Area Forecast Discussion
Issued by NWS Great Falls, MT

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FXUS65 KTFX 220226
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
825 PM MDT Sat Apr 21 2018

CORRECTED AFD TO INCLUDE HYDROLOGY SECTION

.UPDATE...

Most of tonight`s forecast remains valid. One change was
cancelling the High Wind Warning. Surface winds are subsiding and
will continue to do so via weakening flow aloft and nocturnal
boundary layer stabilization. Also decreased POPs to reflect
expectation of fair weather tonight into Sunday morning as our
region resides in between disturbances. This adjustment was based
on latest observed trends and mesoscale model guidance.
Jaszka

&&

.SYNOPSIS...

Strong, gusty winds will diminish this evening as a fast-moving
Pacific cold front moves through the region the remainder of this
afternoon and evening. In areas where flooding is occurring,
these strong winds may lead to increasing water levels on the
eastern edges of flooded areas due to wave activity, which may
overtop sand bags and or levees. A weather system will bring
widespread rain and showers with mountain snow to central and
southwest Montana Sunday evening through Monday morning before
exiting the area.

&&

.AVIATION...
Updated 2303Z.

Westerly to southwesterly flow aloft and VFR expected next 24-hours
as the next Pacific disturbance approaches from the west. Gusty
southwesterly to westerly surface winds will subside between 00Z and
03Z/Sun. In addition, mountain wave-induced turbulence should taper
this evening and especially by daybreak Sunday. As the
aforementioned disturbance approaches the area, a swath of valley
rain and mountain rain/snow is expected to develop over southwest
Montana between about 21Z/Sun and 00Z/Mon, mainly southwest of a
KHLN to KBZN line. Mountain obscuration and MVFR to IFR should occur
within this precipitation.
Jaszka

&&

.PREV DISCUSSION...
/ISSUED 224 PM MDT Sat Apr 21 2018/

Remainder of this Afternoon through Monday Afternoon...Strong
pressure rises and favorable upper-level dynamics behind a
Pacific cold front moving across the area are resulting in
widespread high wind conditions across the plains of north-central
Montana the remainder of this afternoon and early evening.

Winds diminish quickly over north-central Montana this evening,
and subside after midnight, with northerly surface flow
developing by late Sunday afternoon across all of north-central
and southwest Montana Monday morning ahead of a deepening upper
level trough moving through the southwest. With impressive
dynamics and thermodynamics and a favorable low- to mid-level wind
profile, we expect this system to generate periods of moderate
precipitation with embedded heavier showers, mainly in the form
of rain but also high-mountain snow. Snow levels will fall
rapidly with favorable dynamics after midnight Sunday night and
may result in a period of heavy, wet snow at elevations below
5000 feet Monday morning, which could be problematic for the
morning rush hour across southwest Montana, particularly the
Bozeman and Bozeman Pass areas and vicinity. One other important
aspect we will need to watch for is the potential for the rain and
wet snow to produce increased run-off of rain and snowmelt on the
mid-slopes of the mountains, where a significant snowpack still
remains. However, as of right now, we do not anticipate flooding
to be a major concern over southwest Montana.
CC

Monday Night through Saturday...Transitory ridging quickly builds
in behind this departing disturbance on Tuesday, before a weak
clipper system moves in late Wednesday. This clipper system will
bring light precipitation chances, but little to no impacts to the
CWA. Ridging then builds overhead to end the work week, with
temperatures rising well above climatological norms. These warm
temperatures will push freezing (melting) levels into the 8500-ft
to 9500-ft range to end the work week, which could lead to
increased runoff from these mid-slope elevations.
-Moldan

&&

.HYDROLOGY...
/Updated 825 PM, April 21/

Widespread snowmelt flooding and river flooding continue across the
Hi-Line. Attention may then turn to Southwest Montana for snowmelt
flooding next week.

No significant changes have been noted with most river levels or
flooding impacts across the Hi-line. Thus, most Areal Flood
Warnings/Advisories and River Flood Warnings remain in effect.
However, cancelled the Flood Warning for Battle Creek near Chinook
based on the latest observed river level and the new forecast for
that waterway.

The main focus for flooding continues to be parts of Hill and Blaine
Counties due to Lodge Creek and the Milk River levels in the
moderate flood stage. After talking with the River Forecast Center
(RFC), they mentioned they are confident in major flood levels being
reached along the Milk River near Harlem, but are less confident on
exactly when this will happen. It may depend on how quickly the
water works downstream from Lodge Creek/Cypress Hills area.

Lastly, a moderate precipitation event is expected across SW MT and
some of this will be rain on snow. At this time, no flood products
are in effect for that area, but it will be closely monitored.
Eventually, this precip plus warming temps next week may start to
lead to flooding concerns there.
MARTIN/Jaszka

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  33  58  33  51 /   0  10  30  40
CTB  31  54  30  52 /   0  10  20  20
HLN  37  60  36  50 /   0  20  60  70
BZN  35  61  36  44 /  10  50 100 100
WEY  28  52  33  39 /   0  50  90  90
DLN  33  60  34  45 /   0  70 100 100
HVR  31  57  32  54 /   0  10  10  10
LWT  34  57  33  46 /  10  20  70  70

&&

.TFX WATCHES/WARNINGS/ADVISORIES...

Flood highlights continue for portions of North-Central MT.

&&

$$

http://www.weather.gov/greatfalls



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