Boulder Office Area Forecast Discussion

000
FXUS65 KBOU 091102
AFDBOU

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE DENVER CO
400 AM MST TUE MAR 9 2010

.SHORT TERM...RAIN SHIELD ASSOCIATED WITH THE LOW TO OUR EAST IS
DRIFTING AWAY...WITH DRIER AIR UNDERCUTTING THE CLOUDS ACROSS
EASTERN COLORADO. STILL SOME RAIN IN THE CORNER BUT VERY
LIGHT...MODELS SEEM TOO SLOW TO PUSH THIS OUT BUT THERE IS WESTWARD
REDEVELOPMENT OF THE MORE SIGNIFICANT RAIN ACROSS NEBRASKA. SUSPECT
THAT THE HEAVIER RAIN WILL STAY EAST...BUT THERE MAY WELL BE
OCCASIONAL LIGHT RAIN IN THE CORNER THROUGH THE DAY. SHOVED THE BACK
EDGE OF THIS EASTWARD A BIT IN THE NEW FORECAST. FURTHER WEST THERE
SHOULD BE A FAIR AMOUNT OF SUNSHINE THIS MORNING...ENOUGH TO
THREATEN SOME CONVECTIVE CLOUD DEVELOPMENT IN THE LATE AFTERNOON.
RAISED HIGH TEMPS IN THE EAST ABOUT 5 DEGREES BASED ON MORE
SUNSHINE. CLEARING OCCURRING OVER OUR MOUNTAINS NOW AS WELL...THOUGH
THERE ARE SOME SHOWERS AROUND GRAND JUNCTION THAT INDICATE THE
CLEARING MAY NOT BE AS ROBUST AT LEAST ACROSS THE CENTRAL MOUNTAINS
THIS MORNING. HIGH CLOUDS SPREAD BACK OVER THE MOUNTAINS THIS
AFTERNOON AND INSTABILITY MAY PRODUCE SOME SHOWERS THERE AS WELL.
FOR TONIGHT THE MODELS SEEM TO AGREE ON THE BIG PICTURE...THOUGH THE
PRECIP FIELD WINDS UP DIFFERENT. DEFORMATION ZONE MOVES OVER THE
WEST AND THEN SOUTH PARTS OF OUR AREA...WEAK LIFT EVERYWHERE BUT
BETTER CLOSER TO THE LOW. SLIGHT COOLING ALOFT LEADS TO A MOIST
ADIABATIC LAPSE RATE. 06Z NAM GOES NUTS WITH A BAND OF SHOWERS IN
THE DEFORMATION BAND ACROSS BOULDER AND DENVER...UP TO 8 INCHES OF
SNOW THIS EVENING. GFS LOOKS MORE REASONABLE HOLDING AN AXIS OF
LIGHT TO MODERATE SNOW OVER THE MOUNTAINS. I DO NOT SEE ENOUGH WIND
TO GENERATE SUSTAINED STRONG CONVERGENCE ALONG THE FOOTHILLS...SO
NOT TOO WORRIED ALTHOUGH WITH THE INSTABILITY ANYTHING IS POSSIBLE.
FOR THE MOST PART KEPT THE CURRENT FORECAST OVERNIGHT...RAISED POPS
IN THE MOUNTAINS A BIT AS ODDS OF AT LEAST A LITTLE SEEM PRETTY
GOOD. LATEST NAM HAS LESS COOLING TONIGHT THAN EARLIER FORECAST AND
MY HUNCH IS THAT THIS IS RIGHT...MAINLY DUE TO A LITTLE DRYING AND
THE SLIGHT COOLING ALOFT. NAM SOUNDINGS ARE STILL COLD ENOUGH FOR
SNOW OR A MIX EVERYWHERE...THOUGH I HAVE DOUBTS THIS CHANGEOVER WILL
COME QUICKLY. I TOOK OUT SOME OF THE DETAIL AND WENT WITH A MIX
EVERYWHERE ON THE PLAINS THIS EVENING...THEN SNOW ABOVE 4500 FEET BY
LATE TONIGHT. AGAIN AMOUNTS SHOULD BE LIGHT AND FAVOR THE SOUTH AND
WEST.

.LONG TERM...MODELS ARE BACK TO SHOWING A SOLUTION FROM SEVERAL RUNS
AGO.  AT 12Z WEDNESDAY...THERE IS AN UPPER CLOSED LOW OVER EAST
CENTRAL NEW MEXICO.  THERE IS ALSO THE REMAINS OF OUR CURRENT UPPER
LOW (NOW OVER SOUTHWESTERN KANSAS) OVER CENTRAL NEBRASKA.  MODELS
HAVE THE TWO PHASE TOGETHER...WITH THE MAIN CENTER OVER NORTH
CENTRAL OKLAHOMA AT 00Z LATE WEDNESDAY AFTERNOON.  AT 12Z
THURSDAY...THE MAIN CENTER IS OVER NORTHEASTERN KANSAS CONNECTING
BACK TO A WEAKER CENTER OVER CENTRAL NEW MEXICO.  BY THURSDAY
EVENING THE UPPER LOW IS STRENGTHENING AND STILL AROUND EASTERN
KANSAS WITH FAIRLY WEAK NORTHERLY FLOW ALOFT FOR THE FORECAST AREA.
BASICALLY...THE FORECAST AREA IS IN A BROAD UPPER TROUGH THROUGH THE
PERIOD WITH PRETTY WEAK FLOW ALOFT.  THE FORECAST AREA IS MOSTLY IN
UPWARD VERTICAL VELOCITY  WEDNESDAY INTO THURSDAY AFTERNOON...THEN
WEAK DOWNWARD MOTION IS PROGGED AFTER THAT.  THERE IS A BRIEF PERIOD
OF SOMEWHAT STRONGER UPWARD MOTION WEDNESDAY MORNING.  THE BOUNDARY
LAYER FLOW IS PROGGED TO BE WEAK WEDNESDAY MORNING...WITH AN
INCREASE OF NORTHERLIES BY MIDDAY.  IT LOOKS LIKE A BIT OF COOLER
SURGE OF AIR.  THIS UPSLOPE CONTINUES INTO WEDNESDAY EVENING...
BEFORE BECOMING MORE NORTH-NORTHWESTERLY BY MIDNIGHT WEDNESDAY
NIGHT.  BRISK NORTH-NORTHWESTERLY SURFACE FLOW CONTINUE THURSDAY AND
THURSDAY NIGHT.  FOR MOISTURE...THE NAM AND GFS ARE IN POOR
AGREEMENT AGAIN FOR THE FIRST HALF OF WEDNESDAY...THEN DEEPER MORE
WIDESPREAD MOISTURE PROGGED BY LATER AFTERNOON.  THE GFS STILL SHOWS
IT TO BE DEEPER.  BOTH HAVE DEEP MOISTURE IN ALL AREAS WEDNESDAY
NIGHT...AND PRETTY DEEP ALL DAY THURSDAY...SAVE FOR THE LOW LEVELS
OF THE FOOTHILLS AND IMMEDIATE PLAINS WHERE DRY DOWNSLOPING IS
PROGGED.  THE GFS HAS A BIT MORE THAN THE NAM THOSE TWO PERIODS.
THURSDAY NIGHT THERE IS SOME DRYING...WITH DEEPER MOISTURE HANGING
ON OVER THE EASTERN THIRD OF THE FORECAST AREA.  THERE IS SOME
LIMITED CAPE OVER THE FOOTHILLS AND SOUTHERN FORECAST AREA LATE DAY
WEDNESDAY.  THERE IS NONE ON THURSDAY.  THE QPF FIELDS SHOW MINIMAL
AMOUNTS OF MEASURABLE PRECIPITATION OVER THE FORECAST AREA ON
WEDNESDAY.  THE NAM KEEPS IT OVER THE WESTERN 2/3RDS OF THE FORECAST
AREA.  THE GFS ALSO HAS SOME OVER THE EAST.  THERE IS A SIMILAR
PATTERN FOR WEDNESDAY NIGHT...WITH BOTH MODELS HAVING THE BEST
AMOUNTS OVER THE FOOTHILLS AND IMMEDIATE PLAINS.  THERE IS EVEN LESS
INDICATED ON THURSDAY...BUT IT COVERS ALL THE FORECAST AREA EXCEPT
FOR THE FOOTHILLS AND ADJACENT PLAINS.  THURSDAY NIGHT THE
MEASURABLE PRECIPITATION DECREASES.  FOR MOUNTAIN POPS...WILL GO
"LIKELY"S WEDNESDAY AND WEDNESDAY NIGHT.  THERE IS MOISTURE...WEAK
UPWARD MOTION...SOME OROGRAPHIC ENHANCEMENT...SOME COLD AIR
ADVECTION...AND OK LAPSE RATES.  AMOUNTS DO NOT LOOK HIGHLIGHT-ISH.
FOR THE PLAINS/FOOTHILLS...PREVIOUS GRIDS HAD "CHANCE"S GOING
WEDNESDAY INTO THURSDAY...THAT STILL LOOKS GOOD.  WILL MAKE MINOR
CHANGES TO TRY TO DEFINE UPSLOPE AND DOWNSLOPE PERIODS FOR THE
FOOTHILLS AND IMMEDIATE PLAINS.  GOOD LUCK TO ME WITH THE WAY THESE
MODELS HAVE BEHAVED LATELY.  THAT`S WHAT KEEPS ME FROM GOING
"LIKELY"S IN SOME ZONES.  TEMPERATURES AND WET BULB ZERO HEIGHTS
POINT TO SNOW MUCH OF THE TIME EXCEPT FOR THE 18Z TO 00Z TIME FRAMES
OVER THE PLAINS BOTH WEDNESDAY AND THURSDAY AFTERNOONS.  FOR
TEMPERATURES...HIGHS ON WEDNESDAY ARE 3-5 C COOLER THAN TODAY`S
HIGHS.  THURSDAY`S HIGHS ARE SIMILAR TO WEDNESDAY`S.  FOR THE LATER
DAYS...FRIDAY THROUGH MONDAY...MODELS HAVE A HIGH AMPLITUDE UPPER
RIDGE MOVING INTO COLORADO FRIDAY.  THE RIDGE AXIS IS OVER THE
FORECAST AREA ON SATURDAY.  THE GFS INDICATES WEAK UPPER TROUGHING
ON SUNDAY...THEN NORTHWESTERLY FLOW ALOFT ON MONDAY.  THE ECMWF HAS
AN UPPER CLOSED LOW NEAR THE FOUR CORNERS LATE SATURDAY NIGHT INTO
SUNDAY.  ON MONDAY THAT MODEL HAS UPPER RIDGING APPROACHING.   WILL
GO DRY...WITH MINOR POPS IN THE MOUNTAINS SUNDAY INTO MONDAY.

&&

.AVIATION...VFR CONDITIONS EXPECTED TODAY WITH JUST A SLIGHT CHANCE
OF LIGHT RAIN SHOWERS LATE. LOWERING CEILINGS OVERNIGHT WITH
INSTRUMENT APPROACHES LIKELY NEEDED. A CHANCE OF MVFR CEILINGS AND
VISIBILITIES IN LIGHT RAIN OR SNOW...MAINLY AFTER MIDNIGHT. ANY SNOW
ACCUMULATION WILL BE LIGHT.

&&

.BOU WATCHES/WARNINGS/ADVISORIES...NONE.

$$
RJK/GIMMESTAD

NWS BOU Office Area Forecast Discussion