421
FXUS65 KTFX 091803
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
1203 PM MDT Sun Jun 9 2024

...Updated Aviation Section...


.SYNOPSIS...

Showers and thunderstorms increase over Southwest Montana this
afternoon before spreading northward into central and north-
central areas later in the evening and the overnight hours. The
stronger storms may contain strong, gusty winds, hail, and heavy
downpours, particularly for southwestern locations. Shower and
thunderstorm activity diminishes Monday with breezy and much drier
conditions expected heading into the mid-week period.

&&

.Update...

Morning update has been published, with only minor adjustment made
to the on-going forecast. Main adjustment was to increase PoPs an
additional 10-20% across the CWA through tonight as latest Hi-Res
guidance continues to indicate numerous showers and thunderstorms
lifting east and northeast across Southwest through North Central
Montana through the overnight hours. Early morning convection
across Southwest Montana will predominately remain near the Idaho
border through this morning, with additional showers and
thunderstorms developing between 10am-2pm over the higher terrain
of Northeast Idaho/Southwest Montana and then lifting northeast
over lower elevations of Southwest and into Central Montana
through 6pm. Additional showers and thunderstorms will then
overspread the remainder of Central and North Central from west to
east from the late evening through morning hours on Monday as the
shortwave moves overhead. A Marginal Risk for severe thunderstorms
continues southeast of a general Dillon, to Three Forks, to WSS,
to Lewistown line, with damaging wind gusts being the primary
concern. Additionally, bursts of heavier rain over the higher
terrain where an appreciable snowpack remains could lead to rises
on creeks and streams emanating from the mountains. - Moldan

&&

.AVIATION...

9/18Z TAF Period

VFR conditions will prevail for the KGTF, KCTB, KHVR, and KLWT
terminals during this TAF period. For those terminals tonight into
tomorrow morning there is 40 - 70% percent chance for rain showers
which could reduce ceilings to low VFR levels and so a PROB30 group
was included in those TAFs for it. At the KGTF, KHVR, and KLWT
terminals there is a 20% chance for thunder with the rain showers.
Another aviation concern for the KGTF, KHVR, and KLWT terminals is
wind gusts of 19 - 23 kts at the very end of the TAF period. For the
KEKS and KWYS terminals showers and thunderstorms will reduce
visibility and ceilings to MVFR levels for the duration of this TAF
period. These showers and thunderstorms will move north this
afternoon and evening. They will begin affecting the KBZN and KHLN
terminals between 23Z and 00Z. These thunderstorms will have
lightning, gusty to strong winds, small hail, and brief heavy rain.
For the KHLN and KBZN terminals VFR conditions will prevail until
the showers and thunderstorms move in between 23Z and 00Z. MVFR
conditions will then prevail for the KBZN and KEKS terminals through
the end of this TAF period. Due to the showers and thunderstorms
there will be intermittent mountain obscuration primarily across
Southwestern Montana through the duration of this TAF period. -IG

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

&&

.PREV DISCUSSION...
/ISSUED 527 AM MDT Sun Jun 9 2024/

Lower level north/easterly flow has persisted through the
overnight hours for continued breezy conditions and scattered to
broken clouds, mostly over central and north-central MT. Lower
level cloud cover associated with this set up is expected to
retreat to the northeast as a southwesterly flow aloft begins to
develop ahead of a shortwave currently moving into the Pacific
Northwest. An initial wave of weakening showers and isolated
thunderstorms moving through Central Idaho is already being
tracked on radar and should reach Beaverhead County later this
morning in the form of isolated lighter end showers/virga and
perhaps a lightning strike or two.

Southwesterly flow aloft becomes more established ahead of the
approaching shortwave later today, providing ascent, shear, and
modest instability for afternoon and evening shower and
thunderstorm development. The highest CAPE (500 to 1,000 J/kg)
looks to be for areas south of I90, where gusty winds and hail may
accompany the stronger cells. Precipitable water values will
generally range in the 0.75 to 1 inch mark, so localized heavy
downpours can also be expected. This activity will lift northward
into Central/North- central MT during the evening and the
overnight hours, but the loss of diurnal heating will result in a
more benign brand of showers and general rains. While hi-res
guidance does bring a secondary round of showers and storms to the
southwest late tonight, a more stabilized atmosphere should
preclude the potential for stronger development in a similar
fashion as northern areas. Westerly winds increase and things dry
out on Monday following the passage of the shortwave, but another
weak perturbation does move across the region during the afternoon
hours for additional isolated to scattered showers and
thunderstorms, mostly along the Hi-Line and over and near areas of
higher terrain elsewhere.

Ensembles remain in fairly good agreement with a zonal flow aloft
settling in for the mid-week period. H700 to H500 flow will be on
the stronger side for this time of time of year, mostly between
40 and 50 kts or 2 to 3 standard deviations above climatology
according to the NAEFS (North American Ensemble Forecast System).
Deep layer mixing should encourage breezy to windy conditions,
especially over the plains and the Rocky Mountain Front on
Wednesday. Wind exceedence probabilities suggest that peak wind
gusts will mostly fall between the 35 and 55 mph range except
along the Rocky Mountain Front and adjacent high plains where
gusts up to and exceeding 60 mph will be more common. The primary
impact will be to summer outdoor recreation and travel; however,
afternoon temperatures in the 70s and 80s combined with the said
winds and minimum relative humidity values in the teens and 20s is
expected to accelerate the curing/drying of grassland fuels.
While no fire products are anticipated this time around, this may
set up fire weather concerns in the future.

Uncertainty remains in terms of forecast details for the upcoming
weekend, but ensembles continue to highlight a Pacific trough
exerting an influence over the Northern Rockies. At this time,
there is an expectation for breezy conditions, isolate to
scattered shower and thunderstorm activity, and a slight cool down
for Saturday and Sunday. - RCG

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  74  56  80  50 /  10  70  30  10
CTB  69  51  77  49 /  10  60  30   0
HLN  79  59  84  51 /  30  80  30   0
BZN  77  55  78  45 /  60  80  50   0
WYS  74  47  71  39 /  70  80  60   0
DLN  76  51  76  45 /  90  90  20   0
HVR  73  53  77  49 /  10  70  50  10
LWT  73  54  76  46 /  10  70  40  10

&&

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

$$

http://www.weather.gov/greatfalls