546
FXUS65 KTFX 171128
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
525 AM MDT Sat Aug 17 2024

UPDATED AVIATION SECTION

.SYNOPSIS...

Afternoon temperatures warm back above average this weekend
through mid- next week with lower elevation highs generally
running in the upper 80s and 90s. There will be a brief pause in
scattered shower and thunderstorm activity today before the daily
chances return Sunday and Monday. A few storms may be on the
stronger side with erratic gusty winds, hail, and brief heavy
rains in addition to lightning.

&&

.DISCUSSION...

Overnight shower/thunder activity continues to weaken as it moves
eastward and exits the CWA. Partial clearing to the west is
combining with a decoupling boundary layer and lingering moisture
for some patchy fog development, especially over river valleys and
other fog susceptible areas of North-central and Southwest MT.
This patchy fog may persist through mid- morning, but most
locations have a 70% + chance for visibility remaining above 1
mile.

Transient ridging moves in today and temporarily pauses shower
and thunderstorm activity while afternoon temperatures warm back
above the climatological average. The southwesterly flow aloft
becomes more organized again tonight and will send the first of a
series of embedded shortwaves into the Northern Rockies late
tonight into early Sunday. This initial wave doesn`t appear to be
very impactful, but may bring a few weak showers/virga and perhaps
a rumble of thunder or two for locations along the Continental
Divide and far Southwest MT.

Stronger shortwaves and moisture arrive later Sunday into Monday
and will bring multiple rounds of scattered showers and
thunderstorms, mostly during the afternoon thru late evening
period. Forecast soundings highlight ML CAPE values around 500 to
1,000 J/kg, bulk shear around 25 kts, and a sufficiently hot/dry
sub cloud layer, all of which are favorable for the stronger cells
to contain localized strong wind gusts, hail, and cloud to ground
lightning. PWATS around the 0.75 to 1 inch mark will also
maintain the potential for localized heavy downpours.

Slightly cooler and drier air moves in and reduces overall shower
and thunderstorm activity for the Tuesday through Thursday
period. West to southwesterly flow aloft amid deep layer mixing
will make breezy to windy and dry conditions the primary concern
for this timeframe, particularly for Southwest MT and locations
along the Rocky Mountain Front where minimum RHs look to be in the
teens and 20s with probabilities for sustained winds above 20 mph
running above 60%. Southwestern locations will be most
susceptible for stronger wind gusts and currently have the highest
probabilities (~40 to 60%) for gusts above 40 mph.

Forecast uncertainty increases heading toward next weekend with a
bout half of the ensembles moving the Pacific NW trough,
responsible for the southwesterly flow aloft, ejecting eastward
into the Northern Rockies while the other half favors the said
trough retreating to the west or remaining stationary. The NBM was
left as is for now, which depicts seasonably warm temperatures
and breezy conditions with multiple opportunities for isolated to
scattered showers and thunderstorms. - RCG

&&

.AVIATION...
17/12Z TAF Period

VFR conditions are mostly expected to continue through at least
18/12Z across North Central (KCTB, KHVR), Central (KLWT, KGTF,
KHLN), and Southwest (KBZN, KEKS, KWYS) Montana, unless otherwise
mentioned.

Weak high pressure aloft is forecast to bring clearing and light
winds to the area through the period. However, this has allowed
patchy fog to form in some low-lying areas and areas that received
precipitation yesterday, including KCTB. Some other areas may still
see some fog development, but it will generally burn off by 15Z.
Have mentioned the potential for MVFR/IFR conditions at terminals
that received measurable precipitation. A weak disturbance in the
southwesterly flow aloft may bring some scattered to broken mid
level cloudiness to the area after 00Z. -Coulston

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

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  87  56  92  59 /   0  10  10  30
CTB  82  52  86  54 /   0  20  10  30
HLN  89  58  96  59 /   0   0  20  20
BZN  88  55  93  54 /   0  10  10  20
WYS  80  45  80  45 /   0  10  20  40
DLN  83  52  87  52 /   0  10  10  10
HVR  88  58  96  61 /   0   0  20  20
LWT  85  56  92  58 /   0  10  40  20

&&

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

$$

http://www.weather.gov/greatfalls