Area Forecast Discussion
Issued by NWS Great Falls, MT

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FXUS65 KTFX 152054
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
254 PM MDT Sun Apr 15 2018

.SYNOPSIS...

Showers and isolated thunderstorms will develop across central
Montana this evening. Periods of rain and mountain snow become
more widespread overnight, spreading across the mountains into
central and southwest Montana during the day on Monday. A few
stronger showers and thunderstorms are possible Monday afternoon.
Temperatures trend lower into Tuesday as precipitation moves away
to the east.

&&

.DISCUSSION...

Tonight through Tuesday...Modest instability along an advancing
warm front will bring our first fair shot at thunderstorms for the
spring late this afternoon into the evening. Periods of rain and
snow become more widespread over the Continental divide and
advance across western and southern portions of the forecast
area into Monday. Precipitation amounts could exceed 0.25 inches
broadly along and west of I15 in North- central MT, with higher
amounts close to the mountains. Snow levels will drop to around
5000 feet as a cold front crosses the mountains. A few areas of
patchy fog will also be possible ahead of this front tonight into
early Monday morning. The cold front then meets modest instability
from a strong jet stream Monday afternoon. The result will be
chances for more scattered thunderstorm activity, especially
across central Montana. A few stronger storms are possible, with
strong winds and small hail possible. Temperatures on Monday will
be slightly below normal...except for areas ahead of the cold
front in the central and eastern portions. These warmer areas will
also be the better areas for thunderstorms. The cold front then
moves across the region quickly Monday night. Downsloping westerly
flow will tend to limit precip across the lower elevations...with
some mountain snow possible. Eventually this large low moves into
Canada on Tuesday...bringing moist NW flow to the region on
Tuesday. This could result in scattered rain and snow showers.
Snow levels cold even lower to 4000 feet or lower on Tuesday.
Temperatures will also continue to be below normal on
Tuesday...with highs in the 40s to low 50s expected. Anglin/PN

Tuesday Night through Sunday...Scattered, mainly mountain rain and
snow showers will likely linger over the forecast area Tuesday night
under an unsettled northwest flow aloft left in the wake of the
Monday/Tuesday system. Weak high pressure will then keep the area
dry Wednesday into Wednesday night, as temperatures remain below
normal. The next low pressure system to affect the area will stay
well south of the area as it moves east over the Great Basin on
Thursday and over the central/southern Rockies Thursday night.
Moisture wrapping around this system will bring scattered showers to
the southern half of the forecast area during this time, but the
northern half should remain dry. Another weak area of high pressure
will build into Montana Friday into Friday night to keep the area
dry with continued below normal temperatures. Temperatures will then
warm back closer to normal ahead of an approaching low pressure
trough and associated Pacific cold front, but this system will bring
another round of scattered rain and mountain snow showers with gusty
southwest winds for Saturday into Saturday night. Breezy conditions
will continue into Sunday in the wake of the system, but it will be
drier with slightly cooler than normal temperatures once again.
Coulston

&&

.AVIATION...
Updated 1800z.

Scattered showers and isolated thunderstorms could bring brief
MVFR conditions today.

Clouds continue to increase across the North-central and SW
Montana this afternoon. Scattered rain and mountain snow showers
are observed on radar advancing into southwest MT. These showers
will continue to spread across central Montana, possibly reducing
flight conditions to KHLN/KBZN/KLWT. Isolated thunderstorms are
possible. Shower and thunderstorms will then diminish in coverage
this evening. Another round of showers may return late tonight...with
some patchy fog also possible ahead of an approaching cold front.

&&

.HYDROLOGY...
/Updated 345 am, April 15/

Big Sandy Creek near Havre continues to rapidly rise and is now
well into Moderate flood stage...and perhaps could continue to
rise into Major Flood stage. Satellite imagery shows a large area
of snow that likely melted more quickly today, running right into
the creek. This is likely what has contributed to such a rapid
rise. The Hill County Sheriff`s Office has confirmed numerous
reports of flooding across the county which is more evidence of
the rapid runoff from that area of snow. The Flood Warning has
been updated to reflect the more rapid rise being seen. Additional
updates are likely as the level continues to rise.

Elsewhere, flood highlights continue for much of North Central MT.
Warmer temperatures through today will allow the snowmelt to
continue. In addition, there will be an increased risk of showers
and even a few thunderstorms Today/Monday which will add to the
snowmelt, potentially increasing runoff in some areas. River/creek
levels will continue to be closely monitored in case any
additional river flood warnings are required. General lowland
flooding will remain a threat, especially in the Flood Advisory
areas. Martin/Anglin

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  37  54  31  48 /  30  50  30  20
CTB  33  45  30  46 /  70  80  20  30
HLN  38  54  30  46 /  20  40  40  30
BZN  36  56  27  44 /  10  30  60  40
WEY  31  44  22  34 /  20  70  60  50
DLN  35  49  25  41 /  30  50  30  40
HVR  34  47  31  45 /  10  50  50  20
LWT  35  60  30  44 /  10  30  40  30

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
Flood Warning in effect until further notice for Big Sandy Creek
near Havre.

Flood Warning in effect until further notice for the Milk River
near Harlem.

Flood Watch in effect until Tuesday morning for Clear Creek at
Chinook.

Areal Flood Advisory in effect until 130 PM MDT Monday for
Liberty...Hill...Blaine...Choteau Counties.

Areal Flood Advisory in effect until 1215 PM MDT Monday for
Pondera...Teton Counties.

Areal Flood Advisory in effect until 1200 PM MDT Monday for
Glacier...Toole Counties.

Areal Flood Advisory in effect until 715 PM MDT Monday for
Fergus County.

Areal Flood Advisory in effect until 730 PM MDT Monday for Judith
Basin County.
&&

$$

http://www.weather.gov/greatfalls



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