Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 291624
AFDBOU

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE DENVER/BOULDER CO
1024 AM MDT MON SEP 29 2014

.UPDATE...
ISSUED AT 1022 AM MDT MON SEP 29 2014

FORECAST ON TRACK WITH AN ACTIVE WEATHER DAY WITH A FEW SEVERE
STORMS/SQUALL LINES LIKELY LATER THIS AFTERNOON THROUGH THE
EVENING. CURRENT ANALYSIS SHOWS A SCATTERED TO BROKEN STRATUS DECK
IN THE LOW LEVEL COOL POOL SITTING OVER MOST OF THE FRONT RANGE.
THE STRATUS AND COOLER BOUNDARY LAYER IS EXPECTED TO ERODE
GRADUALLY NORTHWARD THROUGH THE DAY AS DEEP LAYER LIFT/MIXING AND
SOUTHERLY FLOW INCREASE AHEAD OF STRONG SHORT WAVE. Q-G LIFT
ANALYSIS SHOWS THE MAX LIFT TURNING EASTWARD AND EJECTING TOWARD
THE FOUR CORNERS AT THIS TIME...AND TARGETED TO REACH THE FRONT
RANGE BY PEAK HEATING NEAR 21Z. CAPES WILL REACH THE 1000-1500
J/KG RANGE WITH T/TD MID 70S/MID 50S. THE HIGHER CAPES WILL BE
WHERE STRONGEST HEATING OCCURS ON THE EASTERN PLAINS. MEANWHILE...
0-6 KM SHEAR REACHES 40-50 KTS WHILE 0-1KM HELICITY REACHES
120-180 M2/S2 AND 0-3KM REACHES 180-220 M2/S2. THOSE HIGHER
HELICITY VALUES ALSO MAX OUT TOWARD THE LIMON TO AKRON TO
JULESBURG AREA TOWARD 00Z...YIELDING HIGHEST RISK OF TORNADOES IN
THOSE LOCATIONS. HOWEVER...SURFACE CONVERGENCE ZONE WHICH COULD
ORGANIZE ALONG THE PALMER DIVIDE AREA OF DOUGLAS/ELBERT COUNTIES
BY MID AFTERNOON COULD ALSO RESULT IN TORNADO THREAT THERE AS
WELL. ORGANIZED SQUALL LINES WITH HIGH WINDS WILL ALSO BE A THREAT
INTO THE EVENING GIVEN THE STRONG MID LEVEL FLOW AT 700 MB
INCREASING TO NEAR 50 KTS OVER THE EASTERN PLAINS TOWARD 00Z.

&&

.SHORT TERM...(TODAY THROUGH TONIGHT )
ISSUED AT 402 AM MDT MON SEP 29 2014

A STRONG STORM SYSTEM WILL RACE ACROSS THE CENTRAL ROCKIES THIS
AFTERNOON AND TONIGHT BRINGING SHOWERS AND THUNDERSTORMS. SOME THE
STORMS COULD BE SEVERE WITH DAMAGING WINDS...HAIL AND TORNADOES. A
BROAD UPPER LEVEL LOW IS OVER THE GREAT BASIN. WATER VAPOR SHOWING
AT LEAST THREE WAVES ROTATING AROUND IT. THE STRONGEST ONE...THE ONE
THAT WILL AFFECT COLORADO...IS NEAR LAS VEGAS NEVADA. THIS STRONG
SHARP WAVE WILL QUICKLY MOVE EASTWARD TODAY. SOUTHERLY FLOW AHEAD OF
IT WILL BECOME STRONG.

THERE WILL BE STRONG ASCENT AHEAD OF THE TROUGH THIS AFTERNOON AND
EARLY EVENING. EXPECT ENOUGH LIFT THIS MORNING TO PRODUCE SHOWERS
AND POSSIBLY A FEW WEAK STORMS. AS LIFT AND INSTABILITY INCREASE...
CONVECTION WILL BECOME WIDESPREAD OVER CENTRAL COLORADO THIS
AFTERNOON AND SPREAD OVER THE EASTERN PLAINS THIS EVENING. BECAUSE
OF THE INSTABILITY...SCATTERED STORMS SHOULD FORM PRIOR TO THE MAIN
SWATH OF SHOWERS AND STORMS. CAPES OF UP TO 1500 J/KG WILL BE
POSSIBLE THIS AFTERNOON OVER THE PLAINS IF TEMPERATURES CLIMB INTO
THE LOWER 70S AND DEW POINTS ARE IN THE MID 50S. THIS INSTABILITY
COMBINED WITH VERY GOOD SHEAR WILL LIKELY PRODUCE A FEW SEVERE
STORMS WITH DAMAGING WIND...HAIL AND POSSIBLY TORNADOES. BEST CHANCE
FOR SEVERE STORMS WILL BE EAST OF A LINE FROM GREELEY TO DENVER
INTERNATIONAL AIRPORT. THOUGH CAN`T RULE OUT SEVERE STORMS WEST TO
THE FOOTHILLS.

MODELS IN GOOD AGREEMENT IN SHOWING A LINE OF CONVECTION MOVING
ACROSS NORTHEAST COLORADO THIS EVENING. WEST PARTS OF THE AREA WILL
START TO SEE DRYING AND SUBSIDENCE LATE THIS EVENING...WHICH WILL
SPREAD EAST OVERNIGHT. BY TUESDAY MORNING...EXPECT DRY CONDITIONS
ACROSS THE AREA.

.LONG TERM...(TUESDAY THROUGH SUNDAY)
ISSUED AT 402 AM MDT MON SEP 29 2014

OPERATIONAL MODELS AGREE IN PINWHEELING THE CLOSED UPPER LOW/NEG
TILT TROUGH NORTHEASTWARD ACROSS WYOMING AND NORTHERN GREAT PLAINS
ON TUESDAY...PLACING NRN COLORADO IN A DRIER ZONAL FLOW. TUESDAY
LOOKS DRY EAST OF THE MTNS WITH PREVAILING WLY/DOWNSLOPE FLOW.
WHEREAS MTN AREAS...PARTICULARLY AREAS WEST OF THE CONTINENTAL
DIVIDE...REMAIN IN A MORE FAVORABLE POSITION TO SEE LIGHT
OROGRAPHIC AND/OR CONVECTIVE PCPN BY LATE IN THE DAY IN ADVANCE
OF A MID-LEVEL PERTABATION FORMING IN THE LEFT FRONT QUAD OF A
90+ KT JET DIVING SEWRD ACRS THE CENTRAL GREAT BASIN. MODELS SHOW
THE JET CARVING OUT A POSITIVE TILT LONGWAVE TROUGH OVER THE
GREAT BASIN TUESDAY NIGHT/WEDNESDAY...AND SWINGING ACRS ERN
COLORADO WEDNESDAY NIGHT. MODEL QG VERTICAL VELOCITY FIELDS
INDICATE WEAK TO MODERATE LARGE SCALE ASCENT OVER THE FORECAST
AREA DURING THE DAY WEDNESDAY AS A 80-100KT JET PASSES JUST TO OUR
SOUTH. COOLING ALOFT WILL STEEPEN LAPSE RATES AND CYCLONIC
UPSLOPE FLOW BEHIND A COLD FRONT THAT PASSES THROUGH THE DENVER
METRO AREA DURING THE MORNING WILL PROVIDE ADDITIONAL LIFT FOR
WIDESPREAD CLOUD COVER AND PRECIPITATION BY THE AFTERNOON. ALL
THIS POINTS TO A COOL...WET DAY WITH MODEL GUIDANCE INDICATING
HIGHS ONLY IN THE 60S AT LOWER ELEVATIONS AND 30S TO LOWER 50S IN
THE HIGH COUNTRY. MTN AREAS ROUGHLY ABOVE TIMBERLINE COULD SEE A
MIX OF RAIN/SNOW DURING THE DAYLIGHT HOURS WITH SPOTTY LIGHT SNOW
ACCUMULATION BY LATE AFTERNOON. BY EVENING WITH THE THERMAL
TROUGH PASSING OVERHEAD...SNOW LEVEL IS EXPECTED TO LOWER TO
AROUND THE 9500 OR 10K FOOT LEVEL. HOWEVER...MOISTURE APPEARS TO
BE ON THE DECLINE AT THAT TIME WITH DRIER STABLE AIR SPREADING
IN BEHIND THE TROUGH. HIGH MTN AREAS COULD STILL END UP WITH A
FEW INCHES OF SNOW BY LATE WEDNESDAY NIGHT. MODEL RAIN TOTALS ON
THE PLAINS VARY ANYWHERE FROM A COUPLE TENTHS OF AN INCH TO AROUND
0.75 INCH BY MID-EVENING WEDNESDAY...WITH THE GREATER AMOUNTS
SCATTERED ACRS THE NERN CORNER OF THE STATE.

BY THURSDAY...TROUGH MOVES EAST AND FLOW ALOFT GOES
NORTHWESTERLY. AS A RESULT EXPECT SLIGHTLY WARMER TEMPERATURES
WITH MUCH MORE SUNSHINE.

FRIDAY THROUGH SUNDAY...LARGE UPPER RIDGE BUILDING OVER THE WEST
COAST GRADUALLY SHIFTS EAST OVER THE GREAT BASIN BY WEEKS END. THE
CWA REMAINS UNDER THE INFLUENCE OF NORTHWEST FLOW ALOFT...BUT
TEMPERATURES ARE FORECAST TO GRADUALLY WARM A FEW DEGS EACH DAY WITH
PLENTY OF SUNSHINE. HOWEVER BY SUNDAY...COULD SEE TEMPERATURES
DIP SOME WITH PASSAGE OF A WEAK MOSTLY DRY COLD FRONT.

&&

.AVIATION...(FOR THE TAFS THROUGH 18Z TUESDAY MORNING)
ISSUED AT 959 AM MDT MON SEP 29 2014

STRATUS DECK AROUND 2000 FT AGL IS EXPECTED TO GRADUALLY BREAK
TOWARD 18Z-19Z WITH INCREASED MIXING AHEAD OF UPPER LEVEL TROUGH.
THUNDERSTORMS WILL BE LIKELY 20Z-01Z...FIRST NEAR THE FOOTHILLS
AFFECTING KBJC AND KAPA AND THEN TOWARD 21Z-22Z AT KDEN. MAIN
THREAT OF SEVERE STORMS WOULD BE JUST SOUTH OF THE FRONT RANGE
AIRPORTS BUT ANY STORMS WILL BE CAPABLE OF PRODUCING STRONG WIND
GUSTS TO AROUND 40-45 KTS GIVEN STRENGTHENING MID LEVEL FLOW.
LITTLE THREAT OF STORMS AFTER 01Z AS DOWNSLOPE AND SUBSIDENCE
FOLLOW MAIN BATCH OF ORGANIZED STORMS.

&&

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

$$

UPDATE...BARJENBRUCH
SHORT TERM...MEIER
LONG TERM...BAKER
AVIATION...BARJENBRUCH



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