Area Forecast Discussion
Issued by NWS Grand Junction, CO

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FXUS65 KGJT 190344
AFDGJT

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

.UPDATE...
ISSUED AT 933 PM MDT MON MAY 18 2015

CURRENT FORECAST REMAINS ON TRACK AS SHOWERS AND THUNDERSTORMS
CONTINUE TO LIFT NORTH ACROSS THE FORECAST AREA AHEAD OF THE MAIN
UPPER LEVEL LOW WHICH CLOSES OFF OVER WESTERN UTAH LATER TONIGHT.
FEW STRONGER CELLS EARLIER THIS EVENING DROPPED AN ESTIMATED HALF
INCH OF RAIN JUST EAST OF VERNAL UTAH AND NEAR ARCHES NATIONAL
PARK. OTHERWISE...WIDESPREAD COVERAGE OF PRECIPITATION GENERALLY
UNDER A QUARTER INCH THUS FAR. HRRR REMAINS ACTIVE OVERNIGHT WITH
HEAVIER SHOWERS LIFTING NORTH INTO THE SOUTHERN VALLEYS AFTER
MIDNIGHT AND INTO THE CENTRAL ZONES AFTER 3 AM. HIGH SURFACE
MOISTURE AND MILD H7 TEMPS WILL KEEP THE SNOW LEVELS HIGH THROUGH
LATE TONIGHT...BEFORE THE LOW MOVES CLOSER AND ALLOWS TEMPS TO
DROP A BIT TO DRIVE LEVELS DOWN TO AROUND 9K BY EARLY TUESDAY
MORNING. BULK OF THE ACCUMULATIONS SNOWFALL WILL BE RIGHT AT THE
HIGHER PASSES AND ABOVE. CURRENT HIGHLIGHTS LOOK GOOD AND NO
ADJUSTMENTS WILL BE MADE AT THIS TIME.

&&

.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 06Z TAFS THROUGH 06Z TUESDAY NIGHT)
ISSUED AT 933 PM MDT MON MAY 18 2015

WIDESPREAD RAIN AND HIGH MOUNTAIN SNOW WILL CONTINUE ACROSS THE
REGION THROUGH TONIGHT...WITH SHOWERS BECOMING MORE SCATTERED IN
NATURE BY TUESDAY AFTERNOON. ISOLATED THUNDERSTORMS WILL ALSO
CONTINUE. EXPECT OCCASIONAL MVFR TO IFR CIGS AND VSBYS NEAR THE
STRONGER SHOWERS...WITH MOUNTAIN TOPS FQTLY OBSCURED THROUGH
TUESDAY AFTERNOON. SHOWERS END TUESDAY EVENING WITH VFR WEATHER
RETURNING TO MUCH OF THE FORECAST AREA.

&&

.GJT WATCHES/WARNINGS/ADVISORIES...
CO...WINTER WEATHER ADVISORY UNTIL NOON MDT TUESDAY FOR COZ010-012-
     013-018-019.

UT...NONE.
&&

$$

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


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