Area Forecast Discussion
Issued by NWS Great Falls, MT

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942
FXUS65 KTFX 230241
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
841 PM MDT Sun Apr 22 2018

.SYNOPSIS...

A rapidly-developing weather system will bring widespread rain
and mountain snow to central and southwest Montana this evening
through Monday morning, with snow levels lowering to some valley
floors over southwest Montana Monday morning. Fair weather with
warming temperatures are expected through the remainder of the
week, with warmest conditions expected Friday.

&&

.UPDATE...

Fast moving shortwave aloft will bring precipitation to southwest
and portions of central Montana tonight and Monday. At this time
it appears that the forecast is right on track as several bands of
light rain have already developed across this region. Only updates
required so far this evening were minor tweaks to wind speed and
direction. Also made rather minor tweaks to overnight lows and
the diurnal temperature curve to be more in keeping with current
observations. Remainder of the forecast, including the Winter
Weather Advisory, have been left as-is. Several updates were also
made to the current batch of flood highlights. Please see the
Hydrology section below for further details. mpj

&&

.AVIATION...
Updated 2350Z.

An approaching weather system will bring scattered to numerous
evening showers and possibly a thunderstorm to southwest Montana
this evening with cloud cover lowering and thickening overnight. By
early morning Monday, widespread IFR/MVFR conditions in rain and
snow will prevail across southwest and portions of central Montana
with mountains generally obscured. Only isolated/scattered showers
are expected over north-central Montana with VFR conditions
prevailing but areas IFR/MVFR possible in showers and/or low clouds.
mpj

&&

.HYDROLOGY...

Updated 753 PM April 22

River flooding continues to impact the Milk River and its
tributaries at this time. An increased risk of flooding may develop
across SW MT later this week.

River levels are finally beginning to crest or fall along the Milk
River Valley this afternoon. With very little snow left to melt out
and no incoming precip over the next few days, the falling trend
should continue as crests move into northeast Montana. But, flooding
impacts will likely persist for a while since it will take time for
all the water to recede. As of this evening, Big Sandy Creek near
Havre had nearly fallen to below flood stage and should fall below
flood stage by around midnight tonight. Thus, the flood warning for
this waterway will likely be cancelled overnight tonight.

Flooding concerns will begin increasing through this week for areas
affecting by snowmelt coming out of the Rockies. Above normal
temperatures by mid to late week should increase the snowmelt rates,
and the latest RFC forecasts show this potential in their 5-10 day
river forecasts. Rises will likely be seen along the Marias River in
Central Montana as well as the Missouri, Jefferson, and Big Hole
Rivers in SW MT. It is recommended that those with interests along
those rivers begin thinking ahead towards the increasing
flows/levels later this week. Lastly, a moderate precip event across
SW MT through Monday may add to any ongoing snowmelt flooding,
especially areas near smaller streams/creeks coming out of the
mountains.
MARTIN/Jaszka

&&

.PREV DISCUSSION...
/ISSUED 753 PM MDT Sun Apr 22 2018/

This Evening through Monday...The forecast appears largely on
track, though with a bit later event starting and ending time
based on latest model and observed trends. This may end up aiding
in raising snow amounts Monday morning over parts of southwest
Montana with more of the precipitation falling during the morning
hours as slightly colder air arrives from the north. A Winter
Weather Advisory has been issued for elevations above 5000 feet,
though snow levels may drop as low as 4000 feet before ending over
parts of southwest Montana. Roads are quite warm currently, but
with fairly unfortunate timing of what could be moderate to heavy
snow rates at times in the early morning hours could result in
roads building up a somewhat significant slush. The one
significant area of concern is Bozeman Pass on Interstate 90,
which looks to receive the most precipitation, with a good amount
of it falling as wet snow. If models are correct with QPF, then
roads there could be snow-covered for a while during the early
and mid-morning hours.

Monday Night through the remainder of the week...Dry and mild
conditions return on Tuesday with a broad, high amplitude ridge
over the region. A minor shortwave system passes to the north on
Wednesday, yielding slightly cooler temperatures on Wednesday. The
overall ridge strengthens through the second half of the week as
temperatures trend much warmer. Clouds and unsettled weather show
signs of returning toward the weekend, with increasing clouds and
precipitation chances along with a downward temperature trend. CC

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  34  50  29  65 /  30  30   0   0
CTB  30  50  29  67 /  10  10   0   0
HLN  36  48  29  63 /  90  90   0   0
BZN  36  42  23  57 / 100 100  10  10
WEY  33  37  17  50 / 100 100  20  10
DLN  34  43  25  55 / 100 100  10   0
HVR  33  53  30  65 /  10  10   0   0
LWT  34  44  26  57 / 100 100  10   0

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory from 3 AM to noon MDT Monday above 5000
feet for Broadwater...Cascade...Gallatin...Jefferson...Judith
Basin...Madison...Meagher.

Flood highlights continue for portions of North-Central and
Southwest MT.

&&

$$

http://www.weather.gov/greatfalls



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