528 
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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