FXUS63 KBIS 252140
AFDBIS

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE BISMARCK ND
340 PM CST WED NOV 25 2009

.SHORT TERM...WEDNESDAY THROUGH SATURDAY NIGHT...

THE MAIN FORECAST CHALLENGE TODAY CONCERNS PRECIPITATION CHANCES 
THIS WEEKEND WITH A PASSING LOW PRESSURE SYSTEM. 

THE PRESENCE OF WEAK REX BLOCKING OVER SOUTHEASTERN CANADA HAS 
INTRODUCED A BIT OF NORTHWESTERLY STEERING FLOW TO THE NORTHERN 
PLAINS...BUT FLOW CONTINUES TO BE MODULATED BY THE POLAR VORTEX OVER 
THE BERING SEA. HEIGHT RISES AHEAD OF THE VORTEX SHOULD MAINTAIN THE 
WESTERN NORTH AMERICAN RIDGE ALOFT AS SHORTWAVE ENERGY PASSES 
THROUGH AND PUSHES THROUGH THE NORTHERN PLAINS. 

ONE PARTICULARLY STRONG SHORTWAVE IS FORECAST TO ERODE THE WESTERN 
NORTH AMERICAN RIDGE AND PROGRESS ALONG THE NORTHERN TIER ON 
SATURDAY. ASSOCIATED PRECIPITATION REMAINS LIGHT...AT LEAST 
ACCORDING TO THE 12 UTC NAM/GFS/GEM...09 UTC SREF MEAN...00/12 UTC 
ECMWF. THE MAIN PROBLEM IS THAT THE ABOVE MODELS STILL DISAGREE ON 
THE POSITION OF PRECIPITATION. THIS APPEARS MAINLY DUE TO THE 
COMPLEX NATURE OF THE MULTI-STREAM FLOW AND THE DEGREE OF PHASING 
THAT WILL DEVELOP. CONSEQUENTLY...GRIDDED FORECASTS SHOW ONLY A 
SLIGHT CHANCE OF PRECIPITATION ASSOCIATED WITH THIS SYSTEM IN THE 
HOPES THAT MODELS WILL CONVERGE ON A SOLUTION BY FRIDAY. 

FINALLY...TEMPERATURES ABOVE AVERAGE PRIOR TO BREAKING DOWN THAT 
WESTERN RIDGE SHOULD DROP TO NEAR OR A LITTLE BELOW AVERAGE WITH 
PASSAGE OF THE WEEKEND SHORTWAVE. 

.LONG TERM...SUNDAY THROUGH WEDNESDAY...

THE ECMWF AND GFS ARE IN GOOD AGREEMENT INITIALLY MOVING AN UPPER 
LEVEL TROUGH EAST OF THE FORECAST AREA AS ANOTHER UPPER LEVEL RIDGE 
MOVES OVER THE AREA SUNDAY AND MONDAY.  WILL KEEP A SLIGHT CHANCE OF 
SNOW IN THE FORECAST SUNDAY IN RESPONSE TO THE EXITING TROUGH BUT 
MONDAY SHOULD BE DRY UNDER THE RIDGE.  WARM AIR ADVECTION WITH THE 
UPPER LEVEL RIDGE SHOULD WARM TEMPS ON MONDAY. FOR NOW WILL GO ABOUT 
5 TO 10 ABOVE NORMAL MOST AREAS.  A STRONG UPPER LEVEL SHORT WAVE 
WILL MOVE OVER THE ROCKIES THROUGH THE RIDGE MONDAY NIGHT AND DIG 
INTO THE PLAINS.  ALTHOUGH THE MODELS DIFFER ON TIMING AND 
STRENGTH...WITH EITHER MODEL THICKNESSES DROP AND A STRONG COLD 
FRONT MOVES ACROSS THE FORECAST AREA AS THE PARENT SURFACE LOW MOVES 
ACROSS THE SOUTHERN CANADIAN PLAINS TUESDAY AND TUESDAY NIGHT. WILL 
KEEP THE CHANCE OF PRECIP LOW AS THE BEST CHANCE OF PRECIP WILL 
REMAIN NORTH OF THE FORECAST AREA BUT SHOULD BE AT LEAST A SLIGHT 
CHANCE ALONG THE FRONT AND WITH THE STRONG COLD ADVECTION BEHIND 
IT.  BOTH MODELS DROP THE THICKNESSES DOWN TO 516 DKM OR LOWER 
WEDNESDAY AND WEDNESDAY NIGHT. HAVE TRENDED TEMPS DOWN BUT HAVE BEEN 
CONSERVATIVE AS THERE MAY BE LITTLE IF ANY SNOW COVER.

&&

.AVIATION...

THE WEST EDGE OF THE CLOUDS COVERING EASTERN AND CENTRAL 
NORTH DAKOTA ARE EAST OF A LINE FROM DICKINSON TO WILLISTON AND THE 
EDGE HAS BEEN NEARLY STATIONARY THROUGH THE AFTERNOON. THE LOW LEVEL 
WINDS HAVE BEEN FROM THE NORTH BUT SHOULD SWING TO THE WEST THIS 
EVENING AND THAT SHOULD MOVE THE CLOUDS EAST.  RECENT SNOW AND RAIN 
IN THE JAMESTOWN AREA TODAY COMBINED WITH LIGHT WINDS AND CLEARING 
SKIES LATER TONIGHT POINT TOWARD POTENTIAL FREEZING FOG TOWARD 
MORNING.  MODEL SOUNDINGS ARE BORDERLINE ON POTENTIAL SO WILL LIKELY 
ADD A TEMPO GROUP WITH VISIBILITIES AROUND 3 MI FROM 4 TO 8 AM CST 
THURSDAY MORNING TO INDICATE POTENTIAL.  OTHERWISE VFR CONDITIONS 
EXPECTED THROUGH THE DAY THURSDAY. 

&&

.BIS WATCHES/WARNINGS/ADVISORIES...
NONE.
&&

$$

SHORT TERM...SCHECK
LONG TERM/AVIATION...TSW