Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 280229
AFDBOU

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE DENVER/BOULDER CO
829 PM MDT WED APR 27 2016

.UPDATE...
ISSUED AT 828 PM MDT WED APR 27 2016

EVENING RUNS OF THE HRRR AND RAP MODELS HAVE BEEN SHOWING
PRECIPITATION SPREADING OVER THE PALMER DIVIDE AND URBAN CORRIDOR
THROUGH THE LATE NIGHT AND MORNING HOURS. PRECIPITATION SEEMS TO
BE THE RESULT OF PERSISTENT MOIST SOUTHEASTERLY FLOW AT LOW LEVELS
OVERNIGHT IN COMBINATION WITH COLORADO LYING UNDER THE LEFT EXIT
REGION OF THE UPPER JET...ASSOCIATED WITH THE APPROACHING UPPER
LOW THAT WILL AFFECT OUR WEATHER THROUGH THE END OF THE WEEK AND
INTO THE WEEKEND. WILL BE UPDATING THE FORECAST TO INCLUDE MORE
PRECIPITATION ON THE PLAINS THURSDAY MORNING. MORNING TEMPERATURES
AND THE PHASE OF PRECIPITATION WILL BE THE TRICKY THINGS TO
FORECAST. MORNING LOWS IN THE DENVER AREA ARE FORECAST TO BE
AROUND 33 DEGREES...CERTAINLY COLD ENOUGH FOR SNOW...BUT
PRECIPITATION INTENSITIES IN THE RAP AND HRRR ARE ONLY A COUPLE
HUNDREDTHS PER HOUR. (.01-.05 INCH/HR) THESE LIGHT INTENSITIES
SHOULD NOT LEAD TO ANY SNOW ACCUMULATION. THAT BEING SAID...THE
PALMER RIDGE COULD SEE SOME LIGHT ACCUMULATION DUE TO THE COOLER
TEMPERATURES AND HIGHER ELEVATION THAN DENVER. ONCE THE RAIN OR
SNOW BEGINS ALONG THE I-25 CORRIDOR TOMORROW MORNING...IT SHOULD
THEN CONTINUE THROUGH THE EVENING AS THE DYNAMIC FORCING FROM THE
APPROACHING STORM STRENGTHENS.

SINCE THE EVENING MODELS ARE JUST NOW STARTING TO ARRIVE...WILL
NOT MAKE ANY CHANGES TO THE WINTER STORM WATCH IN THE MOUNTAINS
AND FOOTHILLS AT THIS TIME.

&&

.SHORT TERM...(THIS EVENING THROUGH THURSDAY)
ISSUED AT 355 PM MDT WED APR 27 2016

LOWER SPECIFIC HUMIDITY/PRECIPITABLE WATER AND A MID-LEVEL CAP
BENEATH A PASSING SHORTWAVE RIDGE PRETTY MUCH SQUELCHED SHOWER
FORMATION AND SUPPRESSED UPDRAFTS TODAY. THE ISOLATED SHOWERS
IN THE FCST WORKED OUT. THE GUSTY N-NWLY WINDS ALSO MATERIALIZED
ON THE PLAINS AND ON THE HIGH MTN RIDGES ON THE BACKSIDE OF THE
DEPARTING UPPER TROUGH.

OVERNIGHT...ATTENTION SHIFTS TO THE LARGE UPPER TROUGH WINDING UP
OVER THE DESERT SOUTHWEST/LOWER GREAT BASIN. IN ADVANCE OF THIS
SYSTEM...MODELS SHOW A DEEPENING SOUTHWESTERLY TRANSPORTING
HIGHER THETA-E AIR NORTHWARD UP INTO COLORADO AND BY LATE TONIGHT
COULD SEE SNOWFALL ON THE INCREASE OVER HIGHER ELEVATIONS ACRS
SWRN PORTIONS OF THE CWA. MEANWHILE...MODELS SHOW EAST-
SOUTHEASTERLY LOW-LEVEL FLOW ON THE PLAINS MOVING MOISTURE
WESTWARD UP ALONG THE PALMER DIVIDE AND BY LATE TONIGHT RESULTING
IN PATCHY TO AREAS OF FOG AS FAR NORTH AS ERN PORTIONS OF THE
DENVER METRO AREA. VSBYS GENERALLY IN THE 3-5 MILE RANGE.
HOWEVER...THIS SAME ELY FLOW IS PROJECTED TO PRODUCE NEAR
SATURATED CONDITIONS IN THE FRONT RANGE FOOTHILLS LATE TONIGHT
THRU MID-MORNING ON THURSDAY. SO ADDED AREAS OF FOG HERE WHERE
SOME VSBYS IN SOME AREAS COULD DROP TO BELOW A MILE AROUND
SUNRISE. LOWS TONIGHT ONLY SLIGHTLY WARMER THAN LAST NIGHT

ON THURSDAY...WITH THE UPPER TROUGH CLOSING IN...SHOW SEE SNOWFALL
COVERAGE EXPANDING NORTHWARD ACROSS THE HIGH COUNTRY AND OVER THE
FRONT RANGE FOOTHILLS. PRECIP MAY START OUT AS A RAIN/SNOW MIX IN
THE FOOTHILLS AND LOWER MTN VALLEYS BUT WITH TIME CHANGE OVER TO
ALL SNOW IN THE AFTERNOON WITH PRECIP BECOMING MORE CONVECTIVE IN
CHARACTER. ON THE PLAINS...NO PRECIP TO START OUT THE DAY AND BY
MID-MORNING THE FOG SHOULD DISSIPATE. BY AFTERNOON SHOWERS FORMING
OVER THE FRONT RANGE FOOTHILLS WITH ENHANCED UPSLOPE FLOW ARE
EXPECTED TO BEGIN SPREADING OUT ONTO THE PLAINS WITH SWLY
TRANSPORT WINDS. WILL SLOWLY FALLING TEMPERATURES...COULD SEE
PRECIP IN AREAS NEAR THE FOOTHILLS AND ON THE PALMER DIVIDE
CHANGING TO A MIX OF RAIN/SNOW. MAY EVEN SEE A STRAY T-STORM OR
TWO ON THE PALMER DIVIDE LATE. HIGH TEMPS TOMORROW PROGGED TO BE
GENERALLY 3-5 DEG F LOWER THAN THOSE OBSERVED TODAY.

.LONG TERM...(THURSDAY NIGHT THROUGH WEDNESDAY)
ISSUED AT 355 PM MDT WED APR 27 2016

PRETTY REMARKABLE AGREEMENT BETWEEN THE MODELS REGARDING THE
SYNOPTIC SCALE EVOLUTION/MOVEMENT OF THE UPPER LOW AND ATTENDANT
QG VERTICAL VELOCITY FIELDS THROUGHOUT THE DURATION OF THE
UPCOMING STORM. THE TRACK IS NOT NECESSARILY PERFECT FOR A MAJOR
STORM...A BIT TOO FAR NORTH...BUT ENOUGH ASCENT FOR A LONG ENOUGH
TIME TO GENERATE DECENT PRECIPITATION AMOUNTS ACROSS MUCH OF THE
CWA. SURFACE EVOLUTION IS ALSO DECENT...PERHAPS NOT PERFECT...WITH
THE LATEST ECMWF SUGGESTING A BIT LESS UPSLOPE THAN THE OTHER
MODELS DURING THE DAY FRIDAY. THE OTHER BIG CONCERN...LIKELY THE
MOST IMPORTANT...IS THE THERMODYNAMIC STRUCTURE OF THE AIRMASS.
LOTS OF DISCREPANCIES BETWIXT THE MODELS WITH THE ECMWF AND NAM
SUGGESTING MORE SNOW AT LOWER ELEVATIONS WHILST THE GFS IS QUITE
A BIT WARMER. LASTLY...THE GFS HAS ABOUT 3-4 TIMES THE QPF ACROSS
PARTS OF THE CWA COMPARED TO THE OTHER SIMULATIONS SO PLENTY OF
UNCERTAINTY STILL LINGERS IN THE DETAILS. FORECAST PRECIPITABLE
WATER NUMBERS ARE SUBSTANTIAL ENOUGH TO SUGGEST THAT 0.5 TO 1.0
INCH OF WATER IS NOT OUT OF THE QUESTION IN MOST AREAS BEFORE ALL
IS SAID AND DONE. WINTER STORM WATCH HAS ALREADY BEEN POSTED FOR
THE FOOTHILLS...SOUTH PARK AND MOST OF THE MOUNTAINS.

LATER ON DOWN THE LINE GENERAL TROUGHINESS REMAINS OVER THE
CENTRAL ROCKIES WITH BELOW NORMAL TEMPERATURES THROUGH THE EARLY
PART OF NEXT WEEK. WARMER DRIER CONDITIONS MAY RETURN BY MIDWEEK.

&&

.AVIATION...(FOR THE 00Z TAFS THROUGH 00Z THURSDAY EVENING)
ISSUED AT 828 PM MDT WED APR 27 2016

WITH PRECIPITATION NOW EXPECTED TO BEGIN ACROSS THE DENVER AREA
EARLIER IN THE MORNING...WILL BE ADDING SHOWERS TO THE TAFS AS
EARLY AS 08Z OR 09Z. WILL KEEP THE MENITON OF FOG AHEAD OF THE
ARRIVAL OF THE PRECIPITATION.

&&

.BOU WATCHES/WARNINGS/ADVISORIES...
WINTER STORM WATCH FROM THURSDAY EVENING THROUGH FRIDAY EVENING
FOR COZ033>037.

&&

$$

UPDATE...DANKERS
SHORT TERM...BAKER
LONG TERM...ET
AVIATION...DANKERS


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