-- Discussion containing changed information from previous version are highlighted. --
989
FXUS65 KTFX 240554
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
1154 PM MDT Thu May 23 2024

...Updated Aviation Section...


.SYNOPSIS...

Unsettled conditions will continue through the end of the work
week and weekend, with temperatures running below normal and
chances for showers, thunderstorms, and at times mountain snow.

&&

.UPDATE...

No significant changes needed to the overnight forecast period.
Main adjustment was to blend more of the high-resolution model
data into the probability of precipitation forecast to better
reflect the overall decrease in shower and thunderstorm coverage
this evening on the Hi-Line, but increase it a bit over Central
Montana. The weak disturbance responsible for this activity
continues to move across the area, but showers have become more
scattered and the threat for additional thunderstorms has
decreased. Not expecting heavy rainfall from these showers and
storms, but they should move across much of North Central and
Central Montana overnight. Other adjustment was to increase
overnight humidity to better reflect additional shower activity.
As a result, have increased the patchy fog coverage in areas where
winds will become light, as partial clearing is likely overnight.
-Coulston


&&

.AVIATION...
-- Changed Discussion --
24/06Z TAF Period

VFR conditions will prevail across all terminals during this TAF
period. From the beginning of this TAF period through early Friday
morning there will be rain showers around the KHVR terminal. At the
KGTF terminal Friday morning there will be rain showers in the area.
Friday morning and afternoon at the KGTF, KCTB, KHLN, and KLWT
terminals there will be winds gusting from 16 to 20 kts. Friday
afternoon at the KCTB, KHVR, and KLWT terminals there is a 50%
chance for thunderstorms and so a PROB30 group was included for
them. Friday afternoon there is a 60% chance for rain showers at the
KGTF terminal and so a PROB30 group was included for it. At the KGTF
terminal there is a 20% chance that some of the rain showers Friday
afternoon could be thunderstorms but there wasn`t enough
probabilistic support to include it in the PROB30 group. At the KEKS
and KBZN terminals there is a 20% chance for thunderstorms Friday
afternoon and evening but there wasn`t enough probabilistic support
to include it in those TAFs. Friday morning at the KEKS, KBZN, KWYS,
and KHLN terminals fog could form and reduce visbility to IFR
levels, but there wasn`t enough proabilistic support to include it
in those TAFs. -IG

Refer to weather.gov/zlc for more detailed regional aviation
weather and hazard information.
-- End Changed Discussion --

&&

.PREV DISCUSSION...
/ISSUED 600 PM MDT Thu May 23 2024/

600 pm update... The Flood Watch for the Little Belt and Highwood
Mountains was cancelled earlier, as rainfall was not quite as
heavy as originally expected, much of the precipitation fell as
snow in the mountains, and there has been very little movement in
river and creek levels so far. Snow has also mostly ended or
decreased to non- impactful levels in the mountains of Central
and Southwest Montana, so the remaining Winter Storm Warnings for
those areas were cancelled after 430 pm. -Coulston

Rest of today through tonight...upper level disturbance over
North Central Wyoming and its associated surface low over Western
South Dakota will continue to slide east and away from the
Northern Rockies through the evening hours tonight. This will
lead to decreasing precipitation across Southwest Montana through
the remainder of the afternoon hours, with additional snow
accumulations of 1-2" and isolated amounts approaching 4" across
the remaining Winter Storm Warnings for the Bridger, Gallatin,
and Madison Ranges. Clear skies during the early morning hours
over the plains of Southern Alberta, North Central Montana, and
northern portions of Central Montana allowed temperatures to warm
into the 50s, which combined with unstable northwesterly flow
aloft and an embedded wave tracking southeast through the flow
has allowed scattered showers and isolated thunderstorms to
develop. These showers and storms will track slowly east through
the overnight hours tonight, bringing brief periods of heavy rain
and the potential for small hail. A brief cold air funnel can not
also be ruled out given H500 temperatures approaching -25C and
the presence of existing vorticity associated with the longwave
trough and secondary closed (H500) over Alberta and Saskatchewan.
Should any cold air funnel form they would be brief and pose no
danger to the public or infrastructure.
-Moldan

Tomorrow through Memorial Day is favored by ensemble model
guidance to continue to see unsettled weather conditions through
the Memorial Day weekend in terms of scattered rain showers and
thunderstorms. Tomorrow is favored along the Hi-Line down to
Great Falls and Lewistown for scattered thunderstorms as an upper
level shortwave is forecasted to move across North Central Montana
tomorrow afternoon and quickly depart our area (Fergus and Blaine
Counties) by Saturday night. Then, models are favoring another
upper level trough to move in for Saturday and Sunday across
Southwest and North Central Montana with more coverage of rain
shower and thunderstorm activity. Sunday afternoon and early
evening can be periodically breezy with wind gusts as high as 40
mph across the Hi-Plains region as our upper level trough begins
to depart to our east. Primary concerns for thunderstorm
development this weekend is going to be lightning, heavy
downpours, small hail size, and gusty erratic winds. Temperatures
this weekend are also going to remain unseasonably cool with highs
in the 50s and 60s and lows in the 30s (20s in mountainous
terrain).

Monday into Tuesday morning, upper level ridging is favored by
latest ensemble model guidance to build back into the region and
bring dry conditions with warm afternoon temperatures. Latest
National Blend of Models 4.2 Probabilistic data suggests high
temperatures reaching over 80 degrees for Tuesday is high (>75
percent) for locations from Great Falls to Havre down to Helena.
Wednesday afternoon and evening into the end of next week,
ensemble model guidance is still uncertain on the exact path of
upper level troughing across our region. Upper level troughing
could come down from Canada and bring more unsettled weather
conditions to our region or it could remain to our north. What is
currently favored is a cold front from the Pacific Northwest to
move across our region Wednesday and Thursday bringing
temperatures back to near normal (60s and low 70s) for the end of
May. Webb

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  39  63  40  65 /  30  30  10  60
CTB  39  60  37  62 /  50  50  10  40
HLN  37  65  42  64 /  10  20  10  70
BZN  26  62  36  62 /   0  20  10  70
WYS  23  54  30  52 /  10  10  10  80
DLN  27  61  36  58 /   0  10  20  70
HVR  42  63  39  69 /  40  60  10  50
LWT  33  60  37  62 /  10  40  10  80

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
None.
&&

$$

http://www.weather.gov/greatfalls