Flash Flood Warning 11, NWS Great Falls, MT

Area Forecast Discussion
National Weather Service Great Falls MT
137 PM MDT Fri Jun 23 2023

.SYNOPSIS...

Shower and thunderstorm coverage will increase through the
remainder of the afternoon, with rain persisting through much of
the overnight hours and into Saturday, for most locations across
Southwest and into portions of North Central Montana. The rain
could be especially heavy over eastern portions of Southwest and
Central Montana, which could lead to instances of Flash Flooding,
especially near the higher terrain. Rain will gradually end
Saturday afternoon and evening and storm departs to the east, with
temperatures rebounding to near normal on Sunday.

&&

.DISCUSSION...

Rest of today through Saturday...primary concern throughout the
period will be the potential for heavy rain as showers and
thunderstorms develop and overspread Southwest through North Central
Montana from south to north. Closed H500 low over the Northern Great
Basin, which was associated with a negatively tilted longwave trough
over the northwestern 1/3rd of the CONUS, will move to over South
Central Montana by midnight tonight and the Central Dakotas by
Saturday afternoon. This track will lead to increasing upper level
ascent overspreading a moistening column as low to mid-level east to
northeast flow transports anomalously "rich" air westward to the
Continental Divide across Southwest and into portions of North
Central Montana. This increasing ascent amidst the moistening column
will lead to an increase in both coverage and intensity of showers
and thunderstorms from the afternoon hours today through late
Saturday morning/early Saturday afternoon across much of the CWA. Hi-
Res models continue to indicate that the best coverage of these
showers and thunderstorms will occur along and south/east of a
Landusky, to Cascade, to Boulder, to Ennis line, as well as along
the Rocky Mountain Front south of US Hwy 2. ECMWF EFI values (for
QPF) of 0.6 to 0.8, with a shift of tails ranging from 0 to 2, exist
across far eastern portions of Southwest and Central Montana from
this evening through Saturday afternoon, which raises concerns that
QPF amounts may exceed the modeled climatology across these areas.
Additionally, NBM probabilistic guidance suggests a 20-30% chance
that rainfall amounts may exceed 1" in the Lewistown and Bozeman
areas over this 24hr period, with higher values in the terrain along
the Billings/Great Falls CWA borders. Given the potential for heavy
rainfall, which could lead to instance of flash flooding, decided to
expand the Flood Watch (originally for the zones that make up Fergus
County) to include the Big Belt, Bridger and Castle Mountains,
Little Belt and Highwood Mountains, Meagher County Valleys, and
Judith Basin County and Judith Gap through Saturday afternoon.
Precipitation coverage and intensity will gradually decrease from
the late afternoon/evening hours on Saturday as the aforementioned
upper level disturbance moves east over the Dakotas. High
temperatures on Saturday will be well below normal across most
locations, with locations along the Hi-Line (where precipitation
will be most limited) being the closest to normal. - Moldan

Sunday through next Friday...weak upper-level ridging propagates
across much of CONUS followed by generally zonal flow with several
embedded shortwave disturbances that may result in daily
opportunities for showers and thunderstorms across the CWA, though
there still remains a fair amount of uncertainty with regards to
timing among other details. Afternoon temperatures trend near
seasonal averages for most locations, sitting in the mid to upper
70s, with some locations approaching or exceeding 80 degrees. -
Pierce

&&

.AVIATION...
1131 AM MDT Fri Jun 23 2023 (23/18Z TAF Period)

Showers and thunderstorms will be the main concern this TAF period,
initially forming around terrain early this afternoon and spreading
to lower elevations through the rest of the afternoon and evening.
Thunderstorms will be heavy at times, but confidence in conditions
directly impacting a terminal were too low to tempo less than 3 SM
visibility at this time. A more widespread rain develops overnight
across Central Montana and along the Rocky Mountain Front before
slowly fading late tonight into tomorrow morning. Lingering
instability could result in a few morning showers or perhaps a weak
thunderstorm across the plains tomorrow morning. -AM

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

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  53  71  48  77 /  70  50  10  30
CTB  50  73  46  77 /  80  40  10  20
HLN  55  72  50  77 / 100  80  30  50
BZN  51  67  44  74 / 100  80  30  40
WYS  41  60  35  68 /  90  80  50  40
DLN  46  66  43  70 /  90  50  30  50
HVR  53  76  51  82 /  40  20  10  10
LWT  51  64  46  74 / 100  70  30  40

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
Flood Watch through Saturday afternoon for Big Belt, Bridger and
Castle Mountains-Fergus County below 4500ft-Judith Basin County
and Judith Gap-Little Belt and Highwood Mountains-Meagher County
Valleys-Snowy and Judith Mountains.

&&

http://www.weather.gov/greatfalls

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