479
FXUS65 KTFX 211620
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
1020 AM MDT Tue May 21 2024

.SYNOPSIS...

Another day of late morning through early evening showers and
isolated thunderstorms will be around today. The next impactful
system arrives late tomorrow, lasting into Thursday evening.
Mountain snow is forecast with this system, along with lower
elevation rain mixing in with snow at times. Confidence is
increasing in another round of widespread precipitation this
weekend.

&&

.UPDATE...

Biggest change to the existing forecast was to add some gusty
winds to showers and thunderstorms across the Plains this
afternoon and evening, as DCAPE values (a measure of the potential
for downdrafts and outflows) will likely be sufficient to produce
a few non severe thunderstorm wind gusts this afternoon.
Otherwise, the forecast remains on track. Ludwig

&&

.AVIATION...
21/12Z TAF Period

Primary concern this morning is for low clouds at KWYS and KHVR and
vicinity, which are forecast to dissipate after 16Z. Attention then
turns to late morning through early evening showers and isolated
thunderstorms. Mainly light winds will be in place through the
forecast period, but will be a bit breezy out of the west at KCTB
and along the Rocky Mountain Front today. -AM

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

&&

.HYDROLOGY...

A Flood Watch remains in effect for the Little Belts Wednesday night
into Friday. The chances for an inch and two inches of precipitation
have increased slightly (Largely around 70% and 30% respectively)
for the Little Belts over this period. There is some uncertainty as
to how low snow levels fall with this system. Nonetheless, if most
precipitation across the Little Belts Falls as rain, there are
concerns for flooding on creeks and streams in the area given recent
precipitation/saturated soils. -AM

&&

.PREV DISCUSSION...
/ISSUED 511 AM MDT Tue May 21 2024/

Through tonight... The Northern Rockies will be between systems
today, which will result in a brief period of transient ridging. The
concern through the rest of the overnight will be for patchy fog
development where clearing skies overlap with areas that have seen
precipitation over the last day or two. High temperatures will
climb a touch more today, getting close to average for this time
of the year. Although transient ridging will be in place, cool
temperatures aloft will be around. Combine this with ample daytime
heating and the result will be for another round of late morning
through early evening showers and isolated thunderstorms. Small
hail and briefly gusty winds will be the concern with any of the
stronger showers/thunderstorms today. By late evening, mid and
upper level clouds will begin to stream in from the west ahead of
the next impactful system, which should help mitigate fog chances
across the region for tomorrow morning.

Wednesday through Thursday night... The next round of impactful
precipitation will begin to move in Wednesday. A closed upper level
low will dive southward from BC into the Pacific NW tonight into
Tuesday, slowly turning eastward Wednesday evening, moving across
southern Idaho and vicinity before exiting eastward Thursday night.
Ensembles continue to favor Southwest through Central Montana as the
areas with the best chance for an inch of liquid precipitation,
though a bit of uncertainty is creeping in.

The first complicating factor to these chances is that the regime
appears to be convective in nature through the day Wednesday, before
transitioning to a more stratiform event Wednesday night. This would
lead to decreased confidence in impacts at any one location through
the day Wednesday. Additionally, there is a subset of guidance that
forms a H7 low much further north than the H5 low that will be
largely over southern ID. This would favor higher precipitation
amounts further north, further adding to a complicated forecast.
Regardless, mountain snow will be around in the afternoon Wednesday,
with snow levels falling through the overnight into Thursday
morning. Given the complicating factors present at this time, I have
elected to keep all Winter Storm Watches as is for now, in hopes
that guidance trends to a less bi-modal solution over the next 12 to
24 hours.

Friday through Sunday... The region looks to get a brief break
Friday, as one system departs and another begins its approach.
Showers will still be around given cool air aloft, but not expecting
many impacts from these showers. Another system looks to work in
quite similarly to the Wednesday and Thursday system for this
weekend. An upper level disturbance will drop southward from BC into
the Pacific NW and then turn eastward. There is a bit more
uncertainty with respect to track and timing with this system
however, which will play a role in impacts. The chance for a half an
inch of precipitation with this system is largely lower than 50%,
owing to the aforementioned uncertainty.

Sunday night into early next week... Ensembles favor upper level
ridging building in behind the departing system Sunday night into
early next week, which will allow temperatures to trend back
closer to normal Monday and then above normal for Tuesday. This
period looks largely dry, but there will be a non-zero chance for
a shower or thunderstorm. -AM

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  63  41  58  40 /  40  20  70  90
CTB  62  39  54  37 /  30  10  80  80
HLN  65  44  60  41 /  30  20  80 100
BZN  62  36  60  36 /  20  20  70  90
WYS  52  30  49  29 /  30  20  80 100
DLN  60  38  58  33 /  20  10  90  90
HVR  63  40  62  41 /  40  20  50  60
LWT  58  36  59  36 /  50  30  60  90

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
Winter Storm Watch from Wednesday evening through Thursday
morning for Elkhorn and Boulder Mountains-Northwest Beaverhead
County-Ruby Mountains and Southern Beaverhead Mountains-Southern
Rocky Mountain Front.

Flood Watch from Wednesday evening through Friday morning for
Little Belt and Highwood Mountains.

Winter Storm Watch from Wednesday evening through Thursday
afternoon for Big Belt, Bridger and Castle Mountains-Gallatin
and Madison County Mountains and Centennial Mountains.

&&

$$

http://www.weather.gov/greatfalls