Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 211040
AFDBOU

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE DENVER/BOULDER CO
340 AM MST FRI NOV 21 2014

.SHORT TERM...(TODAY THROUGH TONIGHT )
ISSUED AT 339 AM MST FRI NOV 21 2014

WEAK UPPER LEVEL TROUGH WILL MOVE THROUGH THIS MORNING INCREASING
CLOUDS OVER THE MOUNTAINS. MOISTURE OVER THE MOUNTAINS IS WEAK
AND SHORT LIVED THIS MORNING...HOWEVER A SLIGHT WIND CHANGE TO A
MORE NW DIRECTION BETWEEN 12 AND 15Z FOR PORTIONS OF THE HIGHER
NORTHERN MOUNTAINS COULD BRING A SLIGHT CHANCE OF SNOWFALL. IF
SNOW DOES FALL AMOUNTS WILL BE VERY LIGHT. ELSEWHERE CEILINGS WILL
STAY OVER THE EXTREME NORTHEAST CORNER FOR MOST OF THE DAY AND
SCATTER OUT CLOSER TO THE FOOTHILLS. THIS WILL ALLOW TEMPERATURES
TO GET SLIGHTLY WARMER THAN YESTERDAY WITH HIGHS IN THE LOWER TO
MID 50S FOR THE PLAINS AND 40S FOR THE MOUNTAINS.

FOR THIS EVENING...A STRENGTHENING SURFACE HIGH PRESSURE OVER THE
WEST AND LEE TROUGH OVER THE EAST WILL ALLOW MOUNTAIN WAVE CLOUDS
TO DEVELOP. THIS WILL KEEP LOWS SLIGHTLY WARMER WHILE STAYING
PARTLY CLOUDY OUT ON THE PLAINS WITH LOWS IN THE MID 20S.

.LONG TERM...(SATURDAY THROUGH THURSDAY)
ISSUED AT 339 AM MST FRI NOV 21 2014

MAIN CHALLENGES CONTINUE TO SURROUND STRONG WEST/NORTHWEST FLOW
ALOFT DEVELOPING ACROSS THE STATE. THIS WILL RESULT IN A LENGTHY
MOUNTAIN SNOW/BLOWING SNOW EVENT AND STRONG GUSTY WINDS OVER MUCH
OF THE FORECAST AREA.

UNTIL THEN...SATURDAY WILL START OFF RATHER MILD. INCREASING
WESTERLY FLOW AND DOWNSLOPE COMPONENT WILL COMBINE WITH WARM
ADVECTION TO BRING ONE MORE ABOVE NORMAL TEMPERATURE DAY TO THE
FORECAST AREA. HOWEVER...THE INCREASE IN CLOUDS THROUGHOUT THE DAY
COULD LIMIT HEATING A BIT MAINLY IN VALLEY LOCATIONS TOWARD
GREELEY AND FORT COLLINS.

BY LATE SATURDAY AFTERNOON...THE PACIFIC MOISTURE PLUME IS
EXPECTED TO REACH THE FORECAST AREA ON THE NOSE OF A STRONG 150+
KT UPPER LEVEL JET. THERE IS DECENT Q-G LIFT PROGGED ON THE NOSE
OF THIS FEATURE AS WELL...SO THE COMBINATION OF UPPER LEVEL
DIVERGENCE...LARGE SCALE LIFT...AND INCREASE MOISTURE WILL BRING
WIDESPREAD SNOW TO THE MOUNTAINS BY LATE SATURDAY AFTERNOON/
EVENING. FLOW IS STILL NOT TERRIBLY FAVORABLE FOR HEAVY SNOW OVER
THE FRONT RANGE/SUMMIT COUNTY AREA HOWEVER UNTIL LATER IN THE
EVENING AND OVERNIGHT AS FLOW EVENTUALLY TURNS NORTHWEST AND
STRONGER COLD ADVECTION ARRIVES. WE DO ANTICIPATE A DECREASE IN
INTENSITY BY SUNDAY MORNING AS SOMEWHAT DRIER AIR ADVECTS INTO
THE REGION FROM THE NORTHWEST...BUT FAVORABLE OROGRAPHIC COMPONENT
SHOULD KEEP SOME ACCUMULATING SNOW GOING. ON THE PLAINS...HAVE
ALSO INTRODUCED LOW POPS SATURDAY NIGHT GIVEN A STRONGER Q-G
SIGNAL NOW SEEN IN THE LATEST MODEL RUNS AND MOIST ADIABATIC LAPSE
RATES JUST ABOVE THE DRIER BOUNDARY LAYER. IN ADDITION...THERE
COULD BE WEAK ANTICYCLONIC TURNING SO DOWNSLOPE DRYING COULD BE
LIMITED ESPECIALLY OVER THE PALMER DIVIDE AREA. SOME POTENTIAL FOR
LIGHT ACCUMULATIONS THERE GIVEN NEUTRAL STABILITY PROFILE.

ANOTHER STRONGER WAVE IN NORTHWEST FLOW ALOFT WILL LIKELY BE
ARRIVING SOMETIME LATE SUNDAY NIGHT OR MONDAY. THIS WOULD BRING
POTENTIAL FOR MORE LOCALLY HEAVY SNOWFALL TO THE HIGH COUNTRY.
LAPSE RATES ARE QUITE HIGH WITH EACH OF THESE PASSING DISTURBANCES
ON THE ORDER OF 6.5 TO 7.5 C/KM. CROSS SECTIONS ALSO SHOW RATHER
STRONG VERTICAL MOTION THROUGH DENDRITIC GROWTH LAYER SO EFFICIENT
GROWTH PROCESS WOULD RESULT IN POTENTIAL FOR SIGNIFICANT
ACCUMULATIONS. HARD TO PIN POINT TIME OF HEAVIEST SNOWFALL
CONSIDERING VARYING LAPSE RATES/WIND DIRECTIONS WITH EACH PASSING
WAVE...BUT AN EXTENDED PERIOD OF ACCUMULATING SNOWFALL IS LIKELY.
STRONG WINDS WILL ALSO OCCUR THROUGH MOST OF THIS PERIOD AND
COMBINE TO PRODUCE A HIGHER IMPACT WEATHER EVENT. AT THIS
TIME...OPTING FOR A LONG DURATION WINTER STORM WATCH FOR THE HIGH
ELEVATIONS SATURDAY EVENING THROUGH LATE MONDAY AFTERNOON.
GIVEN DURATION AND ABOVE MENTIONED PARAMETERS...POTENTIAL STORM
TOTALS OF 8-18 INCHES ABOVE 9000 FEET WITH LOCALLY HIGHER
AMOUNTS.

IF THE MONDAY WAVE IS STRONG ENOUGH...THERE COULD BE ANOTHER
THREAT OF A FEW SNOW SHOWERS ON THE PLAINS. OTHERWISE GUSTY
NORTHWEST WINDS AND COLDER TEMPERATURES WILL PREVAIL SUNDAY
THROUGH MONDAY IN THIS WEATHER PATTERN.

BY TUESDAY...MODELS SEEM TO BE MORE IN AGREEMENT WITH WARM
ADVECTION BEGINNING. THIS WOULD LEAD TO LESS MOUNTAIN SNOWFALL BUT
ALSO INCREASE THE THREAT OF A FRONT RANGE HIGH WIND EVENT. CROSS
SECTIONS SHOW VERY STRONG CROSS MOUNTAIN COMPONENT OF 50-70 KNOTS
AND WARM ADVECTION WOULD RESULT IN FAVORABLE MOUNTAIN WAVE SETUP
WITH MOUNTAIN TOP STABLE LAYER. LARGE SCALE SUBSIDENCE AND STRONG
CROSS MOUNTAIN PRESSURE GRADIENT WOULD ALSO BE FAVORABLE FOR HIGH
WINDS. STILL A LONG WAY OUT BUT LOOKING LIKE A VERY WINDY SETUP
FOR TUESDAY INTO TUESDAY NIGHT ALONG THE FRONT RANGE.

AFTER THAT...EYES TURN TOWARD THANKSGIVING WEATHER. LATEST ECMWF
HAS TAKEN A RATHER DRAMATIC TURN TO RIDGING VERSUS ITS EARLIER
TROUGHINESS WHILE GFS IS TRENDING THE OTHER WAY TO OLDER ECMWF
RUNS WITH A TROUGH AND COLD FRONT APPROACHING FROM THE NORTH. AS A
RESULT...CONFIDENCE OF ANY ONE SOLUTION IS RATHER LOW RIGHT NOW
CONSIDERING THE INTER AND INTRA MODEL VARIABILITY. AT THIS TIME
WILL REFRAIN FROM MAKING BIG CHANGES AND KEEP COOLER TEMPERATURES
AND A SLIGHT CHANCE OF SNOW IN THE FORECAST FOR THANKSGIVING.

&&

.AVIATION...(FOR THE TAFS THROUGH 12Z SATURDAY MORNING)
ISSUED AT 339 AM MST FRI NOV 21 2014

VFR CONDITIONS ARE EXPECTED FOR THE NEXT 24 HOURS WITH DRAINAGE
WINDS CONTINUING INTO THE AFTERNOON BEFORE IT CHANGES TO A MORE
SOUTHEAST DIRECTION. CEILINGS WILL SCATTER OUT EARLY THIS MORNING WITH
JUST AN UPPER LEVEL CIRRUS DECK THIS AFTERNOON. WINDS WILL RETURN
TO DRAINAGE BY THIS AFTERNOON.

&&

.BOU WATCHES/WARNINGS/ADVISORIES...
WINTER STORM WATCH FROM SATURDAY AFTERNOON THROUGH MONDAY
AFTERNOON FOR COZ031-033-034.
&&

$$

SHORT TERM...BOWEN
LONG TERM...BARJENBRUCH
AVIATION...BOWEN


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