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Prognostic Meteorological Discussion
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FXUS02 KWBC 181542
PMDEPD

EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1141 AM EDT TUE JUN 18 2013

VALID 12Z FRI JUN 21 2013 - 12Z TUE JUN 25 2013

...OVERVIEW AND MODEL/ENSEMBLE EVALUATION...

THE MEDIUM RANGE PERIOD WILL TURN QUITE SUMMER-LIKE RIGHT ON CUE
AS THE SUMMER SOLSTICE OCCURS ON FRI/D3... JUNE 21. A BUILDING
SUBTROPICAL RIDGE SHOULD OVERTAKE THE LOWER LATITUDES ALONG 30N
BUT EXPANDING INTO THE INNER MOUNTAIN WEST BY NEXT WEEK. THE
MODELS/ENSEMBLES GENERALLY SHOW GOOD AGREEMENT THROUGHOUT THE
PERIOD WITH ONLY MINOR DEVIATIONS IN TIMING/AMPLITUDE OF MINOR
SHORTWAVE DISTURBANCES ACROSS THE NORTHERN TIER. BOTH THE GEFS AND
EC ENSEMBLES HANDLE THE LONGWAVE PATTERN COMPARABLY... WITH THE
NOTABLE DIFFERENCE IN TIMING /GEFS ABOUT 12 HRS AHEAD OF THE
ECENS/. THE DETERMINISTIC RUNS OF THE ECMWF/GFS GENERALLY FOLLOWED
THEIR PARENT ENSEMBLE SYSTEM. THE 00Z-06Z GFS RUNS APPEARED TOO
STRONG WITH A SFC WAVE ALONG THE FRONT IN THE NORTHEAST THIS
WEEKEND... AND THEREFORE FARTHER SOUTH AND COOLER... COMPARED TO
CONTINUITY AND THE BROADER CONSENSUS. THE 06Z PARALLEL RUN OF THE
GFS WAS CLOSER TO THE 00Z ECMWF/ECENS/GEFS... SO THEIR CONSENSUS
WAS USED AS A BASE IN THE FORECAST THROUGHOUT THE PERIOD. A BIT
MORE WEIGHT WAS GIVEN TO THE ECMWF/ECENS GIVEN CONTINUITY.

...SENSIBLE WEATHER HIGHLIGHT...

WITH THE POLAR JET MEANDERING ACROSS THE NORTHERN TIER OF STATES
THIS PERIOD... EXPECT TEMPERATURES TO REMAIN NEAR OR BELOW NORMAL
THERE... WITH PERIODS OF SHOWERS AS NEW BATCHES OF SHORTWAVE
ENERGY PASS. THE ESSENTIALLY WHOLESALE EJECTION OF THE NORTHWEST
TROUGH ACROSS THE NORTHERN PLAINS AND UPPER MIDWEST SHOULD BE
ACCOMPANIED BY A VIGOROUS BOUT OF CONVECTION... THOUGH
PREDICTABILITY REMAINS LOW PER THE LATEST SPC FORECAST AFTER
FRI/D3 DUE TO TIMING/EVOLUTION DIFFERENCES. BY THE END OF THE
PERIOD... NEW SHORTWAVE ENERGY SHOULD BE APPROACHING THE NORTHWEST
COAST... WITH A FRESH ROUND OF SHOWERS THERE. AMOUNTS COULD TOP AN
INCH OR TWO IN SOME LOCATIONS PER SOME RECENT DETERMINISTIC RUNS
AND THE IN-HOUSE WPC BIAS-CORRECTED ENSEMBLE. THE SOUTHEAST WILL
BE SHOWERY ON THE EQUATORWARD SIDE OF THE ZONAL SUBTROPICAL RIDGE
AXIS THERE... AIDED EARLY IN THE FORECAST BY THE LOW-LEVEL
CONVERGENCE ASSOCIATED WITH FRONTOLYSIS OVER THE REGION. THE
SOUTHWEST STATES MAY SEE SOME HIGH-BASED DRY CONVECTION...
ESPECIALLY DURING THE FIRST HALF OF THE PERIOD WHILE HEIGHTS ARE
STILL RELATIVELY LOW IN THE WAKE OF THE DEPARTING TROUGH TO THE
NORTH. MEASURABLE PRECIPITATION MAY BE CONFINED TO NEW MEXICO
RATHER THAN ARIZONA.


FRACASSO/CISCO

$$
National Weather Service National Headquarters
