Area Forecast Discussion
Issued by NWS Glasgow, MT

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FXUS65 KGGW 220433 AAB
AFDGGW

AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE GLASGOW MT
933 PM MST SUN DEC 21 2014

.SHORT TERM...FOR NORTHEAST MONTANA...TONIGHT THROUGH TUESDAY...

MINOR UPDATE TO REFLECT LATEST CONDITIONS AND GUIDANCE.
OBSERVATIONS UPSTREAM IN SOUTHERN CANADA SHOW TEMPERATURES
DROPPING AND SNOW. LOCAL OBSERVATIONS ONLY SHOW A FEW SPOTS BELOW
FREEZING AND LIGHT RAIN. SO EXPECT A TRANSITION OVER TO SNOW LATER
TONIGHT. ALSO ADJUSTED WIND GRIDS AND ADDED BLSN TO SOUTHEAST
AREAS TOMORROW BASED ON CONTINUING STRONG WIND. SCT

PREVIOUS UPDATE...
EARLY UPDATE WAS MAINLY TO BEGIN WSW SOONER DUE TO CONTINUING RAIN
AND TEMPERATURES GETTING CLOSE TO FREEZING EARLY THIS EVENING.
ADDED MENTION OF ISOLATED AREAS OF FREEZING RAIN BEFORE SNOW BEGIN
LATER TONIGHT. SCT

PREVIOUS DISCUSSION...
SYNOPTIC SET UP...SHORT TERM BEGINS WITH A BROAD TROUGH OVER
ONTARIO...THE GREAT LAKES...THE MISSISSIPPI VALLEY AND
TERMINATING OVER THE SOUTHERN US PLAINS. A MINI RIDGE EXTENDS OVER
THE NORTHERN AND CENTRAL US PLAINS AND THE SOUTHERN US ROCKIES.
WEST OF THIS...ZONAL FLOW RUNS TO THE COAST WITH BROAD RIDGING OFF
SHORE. THE HIGH PLAINS OF CANADA ARE EXPERIENCE A COLD FRONT
RUNNING DOWN THEM WHICH SHOULD HIT THE INTERNATIONAL BORDER SOME
TIME TONIGHT.

THIS AFTERNOON THROUGH MONDAY...WARM FRONT MOVING ACROSS THE AREA
SHOULD TRANSITION RAIN AND FREEZING RAIN DURING THIS AFTERNOON
OVER TO SNOW ACROSS THE NORTHEAST CORNER OF THE CWA BY THIS
EVENING. ONCE A SURFACE LOW MOVES ALONG THE WARM FRONT LIKE A
ZIPPER LINE A COLD FRONT WILL PULL IN BEHIND IT DROPPING
TEMPERATURES OFF AND TRANSITIONING ALL REMAINING RAIN TO SNOW.
TOTAL SNOW AMOUNTS NEAR THE ZONE OF THE TREKKING SURFACE LOW ARE
EXPECT TO REACH AROUND 1 TO 3 INCHES OF TOTAL ACCUMULATION INTO
MONDAY AFTERNOON. A WINTER WEATHER ADVISORY FOR SNOW PUT UP FOR
THIS.

MONDAY NIGHT THROUGH TUESDAY...FLOW WILL TURN NORTHWEST AND THEN
NORTH ALLOWING CAA TO DROP TEMPERATURES ACROSS THE AREA. AS DRY
SURFACE AIR MOVES IN THE STORMS SHOULD EXIT AND SNOW WILL
PROGRESSIVELY BECOME MORE SCATTERED AND LIGHT INTO THE EVENING
HOURS. BY TUESDAY...PRECIPITATION SHOULD BE CLEARED OUT BUT COLD
TEMPERATURES WILL STICK AROUND WITH THE NEW SURFACE HIGH.

GAH

.LONG TERM...TUESDAY NIGHT THROUGH SUNDAY...

MODELS ARE IN GOOD AGREEMENT ABOUT THE DEEPENING OF A LONG WAVE
TROUGH OVER THE CENTRAL TO EASTERN 2/3RDS OF THE CONUS THROUGH
DAY 7. THIS FOLLOWS DIGGING SOUTHWARD OF STRONG MID-LEVEL MOMENTUM
CURRENTLY SEEN ON ANALYSES OVER OREGON/WASHINGTON STATE. THIS
TROUGH LEAVES BEHIND SOME RIDGING OVER THE WESTERN US AND WILL
HELP TO KEEP THE NORTHEAST MONTANA AREA WARMER THAN NORMAL THROUGH
WEDNESDAY.

BY THURSDAY MORNING...EASTERN TROUGH BEGINS TO MOVE EASTWARD
ALLOWING A SHORTWAVE TROUGH TO ADVANCE ON MONTANA FROM THE
NORTHWEST. A FAIR AMOUNT OF UNCERTAINTY EXISTS AS TO THE DETAILS
WITH THIS FEATURE WITH THE EC MODEL BRINGING MOST OF THE SNOW
SOUTH OF THE GGW AREA WHILE THE GFS MODEL HAS AROUND 4 INCHES OF
SNOW ON CHRISTMAS DAY OVER MOST OF THE GGW AREA. SOME SNOW IS
PROBABLE IN THE WEDNESDAY NIGHT THROUGH THURSDAY PERIOD AS ALL
MODELS HAVE A SHORTWAVE PASSAGE OF SOME SORT AT THIS TIME.

SYSTEM ON THURSDAY WILL BE FOLLOWED BY STRONG NORTHERLY FLOW AND
MUCH COLDER TEMPERATURES FRIDAY THROUGH SUNDAY AND BEYOND.  MARTIN

PREVIOUS LONG TERM DISCUSSION...
THE PERIOD BEGINS TUESDAY NIGHT WITH A LONG-WAVE EAST-LEANING
RIDGE OVER THE WESTERN STATES AND A MEDIUM-WAVE TROUGH EXTENDING
FROM HUDSON BAY TO TEXAS. THIS PLACES A NORTHWESTERLY FLOW ALOFT
OVER NORTHEAST MONTANA. A SPLIT TROUGH DISTURBANCE IN NORTHWESTERN
CANADA KNOCKS DOWN THE RIDGE...SENDING A BROAD COLD FRONT SOUTH
TOWARDS THE INTERNATIONAL BORDER. THE NORTHWEST FLOW BECOMES MORE
WESTERLY. THIS WILL SEND PACIFIC MOISTURE OVER THE DIVIDE
OVERRUNNING THE COLD BOUNDARY IN MONTANA ON WEDNESDAY.

THE EC AND GFS HAVE BEEN STRUGGLING TO COME INTO SOME SORT OF
AGREEMENT AS TO WHERE THIS BOUNDARY WILL BE...WITH THE EC
FURTHER SOUTH. THE BOUNDARY LOCATION WILL DETERMINE WX TYPE AND
WHERE THAT PRECIPITATION WILL FALL. THE LATEST SOLUTIONS ARE
CLOSER TOGETHER. SO RIGHT NOW A BLENDED SOLUTION POINTS TO NEMONT
GETTING AN INITIAL SHOT OF MIXED PRECIPITATION THEN TURNING TO
SNOW BY WEDNESDAY EVENING. FOR NOW...EXPECT 1 TO 3 INCHES OF
SNOW....MAINLY SOUTH OF THE MISSOURI RIVER.

AS THE FRONT CONTINUES SOUTH...SO WILL THE PRECIPITATION. THEN
LATE WEDNESDAY NIGHT AND THURSDAY BOTH MODELS POINT TO A HEAVY
SNOW EVENT OVER CENTRAL AND SOUTHEAST MONTANA. THE ORIGINAL SPLIT
TROUGH FEATURE WILL CONTINUE TO BRING IN LIGHT SNOW SHOWERS INTO
NEMONT THROUGH THE REMAINDER OF THE PERIOD. TOWARDS THE WEEKEND
TEMPERATURES CONTINUE TO TREND LOWER AS CAA IS SUSTAINED.


&&

.AVIATION...
SYNOPSIS: A WARM FRONT ACROSS THE AREA WILL MOVE INTO THE DAKOTAS
OVERNIGHT. THEN A COLD FRONT WILL PASS THROUGH THE REGION LATE
TONIGHT THROUGH MONDAY.

FLIGHT CATEGORY: VFR TRANSITIONING TO MVFR AFT 06Z WITH PERIODS
OF IFR CONDITIONS.

CIGS AND VSBY: CIGS LOW VFR 035-080 WILL LOWER TO 015-035 AROUND
06Z...THEN TEMPOS OF 010 EXPECTED AFT 12Z LASTING THROUGH 00Z.

WIND: WEST-SOUTHWEST WIND 10-20 KNOTS SHIFTING NORTHWEST BY 06Z
AND INCREASING TO 15 TO 20 KNOTS AND GUSTY LASTING THROUGH 00Z
BEFORE DIMINISHING.      PROTON

&&

.GLASGOW WATCHES/WARNINGS/ADVISORIES...
WINTER WEATHER ADVISORY UNTIL 11 PM MST MONDAY FOR CENTRAL AND
SOUTHERN VALLEY...DANIELS...DAWSON...EASTERN ROOSEVELT...
MCCONE...NORTHERN VALLEY...PRAIRIE...RICHLAND...SHERIDAN...
WESTERN ROOSEVELT...WIBAUX.

&&

$$

WEATHER.GOV/GLASGOW





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