633 FXUS63 KOAX 022108 AFDOAX AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE OMAHA/VALLEY NE 308 PM CST TUE DEC 2 2008 .SHORT TERM...TONIGHT THROUGH FRIDAY. SNOW CHANCES/AMOUNTS TONIGHT...THEN TEMPERATURES THROUGH FRIDAY...ARE THE PRIMARY SHORT TERM FORECAST CONCERNS. POTENT MID LEVEL SHORT WAVE WAS DROPPING INTO THE CENTRAL ROCKIES THIS AFTERNOON...LED BY 120M HEIGHT FALLS AS PER MORNING RAOB DATA. SIGNIFICANT DRYING WAS OCCURRING IN THE WAKE OF THIS WAVE OVER NORTHERN CALIFORNIA AS SHOWN ON WATER VAPOR LOOPS...AND PLUME OF MID LEVEL MOISTURE WAS BEGINNING TO SURGE NORTHEAST INTO COLORADO AHEAD OF THE SYSTEM. OTHERWISE NOT MUCH AVAILABLE MOISTURE WAS NOTED ON MORNING UPPER AIR CHARTS...CAUSING A BIT OF CONCERN FOR PRECIPITATION GENERATION ACROSS THE CWA TONIGHT. HOWEVER...APPARENT STRENGTH OF SYSTEM AS NOTED ON WATER VAPOR IMAGERY SUGGESTS SUFFICIENT MOISTURE WILL WORK INTO THE SYSTEM PRIOR TO ITS ARRIVAL IN OUR AREA. AT THE SURFACE...PRESSURES HAVE BEEN LOWERING THIS AFTERNOON IN THE HIGH PLAINS WITH SYSTEM APPROACHING. A COLD FRONT WAS SETTLING SOUTH TO THE NEBRASKA/SOUTH DAKOTA BORDER AS OF MID AFTERNOON...WITH SOUTHWEST FLOW AHEAD OF IT HELPING TO WARM TEMPS INTO THE 50S ACROSS MUCH OF THE CWA. HOWEVER 30S AIR WAS SURGING SOUTH THROUGH NORTHERN SOUTH DAKOTA...AND SHOULD ARRIVE IN OUR NORTHERN CWA BY MIDNIGHT. MODELS ARE FAIRLY SIMILAR IN TIMING OF SOUTHWARD MOVING COLD FRONT INTO AND THROUGH THE CWA BY 12Z. AND ATMOSPHERIC TEMPERATURE AND MOISTURE PROFILES ARE PRETTY CLOSE AS WELL. HOWEVER MODELS DIVERGE ON QPF PLACEMENT WITH THE GFS MOST ROBUST IN PAINTING A SWATH OF A TENTH TO A QUARTER INCH OF LIQUID ACROSS THE CWA...WHILE THE NAM KEEPS MOST OF ITS PRECIP ALONG OUR NORTHERN BORDER THROUGH MUCH OF THE NIGHT. THE NAM THEN PAINTS ANOTHER STRIPE FROM FAR SOUTHEAST NEBRASKA TO THE NORTHEAST WEDNESDAY MORNING. BOTH MODELS SUPPORT A TIGHT BAROCLINIC ZONE DEVELOPING THIS EVENING ACROSS SOUTHEAST SOUTH DAKOTA INTO NORTHEAST NEBRASKA. SO EXPECT PRECIPITATION TO GET GOING THERE BY LATE EVENING AND CONTINUE THROUGH EARLY MORNING. FOCUS THEN SHIFTS SOUTHEAST WHERE FRONTOGENETICAL FORCING IN THE 750-600MB LAYER IS MAXIMIZED. NAM AND GFS ARE SIMILAR WITH THIS DEPICTION...BUT THE NAM DOES NOT PRODUCE QPF BETWEEN 06Z AND 12Z DESPITE SIMILAR RELATIVE HUMIDITY PROGS. GIVEN THEIR SIMILAR RH AND FRONTOGENESIS PROGS...HAVE GONE MORE CLOSELY WITH THE GFS QPF SHOWING A BAND OF PRECIPITATION SOUTHEAST OF THE INTERSTATE 80 CORRIDOR LATE TONIGHT AND EARLY WEDNESDAY. A QUICK LOOK AT THE INCOMING 18Z NAM LENDS A BIT MORE SUPPORT TO THIS SOLUTION AS IT PAINTS A SIMILAR STRIPE OF PRECIP FROM SOUTHEAST NEBRASKA TROUGH CENTRAL IOWA. USING A COMBINATION OF COBB AND GARCIA METHODS RUN ON THE GFS MODEL OUTPUT YIELDS A BROAD AREA OF HALF TO ONE INCH SNOW TOTALS ACROSS THE CWA...WITH HIGHER AMOUNTS NEAR 2 INCHES ALONG THE SOUTH DAKOTA BORDER AND JUST SOUTHEAST OF THE OMAHA METRO AREA AND INTO SOUTHWEST IOWA. BELOW ADVISORY CRITERIA...BUT WILL HIT THE HAZARDOUS WEATHER OUTLOOK WITH AMOUNTS AND TIMING...AS WELL AS A CAUTION FOR THE FIRST SIGNIFICANT SNOW OF THE YEAR FOR MOST AREAS. THE SNOW SHOULD END BY NOON ACROSS MOST OF THE CWA...BUT FLURRIES WILL STILL BE POSSIBLE THROUGH THE AFTERNOON UNDER COLD ADVECTION AND SLIGHTLY CYCLONIC FLOW ALOFT. TEMPERATURES SHOULD NOT REBOUND MUCH FROM THE MIDDLE AND UPPER 20S...SO HAVE HIGHS A FEW DEGREES EITHER SIDE OF 30. WE SHOULD STAY IN A COLD PERIOD THROUGH THE END OF THE WEEK AS NORTHWEST FLOW ALOFT BRINGS A COUPLE MORE REINFORCING SHOTS OF COLD AIR TO THE PLAINS. COULD SEE SOME LIGHT SNOW IN OUR NORTHEAST ON THURSDAY AS ONE OF THOSE SYSTEMS DIVES THROUGH THE NORTHERN PLAINS. OTHERWISE AM EXPECTING LOWS IN THE TEENS AND HIGHS IN THE 20S OR 30S THROUGH FRIDAY...ESPECIALLY WHERE SNOW COVER WILL PERSIST. .LONG TERM...FRIDAY NIGHT THROUGH TUESDAY. MEDIUM RANGE MODELS OVERALL DEPICTING NORTHWEST FLOW PREVAILING OVER THE CENTRAL CONUS THROUGH THE PERIOD WITH A SERIES OF SHORTWAVE TROFS AND ATTENDANT COLD FRONTS MOVING ACROSS THE AREA. FIRST WAVE OF CONCERN DIVES THROUGH THE UPPER MIDWEST FRIDAY NIGHT/SATURDAY TIME FRAME WITH COLD FRONT PASSAGE THRU THE CWA SATURDAY MORNING. NIL MOISTURE PROGGED TO TAP INTO THUS DRY FROPA ANTICIPATED. THE ECM IS NOTICEABLY MORE AGGRESSIVE SWEEPING THIS FIRST COLD AIRMASS SWD THRU THE UPPER MIDWEST/GREAT LAKES REGION...AS WELL AS THE REINFORCING SHOT COMING EARLY NEXT WEEK. AT THAT TIME...DPVA/ISENTROPIC UPGLIDE AND AREA OF 700MB-600MB SLANTWISE INSTABILITY ALONG WITH SUBTLE MOISTURE RETURN SUNDAY NIGHT THROUGH MONDAY SUGGESTS SMALL CHANCE FOR LIGHT SNOW REASONABLE WITH THIS SECOND WAVE AND ASSOCIATED FROPA. OTHERWISE...APPEARS BRUNT OF COLD AIRMASSES WILL REMAIN EAST OF THE CWA WITH ONLY WEAK THERMAL ADVECTION ANTICIPATED WITH THE FROPAS. && .AVIATION... FOR TAF SITES KOFK/KOMA/KLNK THROUGH 03/18Z. VFR CONDITIONS EXPECTED AS A WARM FRONT THEN A COLD FRONT PUSH THRU THE AREA THROUGH 06Z. BEHIND THE FRONT...MVFR CIGS DEVELOP ALONG WITH PRECIP. INITIALLY... -DZ/-RA MAY DEVELOP HOWEVER...AS THE COLD AIR MOVES IN THE PRECIP SHOULD CHANGE OVER TO SNOW. HAVE PRECIP STARTING AT KOFK BTWN 05-07Z AND KOMA/KLNK BTWN 08Z-10Z. THE -SN SHOULD DIMINISH TO FLURRIES LATER IN THE MORNING. THE PRECIPITATION WILL BE ACCOMPANIED BY GUSTY NW WINDS FM 15 TO 30KT. && .OAX WATCHES/WARNINGS/ADVISORIES... NE...NONE. IA...NONE. && $$ SHORT TERM...DERGAN LONG TERM...DEE AVIATION...ZAPOTOCNY