening. These storms will
mainly affect areas south and east of KDLN, KHLN, KGTF and KHVR.
Lightning and strong wind gusts will be the primary hazards, though
some hail will also be possible. Gusty west to northwest winds
spread east and southeast through much of central MT by this evening
behind a weak cold front with winds diminishing some overnight.
Gusty westerly surface winds return on Friday afternoon.
-Hoenisch/RCG

Westerly surface winds increase this morning along the Rocky Mtn
Front, spreading east into adjacent western portions of north-
central MT by this afternoon while southwest winds also increase
this afternoon across southwest MT.

Refer to weather.gov/zlc for more detailed regional aviation
weather and hazard information.

&&

.FIRE WEATHER...

Increasing westerly winds associated with an eastward advancing
cold front will continue through this evening for areas along the
Rocky Mountain Front and adjacent plains. Isolated to scattered
showers and thunderstorm remain possible over the southwest and
eastern portions of Central and North-central Montana through this
evening. The near critical to critical fire weather conditions
along the northern divide and adjacent plains will expand
southeastward on Friday as deep mixing brings another round of
gusty westerly winds, with very low afternoon relative humidity
values. The potential for lightning holdover fires to emerge
continues through Friday.

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  53  87  50  89 /  10   0   0   0
CTB  46  82  49  85 /   0   0   0   0
HLN  54  89  55  92 /  20   0   0   0
BZN  52  91  49  91 /  30   0   0   0
WYS  42  84  38  84 /  30   0   0   0
DLN  50  86  48  88 /  20   0   0   0
HVR  52  89  52  89 /  10   0   0   0
LWT  54  86  51  87 /  40   0   0   0

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
Red Flag Warning from noon to 9 PM MDT Friday Central and
Eastern Lewis and Clark National Forest Areas...Chouteau and
Fergus Counties...Helena and Townsend Ranger Districts of the
Helena National Forest...Lincoln Ranger District of the Helena
National Forest.

Red Flag Warning until 9 PM MDT Friday Eastern
Glacier/Toole/Central/Eastern Pondera/Liberty...Hill and Blaine
Counties...Lewis and Clark National Forest Rocky Mountain
District-Rocky Mountain Front.

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

Red Flag Warning from noon to 9 PM MDT Friday Gallatin National
Forest.

&&

http://www.weather.gov/greatfalls

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