Area Forecast Discussion
Issued by NWS Great Falls, MT

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FXUS65 KTFX 201719
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
1119 AM MDT Fri Apr 20 2018

UPDATED AVIATION AND HYDROLOGY SECTIONS

.SYNOPSIS...

Mostly clear skies prevail on Friday, as high temperatures range
from the upper 50s to the mid 60s. Strong winds and a few showers
develop on Saturday associated with a passing frontal system.
Temperatures could reach 70 degrees Saturday afternoon, then cool
back to near normal for Sunday.

&&

.UPDATE...

High pressure aloft builds over the region today for mostly clear
skies and warm temperatures. Going forecast is in pretty good
shape but went ahead and updated to increase winds several knots
this morning and early afternoon to be in keeping with current
observations. Early morning model runs continue to support the
High Wind Watch for the Rocky Mountain Front and Glacier County.
There is some indication in at least one model that this watch may
need to be expanded to a few adjacent counties but will hold off
on making any changes until the afternoon package so that
additional models can be examined. Also, warnings and advisories
for flooding in north-central Montana will continue and I do not
anticipate any major changes will be needed. mpj

&&

.AVIATION...
Updated 1719Z.

VFR prevail next 24-hours as cloud cover increases gradually from
the west due to a Pacific disturbance. Mountain-obscuring snow
and/or rain showers are expected to affect the Rocky Mountain Front
around and especially after 12Z/Sat. Breezy southwesterly to
westerly surface winds develop after 15Z/Sat for the North-Central
MT plains.
Jaszka

&&

.HYDROLOGY...
Updated 1119 AM Friday

Areal flood warnings continue for Toole, Liberty, Hill and Blaine
counties where areas of snowpack linger and will continue to
contribute to runoff and flooding of lowland areas though the next
several days. Areal flood advisories continue for Glacier and
Pondera counties, where some roads are still being impacted by
lowland flooding but remaining lowland snowpack is minimal.
Numerous point flood warnings continue for the Milk River and its
tributaries across Hill and Blaine counties where additional
snowmelt and inflow from upstream tributaries will maintain flooding
through the next several days. However, the flood warning for Battle
Creek near Chinook has been cancelled since no flooding is observed
or forecast. The same can be said for the flood watch for the
Missouri River near Landusky. With lessening snowpack and dry
conditions this weekend slow improvement is expected across the Hi-
Line. A storm system early next week could bring precipitation to
southwest Montana.
Hoenisch/Jaszka

&&

.PREV DISCUSSION...
/ISSUED 409 AM MDT Fri Apr 20 2018/

Snowmelt flooding continues across the HiLine, as described
separately under the Hydrology section below.

Patchy fog is possible over northern portions of North-central
Montana early this morning. Otherwise, ridging aloft will
maintain sunny and warm conditions on Friday. Warm temperatures
continue into Saturday, possibly climbing to near 70 in some
areas.

A strong shortwave trough crosses the northern Rockies on
Saturday afternoon. This feature will cause a period of strong
winds to develop on Saturday, first over the Rocky Mountain
Front, then spreading from west to east across N-central MT with
a surface cold front. As this wave passes during peak afternoon
heating, instability through a deep surface layer could enhance
the downward momentum transfer and bring high wind gusts to the
surface. A high wind watch is in effect to highlight this
potential for damaging winds.

Winds decline quickly Saturday night, with dry but somewhat cooler
temperatures on Sunday. The next shortwave trough develops Sunday
night, producing chances for precipitation across southwest
Montana. Global ensemble forecasts have been consistent in
producing moderate precipitation. However, recent forecasts from
the NAM and SREF models present a weaker wave and much drier
solutions. High chances for moderate precipitation were
maintained in the forecast based on their recent continuity in
solutions. Precipitation amounts on Saturday could exceed 0.5
inches over portions of Southwest Montana. Snow levels could fall
below 6000 feet, with a few inches of wet snow possible at and
above mountain passes. Since this scenario would present rain and
mountain snow, it does not appear to create an immediate flooding
concern.

A broad and high amplitude ridge builds over the region starting
next Tuesday, and persisting through the week ahead. Temperatures
will trend above average, with freezing (melting) levels possibly
climbing above 9k feet and focusing new attention to mid-
elevation snowmelt runoff.

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  64  38  63  32 /   0   0  10   0
CTB  60  36  54  31 /   0   0  10   0
HLN  63  38  64  35 /   0   0  10   0
BZN  60  31  65  33 /   0   0   0  10
WEY  49  21  52  29 /   0   0   0   0
DLN  57  31  62  31 /   0   0   0   0
HVR  57  34  60  32 /   0   0  20  10
LWT  61  36  66  32 /   0   0  10  10

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
High Wind Watch from Saturday morning through Saturday evening
Eastern Glacier...Northern Rocky Mountain Front...Southern Rocky
Mountain Front.

Flood highlights continue for portions of North Central MT.

&&

$$

http://www.weather.gov/greatfalls



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