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000
FXUS65 KBOU 251124
AFDBOU

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE DENVER/BOULDER CO
424 AM MST WED FEB 25 2015

.SHORT TERM...(TODAY THROUGH TONIGHT )
ISSUED AT 424 AM MST WED FEB 25 2015

A STREAK OF DIFFICULT FORECASTS REMAINS IN TACT. A LOT GOING ON
THIS AFTERNOON AND EVENING...AND AGAIN POTENTIAL OF CONVECTION
THROWS A LOT OF VARIABILITY INTO MODEL OUTPUT AND FORECAST
CONFIDENCE FOR SNOWFALL AMOUNTS IN ANY PARTICULAR AREA.

HERE IS WHERE WE ARE...LIGHT SNOW IS OCCURRING IN THE MOUNTAINS
EARLY THIS MORNING BUT THIS SHOULD DECREASE AS SOME DRIER AIR
ALOFT WORKS ACROSS THE AREA THIS MORNING. ON THE PLAINS...WE
SHOULD SEE A FEW RAIN/SNOW SHOWERS SPREADING SOUTHEAST EARLY THIS
MORNING...MAINLY EAST OF A GREELEY TO LIMON LINE. CANT EVEN RULE
OUT A BIT OF FREEZING VERY LIGHT RAIN/SPRINKLES AS A FEW VALLEY
LOCATIONS MAY HOVER JUST BELOW FREEZING AS LIGHT PRECIP BEGINS.
CLOSER TO THE FRONT RANGE...OCCASIONAL GUSTY CHINOOK WINDS BUT
WARM TEMPERATURES WILL REMAIN IN PLACE...AND ENOUGH DOWNSLOPE TO
HAVE SOME BREAKS IN THE CLOUDS.

MAIN FORECAST ATTENTION TURNS TOWARD STRONG COLD FRONT ACROSS THE
PLAINS...WHICH IS ON TIME TO PUSH THROUGH BETWEEN 1 AND 3 PM.
TEMPERATURES WILL DROP BEHIND THE FRONT WITH ANTICYCLONIC UPSLOPE
DEEPENING. UPSTREAM MOISTURE SEEMS PLENTIFUL BASED ON REAL OBS AND
PRECIPITATION OCCURRING TO OUR NORTH. LAPSE RATES BECOME MORE
UNSTABLE...ESPECIALLY IN THE NAM WHERE SOME CAPE IS
INDICATED...WHILE GFS IS SLIGHTLY MORE STABLE. THE NAM...AS WELL
AS SEVERAL MESOSCALE MODELS INCLUDING THE NMM...ARW...AND THE
FINAL HOURS OF THE RAP AND HRRR INDICATE CONVECTION BLOSSOMING
WITH 2 INCH PER HOUR SNOW RATES. BUT THEY ALSO CONFINE THEM TO
AREAS VERY CLOSE TO THE FOOTHILLS...WHICH SEEMS POSSIBLE
CONSIDERING THE STRENGTH OF NORTH/NORTHEAST FLOW. IF THIS VERIFIES
THEN HEAVIER SNOW WOULD CERTAINLY BE MORE FOCUSED TOWARD THE
FOOTHILLS...AND MUCH LESS POTENTIAL FOR MUCH ACCUMULATION TOWARD
FORT COLLINS...GREELEY...AND POINTS JUST EAST OF DENVER. WE HAVE
TRENDED FORECASTS IN THIS DIRECTION SINCE THE STRONGER NORTHERLY
FLOW USUALLY SHUTS THOSE AREAS DOWN PRETTY QUICKLY...AND DRIER LOW
LEVEL AIR WITH COLDER ARCTIC AIR THIS EVENING WOULD HAVE A
TENDENCY TO DO THE SAME.

AT THE SAME TIME...THAT ALSO FOCUSES THE HEAVIER PRECIPITATION
TOWARD THE FOOTHILLS SO HAVE OPTED TO ISSUE WINTER STORM WARNINGS
FOR FOOTHILL LOCATIONS...MAINLY BOULDER COUNTY FOOTHILLS SOUTHWARD
TO JEFFERSON/DOUGLAS COUNTIES. THE SOUTHERN FOOTHILLS HAVE NOT
BEEN DOING VERY WELL WITH REGARD TO SNOW AMOUNTS IN THE PAST
COUPLE STORMS BUT GIVEN THE STRONGER FLOW LATER TODAY WE DO EXPECT
HEAVIER TOTALS THERE. ANOTHER CONCERNING AREA IF CONVECTION
BLOSSOMS AS MESOSCALE MODELS INDICATE WOULD BE THE WESTERN SUBURBS
FOR POTENTIAL WARNING CRITERIA. HARD TO EXACTLY SAY HOW THIS COMES
TOGETHER BUT FOR NOW WILL ISSUE ADVISORIES STARTING AROUND 3 PM
FOR THE DENVER/BOULDER METRO AREA AND PALMER DIVIDE. FORT COLLINS
PER DISCUSSION ABOVE SEEMS TO GET EXCLUDED FROM HEAVIER
PRECIPITATION AGAIN. SOME THREAT WESTERN SUBURBS COULD GET A QUICK
6 INCHES PLUS LATE THIS AFTERNOON/EVENING SO WILL MONITOR FOR
POTENTIAL UPGRADE TO WARNING. OVERALL...THERE WILL BE A VERY LARGE
DISPARITY WITH SNOW TOTALS WITH LITERALLY TRACE TO 10 INCH AMOUNTS
POSSIBLE IN PORTIONS OF THE DENVER METRO AREA...AND HARD TO
PINPOINT WHO GETS EXACTLY HOW MUCH. ONCE SNOW DEVELOPS WE DO
ANTICIPATE RAPID DETERIORATION OF CONDITIONS AS SNOW INTENSITY
COULD TURN DRY AND WARM ROADS INTO A QUICK SHEET OF PACKED
SNOW/ICE. THE LATE AFTERNOON/EVENING COMMUTE FOR THE
DENVER/BOULDER/PALMER DIVIDE/FOOTHILLS AREAS LOOKS DIFFICULT AT
THIS POINT.

FOR OVERNIGHT...SNOW INTENSITY IS EXPECTED TO GRADUALLY DECREASE
WITH UPSLOPE COMPONENT WEAKENING AND AIRMASS STABILIZING WITH
COLDER/DRIER BOUNDARY LAYER FLOW. MOST OF THE ACCUMULATING SNOW
SHOULD BE OVER BY DAYBREAK SO WILL RUN WARNINGS AND ADVISORIES TIL
6 AM.

.LONG TERM...(THURSDAY THROUGH TUESDAY)
ISSUED AT 424 AM MST WED FEB 25 2015

THE MEDIUM RANGE MODELS CONTINUE TO INDICATE THAT THIS EVENINGS
SNOWFALL IS THE BEGINNING OF A PROLONGED PERIOD OF LIGHT SNOW
ACROSS MUCH OF THE FORECAST AREA...ESPECIALLY THE MOUNTAINS. THE
FLOW PATTERN ALOFT IS FORECAST TO BEGIN AS A CYCLONICALLY CURVED
NORTHWESTERLY FLOW WITH A COUPLE EMBEDDED SHORT WAVE TROUGHS. BY
FRIDAY EVENING...FLOW ALOFT ACROSS COLORADO WILL BECOME MORE
WESTERLY AS AN UPPER TROUGH BEGINS CARVING OUT OVER THE PACIFIC
NORTHWEST AND THEN GREAT BASIN. THE WESTERLY FLOW WILL BE MOIST
WITH ENOUGH CYCLONIC CURVATURE TO MAINTAIN WEAK QUASI-GEOSTROPHIC
FORCING FOR ASCENT ACROSS THE STATE. THIS WILL RESULT IN A
CONTINUATION OF LIGHT SNOWFALL THROUGH FRIDAY AND SATURDAY. AT
LOWER LEVELS...A VERY COLD AIRMASS WILL MOVE INTO THE REGION
BEHIND THIS AFTERNOONS COLD FRONT AND TAKE UP RESIDENCE OVER
NORTHEAST COLROADO THROUGH ABOUT SUNDAY. A LARGE SURFACE HIGH OVER
THE CENTRAL PLAINS WILL MAINTAIN SOUTHEASTERLY LOW LEVEL FLOW
ACROSS EASTERN COLORADO WELL INTO THE WEEKEND. THIS SHOULD
CONTINUE FEEDING MOISTURE INTO THE REGION WHICH COULD BE HELD
AGAINST THE FOOTHILLS BY A DENVER CYCLONE THAT WILL DEVELOP
THURSDAY NIGHT AND LAST THROUGH SATURDAY. WITH WEAK DYNAMICS
REMAINING OVERHEAD AND MOIST AIR AT LOWER LEVELS...LIGHT SNOW WILL
BE POSSIBLE WHILE THE UPPER FLOW PATTERN REMAINS IN PLACE.

BY LATE IN THE WEEKEND...THE UPPER TROUGH WILL SINK TO THE DESERT
SOUTHWEST...LEAVING COLORADO UNDER THE DEFORMATION ZONE DOWNSTREAM
OF THE UPPER CIRCULATION. THIS WILL PRODUCE THE HEAVIEST SNOWFALL
MORE OVER SOUTHWESTERN COLORADO...SIMILAR TO WHAT OCCURRED THIS
PAST MONDAY. BUT THE NORTH-CENTRAL COLORADO MOUNTAINS WILL BE ON
THE NORTHERN FRINGES OF THIS SYSTEM AND SHOULD RECEIVE AT LEAST
LIGHT SNOWFALL. NO FORECAST PERIOD IS EXPECTED TO CONTAIN
PARTICULARLY HEAVY SNOWFALL AMOUNTS...BUT SEVERAL CONSECUTIVE
PERIODS WITH 2 TO 4 INCHES OF NEW SNOW COULD ADD UP AFTER AWHILE.

TEMPERATURE-WISE...THE COLDEST TEMPERATURES FOR NORTHEAST COLORADO
WILL BE THURSDAY AND FRIDAY WITH ONLY SLIGHT MODERATION THROUGH
THE WEEKEND. DURING THE FIRST HALF OF NEXT WEEK...THE GFS AND
ECMWF HAVE SLIGHTLY DIFFERENT SCENARIOS INVOLVING THE WESTERN U.S.
TROUGH MOVING ACROSS COLORADO. THIS WILL KEEP THE RATHER UNSETTLED
WEATHER OVER THE STATE THROUGH THAT TIME. LITTLE WARMING CAN BE
EXPECTED UNTIL AFTER THE MAIN TROUGH CLEARS THE AREA. ALL IN
ALL...SNOWPACK CONDITIONS WILL ONLY IMPROVE THROUGH THE NEXT WEEK.

&&

.AVIATION...(FOR THE TAFS THROUGH 12Z THURSDAY MORNING)
ISSUED AT 424 AM MST WED FEB 25 2015

VFR CONDITIONS WILL PREVAIL THROUGH ABOUT 20Z. WINDS SHOULD TREND
FROM WEST/SOUTHWEST TO NORTHWEST/NORTHERLY THROUGH 18Z...AND THEN
BECOME GUSTY FROM THE NORTH/NORTHEAST WITH FRONTAL PASSAGE 20Z-
21Z WITH GUSTS AROUND 25 KNOTS EXPECTED. EXPECT IFR CONDITIONS AND
HEAVIER SNOW SHOWERS DEVELOPING 21Z-23Z WITH POTENTIAL FOR 1-2
INCH PER HOUR SNOW RATES AND TEMPO 1/4SM VISIBILITY 23Z-04Z.
HIGHEST THREAT WOULD BE AT KBJC AND KAPA...BUT SOME THREAT AT KDEN
TOO. OVERALL INTENSITY SHOULD DECREASE AFTER 04Z BUT IFR
CONDITIONS WILL LIKELY LINGER WITH LIGHT SNOW THROUGH EARLY
THURSDAY MORNING.

&&

.BOU WATCHES/WARNINGS/ADVISORIES...
WINTER WEATHER ADVISORY FROM 3 PM THIS AFTERNOON TO 6 AM MST
THURSDAY FOR COZ039>041.

WINTER STORM WARNING FROM 2 PM THIS AFTERNOON TO 6 AM MST
THURSDAY FOR COZ035-036.

WINTER WEATHER ADVISORY FROM 2 PM THIS AFTERNOON TO 6 AM MST
THURSDAY FOR COZ033-034.

&&

$$

SHORT TERM...BARJENBRUCH
LONG TERM...DANKERS
AVIATION...BARJENBRUCH