FXUS65 KGGW 030939
AFDGGW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE GLASGOW MT
239 AM MST THU DEC 3 2009

.SHORT TERM...FOR NORTHEAST MONTANA...TODAY THROUGH SATURDAY...
UPPER LOW CENTERED OVER THE DAKOTAS WILL MOVE EAST TO IOWA AND
MINNESOTA TODAY. CYCLONIC CIRCULATION AROUND THIS LOW EXTENDS INTO
THE EASTERN MONTANA AT THIS TIME. THIS CYCLONIC CIRCULATION WILL
EXIT THE STATE THIS AFTERNOON. UNTIL THEN...MOST OF THE FORECAST
AREA WILL SEE LIGHT SNOW SHOWERS OR FLURRIES WITH THE BEST CHANCE
ALONG THE NORTH DAKOTA BORDER. 

WITH THE UPPER LOW MOVING TO THE EAST AND RIDGE APPROACHING FROM
THE WEST...SKIES SHOULD CLEAR TONIGHT ALLOWING FOR A COLD NIGHT
WITH SOME BELOW ZERO TEMPERATURES LIKELY. 

SHORTWAVE RIDGE WILL MOVE THROUGH THE AREA ON FRIDAY WITH DRY
WEATHER AND SOMEWHAT WARMER TEMPERATURES.

UPPER LOW ENTERS WESTERN MONTANA FRIDAY NIGHT FROM THE NORTHWEST.
THE LOW MOVES SOUTH OR SOUTHEAST DURING THE DAY ON SATURDAY AND
BRINGS A COLD FRONT THROUGH THE AREA. THE NAM AND ECMWF FAVOR TAKING
THE LOW INTO IDAHO ON SATURDAY WHILE THE OPERATIONAL GFS WANTS TO
TAKE THIS LOW TOWARDS EASTERN MONTANA. GIVEN THAT MOST OF THE GFS
ENSEMBLES ARE CLOSER TO THE NAM AND ECMWF THAT IT IS TO THE
OPERATIONAL GFS...WILL FAVOR THE NAM...ECMWF SOLUTION AT THIS
POINT. THIS WILL KEEP MOST OF THE MOISTURE WEST OF THE FORECAST
AREA. WILL MENTION A CHANCE OF SNOW IN THE SOUTHWEST AND A SLIGHT
CHANCE OF SNOW OVER MOST OF THE AREA ON SATURDAY. FORRESTER

.LONG TERM...SATURDAY NIGHT THROUGH THURSDAY...
THE LONGWAVE PATTERN LATE THIS WEEKEND AND EARLY NEXT WEEK FEATURES
A CLOSED UPPER HIGH OVER ALASKA WITH A BROAD LONGWAVE TROUGH OVER
THE INTERIOR OF NORTH AMERICA. AN UPPER VORTEX...OR TWO DROP INTO
THE NORTHWEST UNITED STATES LATE IN THE WEEKEND. HOWEVER...MODEL
SOLUTIONS DISAGREE ON THE ORGANIZATION OF THE FEATURES. THE ECMWF
SOLUTION HAS A CLOSED LOW CENTER OVER THE PACIFIC NORTHWEST WITH
MOST OF THE SNOW FALLING ALONG THE CONTINENTAL DIVIDE. GFS...GFS
ENSEMBLE MEAN...AND GEM HAVE THE WESTERN LOW CENTER OVER THE
PACIFIC NORTHWEST AS WELL...BUT ALSO AN EASTERN LOW CENTER OVER
THE DAKOTAS. AS A RESULT LIGHT QPF IS GENERATED SATURDAY NIGHT AND
SUNDAY WITH THE LIFT PROVIDED BY THE EASTERN UPPER LOW. OF
NOTE...THE ECMWF DOES HAVE AN EASTERN PIECE OF ENERGY...BUT IT IS
IN THE FORM OF A PROGRESSIVE OPEN WAVE. A 20 PERCENT CHANCE OF
LIGHT SNOW WAS INCLUDED FOR SATURDAY NIGHT AND SUNDAY BASED ON THE
STRONGER CONSENSUS FOR THE DUAL UPPER LOWS. COLD TEMPERATURES WILL
PREVAIL AS 850MB TEMPERATURES OF -15 TO -20C OVERSPREAD THE REGION
LATE THIS WEEKEND AND EARLY NEXT WEEK. BY THE MIDDLE OF NEXT WEEK
A POLAR VORTEX SETTLES OVER NUNAVUT AND TRIGGERS THE UPPER LOW
OVER THE PACIFIC NORTHWEST TO MORPH INTO AN OPEN WAVE THAT RACES
EASTWARD. THIS WILL BRING ANOTHER SURGE OF PERHAPS EVEN COLDER AIR
WITH SUB -20C 850MB TEMPERATURES POSSIBLE. BEYOND THIS POINT MODEL
SOLUTIONS AGREE THAT AN ACTIVE WESTERLY PACIFIC JET DEVELOPS
AROUND 35-40N LATITUDE. HOWEVER...THEY DISAGREE SIGNIFICANTLY ON
HOW THE POLAR VORTEX AFFECTS THE NORTHERN UNITED STATES. THE ECMWF
HAS IT AN ACTIVE FEATURE IN THE LONGWAVE FLOW WHILE THE GFS KEEPS
IT RATHER PASSIVE AND ADVECTS MILD PACIFIC AIR OVER THE DIVIDE. AT
THIS POINT THE COLDER ECMWF SEEMS MORE REASONABLE AND HAS SUPPORT
FROM THE GFS ENSEMBLE MEAN. THE PAST FEW COMPILATIONS OF THE ECMWF
HAVE BEEN HINTING AT THE POSSIBILITY OF SNOW LATER NEXT WEEK...BUT
WILL HOLD WITH DRY PERSISTENCE FOR NOW GIVEN THAT MANY LOCATIONS
HAVE HAD LITTLE TO NO PRECIPITATION FOR OVER ONE MONTH. AJZ

&&

.AVIATION...
AN OVERCAST CEILING AROUND 4-6K FT WILL PREVAIL THROUGH THIS
MORNING AND THEN BECOME BROKEN THIS AFTERNOON. A LOWER CEILING OF
2-3KT FT WILL OCCASIONALLY PASS OVER THE TERMINALS THIS MORNING
ALONG WITH PERIODIC FLURRIES WITH MINIMAL IMPACT ON VISIBILITY.
THE SKY SHOULD BECOME MOSTLY CLEAR BY TONIGHT. A NORTHWESTERLY
WIND AROUND 8-12KT WILL CONTINUE THROUGH THIS AFTERNOON AND THEN
BECOME LIGHT AND VARIABLE TONIGHT. AJZ

&&

.GLASGOW WATCHES/WARNINGS/ADVISORIES...NONE.

&&

$$

WEATHER.GOV/GLASGOW