Area Forecast Discussion
Issued by NWS Grand Junction, CO

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000
FXUS65 KGJT 270729
AFDGJT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE GRAND JUNCTION CO
1229 AM MST FRI FEB 27 2015

.UPDATE...
ISSUED AT 1227 AM MST FRI FEB 27 2015

ADDED A WINTER WEATHER ADVISORY FOR THE I-70 CORRIDOR FROM
GLENWOOD SPRINGS EAST TO VAIL. 2 INCHES OF SNOW HAS ALREADY BEEN
REPORTED JUST EAST OF GLENWOOD SPRINGS...AND SNOW ACCUMULATION IS
EXPECTED TO CONTINUE THROUGH THE EARLY MORNING HOURS.

UPDATE ISSUED AT 840 PM MST THU FEB 26 2015

SHORT WAVE IN NORTHWEST FLOW HAS REACHED THE FOUR CORNERS REGION
WITH WIDESPREAD SNOW. THIS SHOULD LAST FOR THE NEXT 2-4 HOURS FROM
CORTEZ TO PAGOSA SPRINGS BEFORE DEPARTING. SPOTTER REPORT FROM
CORTEZ MEASURED 1.5 INCHES...SO ACCUMULATING SNOWFALL ACROSS THIS
REGION IS EXPECTED FOR LA PLATA AND ARCHULETA COUNTIES. FOR ALL OF
WESTERN COLORADO...SNOW SHOULD BE DIMINISHING LATE TONIGHT AS THE
REGION EXPERIENCES A SHORT TERM LULL.

A QUICK LOOK AT THE 00Z NAM12 SHOW PROMISING STORM ON TRACK. THE
NAM12 QPF IS MORE THAN IMPRESSIVE WITH A SWATH OF 4+ INCHES OVER
THE SWRN SAN JUAN MOUNTAINS AND FOOTHILLS. CANNOT TAKE THIS FOR
VERBATIM AS THE NAM12 SOMETIME OVERDOES QPF...BUT WONDER IF THE
RANGE OF 2-4 FEET FOR STORM SNOW TOTALS OVER THE SAN JUANS IS TOO
LOW.

&&

.SHORT TERM...(TONIGHT THROUGH FRIDAY NIGHT)
ISSUED AT 314 PM MST THU FEB 26 2015

THE FINAL OPEN WAVE IN NW FLOW PASSES TONIGHT BRINGING SOME
ACCUMULATIONS TO THE MOUNTAINS ESPECIALLY ALONG NW-FACING SLOPES.
MORNING AMOUNTS ARE GENERALLY EXPECTED TO BE IN THE 1-4 INCH
RANGE. SNOW SHOWERS WILL CONTINUE INTO FRIDAY MORNING IN SW
COLORADO. ATTENTION THEN TURNS TO THE ONSET OF THE LONG DURATION
SNOW STORM FOR AREAS SOUTH OF I-70.

A NORTHERN PACIFIC LOW DROPS ALONG THE WA-OR COAST FRIDAY AND INTO
NORTHERN CA FRIDAY NIGHT. THE 110KT JET NOSE FLATTENS INTO NW NEW
MEXICO INTO FRIDAY NIGHT. MOISTURE INCREASES THROUGH FRIDAY
AFTERNOON WITH PRECIP WATER VALUES RISING OVER 0.4 INCHES IN THE
FOUR CORNERS...A GOOD BUT NOT GREAT VALUE FOR FEBRUARY. LATE
AFTERNOON LAPSE RATES LOOK SUFFICIENT FOR SOME POTENTIAL OF
ISOLATED THUNDERSTORMS AND ASSOCIATED LOCAL HEAVY SHOWERS.

SNOW WILL INCREASE OVER THE SOUTHERN ZONES FRIDAY NIGHT. THE NAM
STILL FAVORS A HEAVY BAND JUST NORTH OF THE NM BORDER ASSOCIATED
WITH FORCING IN THE NW QUADRANT OF THE JET. THE ABAJO AND
FOOTHILLS TOO MAY BE FAVORED TO GET SOME HEAVY SNOWFALL RATES
OVERNIGHT. OROGRAPHIC SNOWFALL WILL ENHANCE FAVORING SW- FACING
SLOPES. SNOW LEVELS ARE CRITICAL FOR THIS STORM. SOME WARM
ADVECTION OCCURS FRIDAY NIGHT LIMITING SE UTAH SNOW AMOUNTS...BUT
IT SHOULD TURN TO ALL SNOW ABOVE 6000 FEET.

.LONG TERM...(SATURDAY THROUGH THURSDAY)
ISSUED AT 314 PM MST THU FEB 26 2015

IF THERE IS ANYTHING TO TAKE AWAY FROM THE DISCUSSION BELOW...A
PROLONGED PERIOD OF MOIST...UNSTABLE....SOUTHWEST FLOW WILL BRING
SOME IMPRESSIVE SNOW TOTALS TO MUCH OF SOUTHWEST AND CENTRAL
COLORADO WITH A STORM STARTING FRIDAY AFTERNOON AND ENDING MONDAY
EVENING. MUCH OF THIS WILL BE MEASURED IN FEET.

THE MAIN ENERGY DRIVING THIS STORM IS STILL WELL UPSTREAM JUST OFF
THE BRITISH COLUMBIA COAST. THERE IS SO MUCH TIME BEFORE NOW AND
MONDAY FOR MODELS TO GRAB ONTO SOMETHING AND IMPACT THE FINAL
OUTCOME. THE BROAD PICTURE ABOVE IS FAIRLY CERTAIN BUT THE
FORECAST...LIKE THIS STORM...WILL MOST LIKELY BE VERY DYNAMIC. THE
MAIN REASONS I SEE FOR THIS IS CONVECTION...WARM BOUNDARY LAYER
CONDITIONS IN THE SOUTHERN VALLEYS...AND WHAT SEVERAL MODELS AGREE
UPON IS A GENERAL LULL IN THE PROLIFIC SNOW RATES SATURDAY EVENING
INTO SUNDAY. OUR FIRST GUESS FOR WARNINGS/ADVISORIES AND AND SNOW
AMOUNTS WAS WELL THOUGHT OUT BUT I EXPECTED TO BE HUMBLED ONCE AGAIN
BY MONDAY EVENING AS SNOW TOTALS ARE TALLIED. THE SPINE OF THE SAN
JUANS FROM THE NEW MEXICO BORDER WRAPPED AROUND TO THE NORTHWEST
NEAR SILVERTON WILL BE GROUND ZERO. THERE IS STILL SOME UNCERTAINTY
FOR THE CENTRAL AND NORTHERN CWA AS FAR AS SIGNIFICANT IMPACTS. SNOW
WILL FALL ACROSS THESE AREAS...WITH A MIX IN THE LOWER VALLEYS
DURING THE AFTERNOONS. THE BIGGEST IMPACTS TO THE AREAS OUTSIDE THE
SOUTHWEST LOOK TO BE DURING THE SECOND HALF OF THIS FROM SATURDAY
EVENING ONWARD...BUT ESPECIALLY SUNDAY NIGHT INTO MONDAY AS THE MAIN
TROUGH SWINGS INTO THE CENTRAL BASIN THEN ACROSS THE ROCKIES.

A FEW DETAILS OF NOTE OUR PWAT IS SITTING AT .15 ON THE 12Z RAOB
OR ABOUT 60 PERCENT OF NORMAL. GOES DERIVED IMAGERY SHOWS A STRONG
PACIFIC MOISTURE PLUME BEING WRAPPED INTO THE SYSTEM OFF THE COAST
WITH 150 PERCENT OF PWAT ALONG THE COAST. A FAIRLY DRY LAYER IS
PRESENT TO SOUTH DIVIDING US FROM A SUB-TROPICAL TAP. 295-300K
SURFACES STILL SHOW STRONG MOISTURE TRANSPORT BY SATURDAY MORNING
AS THE UPSTREAM JET ROUNDS THE BASE OF THE TROUGH DIGGING ACROSS
NORTHERN CALIFORNIA. PWAT RESPONDS BY CLIMBING TO NEAR HALF OF AN
INCH BY SUNDAY WHICH WILL BE NEAR 2 STD DEV ABOVE NORMAL. STRONG
UPGLIDE AND UPSLOPE INTO THE SOUTHERN CWA BY SUNRISE ON
SATURDAY...COMBINED WITH A TIGHTENING MID LEVEL FRONTO BAND
ASSOCIATED WITH THE PASSING JET...AND FINALLY OCTANE IN THE FORM
OF UPRIGHT INSTABILITY (ISOLATED THUNDERSTORMS) WILL BE THE
INGREDIENTS IN PLACE FOR SNOW RATES IN EXCESS OF AN INCH PER HOUR.
THIS AREA OF HEAVY SNOW WILL SETTLE OVER THE SOUTHERN VALLEYS VERY
EARLY SATURDAY...THEN LIFT ACROSS THE FOOTHILLS INTO MOUNTAINS OF
THE SAN JUANS THROUGH SATURDAY AFTERNOON AS THE JET LIFTS
NORTHWARD DUE TO FALLING HEIGHTS TO OUR SOUTHWEST. WITH THE JET
LIFTING NORTHWARD...SNOW RATES WILL MODERATE TO THE SOUTH AND MAY
BE ENHANCED TO THE NORTH THROUGH SUNDAY. BOUNDARY LAYER
TEMPERATURES ARE LIKELY TO WARM IN THE SOUTHERN VALLEYS ON SUNDAY
AND WOULD PRODUCE BOTH SOME MELTING OF NEW SNOW AND A MIX OF
PRECIPITATION. ANOTHER PUSH OF MOISTURE AND A REORGANIZATION OF
LIFT OCCURS SUNDAY NIGHT INTO MONDAY AS THE TROUGH PUSHES THROUGH.
SNOW RATES AGAIN INCREASE IN THE SOUTH. INTERESTINGLY THIS TROUGH
IS PUSHED OUT BY YET ANOTHER SYSTEM...WHICH IS DRIER BUT COLDER.
SNOW CONTINUES INTO WEDNESDAY. DETAILS ON THIS WILL COME AT A
LATER TIME. TEMPERATURES STAY BELOW NORMAL.

&&

.AVIATION...(FOR THE 06Z TAFS THROUGH 06Z FRIDAY NIGHT)
ISSUED AT 1041 PM MST THU FEB 26 2015

PASSING BANDS OF LIGHT SHOWERS WILL CONTINUE TONIGHT ACROSS
WESTERN COLORADO. A BREAK IS EXPECTED FRIDAY MORNING BEFORE SNOW
SHOWERS INCREASE AGAIN AROUND 00Z FRIDAY FROM THE
SOUTHWEST...AND AFFECTING SOUTHERN COLORADO TAF SITES BY 06Z.
CIGS/VIS WILL LOWER FROM THE SOUTHWEST TO NORTHEAST FRIDAY NIGHT.


&&

.GJT WATCHES/WARNINGS/ADVISORIES...
CO...WINTER WEATHER ADVISORY FROM 6 PM THIS EVENING TO 8 PM MST
     MONDAY FOR COZ014-021.

     WINTER STORM WARNING FROM NOON TODAY TO 8 PM MST MONDAY FOR
     COZ009-012-017>019.

     WINTER WEATHER ADVISORY UNTIL 8 AM MST THIS MORNING FOR COZ008.

     WINTER STORM WARNING FROM 6 PM THIS EVENING TO 8 PM MST MONDAY
     FOR COZ022-023.

UT...WINTER STORM WARNING FROM NOON TODAY TO 8 PM MST MONDAY FOR
     UTZ028.

&&

$$

UPDATE...JAM
SHORT TERM...JOE
LONG TERM...15
AVIATION...JAM



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