Area Forecast Discussion
Issued by NWS Grand Junction, CO

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000
FXUS65 KGJT 280459
AFDGJT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE GRAND JUNCTION CO
1059 PM MDT SAT SEP 27 2014

.SHORT TERM...(TONIGHT THROUGH SUNDAY NIGHT)
ISSUED AT 300 PM MDT SAT SEP 27 2014

THE DEEP CLOSED LOW SYSTEM WAS CENTERED OVER THE WESTERN GREAT
BASIN EARLY THIS AFTERNOON. STRONG DIVERGENCE IN THE ENTRANCE
REGION OF A NORTHWARD FLOWING JET STREAK ON THE LEADING FLANK OF
THE CLOSED LOW WAS RESPONSIBLE FOR A BROAD BAROCLINIC LEAF
EXTENDING FROM NORTHWEST ARIZONA ACROSS WESTERN AND CENTRAL UTAH.
STRONG STORMS...SOME SEVERE...HAVE BEEN REPORTED BENEATH THE LEAF
STRUCTURE. MEANWHILE...A FEW STORMS HAD DEVELOPED OVER THE FLAT
TOPS AND SAN JUAN MOUNTAINS OF COLORADO WITH A FEW SHOWERS OVER
EASTERN UTAH.

EXPECT MOIST CONVECTION WILL INCREASE ACROSS EASTERN UTAH AND
WESTERN COLORADO TONIGHT AS THE CLOSED LOW EASES SLOWLY EASTWARD
DURING THE NIGHT. MODELS INDICATED 100 KT JET MOVING THROUGH THE
BASE OF THE TROUGH WILL GENERATE IMPRESSIVE DIVERGENCE OVER
EASTERN UTAH THIS EVENING THEN WESTERN COLORADO OVERNIGHT. A DEEP
STREAM OF SUBTROPICAL MOISTURE CONTAINING ABOUT 1 TO 1.2 INCHES
OF PRECIPITABLE WATER MOVING BENEATH THIS FORCING SHOULD YIELD
WIDESPREAD MODERATE TO HEAVY RAIN. DIFFLUENT PATTERN OVER THE
FORECAST AREA FAVORS TRAINING OF THUNDERSTORMS RAISING THE
POTENTIAL FOR FLASH FLOODING. CONSEQUENTLY...WILL HOIST A FLASH
FLOOD WATCH FOR MUCH OF EASTERN UTAH BEGINNING AT 6 PM...AND A
GOOD PORTION OF WESTERN COLORADO AT MIDNIGHT TO ACCOUNT FOR
EASTWARD TRANSIT OF THE STORM.

A DRY SLOT BEGINS TO WORK INTO EASTERN UTAH EARLY ON SUNDAY SO
WILL END THE WATCH THERE BY 6 AM AS SHOWERS BECOME MORE SCATTERED
AND LESS INTENSE. FOCUS THEN SQUARELY ON WESTERN COLORADO WHERE
MOISTURE REMAINS DEEP AND DYNAMICS IMPRESSIVE. BROAD AREAS OF
MODERATE TO HEAVY RAINFALL HAVE A GOOD CHANCE OF CREATING FLASH
FLOODING OR MUD AND DEBRIS FLOWS...ESPECIALLY ACROSS BURN SCARS
AND WHERE SOILS ARE UNSTABLE IN AREAS OF STEEP TERRAIN. BY MIDDAY
HOWEVER...DRIER AIR BEGINS TO INFILTRATE WESTERN COLORADO AND
DYNAMIC FORCING WEAKENS LIMITING COVERAGE AND INTENSITY OF
THUNDERSTORMS. CONCURRENTLY...COLDER AIR MOVING ACROSS EASTERN
UTAH WILL LOWER SNOW LEVEL TO NEAR 10000 FEET...SO EXPECT SOME
SNOW IN THE HIGHER ELEVATIONS. SNOW LEVEL ALONG THE DIVIDE
APPEARS TO BE A BIT HIGHER AT 12000 FEET.

THE LOW WILL SLOWLY EASE OVER EASTERN UTAH SUNDAY NIGHT. PW VALUES
CONTINUE TO FALL SO SHOWERS AND EMBEDDED THUNDERSTORMS WILL
CONTINUE TO DECREASE. HOWEVER...ENOUGH DYNAMIC FORCING TO KEEP
SOME NOCTURNAL ACTIVITY GOING LATE INTO THE NIGHT.

OVERNIGHT LOWS WILL BE JUST A BIT MILDER TONIGHT DUE TO INCREASED
CLOUDS. HOWEVER...AFTERNOON HIGHS WILL BE MARKEDLY COOLER SUNDAY
AS COLDER AIR...COMBINED WITH CLOUDY SKIES AND WIDESPREAD
PRECIPITATION LIMIT INSOLATION. CHILLY SUNDAY NIGHT...BUT NOT
ANTICIPATING FREEZING TEMPERATURES FOR ANY OF THE VALLEYS.

.LONG TERM...(MONDAY THROUGH SATURDAY)
ISSUED AT 300 PM MDT SAT SEP 27 2014

THE LOW ELONGATES NORTH-SOUTH AND LIFTS NORTHEAST ACROSS THE ROCKIES
ON MONDAY WITH SOUTHERLY FLOW TURNING WESTERLY BY EVENING. THE AIR
DRIES WITH PRECIPITABLE WATER VALUES SLATED TO DROP BELOW 1/2 INCH.
BUT...MOISTURE/Q-G FORCING/INSTABILITY STILL LOOK SUFFICIENT FOR
CONVECTIVE SHOWERS AND POSSIBLY A SHORT-LIVED THUNDERSTORM. COOLER
AIR OVERSPREADS THE AREA AS THE -14C TO -16C MID-LEVEL COLD CORE
ASSOCIATED WITH THE TROUGH AXIS PASSES LATE IN THE DAY...WITH SNOW
LEVELS RANGING FROM 9500 TO 10K FT MONDAY.

A CHANCE OF SHOWERS TO LINGER MAINLY ACROSS THE NORTH MONDAY NIGHT
AND TUESDAY. PACIFIC ENERGY KEEPS TROUGHING OVER THE ROCKIES WED.
EMBEDDED JET STREAK WILL BRING SCATTERED SHOWERS TO THE MTNS WED
WITH SNOW LEVELS AROUND 10K FT... LOCALLY LOWER IN CONVECTIVE
SHOWERS.

COOLER MORE AUTUMN-LIKE TEMPS ARE EXPECTED NEXT WEEK. MAY END UP
WITH LOCAL LIGHT FREEZES ON SOME NIGHTS IN THE HIGHER VALLEYS.

PACIFIC RIDGING BEGINS TO BUILD INTO THE WEST LATE NEXT WEEK FOR DRY
WEATHER AND A WARMING TREND.

&&

.AVIATION...(FOR THE 06Z TAFS THROUGH 06Z SUNDAY NIGHT)
ISSUED AT 1055 PM MDT SAT SEP 27 2014

-SHRA/-TSRA WILL CONTINUE ACROSS THE AREA THROUGH MIDDAY SUNDAY.
WHILE FLIGHT CATEGORIES WILL GENERALLY REMAIN VFR...AT TIMES
CIGS/VSBYS MAY DROP INTO MVFR OR EVEN BRIEFLY IFR CATEGORIES IN
THE HEAVIER SHOWERS. MTNS WILL BE OBSCURED. ALSO FOG WILL BE
POSSIBLE SUNDAY MORNING IN VALLEYS WHERE THE BETTER RAINFALL
ACCUMULATIONS WILL HAVE OCCURRED. ACTIVITY WILL DIMINISH FROM THE
WEST SUNDAY AFTERNOON.

&&

.GJT WATCHES/WARNINGS/ADVISORIES...
CO...FLASH FLOOD WATCH THROUGH SUNDAY MORNING FOR COZ001>003-006-007-
     009-011-013-017>022.

UT...FLASH FLOOD WATCH UNTIL 6 AM MDT SUNDAY FOR UTZ022-024-025-
     027>029.

&&

$$

SHORT TERM...NL
LONG TERM...JAD
AVIATION...JDC



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