FXUS65 KPIH 220444
AFDPIH

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE POCATELLO ID
944 PM MST MON DEC 21 2009

.UPDATE...WE ARE MAKING SOME MAJOR UPDATES TO THE FORECAST DUE TO
SLOWER ONSET OF PRECIPITATION AND SPLITTING NATURE OF THE SYSTEM
MAKING FOR SOME SLIGHTLY LOWER SNOW AMOUNTS. HEADLINES ARE STILL
IN PLACE BUT WE HAVE SHIFTED THE STARTING AND ENDING TIMES FOR ALL
OF THEM. PLEASE SEE THE LATEST WSWPIH FOR MORE DETAILS. WE HAVE
ALSO ADJUSTED TEMPERATURES UP OVERNIGHT DUE TO CLOUD COVER AND
WARMER OBSERVED TEMPERATURES AT THIS HOUR. EVEN WITH WARMER AIR
TEMPS AND DRY AIR IN PLACE...ONCE THE PRECIP BEGINS WE SHOULD SEE
EVERYONE DROPPING TO NEAR FREEZING AND BELOW. THIS WILL TILT THE
FORECAST TOWARD ALL SNOW EXCEPT FOR A MIX ON THE SOUTHERN AND
WESTERN ENDS OF THE MAGIC VALLEY. WE ARE EXPECTING THE MIDNIGHT
SHIFT TO MAKE EVEN MORE CHANGES AS THIS COMPLEX SYSTEM CONTINUES
TO EVOLVE OVERNIGHT.  KEYES

&&

.PREV DISCUSSION... /ISSUED 120 PM MST MON DEC 21 2009/ 

SHORT TERM...TONIGHT THROUGH THURSDAY. NEXT PAC LOW POSITIONED 
OVER/NEAR VANCOUVER ISLAND THIS AFTERNOON EXPECTED TO DIG SSE 
THROUGH WASHINGTON/OREGON TONIGHT AND THEN INTO/THROUGH THE GREAT 
BASIN TUESDAY. THE WRN AND SRN ZONES SHOULD BARE THE BRUNT OF THE 
STORM AS IT PASSES JUST TO OUR WEST. THUS HAVE ISSUED A NUMBER OF 
HIGHLIGHTS (SEE BELOW) TO COVER THE EVENT. IF THE LOW GETS HUNG UP 
NEAR SRN UTAH TUESDAY AFTERNOON...WE MAY NEED TO UPGRADE CURRENT 
ADVISORIES IN THE BEAR LAKE REGION TO WARNINGS BUT WE WILL TAKE A 
SECOND LOOK AT THIS WITH FOLLOWING SHIFTS. SECONDARY LOW EXPECTED TO 
DIG SSE OUT OF ALBERTA AND ACROSS MONTANA WEDNESDAY NIGHT WHICH 
SHOULD HELP DRY OUT OUR WRN ZONES WHILE MAINTAINING WEAK UPSLOPE 
FLOW OVER THE ERN HIGHLANDS. UPPER RIDGE AMPLIFIES FURTHER ALONG THE 
COAST THURSDAY FOR DRIER CONDITIONS. TEMPERATURES SHOULD REMAIN MILD 
TONIGHT AHEAD OF APPROACHING SYSTEM AND THEN DROP SIGNIFICANTLY BY 
WEDNESDAY NIGHT WITH FRESH SNOW COVER AND CLEARING SKIES. HUSTON

LONG TERM...THURSDAY NIGHT THROUGH MONDAY. UPPER RIDGE BUILDS INTO 
THE GREAT BASIN AREA THROUGH THE WEEKEND...LEAVING EASTERN IDAHO 
UNDER DRY CONDITIONS.  UNSEASONABLY COLD TEMPERATURES EXPECTED INTO 
SUNDAY...WITH HIGHS PEAKING IN THE TEENS AND LOWS BELOW ZERO ON 
CHRISTMAS AND SATURDAY.  WARMING TREND WILL BE GRADUAL THROUGH THE 
PERIOD.  FOR THE LATER HALF OF THE PERIOD...ECMWF DEPICTS A MORE 
BLOCKING RIDGE THAN THE GFS AND DEFLECTS ALL SYSTEM NORTH OR SOUTH 
OF EASTERN IDAHO INTO EARLY NEXT WEEK.  THE GFS HOWEVER BREAKS DOWN 
THE RIDGE SUNDAY NIGHT AS A LARGE BUT SPLIT SYSTEM MOVES THROUGH THE 
AREA. HAVE KEPT ISOLATED POPS IN THE HIGHER ELEVATIONS WITH THIS 
POTENTIAL.  HANKO

AVIATION...VFR CONDITIONS EXPECTED THIS AFTERNOON...HOWEVER THE 
NEXT SYSTEM WILL BRING SNOW TO THE AREA BEGINNING THIS EVENING.  
SNOW SPREADING WEST TO EAST WILL DROP CEILINGS TO MVFR AT ONSET THEN 
LOWER TO IFR AFTER MIDNIGHT. LOW CONDITIONS WILL PERSIST THROUGH 
MUCH OF THE NIGHT WITH SNOW SHOWERS IN PLACE AND WILL BECOME LESS 
WIDESPREAD AFTER 18Z TUESDAY.

&&

.PIH WATCHES/WARNINGS/ADVISORIES...
WINTER WEATHER ADVISORY FROM 5 AM TUESDAY TO 5 AM MST WEDNESDAY 
FOR IDZ017-021-023>025.

WINTER WEATHER ADVISORY UNTIL 5 PM MST TUESDAY FOR IDZ032.

WINTER STORM WARNING FROM 5 AM TUESDAY TO 5 AM MST WEDNESDAY FOR 
IDZ022.

WINTER STORM WARNING UNTIL 5 PM MST TUESDAY FOR IDZ018-031.

&&

$$