FXUS65 KBOU 281048
AFDBOU
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE DENVER CO
345 AM MST THU DEC 28 2006
.SHORT TERM...LOTS OF UNCERTAINTY IN THE DETAILS...BUT A LARGE STORM
TO OUR SOUTH WILL BRING SIGNIFICANT SNOWFALL OVER THE NEXT FEW DAYS.
DESPITE BROADLY SIMILAR PATTERNS...NAM HAS BEEN FLIP-FLOPPING WITH
DETAILS AND SNOW AMOUNTS...WHILE GFS HAS BEEN MORE CONSISTENT AND
MATCHES THE ECMWF FAIRLY WELL. THE GENERAL PATTERN IS AGREED ON...
WITH WARM ADVECTION AHEAD OF THE LOW PRODUCING CYCLOGENESIS SOUTH OF
US AND WRAPPING WARM MOIST AIR AROUND THE LOW. COMBINATION OF WARM
ADVECTION WRAPPING NORTH OF THE LOW AND THE SECOND JET STREAK
DROPPING THROUGH THE SOUTH SIDE STALL THE LOW TO OUR SOUTH AND
POSSIBLY ELONGATE IT NORTH-SOUTH...THEN THE DEEPENING LOW PUSHES A
POSSIBLY STRONGER SURGE OF MOISTURE AROUND THE LOW AND STRENGTHENS
THE SURFACE DEVELOPMENT STARTING LATE FRIDAY. DECENT AGREEMENT ON
THE PATTERN THROUGH TONIGHT...THOUGH STILL DIFFERENT RESULTS IN SNOW
AMOUNTS...MAINLY HAVING TO DO WITH HOW STRONG THE LOW/MID LEVEL FLOW
GETS THIS AFTERNOON AND THEN HOW MUCH IT DECREASES LATER TONIGHT.
18Z/00Z NAM RUNS WENT NUTS...PRESUMABLY WITH SOME CONVECTION AND
PRODUCED 18-36 INCHES IN AND NEAR THE FOOTHILLS BY FRIDAY MORNING.
GFS SUGGESTING ABOUT HALF THAT...AND OROGRAPHIC SNOW MODEL PRODUCES
ABOUT HALF THAT MUCH BUT THERE WILL BE DYNAMIC LIFT AND POSSIBLY
SOME CONVECTION THIS AFTERNOON...THEN IT SHOULD BE MORE STABLE AFTER
THAT. 06Z NAM BRINGS THE LOW AS FAR EAST AS WESTERN OKLAHOMA AND
THIS LOOKS TOO FAR EAST...IT GIVES 4 INCHES TONIGHT AND THEN IS
DONE. WILL TREND TOWARD THE MIDDLE OF THE ROAD GFS AND BACK OFF
AMOUNTS A BIT TONIGHT ALONG THE FRONT RANGE. HOWEVER...THERE SHOULD
BE GOOD LIFT ON THE PLAINS AND RAISED AMOUNTS THERE A BIT. CONSENSUS
MINUS THE LATEST NAM GIVES AROUND 6 INCHES ON THE PLAINS TONIGHT.
THERE WILL BE SOME WIND...PROBABLY A BIT MORE THAN MOS NUMBERS...
SOME DRIFTING BUT NOT EXPECTING BLIZZARD CONDITIONS UNTIL LATER.
TIMING OF ALL THIS NOT REAL CLEAR EITHER BUT CURRENT PLAN OF UPSLOPE
WINDS STRENGTHENING BY MIDDAY WITH A FEW HOURS TO ACHIEVE SATURATION
STILL LOOKS OK. RAISED LOWS TONIGHT EXPECTING A WARMER/MOISTER
ENVIRONMENT.
.LONG TERM...ON FRIDAY THE NAM HAS THE SOUTHERN UPPER CLOSED LOW
FURTHER NORTH THAN THE GFS...WHICH IS A BIT NORTH OF THE ECMWF
CLOSED LOW POSITION. BY FRIDAY NIGHT...THEY COME IN LINE BETTER
WITH EACH OTHER PUTTING THE CENTER OVER TEXAS-OKLAHOMA PANHANDLES
/NORTHEAST CORNER OF NEW MEXICO AREA. ON FRIDAY NIGHT INTO SATURDAY
THE NAM SCOOTS THE NORTHERN FEATURE DOWN INTO THE SOUTH ONE BY 18Z.
THE GFS DOES NOT...IT KIND OF ATROPHIES IT AWAY BY SUNDAY MORNING.
BOTH MODELS LINGER THE MAIN SOUTHERN CLOSED FEATURE THROUGH SUNDAY
MORNING...THE NAM MOVING IT AWAY AT A SLOWER PACE. THE ECMWF...NAM
...AND GFS SHOW THE CENTER OF THE 500 MB HEIGHT FEATURE WITHIN ABOUT
175 MILES OF EACH OTHER IN THE TEXAS-OKLAHOMA PANHANDLES/SOUTHWEST
KANSAS AREA AT 00Z SUNDAY. THAT'S PRETTY CLOSE FOR 72 HOURS OUT.
AGAIN...IT IS A CUT OFF SYSTEM AND THAT 72 HOURS IS SIX PERIODS
AWAY.
THE CURRENT FORECAST...INCLUDING THE HIGHLIGHTS OR LACK THERE OF...
IS RATHER TROUBLED FOR FRIDAY. FOR A FEW RUNS NOW...NEARLY ALL THE
MODELS HAVE HAD TWO PIECES TO THIS UPPER TROUGH. IN DEFENSE OF
HIGH SNOWFALL POPS FOR FRIDAY...THERE IS FAIRLY DECENT UPSLOPE ON
BOTH MODELS...DEEPER ON THE GFS. THERE IS VERY DEEP MOISTURE ON THE
GFS...CONSIDERABLY LESS ON THE 00Z/06Z NAM (BUT SOME). BOTH HAVE
WEAK TO MODERATE UPWARD SYNOPTIC SCALE FORCING/MOTION ON FRIDAY/
FRIDAY EVENING. PERHAPS BEING IN BETWEEN THE TWO PIECES OF
ENERGY...I WILL DECREASE SNOWFALL AMOUNTS AND POPS A BIT FRIDAY.
BUT IT SHOULD STILL SNOW. THE FIRST BATCH OF UPSLOPE TONIGHT INTO
FRIDAY COMES MAINLY FROM ANTICYCLOGENESIS...SATURDAY/SATURDAY
NIGHT'S COMES MORE FROM CYCLOGENESIS. FRIDAY AND FRIDAY EVENING ON
THE GFS SHOW'S WARNING CRITERIA SNOW...THE NAM HAS ADVISORY SNOW AT
BEST. MAYBE A WATCH IS BEST FOR FRIDAY?
FROM FRIDAY NIGHT MIDNIGHT THROUGH DAWN SUNDAY...THE MAIN PARAMETERS
POINTING TO A SIGNIFICANT SNOWFALL INCLUDE...A DEEP UPSLOPE
CIRCULATION COMPLETE WITH DEEP MOISTURE...SOME MID LEVEL WARMING
...CROSS SECTIONS POINTING TO A BARRIER JET SET-UP...AND STRONG
UPWARD MOTION ON THE QG VERTICAL VELOCITY FIELDS. THE NAM AND GFS
HAVE ALL OF THIS. THE ECMWF DEFINITELY HAS THE UPPER FEATURE IN THE
PROPER SPOT FOR ALL OF THIS AS WELL. THE BOUNDARY LAYER FLOW ALSO
POINTS TO BLIZZARD CONDITIONS FOR NEARLY ALL THE PLAINS FROM
MIDNIGHT FRIDAY NIGHT INTO EARLY SUNDAY MORNING FOR SURE. SOME
PROBLEMS WITH IT INCLUDE DOWNSLOPE DRYING ON THE NAM SATURDAY
AFTERNOON AS WELL AS SIGNIFICANT DIFFERENCES BETWEEN QPF/SNOWFALL
FIELDS BETWEEN THE TWO MODELS FOR MANY OF THE PERIODS.
THE DIFFICULTY OF DISCERNING THE WEATHER FROM THIS PATTERN ITSELF IS
GREAT. NOW CONSIDERING A BIG SNOWFALL COMING TO FRUITION FOR
PERIODS FOUR...FIVE...AND SIX ONLY A WEEK AND HALF AFTER A LARGE
STORM IS YET MORE DIFFICULT. SO WILL GO FOR LONG...SEVERAL PERIOD
WATCH FOR MOST AREAS EAST OF THE DIVIDE MAINLY TO POINT TO THOSE
INTERESTED THAT ANOTHER SIGNIFICANT SNOWFALL IS QUITE POSSIBLE. IT
WOULDN'T SURPRISE ME AT ALL IF THE VARIOUS MODELS WOULD SLOW DOWN
THIS CLOSED/CUT-OFF SYSTEM EVEN FURTHER. IT ALSO NEEDS TO BE
MENTIONED THAT CLIMATOLOGICALLY TWO SUCH EXTREME SNOWFALLS FOR US
ONE RIGHT AFTER ANOTHER ARE VERY RARE.
TEMPERATURES LOOK 4-6 C COOLER FOR HIGHS FRIDAY...THEN 1-2 C WARMER
FOR SATURDAY. FOR THE LATER DAYS...SUNDAY THROUGH WEDNESDAY...MAY
CHANGE SUNDAY SOMEWHAT A BIT WITH THE DEPARTING STORM IN MIND...THE
REST WILL STAY AS IS.
&&
.BOU WATCHES/WARNINGS/ADVISORIES...WINTER STORM WARNING THIS
AFTERNOON AND TONIGHT FOR THE FRONT RANGE MOUNTAINS...
FOOTHILLS...AND PLAINS...ZONES 33>36..38>51.
SNOW ADVISORY THIS AFTERNOON AND TONIGHT FOR SOUTH PARK...
ZONE 37.
WINTER STORM WATCH FRIDAY THROUGH SATURDAY NIGHT FOR ZONES
33>36..38>51.
&&
$$
GIMMESTAD/RJK
FXUS65 KBOU 282135
AFDBOU
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE DENVER CO
235 PM MST THU DEC 28 2006
.SHORT TERM...FOR TNGT...THE UPPER TROF WHICH IS OVER ECNTRL
AZ/WCNTRL NEW MX...WL MOVE SLOWLY ENE THRU 12Z FRIDAY...LOCATED OVER
CNTRL NEW MX AT THAT TIME. ALTHOUGH THE CIRCULATION ASSOCIATED WITH
THIS SYSTEM IS FURTHER S THAN PRVS MDL RUNS...THERE WILL BE PLENTY
OF QG OMEGA AND DEEP UPSLOPE TO WORK WITH IN AND NEAR THE FOOTHILLS
AND PALMER DIVIDE. SELY FLOW THIS AFTN HAS BENEFITED ZONE 37 THIS
AFTN WITH 6 INCHES ALREADY. WITH THIS IN MIND...HAVE DECIDED TO
UPGRADE THE ADVISORY TO A WINTER STORM WARNING THERE TNGT. SPATIAL
AND TIME CROSS-SECTIONS HOLD ONTO DEEP MSTR THROUGH 12Z. MAIN
CONCERN TNGT WL BE THE NERN PLAINS....WHERE THERE IS LESS CONFIDENCE
CONCERNING THE WINTER STORM WARNING...BUT WITH STRONG DYNAMICS IN
PLACE WL CONTINUE THE THE WARNING TNGT. MARGINAL ZONES INCLUDE
48..50 AND 51...WITH THE MOST FAVORED AREAS EXPECTED TO BE ALONG THE
PALMER DIVIDE. THERE IS STILL PLENTY OF UNCERTAINTY TO CONTEND WITH
ON FRIDAY. THE MDLS SHIFT THE STRONGEST DYNAMICS/OMEGA E INTO WRN
KS...BUT STILL SOME STRONG NLY WINDS AND MSTR IN PLACE THRU 00Z
SATURDAY. MOST FAVORED AREAS WILL AGAIN BE ALONG THE PALMER
DIVIDE. WL HOLD TO WINTER STORM WATCHES IN PLACE FOR FRIDAY AND LET
LATER SHIFTS DECIDE ON WHETHER TO EXTEND/DOWNGRADE OR DELAY THE
WINTER STORM WATCHES.
.LONG TERM...RATHER COMPLEX WEATHER SCENARIO SHAPING UP ACROSS THE
FORECAST AREA THRU THE WEEKEND AS THE BIG STORM SYSTEM COMES THRU IN
SEVERAL PIECES. FOR THE WEEKEND FORECAST...OVERALL STORM TRACK HAS A
GENERAL CONSENSUS OF TRACKING THE STORM SLOWLY EASTWARD INTO THE
EASTERN NEW MEXICO BEFORE LIFTING IT NORTHEAST INTO CENTRAL KS ON
SUNDAY. THE DETAILS BETWEEN THE MODELS ARE SIGNIFICANT. THE MAIN
DIFFERENCES IS THE HANDLING OF THE NRN BRANCH TROF AND HOW IT
EFFECTS THE BIG SW LOW. BOTH THE GFS/WRF INFILTRATE THE NRN BRANCH
TROF INTO THE MAIN LOW WHICH BRINGS IN DRIER AIR ON THE WEST SIDE OF
THE LOW AND PUSHES IT FURTHER EAST QUICKER. THE GFS IS THE MOST
EXTREME IN THE DRIER AIR ENTRAINMENT AND APPEARS THIS IS OVERDONE.
ON THE OTHER SIDE....THE UKMET AND EUROPEAN MODELS KEEP THE NRN
BRANCH TROF MORE SEPERATED FROM THE FLOW. THIS WILL ALLOW FOR THE
MSTR TO STAY IN PLACE AND KEEP THE LOW MOVING MORE SLOWER. AT THIS
TIME...HAVE A PREFERENCE FOR THE EUROPEAN/UKMET SOLUTIONS AND CONCUR
WITH HPC THINKING. IF THE SLOWER SOLUTIONS PAN OUT...THE HEAVY SNOW
COULD EXTEND WELL INTO SUNDAY. THERE STILL COULD BE A LULL IN THE
SNOW ON FRI NIGHT AND SAT BEFORE THE CYCLONE INTENSIFIES LATE SAT
AND SUNDAY. MODELS STILL MAXING THE SNOWFALL OVER THE FRONT RANGE
FOOTHILLS AND ACROSS THE PALMER DIVIDE WITH UPWARDS OF 30 INCHES BY
LATE SUNDAY. THIS DOES NOT SEEM OUT OF THE QUESTION GIVEN ABOVE
PARAMETERS.
CONSENSUS AMONST FORECASTERS HERE IS TO KEEP THE WINTER STORM WATCH
GOING THRU SATURDAY AND EXTEND THE WATCH THRU SUN AFTN AS THE CYCONE
INTENSIFIES IN KANSAS. UNDERSTANDLY THIS A VERY LONG WATCH BUT WITH
UNCERTAINTIES FEELS THIS IS THE BEST COURSE FOR NOW. LATER SHIFTS
CAN DELAY THE WATCH AS NECESSARY AS THE SITUATION BECOMES A BIT MORE
CLEAR.
&&
.BOU WATCHES/WARNINGS/ADVISORIES...WINTER STORM WARNING THROUGH
TONIGHT FOR THE FRONT RANGE MOUNTAINS...FOOTHILLS...SOUTH PARK
AND PLAINS...ZONES 33>37..38>51.
WINTER STORM WATCH FRIDAY THROUGH SATURDAY NIGHT FOR ZONES
33>36.
WINTER STORM WATCH FRIDAY THROUGH SUNDAY FOR ZONES..38>51.
&&
$$
FMC/SWE