AREA FORECAST DISCUSSION

 
 FXUS65 KTWC 092102
 AFDTWC
 
 AREA FORECAST DISCUSSION
 NATIONAL WEATHER SERVICE TUCSON AZ 
 202 PM MST TUE FEB 9 2010
 
 .SYNOPSIS...ANOTHER WINTER STORM WILL BRING VALLEY RAIN AND MOUNTAIN 
 SNOW BY LATE WEDNESDAY INTO THURSDAY. THIS NEXT STORM WILL BE COLDER 
 THAN RECENT ONES...SO THE SNOW LEVEL WILL BE MUCH LOWER WEDNESDAY 
 NIGHT AND EARLY THURSDAY WITH SIGNIFICANT ACCUMULATIONS POSSIBLE 
 ABOVE 4000 FEET OR EVEN LOWER. DRY AND WARMER FRIDAY INTO THE 
 WEEKEND UNDER HIGH PRESSURE.
 
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 .DISCUSSION...WATER VAPOR IMAGERY SHOWS THE STRONG LOW PRESSURE 
 SYSTEM SPINNING JUST OFFSHORE OF CENTRAL CALIFORNIA WITH IR AND 
 VISIBLE SATELLITE IMAGERY SHOWING PLENTY OF CLOUDINESS AHEAD OF THE 
 LOW OVER CALIFORNIA...NEVADA AND DEVELOPING RAPIDLY ACROSS UTAH AND 
 ARIZONA. MEANWHILE...A LARGE PLUME OF MOISTURE OVER THE SOUTHEASTERN 
 PACIFIC IS STREAMING NORTHEAST ACROSS SRN BAJA AND INTO NORTHERN 
 MEXICO. AS THE EVENING PROGRESSES...EXPECTING CLOUDINESS TO CONTINUE 
 TO FILL IN ACROSS THE DESERT SOUTHWEST AS THE LOW MOVES SOUTH ALONG 
 THE CALIFORNIA COAST TOWARD THE SAN DIEGO AREA BY EARLY WEDNESDAY 
 MORNING. 
 
 THE MODELS SHOW A BAND OF SHOWERS MAINLY ALONG THE EASTERN PERIPHERY 
 OF THE LOW EARLY WEDNESDAY ALONG THE COLORADO RIVER BASIN. THE LOW 
 IS PROGGED TO BE OVER SOUTHWESTERN ARIZONA OR NORTHWESTERN SONORA BY 
 MID TO LATE AFTERNOON ON WEDNESDAY. AS A RESULT...EXPECT SHOWERS TO 
 INCREASE FROM WEST TO EAST DURING THE DAY. FOR NOW THE MODELS SHOW A 
 BIT OF DISAGREEMENT WITH REGARD TO QPF VALUES AND THUS SNOW AMOUNTS. 
 HOWEVER...ALL OF THE MODELS ARE SHOWING A STRONG JET AT AROUND 
 200-300MB...GENERALLY GREATER THAN 150 KNOTS DEVELOPING ACROSS THE 
 SE CORNER OF ARIZONA...THUS SRN PORTIONS OF ARIZONA WILL BE UNDER 
 THE LEFT FRONT QUADRANT (LEFT EXIT REGION) OF THE JET...SO DYNAMICS 
 WITH THIS SYSTEM LOOK QUITE GOOD. GFS MODEL CROSS-SECTIONS ARE 
 SHOWING OMEGA VALUES OF 15 TO 20 MICROBARS PER SECOND BY LATE 
 WEDNESDAY AFTERNOON OVER MAINLY CENTRAL PORTIONS OF THE FORECAST 
 AREA. THESE VALUES SHIFT EAST DURING THE EVENING AND OVERNIGHT 
 HOURS. 
 
 I KEPT THE INHERITED QPF/SNOW AMOUNT FORECAST FROM THE NIGHT SHIFT. I 
 ALSO INHERITED A WINTER STORM WATCH FOR ALL OF THE HIGHER ELEVATIONS 
 OF SOUTHEAST ARIZONA FOR WEDNESDAY AFTERNOON THROUGH LATE THURSDAY 
 MORNING. WITH SOME CONTINUED UNCERTAINTY WITH REGARD TO THE 
 TIMING...POSITION AND STRENGTH OF THIS SYSTEM I DECIDED TO HOLD OFF 
 ON ANY UPGRADES TO THE WATCH...SINCE TRYING TO FINE TUNE AREAS THAT 
 MAY NEED A WARNING ARE STILL A BIT DIFFICULT AND ALL DEPEND ON THE 
 TRACK OF THE STORM AS WELL AS HOW MUCH OF THE TROPICAL MOISTURE THE 
 LOW IS ABLE TO TAP INTO AND PULL NORTHWARD INTO ARIZONA.    
 
 AS THE LOW MOVES OVER SRN ARIZONA AND NORTHERN SONORA THE MODELS 
 PROG THIS SYSTEM TO BE COLD WITH 500MB TEMPS OF AROUND -24 TO -26 
 DEGS CELSIUS. THEREFORE...I INCLUDED A MENTION OF A SLIGHT CHANCE OF 
 THUNDERSTORMS TO ACCOMPANY THE SHOWERS...MAINLY FOR WEDNESDAY 
 AFTERNOON AND THROUGHOUT THE OVERNIGHT HOURS. SNOW LEVELS ARE 
 EXPECTED TO BE RATHER LOW WITH SNOW LEVELS GENERALLY AROUND 
 3500-4500 FEET DURING THE OVERNIGHT HOURS WEDNESDAY NIGHT INTO 
 THURSDAY. THIS COULD ALSO CHANGE DEPENDING ON WHAT SUBSEQUENT MODEL 
 RUNS HAVE TO SHOW WITH REGARD TO SNOW LEVELS.    
 
 THE LOW SHOULD QUICKLY KICK TO THE EAST THURSDAY WITH PRECIPITATION 
 CHANCES DECREASING CONSIDERABLY DURING THE DAY. FOR THE REMAINDER OF 
 FORECAST PERIOD EXPECTING RIDGING TO GENERALLY INFLUENCE THE WEATHER 
 ACROSS THE DESERT SOUTHWEST WITH SOME SYSTEMS DROPPING OVER THE TOP 
 OF THE RIDGE AND PERHAPS CLIPPING THE EASTERN SECTIONS OF THE 
 FORECAST AREA FROM TIME TO TIME. HOWEVER...FOR NOW I KEPT A DRY 
 FORECAST GOING.
 
 &&
 
 .AVIATION...INCREASING HIGH CLOUDS FROM THE SOUTHWEST TODAY. SURFACE 
 WINDS ARE EXPECTED TO BE LESS THAN 15 KTS AND ADHERE TO TYPICAL 
 DIURNAL PATTERNS. HIGH PRESSURE WILL GIVE WAY TO A STRONG STORM 
 MOVING IN FROM THE WEST BY LATE WEDNESDAY WITH LOWERING CEILINGS AND 
 INCREASING CHANCES FOR PRECIPITATION THROUGH THE DAY ON WEDNESDAY. 
 AVIATION DISCUSSION NOT UPDATED FOR TAF AMENDMENTS.
 
 &&
 
 .FIRE WEATHER...NO FIRE WEATHER CONCERNS AT THIS TIME.
 
 &&
 
 .TWC WATCHES/WARNINGS/ADVISORIES...WINTER STORM WATCH FROM 3 PM MST  
      WEDNESDAY THROUGH 11 AM MST THURSDAY ABOVE 4000 FEET FOR        
      AZZ019-029-030-032>035.
     
 

 
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