Area Forecast Discussion
Issued by NWS Grand Junction, CO

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000
FXUS65 KGJT 182123
AFDGJT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE GRAND JUNCTION CO
323 PM MDT MON MAY 18 2015

.SHORT TERM...(TONIGHT THROUGH TUESDAY NIGHT)
ISSUED AT 323 PM MDT MON MAY 18 2015

SHOWERS WERE INCREASING OVER THE AREA EARLY THIS AFTERNOON AHEAD
OF THE LATEST IN A SERIES OF PACIFIC STORMS WHICH WAS CENTERED
OVER CENTRAL CALIFORNIA. EXPECT THIS TREND TO CONTINUE DURING THE
AFTERNOON AS TEMPERATURES PEAK AND THE CENTER OF THE PACIFIC STORM
MOVES TOWARD SOUTHERN NEVADA.

A VORT LOBE ROTATING THROUGH THE BASE OF THE PACIFIC LOW/S CENTER
WILL ENHANCE LIFT OVER THE REGION TONIGHT. ADDITIONAL LIFT WILL BE
SUPPLIED BY THE A 110 KT JET ROTATING THROUGH THE BASE OF THE
TROUGH WHICH WILL GENERATE CONSIDERABLE DIVERGENCE ALOFT. SINCE
THE MOISTURE FEED IS ALREADY IN PLACE...EXPECT PRECIPITATION WILL
BECOME WIDESPREAD OVER THE MOUNTAINS WITH ACTIVITY MORE SCATTERED
OVER THE VALLEYS. MODEL QPF VALUES ARE PRETTY IMPRESSIVE WITH A
SIGNIFICANT SWATH OF AREAS IN EXCESS OF 0.50 OF AN INCH BY
12Z/TUE.

CAA COMBINED WITH DIURNAL COOLING WILL CAUSE 7H TEMPS TO COOL TO
BETWEEN 1 AND 2 DEG C WHICH TRANSLATES TO SNOW LEVELS OF AROUND
9000 TO 9500 FEET TONIGHT. THIS WILL RESULT IN SOME IMPACTS TO
TRAVEL OVER THE HIGHER PASSES WITH ROADS LIKELY BECOMING ICY AND
SNOWPACKED. MODELS HIT SNOW ACCUMULATIONS PRETTY HARD...BUT EXPECT
SIGNIFICANT SNOWFALL WILL BE LIMITED TO AREAS ABOVE THE
TIMBERLINE. THEREFORE...OPTED TO HOIST A WINTER WEATHER ADVISORY
ABOVE 9500 FEET FOR MOST MOUNTAINS ALONG THE CONTINENTAL DIVIDE IN
LIEU OF WINTER STORM WARNINGS.

TUESDAY...MOISTURE GETS WRAPPED UP AROUND THE UPPER LOW (IN
ALIGNMENT WITH QG ASCENT) THAT IS PROGGED TO BE OVER SALT LAKE
CITY. THE DEEPER MOISTURE WILL BE LIFTING NORTHWARD WITH
WIDESPREAD RAINFALL EXPECTED TO SHIFT TO THE NORTH. ISENTROPIC
FIELDS SHOW COLD ADVECTION FOLLOWING THE CLOUD MASS/RAIN SHIELD.
CONVECTIVE POTENTIAL IS NOT VERY HIGH WITH CAPE VALUES BELOW 500
J/KG...BUT MID LEVEL COLD ADVECTION COMBINED WITH POTENTIAL
VORTICITY ALOFT WILL ENHANCE DESTABILIZATION IN SPITE OF A RAIN
COOLED AIR MASS. TOTAL RAIN AMOUNTS WILL BE SUBSTANTIAL BUT
VARIABLE...RANGING FROM AROUND ONE HALF INCH /0.50/ IN MANY
VALLEYS AND UPWARD TO 2 INCHES IN THE HIGH COUNTRY. AS THE UPPER
LOW CONTINUES TO SHIFT NORTHWARD (AND SPLITS APART)...SHOWERS WILL
BE DIMINISHING FROM SOUTH TO NORTH ON TUESDAY NIGHT.

.LONG TERM...(WEDNESDAY THROUGH MONDAY)
ISSUED AT 323 PM MDT MON MAY 18 2015

NEARLY DEJA VU IN THE LONG TERM PERIOD AS THE "RINSE/REPEAT" CYCLE
GETS PLAYED OUT ONCE AGAIN. THE NEXT UPSTREAM WAVE CARVES OUT A
DEEP TROUGH ALONG THE CALIFORNIA/BAJA COAST. AS THIS EVOLVES...A
RIDGE AXIS ALIGNS ITSELF WITH THE CENTRAL ROCKIES SPINE AND DRIER
SOUTHWEST FLOW DEVELOP. IN THIS CASE...DRIER DOES NOT MEAN NO RAIN
AS DIURNAL DRIVEN SHOWERS/THUNDER FORM OVER THE HIGHER TERRAIN ON
WEDNESDAY.

THURSDAY AFTERNOON/NIGHT...THE SRN CALIFORNIA UPPER LOW EJECTS A
SHORT WAVE THAT REACHES THE FOUR CORNERS AREA THURSDAY AFTERNOON
AND CONVECTIVE SHOWERS/THUNDER WILL BE ON THE INCREASE.

FRIDAY AND SATURDAY...THE MAIN UPPER LOW MOVES TOWARD WRN COLORADO
WITH GOOD AGREEMENT BETWEEN THE GFS/ECMWF MODELS THAT THE CORE
WILL BE OVERHEAD ON SATURDAY AFTERNOON. ANOTHER OPPORTUNITY FOR
WIDESPREAD CONVECTIVE SHOWERS/THUNDER. AHEAD OF THIS FEATURE IS QG
FORCING THAT BOOSTS THE THREAT OF ROBUST SHOWERS (ISOLATED STRONG
STORMS). SATURDAY WILL BE THE COOLEST DAY OF THE HOLIDAY WEEKEND
WITH 40S IN THE MOUNTAINS AND 60S IN THE VALLEYS.

THE REMAINDER OF THE MEMORIAL DAY WEEKEND CONTINUES TO BE
UNSETTLED...IN SPITE OF THE UPPER LOW MOVING DOWNSTREAM.
CONDITIONAL INSTABILITY WILL EXIST SUNDAY AND MONDAY AFTERNOON AND
LEADS TO MOUNTAIN CONVECTION...WITH SUFFICIENT SHEAR ALLOWING FOR
SHOWERS/STORMS TO ROLL OFF THE HIGHER TERRAIN AND GET VALLEY
LOCATIONS. AFTERNOON TEMPS WILL BE WARMER ON SUNDAY AND MEMORIAL
DAY...BUT STAY BELOW NORMAL. POSSIBLY NO 80 DEGREE WEATHER
ANYWHERE FOR THE WEEKEND...HOW OFTEN DOES THAT HAPPEN FOR MEMORIAL
DAY WEEKEND?

&&

.AVIATION...(FOR THE 18Z TAFS THROUGH 18Z TUESDAY AFTERNOON)
ISSUED AT 1039 AM MDT MON MAY 18 2015

SHOWERS AND THUNDERSTORMS WILL INCREASE IN AREAL COVERAGE AND
INTENSITY FROM 18Z TO 21Z...IMPACTING MAINLY THE MOUNTAINS. THIS
WILL RESULT IN AREAS OF MOUNTAIN OBSCURATION ALONG WITH GUSTY
OUTFLOW WINDS. SHOWERS AND THUNDERSTORMS WILL BEGIN TO MOVE OFF
THE HIGHER TERRAIN AND HAVE A GOOD CHANCE OF IMPACTING AIRPORTS
FROM 23Z TO 04Z/TUES. THIS ACTIVITY WILL BE CAPABLE OF PRODUCING
BRIEF MVFR VSBY THOUGH OUTFLOW WINDS TO 35 TO 40 MPH ARE A GREATER
THREAT. THE STORM SYSTEM APPROACHING FROM THE WEST WILL CONTINUE
TO BRING SCATTERED SHOWERS AND EMBEDDED THUNDERSTORMS THROUGHOUT
THE NIGHT. COOLING TEMPERATURES WILL BRING SNOW LEVELS DOWN TO
NEAR 10000 FEET.


&&

.GJT WATCHES/WARNINGS/ADVISORIES...
CO...WINTER WEATHER ADVISORY FROM 6 PM THIS EVENING TO NOON MDT
     TUESDAY FOR COZ010-012-013-018-019.

UT...NONE.
&&

$$

SHORT TERM...NL/PF
LONG TERM...PF
AVIATION...NL



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