599
FXUS65 KTFX 130913
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
313 AM MDT Thu Jun 13 2024

.SYNOPSIS...

Warm and dry weather is expected through Friday. Then a cold front
will move through the Northern Rockies this weekend, initially
bringing scattered showers and thunderstorms followed by cooler,
windy, and showery conditions. The unsettled pattern continues into
early next weak with increasing chances for widespread rain and
higher mountain snow.

&&

.DISCUSSION...

Transient ridging aloft will bring calmer winds and continued dry
conditions through early Friday before a Pacific trough and
attendant cold front moves through the region on Friday afternoon
and ushers in an extended period of unsettled weather for the
weekend through at least the first half of next week. Mid- and
upper level moisture already begins to increase tonight into early
Friday in response to southwesterly flow aloft ahead of the
approaching Pacific trough. This will keep temperatures a little
warmer than the prior night and there also may be few light
showers or virga during this timeframe.

The initial cold front passes through during the afternoon hours
on Friday and brings increased westerly winds and isolated to
scattered showers and thunderstorms. Model guidance has been a
little shaky with the timing of the front and severe weather
potential, but now are showing favorable forecast soundings for a
few stronger storms Friday afternoon and evening for central and
southwest locations south and east of Helena to Lewistown line.
Inverted V profiles with ML CAPE levels around 500 to 1,000 J/kg
and bulk shear north of 25 kts suggests that the healthier cells
will contain stronger winds and hail.

The trough then swings northeastward out of the Pacific Northwest
into Alberta and Saskatchewan, leaving the Northern Rockies in a
brisk westerly flow aloft for the weekend. There will be an
expectation for cooler, windy, and showery weather for most
locations. The windiest conditions look to occur Saturday
afternoon and evening when H700 flow increases to around 45 to 55
kts or about 2 to 3 standard deviations above average. There will
also be a fairly strong cold frontal passage with convective
showers capable of briefly transferring stronger gusts to the
surface. At this time, higher end probabilities (70% or greater)
for gusts over 47 mph are mostly confined to locations along the
Continental Divide, the western high plains, and southwestern
locations south of I90.

Aside from a period of more concentrated precipitation near the
cold front Saturday afternoon/evening, shower activity should more
hit or miss for much of the weekend; however, with cold air aloft
moving in , some light mountain snow can be expected Saturday
night into Sunday, mostly over the higher terrain of Glacier
National Park.

Nearly all ensembles now feature a cold and wet late season
trough impacting the Northern Rockies during the first half of
next week, though there continues to be differences in terms of
timing, depth, and residence time of this weather system and the
newest 13/00Z data has shifted the core of heaviest precipitation
a little farther to the north compared to previous runs.
Regardless, individual ensemble members continue to trend upward
with QPF amounts and now a majority have storm totals running in
the 1 to 2 inch range for most locations north of I90. NBM
exceedence probabilities have been slower to increase, but are
displaying an increasing trend as well with the chances for 1 inch
or more of precip approaching the 50% mark for the said areas.

In addition to the rainfall, temperatures as much of 25 degrees
below average should be sufficient for slushy mountain snow,
generally above pass level. Needless to say, this system will be
closely monitored over the coming days for any necessary
highlights to address winter weather and or hydrology impacts.
Those with outdoor plans from Sunday through mid-next week should
be on alert for the potential cold, wet, and raw conditions,
especially over higher terrain. - RCG

&&

.AVIATION...

13/06Z TAF Period

VFR conditions continue through the TAF period for North Central and
Southwest Montana airfields. Winds for KCTB continue to decrease for
the overnight period becoming lighter and variable. KWYS is still
experiencing breezy conditions but should continue to diminish as
well through the overnight hours. Webb

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

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  77  50  85  51 /   0   0  10  10
CTB  75  47  78  46 /   0   0   0   0
HLN  83  54  86  54 /   0   0  10  20
BZN  82  50  87  50 /   0   0  40  30
WYS  79  47  78  43 /   0  10  40  10
DLN  83  51  83  50 /   0   0  20  10
HVR  77  53  85  52 /   0   0   0  10
LWT  76  52  83  49 /   0   0  20  40

&&

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

$$

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