Winter Storm in Montana, December 13 to 15, 2015

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE GREAT FALLS MT
1052 AM MST SUN DEC 13 2015

AVIATION SECTION UPDATED

.UPDATE...

THE FORECAST THROUGH THIS AFTERNOON LOOKS GOOD. THE MAIN CHANGE
WAS TO REMOVE PRECIP FROM THE HI-LINE FOR THE REST OF THE MORNING
AS THE LIGHT SNOW FROM EARLIER HAS MOVED OUT OF THE AREA. HIGHS
WERE BUMPED UP A BIT IN THAT AREA AS WELL WITH SOME SUN POKING
THROUGH AND SOME HELP FROM DOWNSLOPE/WARMING EFFECTS. MARTIN

&&

.AVIATION...UPDATED 1751Z.

AN UPPER LOW PRESSURE TROF WILL BRING WIDESPREAD SNOW ACROSS MOST OF
CENTRAL AND SOUTHWEST MONTANA. SNOW WILL DEVELOP AFTER 21Z OVER
SOUTHWEST MONTANA AND MOVE NORTH AND EAST OVERNIGHT. WIDESPREAD
IFR/MVFR CONDITIONS AND MOUNTAIN OBSCURATION ARE EXPECTED AS SNOW
DEVELOPS. TO THE NORTH, A WEAK COLD FRONT WILL CROSS THE CANADIAN
BORDER AFTER MIDNIGHT AND BRING LIGHT SNOW AND AREAS OF IFR CIGS
ACROSS THE NORTHERN PORTIONS OF CENTRAL MONTANA, INCLUDING KCTB AND
KHVR.

&&

.PREV DISCUSSION...
/ISSUED 500 AM MST SUN DEC 13 2015/
TODAY THROUGH MONDAY NIGHT...OVERALL...THE MAIN STORM SYSTEM FOR
TONIGHT THRU MONDAY NIGHT IS ON TRACK FROM PREVIOUS FORECASTS.
THUS THE WINTER STORM WATCH HAS BEEN CHANGED TO A WINTER STORM
WARNING...WITH A START TIME VARYING FROM MID AFTERNOON IN THE
SOUTH TO EVENING IN THE CENTRAL AREAS AS IMPACTS SHOULD START TO
BECOME A BIT MORE WIDESPREAD NEAR THE START TIME. A BAND OF LIGHT
SNOW CURRENTLY FALLING FROM HAVRE TO LEWISTOWN CONTINUES TO PUSH
EASTWARD. MOST OF THE SNOW WILL BE DONE BY 5 AM...BUT A FEW SNOW
SHOWERS WILL REMAIN IN THIS REGION THIS MORNING. STEADIER SNOW
CONTINUES TO FALL OVER THE ROCKY MOUNTAIN FRONT...ALONG WITH GUSTY
WINDS OVER THE ADJACENT PLAINS. THE WINDS WILL DIMINISH A BIT AND
SHIFT TO THE NORTH TONIGHT AS AN AREA OF LOW PRESSURE MOVES ACROSS
SOUTHERN MT. LIGHT SNOW WILL DEVELOP OVER THE MOUNTAINS OF
SOUTHWEST MT THIS MORNING...AND BECOME MORE WIDESPREAD THIS
AFTERNOON. THE SNOW SHOULD MOVE INTO THE BOZEMAN/DILLON AREAS THIS
EVENING. THE SNOW WILL THEN SPREAD NORTH INTO THE HELENA/GREAT
FALLS/LEWISTOWN AREAS BY LATE EVENING. THE SNOW WILL BE HEAVY AT
TIMES ON MONDAY...POSSIBLY DURING BOTH THE MORNING/EVENING
COMMUTES. THE SNOW WILL LET UP IN INTENSITY/DIMINISH LATE MONDAY
NIGHT. (A BRIEF PERIOD OF RAIN IS POSSIBLE WHEN THE PRECIPITATION
BEGINS AT LOWER ELEVATIONS...BUT IT SHOULD CHANGE OVER TO ALL SNOW
FAIRLY QUICKLY.) TEMPERATURES TODAY WILL BE MILD (IN THE
40S)...ESPECIALLY OVER NORTH CENTRAL MT...BUT THEN REMAIN FAIRLY
STEADY IN THE UPPER 20S/LOWER 30S ON MONDAY...MOSTLY IN THE 20S ON
TUESDAY. BRUSDA

KEY MESSAGE SHORT TERM:
1. SNOW BECOMES WIDESPREAD AND HEAVIER TONIGHT OVER CENTRAL AND
SOUTHWEST MT.
2. GENERALLY LESS THAN AN INCH OVER THE NORTHERN PLAINS.
3. AN UPDATED SNOWFALL GRAPHIC IS ON THE WEB.

TUESDAY NIGHT THROUGH SUNDAY...MEDIUM RANGE MODELS CONTINUE TO BE IN
GENERAL AGREEMENT. FAST MOVING SHORT WAVE ENERGY MOVES ACROSS THE
AREA WEDNESDAY BRINGING SCATTERED LIGHT SNOW THAT WILL TAPER OFF
WEDNESDAY NIGHT. THE SPEED OF THIS FEATURE EMBEDDED WITHIN THE
NORTHWESTERLY FLOW ALOFT WILL KEEP SNOW AMOUNTS ON THE LIGHT SIDE.
UPPER RIDGING OVER THE WEST COAST THURSDAY WILL BRING DRIER AIR TO
THE AREA THOUGH ISOLATED SNOW SHOWERS ARE POSSIBLE OVER THE WESTERN
AND SOUTHWEST MOUNTAINS. UPPER RIDGE AXIS SHIFTS TO WESTERN MONTANA
FRIDAY BUT ENOUGH PACIFIC MOISTURE MOVING THROUGH THE RIDGE TO
ACCOUNT FOR SCATTERED MOUNTAIN SNOW AND ISOLATED SNOW SHOWERS OVER
THE PLAINS OF NORTH CENTRAL MONTANA. THE UPPER RIDGE FLATTENS BY
SATURDAY WITH ISOLATED PRECIPITATION CONFINED TO SOUTHWEST MONTANA.
BY SUNDAY, THE NEXT UPPER TROF WILL BE INFLUENCING THE AREA. THE GFS
IS FASTER WITH THIS SYSTEM AND HAS IT CROSSING THE DIVIDE SUNDAY
AFTERNOON WITH A CHANCE OF SCATTERED SNOW SHOWERS. THE ECMWF HAS THE
TROF PUSHING INTO THE PACIFIC NORTHWEST DURING THE SAME TIME FRAME.
TEMPERATURES WILL REMAIN COOL AND BELOW SEASONAL AVERAGES TO START
THE PERIOD BUT WILL WARM TO ABOVE SEASONAL AVERAGES BY THE
WEEKEND.

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  44  25  28  15 /  10  80  90  90
CTB  43  22  27  14 /  10  10  20  20
HLN  40  26  31  14 /  30 100 100  90
BZN  41  26  32  14 /  30 100 100 100
WEY  29  22  26  12 / 100 100 100  90
DLN  37  23  28  12 /  50 100 100 100
HVR  43  21  29  16 /  10  10  30  30
LWT  42  26  29  13 /  10 100 100 100

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
WINTER WEATHER ADVISORY UNTIL 11 AM MST MONDAY FOR MTZ009-048.

WINTER STORM WARNING FROM 8 PM THIS EVENING TO 5 AM MST TUESDAY
FOR MTZ012-014-050>054.

WINTER STORM WARNING FROM 2 PM THIS AFTERNOON TO 5 AM MST
TUESDAY FOR MTZ008-015-055.

&&

WEATHER.GOV/GREATFALLS