Special Weather Statement, NWS Great Falls, MT

Area Forecast Discussion
National Weather Service Great Falls MT
1145 AM MDT THU JUN 30 2016

AVIATION SECTION UPDATED

.UPDATE...
Latest analysis shows fair amount of clear skies across the
forecast area except for broad coverage of mid-level clouds from
around across much of southwest MT. There are also some weak rain
showers in that same area that are moving steadily eastward.
Forecast on track for more widespread thunderstorm activity today
as a shortwave trof entering northwest MT now moves east of the
Divide this aftn. Very-short range models continue to indicate an
earlier start to the storms today, with initial storms forming
along the Rocky Mtn Front and southwest mtn ranges between noon
and 3pm. Storm development moves east across the central counties
this aftn/eve. A few strong storms (hail to 0.5 inch, heavy
downpours, and wind gusts of 35-50 mph) are likely; cannot rule
out a severe storm or two, though current wind shear projections
are on the low to marginal side for severe storms.
Waranauskas

&&

.AVIATION...
UPDATED 1745Z.

Most of central MT still under clear skies at 1730Z, with SCT-BKN
mid-level clouds and isolated -SHRA from KHLN south to KWYS. Upper-
level trough in NW MT will track eastward this aftn and be the
trigger for scattered thunderstorms, starting initially along the
Continental Divide by 18-19Z and moving east through the aftn/early
evening period. Good bet that some storms will reach moderate (TSRA)
intensity with locally heavy downpours, small hail, and outflow
gusts of 30-40 kts (possibly higher). Timing of storms at individual
TAF sites will be a bit tricky, so amendments later this aftn may be
needed to fine tune storm occurrence times. Convective activity
tapers off from west to east starting around 23-00Z, with gradually
clearing skies through the evening/overnight hours.
Waranauskas

&&

.PREV DISCUSSION...
/ISSUED /

Today...An upper level ridge with axis north through the US and
Canadian Rockies remains in place across the region with satellite
imagery early this morning showing a shortwave disturbance upstream
over Southern BC/WA while additional energy is moving NE across SW
ID. Isolated showers/thunderstorms are ongoing along/just north of
the AB border as well as in the vicinity of the Bears Paw Mtns and
an area of somewhat more organized shower/thunderstorm activity
moving east between Helena and Bozeman early this morning. This
activity should propagate east/southeast but diminish in intensity
through the early morning period. Previously mentioned shortwave
energy to the W and SW will move into the forecast area this
afternoon setting up favorable conditions for more widespread
thunderstorm activity today across much of the forecast area,
diminishing this evening as the shortwave energy tracks east into
eastern MT. An inverted surface trough of low pressure also deepens
along the Continental Divide today as the shortwave energy
approaches, setting up a SE Low level flow across the plains through
late this afternoon. Instability/CAPE is moderate and while shear is
somewhat marginal at around 30kts, the SE low level flow across N-
central MT could set up a favorable environment for some storms to
approach severe levels this afternoon, particularly across western
portions of N-central MT where shear will be highest. Isolated
damaging wind and hail will be the main threats. PW values around an
inch indicate heavy rain is also possible, but with storm motion
around 15-20kts it should not be excessive.

Tonight through Saturday...Drier air moves in from the west behind
todays shortwave energy with westerly flow aloft increasing late
Friday through Saturday as the upper level ridge de-amplifies.
This will limit chances for convection across northern portions
of the forecast area though a small chance for afternoon/evening
thunderstorms will continue across SW MT Friday and Saturday.
Temperatures remain warm and above seasonal averages with surface
winds becoming W/SW friday and increasing some on Saturday.
Hoenisch

Saturday night through Thursday...West-southwest flow aloft
generally prevails through the period with seasonal temperatures.
Moisture is rather limited in this pattern as total precipitable
water trends near to below average. Isolated, mainly diurnal
convection is possible, enhanced by the periodic passage of
shortwave energy rotating through the flow. A rather persistent lee
trough sets up along the east slope of the Rockies, modulated by
passing shortwave energy aloft. The associated pressure gradient and
deep layer mixing will produce rather breezy conditions each
afternoon. Details in the timing and amplitude of shortwave features
vary widely among model solutions, as is typical of a weakly forced
mid-summer weather pattern. Hence, confidence is rather low
regarding specific details of the forecast. PN

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  84  54  87  56 /  50  30  10  10
CTB  81  49  83  52 /  30  30  10  10
HLN  82  57  89  59 /  40  30  10  10
BZN  82  51  88  53 /  40  40  20  10
WEY  74  42  74  44 /  40  30  20  20
DLN  80  51  86  52 /  30  20  10  10
HVR  83  54  86  57 /  20  30  10  10
LWT  79  54  84  56 /  20  40  10  20

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
None.
&&

weather.gov/greatfalls

SPECIAL WEATHER STATEMENT
NATIONAL WEATHER SERVICE GREAT FALLS MT
121 PM MDT THU JUN 30 2016

MTZ048-302000-
SOUTHERN ROCKY MOUNTAIN FRONT MT-
121 PM MDT THU JUN 30 2016

...SIGNIFICANT WEATHER ADVISORY FOR NORTHWESTERN TETON COUNTY UNTIL
200 PM MDT...

AT 119 PM MDT...DOPPLER RADAR WAS TRACKING A STRONG THUNDERSTORM 18
MILES WEST OF CHOTEAU...MOVING NORTHEAST AT 20 MPH. A SECOND STRONG
THUNDERSTORM WAS LOCATED 18 MILES WEST OF PENDROY...MOVING NORTHEAST
AT 20 MPH.

DIME SIZE HAIL AND WINDS IN EXCESS OF 30 MPH WILL BE POSSIBLE WITH
THESE STORMS.

THIS STORM WILL REMAIN OVER MAINLY RURAL AREAS OF NORTHWESTERN TETON
COUNTY.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

TORRENTIAL RAINFALL IS ALSO OCCURRING WITH THIS STORM...AND MAY LEAD
TO LOCALIZED FLOODING. DO NOT DRIVE YOUR VEHICLE THROUGH FLOODED
ROADWAYS.

THIS STORM MAY INTENSIFY...SO BE CERTAIN TO MONITOR LOCAL RADIO
STATIONS AND AVAILABLE TELEVISION STATIONS FOR ADDITIONAL INFORMATION
AND POSSIBLE WARNINGS FROM THE NATIONAL WEATHER SERVICE.

&&

LAT...LON 4781 11264 4809 11276 4813 11243 4785 11230
TIME...MOT...LOC 1919Z 244DEG 15KT 4789 11257

MARTIN