Severe Thunderstorm Warning 89, NWS Great Falls, MT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE GREAT FALLS MT
235 PM MDT Thu Aug 1 2013

.DISCUSSION...

Tonight through Saturday...Thunderstorms are developing across
southwest and central Montana this afternoon and some storms may
be severe through this evening. A Severe Thunderstorm Watch is in
effect for much of central and southwest Montana until 10 pm this
evening. Large hail, damaging winds, frequent dangerous cloud-to-
ground lightning and heavy rain are possible with these storms. As
moisture continues to spread into north-central Montana and
easterly winds promote upslope flow tonight, precipitation will become
more widespread and heavier, especially along eastern slopes of
the Rocky Mountain Front including Glacier National Park. Heavy
rain could cause ponding on roadways, rapid runoff and possible
debris flows and flash flooding in recent burn areas and as a
result have issued a Flood Watch for areas along the Rocky
Mountain Front and adjacent plains. The heaviest rainfall is
expected tonight through Friday. Rain will continue, heavy at
times across north-central Montana through Friday as an upper-
level area of low pressure continues to move into the Northern
Rockies. Isolated thunderstorms are also possible on Friday,
however severe weather is not anticipated. Across southwest
Montana, precipitation chances will be lower as a dry slot moves
over this area. Cooler temperatures 10 to 15 degrees below
seasonal averages are also expected on Friday. Rain will gradually
taper Friday night from south to north with lingering showers on
Saturday along the Hi-Line and mountains along the Rocky Mountain
Front and central Montana. Temperatures will remain 5 to 10
degrees below seasonal averages on Saturday. MLV

Saturday Night through Thursday...GFS/ECMWF/GEM models are in good
agreement with the forecast solution through Monday. However, after
that point the GFS/ECMWF models keep the area under a west-northwest
flow aloft, while the GEM model keeps weak high pressure in place.
With the GEM consistently being the outlier for the past few nights,
will continue to rely more on a GFS/ECMWF compromise solution. The
shortwave trough over northern Montana Saturday night will move east
of the area on Sunday, allowing the upper level ridge of high
pressure to rebuild into the region. Therefore, will keep a good
chance of showers/thunderstorms over the forecast area for Saturday
night, but decrease to a slight chance for most of the area on
Sunday. An area of low pressure will swing southwest out of Hudson
Bay onto the Canadian Prairies, shifting the flow aloft more
west-northwesterly over the forecast area. This will keep the
pattern unsettled for the remainder of the period. A series of weak
disturbances with associated frontal boundaries will move through
the flow. This will keep winds light and northerly or easterly for
the bulk of the period. These upsloping winds will keep a moderate
chance of showers/thunderstorms over north central and central
Montana, but there will be a lesser chance over southwest Montana
due to less of an upsloping effect there. Temperatures will warm to
near seasonal averages through Monday with the rebuilding ridge, but
they will cool back to around 5 degrees below normal for the
remainder of the period due to the shift in the flow aloft.  Coulston

&&

.AVIATION...
UPDATED 1755Z.
VFR conditions are generally expected across the forecast through
around 00Z, but ceilings will likely become closer to MVFR after
00Z, especially over north central and central Montana. Mid/lower
level moisture will increase from the southwest through 12Z Friday
as a low pressure system approaches Montana from the west. Scattered
showers across the area will become more widespread with embedded
thunderstorms this afternoon as the flow aloft becomes more
unstable.  A jet streak will move into southwest Montana (KHLN KBZN)
for the 18Z to 06Z period as well, which may cause some
thunderstorms to become severe with strong wind gusts and hail.
After 00Z, precipitation will become less showery and more
persistent over north central and central Montana (along and north
of a KHLN-KLWT line), but the airmass will remain unstable enough to
possibly support continued embedded thunderstorms. Drier air moves
into southwestern MT after 12Z Friday while showers continue over
north central MT.  Coulston/Nutter

&&

.HYDROLOGY...
A significantly wetter and stronger than seasonal normal storm
system will move through the area Thursday through
Saturday...bringing the potential for over an inch of rain across
the Rocky Mountain front and adjacent high plains. Some
areas...especially in the mountains may see significantly higher
amounts...leading to a potential for flash flooding.

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  52  68  48  73 / 100  80  60  50
CTB  51  61  46  69 /  90  90  90  60
HLN  55  74  52  79 /  90  70  60  30
BZN  50  76  46  79 /  60  40  30  30
WEY  39  70  34  75 /  40  20  20  10
DLN  48  75  45  79 /  70  20  30  20
HVR  55  64  48  73 /  80  80  60  60
LWT  52  66  46  70 /  70  70  60  50

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
FLOOD WATCH from 6 PM MDT this evening through late Friday night
Cascade...Central and Southern Lewis and Clark...Eastern
Glacier...Eastern Pondera...Eastern Teton...Northern Rocky
Mountain Front...Southern Rocky Mountain Front.

SEVERE THUNDERSTORM WATCH until 10 PM MDT this evening
Beaverhead...Broadwater...Cascade...Central and Southern Lewis
and Clark...Fergus...Gallatin...Jefferson...Judith Basin...
Madison...Meagher...Southern Rocky Mountain Front.

&&

weather.gov/greatfalls

HAZARDOUS WEATHER OUTLOOK
NATIONAL WEATHER SERVICE GREAT FALLS MT
212 PM MDT THU AUG 1 2013

MTZ009-010-012-014-046-048-049-022015-
NORTHERN ROCKY MOUNTAIN FRONT-EASTERN GLACIER-CASCADE-
CENTRAL AND SOUTHERN LEWIS AND CLARK-EASTERN PONDERA-
SOUTHERN ROCKY MOUNTAIN FRONT-EASTERN TETON-
212 PM MDT THU AUG 1 2013

THIS HAZARDOUS WEATHER OUTLOOK IS FOR PORTIONS OF CENTRAL
MONTANA...NORTH CENTRAL MONTANA AND SOUTHWEST MONTANA.

.DAY ONE...THIS AFTERNOON AND TONIGHT

SHOWERS AND THUNDERSTORMS ARE EXPECTED THIS AFTERNOON INTO THE
NIGHT. A FEW THUNDERSTORMS COULD BECOME STRONG.

A FLOOD WATCH FOR HEAVY RAIN POTENTIAL IS IN EFFECT.

.DAYS TWO THROUGH SEVEN...FRIDAY THROUGH WEDNESDAY

A FLOOD WATCH FOR HEAVY RAIN POTENTIAL IS IN EFFECT THROUGH
FRIDAY NIGHT.

.SPOTTER INFORMATION STATEMENT...

ANY REPORTS OF SEVERE WEATHER OR FLOODING ARE GREATLY APPRECIATED.

MTZ008-015-050>055-022015-
BEAVERHEAD-MADISON-JUDITH BASIN-FERGUS-JEFFERSON-BROADWATER-
MEAGHER-GALLATIN-
212 PM MDT THU AUG 1 2013

THIS HAZARDOUS WEATHER OUTLOOK IS FOR PORTIONS OF CENTRAL MONTANA
AND SOUTHWEST MONTANA.

.DAY ONE...THIS AFTERNOON AND TONIGHT

SHOWERS AND THUNDERSTORMS ARE EXPECTED THIS AFTERNOON INTO THE
NIGHT. A FEW THUNDERSTORMS COULD BECOME STRONG.

.DAYS TWO THROUGH SEVEN...FRIDAY THROUGH WEDNESDAY

THE PROBABILITY FOR WIDESPREAD HAZARDOUS WEATHER IS LOW.

.SPOTTER INFORMATION STATEMENT...

ANY REPORTS OF SEVERE WEATHER OR FLOODING ARE GREATLY APPRECIATED.

MTZ011-013-044-045-047-022015-
HILL-CHOUTEAU-TOOLE-LIBERTY-BLAINE-
212 PM MDT THU AUG 1 2013

THIS HAZARDOUS WEATHER OUTLOOK IS FOR PORTIONS OF NORTH CENTRAL
MONTANA.

.DAY ONE...THIS AFTERNOON AND TONIGHT

SCATTERED SHOWERS AND THUNDERSTORMS ARE EXPECTED THIS AFTERNOON
INTO THE NIGHT.

.DAYS TWO THROUGH SEVEN...FRIDAY THROUGH WEDNESDAY

THE PROBABILITY FOR WIDESPREAD HAZARDOUS WEATHER IS LOW.

.SPOTTER INFORMATION STATEMENT...

ANY REPORTS OF SEVERE WEATHER OR FLOODING ARE GREATLY APPRECIATED.

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