Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 161100
AFDBOU

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE DENVER/BOULDER CO
500 AM MDT THU APR 16 2015

.SHORT TERM...(TODAY THROUGH TONIGHT )
ISSUED AT 500 AM MDT THU APR 16 2015

UPPER LOW DROPPING SOUTH ACROSS EASTERN UTAH AT THIS TIME. MODELS
ARE NOT IN TOO BAD OF A SHAPE TO START WITH BUT THEIR DIFFERENCES
GROW WITH TIME. SOME SMALL TRENDS SEEM PRETTY CLEAR THOUGH AND
MAIN ADJUSTMENTS FOR THIS PACKAGE ARE MAKING THE FORECAST A BIT
WETTER AND WARMER.

UPPER LOW APPEARS TO BE CLOSING OFF MORE THAN THE NAM SHOWS...AS
IT LIKES TO MAINTAIN A N-S ELONGATED UPPER TROUGH THROUGH FRIDAY.
THE DIVERGENCE DEVELOPING OVER THE MIDDLE OF COLORADO SUGGEST THIS
IS NOT HAPPENING THAT MUCH YET. LEANING TOWARD THE MORE CLOSED OFF
GFS SOLUTION...THOUGH HESITANT ABOUT THE EXACT PLACEMENT/TIMING OF
ITS MOTION. THE LOW ALSO APPEARS TO BE DROPPING FURTHER WEST AND
POSSIBLY A BIT FURTHER SOUTH THAN WHAT THE 00Z GFS SHOWED. THIS
COULD LEAD TO A BIT FURTHER SOUTH/SLOWER TRACK...BUT THIS WOULD
ALSO FAVOR A WETTER SOLUTION FOR OUR AREA. DEPENDING ON THE
DETAILS...THE TRACK COULD WIND UP FAVORING LARIMER/BOULDER
COUNTIES...OR IF IT IS FURTHER SOUTH AND HAS A SHARPLY CLOSED
CIRCULATION...COULD STILL FAVOR AREAS WEST/SOUTHWEST OF DENVER. IN
ANY EVENT...AREAS ABOVE 7000 FEET SHOULD GET DUMPED ON.

THE PLAINS AIRMASS IS REALLY MOIST AND WILL BE HARD TO COOL.
MODELS CONTINUE TO SHOW THE LACK OF MUCH MORE COOLING WITH THIS
SYSTEM...AND A SLOWER MOTION WOULD GIVE MORE TIME FOR THIS WARM
AIR TO MOVE IN. SO THESE SOLUTIONS ARE WETTER...BUT WARMER. COLD
ENOUGH FOR SNOW IN DENVER NOW...BUT WITH THE WARM ADVECTION IN THE
HEART OF THE SYSTEM TONIGHT THERE IS SOME DOUBT ABOUT IT STILL BE
COLD ENOUGH WHEN THE HEAVY PRECIP FALLS...AND THERE COULD BE
WARMING OVERNIGHT. FOR NOW I JUST NUDGED THE TEMPERATURE UP A
LITTLE BIT...THE STRONG LIFT MAY HELP WITH THE COOLING BUT NO
EVAPORATION TO HELP IN THIS SITUATION. GRIDS SHOW A SHARP GRADIENT
IN SNOWFALL BETWEEN 5500 AND 6500 FEET...WHICH COULD BE OFF BY 500
FEET OR SO EITHER WAY.

MOVED UP THE START TIME OF THE WINTER STORM WARNING TO 8 AM DUE TO
THE SHOWERS DEVELOPING OVER THE MOUNTAINS AT THIS TIME...REPORT
JUST CAME IN THAT I-70 IN SUMMIT COUNTY IS STARTING TO SLUSH UP
ALREADY. CONSIDERED ADVISORIES FOR SOME AREAS BUT TOO MUCH
UNCERTAINTY. SOUTH PARK WOULD DO WELL WITH A SLOWER/MORE
SOUTHERLY TRACK...BUT WILL SHUT DOWN AS THE WINDS GO MORE
NORTHEASTERLY. FOR NOW RAISED POPS AND AMOUNTS THERE A BIT. FOR
FRINGE ZONES NEAR THE FOOTHILLS...DOUGLAS/JEFFERSON/LARIMER
COUNTIES IT IS POSSIBLE PARTS OF THESE AREAS WILL GET QUITE HEAVY
SNOW AND OTHER PARTS WILL SEE LITTLE IMPACT DUE TO THE
TEMPERATURES. GIVEN THAT THE WARM GROUND WILL ALSO LIMIT IMPACT
EVEN WHERE THERE IS HEAVY SNOW...I HAVE CHOSEN NOT TO ISSUE
ADVISORIES AT THIS TIME. IF IT TURNS OUT TO BE SLIGHTLY COOLER
PLACES LIKE CASTLE ROCK...BOULDER...AND AREAS NORTH OF FORT
COLLINS COULD NEED ADVISORIES OR EVEN WARNINGS...BUT THERE COULD
WIND UP BEING LITTLE IMPACT AT ALL.

.LONG TERM...(FRIDAY THROUGH WEDNESDAY)
ISSUED AT 500 AM MDT THU APR 16 2015

THE UPPER LOW BY 12Z FRI WILL BE ALONG THE CO/NM BORDER NEAR
ALAMOSA.  AFTER THAT I HAVE LITTLE CONFIDENCE WITH REGARD TO WHERE
THIS LOW WILL END UP BY FRI AFTN INTO FRI EVENING.  BASED ON LOOKING
AT HEIGHT FALLS AND MID LVL COLD POOL IT SHOULD MOVE ENE TOWARDS
PUEBLO BY 18Z.  AFTER THAT GRIDDED DATA SHOWS THE MID LVL COLD POOL
BY LATE AFTN/EARLY EVENING COULD BE CENTERED ANYWHERE FM NR DENVER
OR SE OF LIMON.  NATURALLY A SHIFT NORTHWARD OR SOUTHWARD BY 100-200
MILES COULD HAVE A DRAMATIC AFFECT ON ADDITIONAL PCPN AMOUNTS FRI
AFTN INTO FRI EVENING OVER NRN CO.

AT THIS POINT IT APPEARS THE MAIN UPPER LEVEL LOW WILL STAY OVER SRN
COLORADO THROUGH FRIDAY MORNING WITH ADDITIONAL PCPN OVER PORTIONS
OF NRN CO THRU MIDDAY.  BUFFER DATA SHOWS PCPN WILL FALL AS RAIN
OVER MOST OF NERN CO EXCEPT POSSIBLY NR THE FOOTHILLS AND OVER THE
PALMER DIVIDE WHERE A MIX MAY OCCUR.  IN THE FOOTHILLS AND MTNS THE
THREAT FOR HEAVY SNOW WILL CONTINUE THRU MIDDAY.

BY LATER IN THE AFTN INTO THE EARLY EVENING HOURS THE BEST CHC OF
PCPN MAY SHIFT TO NRN AREAS OF THE FCST AREA MAINLY NORTH OF A
DENVER TO AKRON LINE. WITH STEEP LAPSE RATES WILL LIKELY SEE SOME
CONVECTION AS WELL BY AFTN IN SOME AREAS. ALSO THERE COULD BE SOME
THREAT FOR SVR STORMS OVER THE PLAINS ALONG AND SOUTH OF I-70 BY
AFTN ALONG A CONVERGENCE ZN THAT DVLPS AS SFC LOW PRES FORMS NEAR
PUEBLO. THE HIGHEST THREAT FOR ADITIONAL HEAVY SNOW WILL PROBABLY
FOCUS MORE IN THE NRN FOOTHILLS AND MTN AREAS NORTH OF I-70 IN THE
AFTN.

FOR FRI NIGHT AS MENTIONED ABV THE FCST WILL HINGE ON WHERE THE MAIN
UPPER LEVEL LOW IS.  SO FAR MOST OF THE MODELS KEEP IT NR DENVER
HOWEVER IT WOULD NOT BE A TOTAL SURPRISE IF IT ENDS UP EAST OR SE OF
DENVER.  AT THIS POINT WILL KEEP THE HIGHEST POPS IN THE NRN
FOOTHILLS AND MTN AREAS WITH CHC POPS OVER THE PLAINS.  TOTAL
ACCUMULATIONS IN THE FOOTHILLS AND MTNS AREAS WILL RANGE FM 1 TO
2 FEET RANGE ALTHOUGH SOME MTN AREAS OF LARIMER AND BOULDER COULD
SEE UP TO 3 FEET.

BY SAT THE MAIN UPPER LEVEL LOW IS FCST TO MOVE INTO SERN CO AFTER
SUPPOSEDLY BEING OVER NRN COLORADO FRI NIGHT.  THIS WOULD LEAVE NRN
CO ON THE WESTERN SIDE THE LOW WITH SOME POTENTIAL FROM PCPN TO
OCCUR ON THE BACKSIDE.  FOR NOW HAVE KEPT POPS IN THE CHC CATEGORY
IN ALL AREAS HOWEVER THESE MAY NEED TO BE INCREASED AT SOME POINT
DEPENDING ON HOW THINGS EVOLVE.  OVERALL TEMPS WILL REMAIN RATHER
WRM SO PCPN WOULD CONTINUE AS RAIN ACROSS THE PLAINS WITH SNOW
CONFINED TO THE FOOTHILLS AND MTNS.

FOR SUN INTO MON THE FLOW ALOFT WILL BE NWLY AS THE UPPER LEVEL
SYSTEM MOVES AWAY FM THE AREA.  THERE WILL BE A COUPLE OF SURGES
MOVING ACROSS WHICH WILL ENHANCE UPSLOPE FLOW ESPECIALLY SUN NIGHT
INTO MON. IN ADDITION A WK SYSTEM EMBEDDED IN THE FLOW MAY MOVE
ACROSS AS WELL WHICH COULD LEAD TO ANOTHER CHC OF PCPN SUN NIGHT
INTO MON.  CURRENT TEMP PROFILE SUGGEST AIRMASS WILL BE COLD ENOUGH
FOR PCPN TO FALL AS SNOW EVEN AT LOWER ELEVATIONS.  HIGH BOTH
DAYS WILL STAY BLO SEASONAL NORMALS.

BY TUE THERE ARE SOME DIFFERENCES AS THE GEM MODEL SHOWS ANOTHER
SURGE OF COOLER AIR AND WK TROUGH MOVING ACROSS WITH A CHC OF PCPN.
MEANWHILE THE ECMWF AND GFS DO NOT SHOW THIS FEATURE AND HAVE DRIER
NWLY FLOW ALOFT ACROSS THE AREA.  FOR NOW WILL JUST KEEP IN SOME LOW
POPS AND KEEP TEMPS NR SEASONAL NORMALS.

&&

.AVIATION...(FOR THE TAFS THROUGH 12Z FRIDAY MORNING)
ISSUED AT 500 AM MDT THU APR 16 2015

MVFR CEILINGS WILL BE WIDESPREAD THIS MORNING WITH AREAS OF LIGHT
SNOW AND RAIN AND LOCALIZED IFR CONDITIONS. HEAVIER PRECIPITATION
IS EXPECTED TONIGHT WITH IFR PREVAILING MOST OF THE NIGHT. SNOW
ACCUMULATION WILL BE HEAVILY DEPENDENT ON AIR AND SURFACE
TEMPERATURES. KDEN IS EXPECTED TO HAVE LITTLE OR NO RUNWAY
ACCUMULATION...WHILE KAPA/KBJC COULD SEE PERIODS OF SLUSH ON THE
RUNWAYS DUE TO HEAVY SNOWFALL RATES. THIS IS MOST LIKELY BETWEEN
02Z AND 08Z.

&&

.BOU WATCHES/WARNINGS/ADVISORIES...
WINTER STORM WARNING FROM 8 AM MDT TODAY TO 6 AM MDT SATURDAY FOR
COZ033>036.

&&

$$

SHORT TERM...GIMMESTAD
LONG TERM...RPK
AVIATION...GIMMESTAD


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