Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 241801
AFDBOU

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE DENVER/BOULDER CO
1101 AM MST WED DEC 24 2014

.SHORT TERM...(TODAY THROUGH TONIGHT )
ISSUED AT 412 AM MST WED DEC 24 2014

A RELATIVELY PLEASANT DAY IN PROGRESS ACROSS THE AREA. HIGH LEVEL
CIRRUS WAVE CLOUDS MAY CUT DOWN THE SUN A BIT AS THE AFTERNOON
PROGRESSES. GUSTY WEST WINDS IN THE 30-40MPH RANGE ARE LIKELY TO
DEVELOP IN THE FOOTHILLS. THOSE CHINOOK WINDS MAY SPREAD INTO
PORTIONS OF THE PLAINS NEAR THE FOOTHILLS LATE TODAY AND BUMP THE
TEMPERATURE UP A COUPLE DEGREES FROM THE CURRENT MAX FORECAST.

THE EFFECTS OF THE DEVELOPING UPPER LOW WILL START TO TURN UPPER
WIND FLOW TO MORE SOUTHWESTERLY AFTER 00Z AND INCREASING QG
ASCENT SHOULD START MOVING IN FROM THE WEST OVERNIGHT. INCREASED
CLOUD COVER BETWEEN 00Z AND 12Z WITH SUFFICIENTLY HIGH LOWER
LEVEL MOISTURE AND QG ASCENT TO WARRANT SOME PRECIPITATION CHANCES
ACROSS THE FAR NORTHWEST REACHES OF THE CWA TOWARDS THE END OF THE
PERIOD. MORE OVERNIGHT CLOUDS AND THE CONTINUED LEE TROUGHING WILL
RESULT IN MILDER MINIMUM TEMPERATURES TONIGHT.

.LONG TERM...(THURSDAY THROUGH TUESDAY)
ISSUED AT 338 AM MST WED DEC 24 2014

AN UPPER LEVEL TROUGH WILL MOVE INTO THE GREAT BASIN EARLY ON THU
AND THEN DEVELOP INTO A STG CLOSED LOW BY THU EVENING NR THE FOUR
CORNERS AREA.  ON FRI THE ECMWF KEEPS THE UPPER LEVEL LOW NR THE
FOUR CORNERS THRU THE AFTN WHILE THE GFS AND NAM BEGIN TO MOVE IT
EASTWARD INTO ERN CO WITH IT BECOMING A MORE OPEN TROUGH. HOWEVER
A FEW OF THE ENSEMBLE MEMBERS KEEP IT CLOSED OFF AS IT MOVES
EASTWARD ACROSS THE PLAINS. MEANWHILE AT THE SFC LOW PRESSURE WILL
INTENSIFY OVER ECNTRL CO THU AFTN AND THEN MOVE INTO THE TX PNHDL
THU NIGHT AS A COLD FNT MOVES ACROSS NERN CO BY THU AFTN. THIS
WILL LEAD TO INCREASING UPSLOPE FLOW BY LATE THU AFTN INTO THU
NIGHT. IN ADDITION MOISTURE WILL GRADUALLY INCREASE AS WELL WITH
MID LVL QG ASCENT FCST FM THU AFTN INTO FRI MORNING. LATEST CIPS
ANALOG GUIDANCE AND SREF PLUME DATA INDICATE THE POTENTIAL FOR
HEAVY SNOW IN AND NR THE FOOTHILLS THU NIGHT. THUS HAVE DECIDED TO
ISSUE A WINTER STORM WATCH FOR THU NIGHT INTO EARLY FRI IN THE
FOOTHILLS AND ALONG THE FRONT RANGE URBAN CORRIDOR. SNOW AMOUNTS
WILL PROBABLY END UP ON THE LOWER END OF WARNING CRITERIA IN MOST
AREAS UNLESS THE UPPER LEVEL LOW COMES ACROSS MORE WOUND UP AS IT
MOVES INTO ERN CO ON FRI.

ON FRI LIGHT SNOW WILL CONTINUE THROUGH THE MORNING HOURS BASED ON
LATEST TRENDS WITH STILL SOME LINGERING MID LVL ASCENT AND WEAK
UPSLOPE FLOW SHOWN BY CROSS-SECTIONS. THUS HAVE UPPED POPS THRU 18Z
IN MOST AREAS. AS MENTIONED ABV IF THE SYSTEM WERE TO COME ACROSS
MORE CLOSED OFF AS SHOWN BY A FEW OF THE ENSEMBLE MEMBERS THEN
HEAVIER SNOW COULD STILL AFFECT SOME AREAS THRU MIDDAY FRI. AS FOR
HIGHS ON FRI WILL KEEP READINGS IN THE UPPER TEENS TO LOWER 20S
OVER NERN CO.

BY FRI NIGHT THE NAM AND GFS MOVE THE UPPER LEVEL TROUGH EAST OF THE
AREA WHILE THE ECMWF STILL HAS IT OVER CENTRAL COLORADO.  HOWEVER
WITH NO UPSLOPE COMPONENT AND DECREASING MOISTURE SNOW CHANCES
SHOULD GRADUALLY END FRI EVENING IN MOST AREAS EXCEPT POSSIBLY IN
THE MTNS WHERE WEAK OROGRAPHICS MAY STILL ALLOW FOR PERIODS OF LIGHT
SNOW.  ON SAT DIRER AIR IN NW FLOW ALOFT WILL BE OVER THE AREA SO
WILL KEEP FCST DRY EXCEPT FOR A SLIGHT CHC OF -SHSN IN THE MTNS.
TEMPS WILL REMAIN COLD WITH AFTN HIGHS ONLY IN THE 20S OVER NERN CO.

BY SUN THE FLOW ALOFT WILL BECOME MORE WLY AS ANOTHER UPPER LEVEL
TROUGH BEGINS TO DIG SSE FM THE PACIFIC NW.  MOISTURE WILL BEGIN TO
INCREASE IN THE MTNS ESPECIALLY BY AFTN WHICH WILL LEAD TO A CHC OF
SNOW.  OVER NERN CO IT WILL BE DRY WITH SOME MODERATION IN TEMPS AS
A WK LEE TROUGH DEVELOPS OVER ERN CO.  HOWEVER POTENTIAL SNOWPACK
AND ASSOCIATED LOW LVL INVERSIONS COULD LIMIT AMOUNT OF WARMING AND
KEEP HIGHS MORE IN THE 25 TO 30 DEGREE RANGE INSTEAD OF THE 30S
WHICH IS BEING SHOWN BY CURRENT GUIDANCE NUMBERS.

FOR SUN NIGHT INTO MON THE ECMWF AND GFS HAVE REVERTED BACK TO THEIR
PREVIOUS SOLUTIONS OF 24 HOURS AGO AS THEY SHOW AN UPPER LEVEL
TROUGH CARVING OUT OVER THE WCNTRL US WITH AN ARCTIC FNT MOVING INTO
NERN CO SUN NIGHT INTO MON.  IF THIS COMES TO FRUITION THERE WOULD
BE ANOTHER GOOD CHC OF SNOW ACROSS THE AREA SUN NIGHT INTO MON.  IN
ADDITION HIGHS ON MON WOULD BE MUCH COLDER WITH READINGS STAYING IN
THE TEENS OVER NERN CO.  FOR MON NIGHT INTO TUE BOTH THE ECMWF AND
GFS SHOW A DEVELOPING POSITIVELY TILTED UPPER LEVEL TROUGH FM
SCNTRL CANADA EXTENDING SW BACK ACROSS THE CNTRL ROCKIES WITH
ANOTHER REINFORCING BLAST OF ARCTIC AIR. IF THIS COMES TRUE HIGHS
NEXT TUE WOULD ONLY BE IN THE SINGLE DIGITS ACROSS NERN CO WITH
OVERNIGHT LOWS WELL BLO ZERO.

&&

.AVIATION...(FOR THE TAFS THROUGH 12Z THURSDAY MORNING)
ISSUED AT 412 AM MST WED DEC 24 2014

MAINLY LIGHT SOUTHWESTERLIES IN PLACE NOW ARE LIKELY CONTINUE
THROUGH THE AFTERNOON. EXPECT NORMAL DRAINAGE PATTERNS TONIGHT,
EXCEPT WITH SOME AREAS OF GUSTY WESTERLY WINDS IN AND CLOSE TO
THE FOOTHILLS BETWEEN 00 AND 12Z. NO ISSUES WITH CEILINGS OR
VISIBILITIES TODAY THROUGH THURSDAY MORNING.

LOOKING AHEAD BRIEFLY...THE PRESENT THINKING FOR A DOWNTURN IN
THE WEATHER AT THE AREA TERMINALS IS THURSDAY AFTERNOON WITH A
COLD FRONTAL PASSAGE LIKELY IN THE 17-18Z TIME PERIOD. CEILINGS
AND VISIBILITIES WILL DETERIORATE MOST LIKELY AFTER 21Z AS LOW
STRATUS AND SNOW DEVELOP.

&&

.BOU WATCHES/WARNINGS/ADVISORIES...
WINTER STORM WATCH FROM THURSDAY EVENING THROUGH FRIDAY MORNING
FOR COZ035-036-038>041.

&&

$$

SHORT TERM...ET
LONG TERM...RPK
AVIATION...ET



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