Area Forecast Discussion
Issued by NWS Great Falls, MT

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FXUS65 KTFX 221609
AFDTFX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE GREAT FALLS MT
1009 AM MDT Tue Apr 22 2014

.UPDATE...
Models are showing sufficient shear and instability over a small
portion of southwest and central Montana this afternoon for the
threat of isolated severe thunderstorms. The main threat from
these storms would be winds but could not rule out a a few cells
producing hail to near one inch in diameter. The best dynamics
will be across Gallatin...Meagher...and southern Judith basin and
Fergus counties. Will be adding mention of severe thunderstorms to
much of this area. There is a threat for weaker thunderstorms
across much of the remainder of the CWA...especially along and
east of intestate 15.

The remainder of the forecast is on track, only minor change was
to increase winds over southwest Montana with the passage of the
cold front this evening. Langlieb

&&

.AVIATION...
UPDATED 1150Z.
A cold front is expected to move to near a KCTB-KBTM line around 21z
and then to near a KHVR-KLWT line near 02z. Showers and
thunderstorms are likely near and behind the front. Expect scattered
MVFR ceilings and visibilities in the showers and thunderstorms with
local IFR ceilings and visibilties. A few of the thunderstorms will
have wind gusts to 40 kts and small hail. Showers and thunderstorms
will end from the west near and after midnight over the plains but
showers are expected to linger over southwest Montana past midnight.
Gusty westerly winds 20 to 30 kts will develop behind the front and
there is a risk of wind gusts to 50 kts along the Rocky Mountain
Front after midnight. Snow levels will be dropping overnight and
could see brief MVFR conditions in snow for the southwest valleys.
Also expect areas of mountain obscurement tonight over the Rocky
Mountain Front and southwest Montana in snow. Blank

&&

.PREV DISCUSSION...
/ISSUED 550 AM MDT Tue Apr 22 2014/
Today through Thursday...A deep upper level trough along the west
coast this morning will shift inland, bringing multiple weather
impacts to the region through the next 24 hrs (see specific info
below). Upper level jet and diffluent flow on the front side of the
trough moving into northern CA/OR early this morning will move east
into the northern Rockies later today while a surface trough
intensifies over central MT this afternoon ahead of a Pacific cold
front associated with the upper trough. Surface trough/cold front
and main forcing associated with the trough move across the region
tonight with showers and thunderstorms developing from SW to NE
across the forecast area late this afternoon and evening. Cooler and
windy conditions move in behind the front tonight through Wednesday
with precipitation diminishing to scattered showers by Wednesday
morning. Westerly flow will limit precipitation chances for areas
east of the Mtns Wednesday and Thursday with temperatures 5-10
degrees below seasonal averages.

Thunderstorms...Convective precipitation developing ahead of the
front this afternoon over western and SW MT will shift east across
north-central and the remainder of SW MT this evening. Upper
level forcing and instability will provide ample lift for
thunderstorm development, though cloud-cover and dewpoints in the
20s/30s will likely be a limiting factor in the strength of
thunderstorm activity. Main concern with thunderstorms this
afternoon/evening is the potential for linear organization and
strong winds with locally heavy rain and small hail a secondary
threat.

Winds....Strong pressure rises behind the front will contribute to
gusty post frontal west winds this evening with strong lower level
westerly flow developing later tonight and continuing through
Wednesday as low pressure deepens over southern AB beneath the
closed mid-upper level low. Area most likely to see winds 30kts with
gusts near 50 kts will be along and adjacent to the Rocky Mtn front
where a a high wind watch remains in effect for tonight through
Wednesday.

Precipitation/Hydrology...Total precipitation amounts through
tonight still look to range from around 1/4 to 1/2 inch for most of
the area with highest amounts likely along a swath from Madison
County north-northeast through Cascade/Judith Basin counties and
north to western Hill county where localized amounts around 3/4" are
possible. Main concern from a hydrologic standpoint is the potential
for embedded heavier precipitation late this afternoon/evening to
fall on mountain snowpack resulting in rapid snowmelt runoff and
flooding of creeks/rivers originating in the mtns. The potential for
this situation exists across most mountain rages of central and
southwest MT with streams emptying into the Gallatin River most
likely to see locally moderate precipitation on a widespread
mountain snowpack.

Winter Weather...Strong cooling associated with the upper level
trough will cause snow levels to fall rapidly tonight behind the
cold front. Expect a change-over to snow on most passes overnight
though precipitation largely diminishes as the cooler air moves
in, resulting in around an inch of snow at pass levels, though
road surfaces may remain at or above freezing, limiting
accumulation on roadways. Higher elevations above 7500 feet in
southwest MT could see 4 to 6 inches of snow accumulation late
tonight through Wednesday morning. Hoenisch

Thursday night through Tuesday...In a general sense models are on
the same page with trends during the period. Expect scattered
showers to diminish Thursday night as an upper ridge moves through
western Montana and into central Montana. Then a wet period is
expected. A strong upper trough will move through the western U.S.
Friday through Saturday. Several weather disturbances embedded in
the southerly flow aloft ahead of the upper trough will bring
periods of rain to the forecast area during this time frame. Then
Sunday the main portion of the upper trough is expected to move into
the central Plains. Southeast flow aloft ahead of the upper trough
will continue to feed in moisture to an area possibly as far west as
a Havre to Great Falls to Bozeman line. Since the models are in
better agreement with this wet period have increased the chances of
precipitation for Friday night through Saturday and also east of the
line mentioned above for Sunday. For the period Friday afternoon
through Sunday precipitation amounts ranging from a half inch to an
inch from the western mountains to the eastern plains could occur.
Models are also in better agreement with forecast temperatures for
Saturday and Sunday as the ECMWF is not as cold as previously while
the GFS is quite a bit colder. Now expect highs only in the 40s over
the plains and 50 to 55 for lower elevations of southwest Montana
over the weekend. In line with the colder temperatures expect snow
over the mountains with lower elevations possibly seeing a little
snow as well during the overnight periods. Looking ahead to early
next week models are pointing to warmer temperatures with decreasing
chances of precipitation as an upper ridge develops. Blank

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  70  37  51  33 /  70 100  30  10
CTB  67  36  50  32 /  60  90  20  10
HLN  67  36  52  35 /  80 100  30  20
BZN  70  32  51  30 /  60  90  40  30
WEY  60  28  43  24 /  70  80  60  40
DLN  64  30  49  30 /  80  80  20  30
HVR  76  40  57  33 /  20 100  40  30
LWT  71  34  50  29 /  30  90  30  20

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
HIGH WIND WATCH from late tonight through Wednesday afternoon
Eastern Glacier...Eastern Pondera...Northern Rocky Mountain
Front...Southern Rocky Mountain Front...Toole.

FLOOD WATCH from 6 PM MDT this evening through Wednesday
afternoon Gallatin.

&&

$$


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