364
FXUS65 KTFX 291800
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
1155 AM MDT Mon Apr 29 2024


.SYNOPSIS...

A slow moving and strong upper level disturbance will bring
unsettled conditions to the Northern Rockies through the middle of
the upcoming work week, with accumulating snow becoming more
likely along the Rocky Mountain Front and across the Island Ranges
of Central Montana from Tuesday evening through Thursday morning.

&&

.UPDATE...

The morning update has been published. Large scale ascent and an
eastward advancing Pacific cold front will increase shower and
isolated thunderstorm activity this afternoon and evening, mostly
for Southwest MT and central/north-central locations south and
east of a Helena to Havre line. Primary concern will be the
potential for isolated instances of stronger wind gusts up to 50
mph mixing down to the surface during any shower or storm
activity, especially for the north to south oriented southwest
valleys. This in addition to a few lightning strikes, brief
downpours, and perhaps some small hail or graupel given mid level
lapse rates around 7 to 7.5C/km. H700 temperatures fall to -10C
behind the front, so a period of wet snow can also be expected
this evening over and near the higher terrain of Central and
Southwest MT. Although this may be a quick burst of heavier snow,
impacts should remain brief and minor. This situation will be
monitored as the day progresses. The main update this morning was
to adjust pops, temperatures, and winds to current trends. - RCG

&&

.AVIATION...
1155 AM MDT Sun Apr 28 2024 (29/18Z TAF Period)

Expect VFR conditions to give way to periods of MVFR to IFR
conditions today as showers and isolated thunderstorms move in and
develop across portions of Central and Southwestern Montana today.
These showers and storms will bring periods of IFR to even LIFR
conditions in the heaviest storms, particularly near and south of a
line from KHLN to KLWT. Periods of reduced sigs and vis will
continue into the overnight hours as rain changes to snow across SW
MT (including KBZN and KEKS), with periods of visibility less than
what the TAFs show. Ludwig

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

&&

.PREV DISCUSSION...
/ISSUED 433 AM MDT Mon Apr 29 2024/

Today through Tuesday morning... A shortwave trough ahead of the
main event will bring rain to lower elevations and snow to the
mountains. Precipitation amounts over a quarter of an inch are
most likely to the southeast of a line from Chinook to Great Falls
with amounts up to a third of an inch possible around Lewistown
and the Judith mountains. There is some convective potential this
afternoon in central and southwestern Montana which may produce
locally higher amounts.

Otherwise, the other main threat this afternoon into Tuesday
morning will be gusty winds up to 50 mph along the Rocky Mountain
front and across southwestern Montana. The timing of the highest
wind gusts in southwestern Montana will be this afternoon while
the highest wind gusts are expected Tuesday morning for the Rocky
Mountain front.

Tuesday afternoon through Thursday morning... The timing of the
snow has shifted as the trough is now expected to move slower
through the region, delaying the onset of the heaviest snowfall by
about 6-8 hours compared to the previous day. This means that the
highest impacts will be generally confined to all day Wednesday.
When considering highlights, the start time on the winter storm
watch along the East Glacier Park Region was kept the same but
extended to Thursday morning to account for the increased time
window. Additionally, a watch was added for the Little Belt and
Highwood mountains. Snow totals increased a bit from the previous
forecast and there is now high enough confidence in at least 10
inches of snow across a wider portion of the zone to warrant
adding it to the watch. This segment of the watch has a 12 hour
delay to the start time compared to the East Glacier watch to
account for the delay in when snow is expected to start further
east but both watches end Thursday morning.

When considering other zones, there was not enough confidence or
widespread impact to add additional zones to the watch. There are
a couple zones, particularly along the Rocky Mountain front and
the Big Belts, where limited areas might be near criteria. But
when looking for more widespread impacts and higher snow amounts
over a larger portion of the zone, there was not enough confidence
to warrant adding them at this time.

Thursday afternoon through next weekend... Ensemble guidance
currently points towards a shortwave moving through the area
between Friday morning and Saturday afternoon which will keep
precipitation chances up. Beyond that, ensembles point towards a
trough coming onshore along the western CONUS early next week
which could continue a wetter pattern across the state.

-thor

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  32  52  32  46 /  60  20  30  80
CTB  31  50  32  42 /  10  40  50  70
HLN  32  52  30  50 /  60  30  10  60
BZN  24  49  24  49 /  80  40  40  50
WYS  20  40  19  39 /  70  50  50  60
DLN  24  47  23  46 /  60  30  20  30
HVR  35  56  33  48 /  40  40  50  70
LWT  28  49  27  45 /  90  20  10  70

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
Winter Storm Watch from Tuesday evening through Thursday morning
for East Glacier Park Region.

Winter Storm Watch from Wednesday morning through Thursday
morning for Little Belt and Highwood Mountains.

&&

$$

http://www.weather.gov/greatfalls