Prognostic Meteorological Discussion
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000
FXUS02 KWBC 051513
PMDEPD

EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1112 AM EDT SUN JUL 05 2015

VALID 12Z WED JUL 08 2015 - 12Z SUN JUL 12 2015

...PATTERN OVERVIEW...

AN UPPER LOW OFF THE WEST COAST WILL DRIFT SLOWLY EASTWARD IN THE
MEDIUM RANGE BEFORE LIFTING NORTHEASTWARD ONCE IT MOVES INTO
CALIFORNIA AND OPENS UP AS IT MOVES ACROSS THE INTERIOR.  ONCE IT
PASSES INLAND...THERE ARE SEVERAL SOLUTIONS THAT SUGGEST THAT IT
WILL ALSO BE ASSOCIATED WITH HEIGHT RISES CENTERED OVER THE ROCKY
MOUNTAINS.  SOME OF THE ENSEMBLE MEANS...INCLUDING THE GFS/NAEFS
HAVE THE TROUGH MOVING LIFTING NORTHEASTWARD AND THEN DROPPING
SLIGHTLY OVER THE NORTHERN PLAINS.  MEANWHILE THE EUROPEAN
ENSEMBLES SUGGEST THAT THE TROUGH CONTINUES NORTHWARD ALONG THE
WEST COAST...ALLOWING A STRONGER RIDGE TO DEVELOP OVER THE
SOUTHWEST.  THESE DIFFERENCES ARE IMPORTANT SINCE THE EVOLUTION OF
HOW THIS LIFTING TROUGH EITHER AMPLIFIES A RIDGE OVER THE WEST OR
RESULTS IN A FLATTER FLOW OVER THE US WILL HAVE A LARGE IMPACT ON
HOW TEMPERATURES/PRECIPITATION EVOLVE LATER IN THE MEDIUM RANGE.

EARLY IN THE FORECAST...THE PATTERN REMAINS CONSISTENT WITH THE
FORECASTS OVER THE PAST FEW DAYS WITH A COLD FRONT MOVING INTO THE
OHIO VALLEY/NORTHEAST ON WEDNESDAY WITH A TRAILING FRONT FROM THE
SOUTHERN PLAINS NORTHEASTWARD ACROSS THE MID MISSISSIPPI VALLEY
INTO THE OHIO VALLEY BEING A FOCUS FOR SHOWERS AND THUNDERSTORMS.
THIS COLD FRONT APPEARS TO STALL OUT FROM THE CENTRAL PLAINS
ACROSS THE OHIO VALLEY INTO THE MID ATLANTIC ON THURSDAY...AGAIN
PROVIDING A FOCUS FOR SHOWERS/THUNDERSHOWERS...WITH A POSSIBLE
WAVE ALONG THE FRONT ALTHOUGH AN EXACT LOCATION IS REALLY HARD TO
PINPOINT YET.  BY FRIDAY...THE WEAK STATIONARY BOUNDARY SEEMS TO
PERSIST AND EXAMINATION OF MANY DIFFERENT SOLUTIONS SUGGEST THAT
THE BOUNDARY MAY CONTINUE TO EXIST THROUGH THE END OF THE
FORECAST...INCLUDING SUNDAY.  OF COURSE...BY DAY 7...SUCH WEAK
FEATURES ARE VERY DIFFICULT TO DISCERN WITHIN THE INCREASING
SPREAD OF THE MODEL SOLUTIONS.

...GUIDANCE EVALUATION/PREFERENCES...

IT APPEARS THAT THE EVOLUTION OF THE WEST COAST CLOSED LOW IS AN
IMPORTANT HARBINGER IN HOW THE WEATHER PATTERNS COULD EVOLVE LATER
IN THE FORECAST.  COMPARING THE MEANS...THE CANADIAN IS THE
OUTLIER THAT FAVORS A BUILDING RIDGE IN THE EAST...SUGGESTING A
RETURN TO HOT WEATHER.  HOWEVER...THE OTHER MODELS ARE MUCH LESS
OBVIOUS AND WHILE THE LIFTING OF THE TROUGH ACROSS THE WEST COULD
LEAD TO A GENERAL RAISING OF HEIGHTS DOWNSTREAM FROM THE ROCKIES
EASTWARD...THE MEANS AND CERTAINLY THE OPERATIONAL RUNS ARE MORE
NUANCED.  IN GENERAL...THE ECMWF MEMBERS TREND SLOWER WITH THE
MOVEMENT OF THE TROUGH TOWARD THE WEST COAST BY DAY 5/FRIDAY WITH
THE MEAN UPPER LOW OVER NORTH CENTRAL CALIFORNIA WHILE THE
GEFS/NAEFS MEMBERS ARE FASTER WITH THE MEAN LOW ONE STATE EAST
OVER NEVADA.  AT THIS POINT IN THE FORECAST...THE DIFFERENCES DO
NOT APPEAR TO IMPACT THE RESULTING FORECAST MUCH BUT LATER
FORECASTS FOR DAYS 6 AND 7 REMAIN HIGHLY UNCERTAIN AS THE IMPACT
OF THE LIFTING TROUGH AND DOWNSTREAM HEIGHTS HAVE A LARGE SPREAD
OF SOLUTIONS.

WITH ALL THE UNCERTAINTY...THERE DOES APPEAR TO BE A TREND IN THE
DAY 7/SUNDAY FORECASTS SHOWING A BUILDING RIDGE NOT ONLY IN THE
EUROPEANS SOLUTIONS... WITH A HIGH AMPLITUDE RIDGE IN THE 00Z
ECMWF OPERATIONAL RUN.  BOTH THE GEFS/NAEFS MEANS ALSO SHOW A
BUILDING RIDGE OVER THE NORTHERN ROCKIES WITH A REMNANT OF THE
WEST COAST TROUGH MOVING EAST OF THE RIDGE AXIS INTO THE NORTHERN
PLAINS STATES.  IF THE TREND FOR THE BUILDING RIDGE PROVES TO BE
SOMEWHAT ACCURATE...THE LIKELIHOOD A BUILDING RIDGE IN THE EAST
BECOMES LESS AND LESS.  AT PRESENT...THE GEFS/NAEFS SHOW A FAIRLY
FLAT FLOW IN THE EAST WHILE THE EUROPEAN MEANS CONTINUE TO POINT
TO A WEAK TROUGH IN THE EAST.  THESE SORT OF DIFFERENCES WILL
HOPEFULLY COME MORE CLEAR AS THE PERIOD DRAWS CLOSER.  THE
FORECAST TO KEEP THE WEAK STATIONARY FRONT IN THE EAST RELIES ON
THE FLAT FLOW REMAINING IN THE EAST AS A COMPROMISE BETWEEN THE
EUROPEAN AND GEFS/NAEFS FORECASTS.

CYCLONIC FLOW NEAR HUDSON BAY MAINTAINS A FRONTAL SYSTEM CLOSE TO
THE US CANADIAN BORDER BUT THE FORECASTS CURRENTLY DO NOT SUGGEST
THAT THE FRONT MAKES IT FARTHER SOUTH THAN THE US BORDER FOR THE
TIME BEING.  A STRONGER COLD FRONT OVER SOUTH CENTRAL CANADA THAN
SUGGESTED BY THE MEANS ALLOWED US TO PREFER THE 00Z GFS OVER THE
06Z VERSION IN GENERATING THE GRAPHICS.  IN
GENERAL...THOUGH...USED A COMPOSITE BLEND OVER THE ENTIRE FORECAST
WITH ONLY A MAJORITY OF OPERATIONAL RUNS USED ON DAY 3/WEDNESDAY
AND A 50/50 BLEND OF OPERATIONAL/MEANS ON DAYS 4 AND 5/THURSDAY
AND FRIDAY WITH LITTLE PREFERENCE FOR ANY ONE MODEL OVER ANOTHER.
A SLIGHT PREFERENCE FOR THE ENSEMBLE MEANS OVER THE OPERATIONAL
ECMWF/GFS WAS USED FOR DAY 6/SATURDAY WHILE MOST OF THE DAY 7
FORECAST WAS GENERATED BY A COMBINATION OF THE EUROPEAN/GEFS
ENSEMBLE MEANS.


...SENSIBLE WEATHER HIGHLIGHTS...

HAVE CONTINUED TO EMPHASIZE A FAVORED AREA FOR RAINFALL NEAR THE
SURFACE FRONTAL BOUNDARY THAT MAY/MAY NOT CONTINUE INTO NEXT
WEEKEND.  THIS OCCURS WITH THE FOCUS PROVIDED BY THE PRESENCE OF
ONE OR MORE LOW AMPLITUDE SHORTWAVE TROUGHS ALOFT. NORTH OF THE
FRONTAL BOUNDARY...COOLER AIR AND THE POSSIBILITY OF RAIN COOLED
AIR AS WELL COULD KEEP TEMPERATURES BELOW SEASONAL AVERAGES.  HOT
AND HUMID CONDITIONS SHOULD PERSIST SOUTH OF THE BOUNDARY BUT
SHOULD NOT BE EXCESSIVE AT THIS POINT.

THE BIGGER CONCERN IS LATER IN THE FORECAST WHEN THERE IS STILL
LARGE UNCERTAINTY.  ANY AMPLIFICATION OF A RIDGE WHETHER OVER THE
ROCKY MOUNTAINS OR THE SMALLER CHANCE IT OCCURS FARTHER EAST WOULD
CERTAINLY INFLUENCE WHERE ANY EXCESSIVE HEAT COULD DEVELOP.  GIVEN
THE TREND FOR THE MEAN RIDGE TO BUILD RECENTLY OVER THE WEST WOULD
MEAN THAT ANY ADDITIONAL RIDGING COULD SEE THE EXTREME HEAT RETURN
TO PORTIONS OF THE WEST...ALTHOUGH IT IS MUCH TOO EARLY TO JUMP ON
THAT BANDWAGON.

MEANWHILE THE UPPER LOW MOVING INTO THE WEST FROM JUST OFF THE CA
COAST IS STILL EXPECTED TO ENHANCE RNFL OVER FAVORED TERRAIN
INLAND FROM THE CENTRAL WEST COAST AND THEN INTO THE NORTHERN
ROCKIES AND POSSIBLY THE PLAINS.  ALSO ANTICIPATE ONE OR MORE
EPISODES OF CONVECTION WITH VARYING INTENSITY OVER THE CNTRL-SRN
ROCKIES AND DIURNAL ACTIVITY OVER PARTS OF THE SOUTHEAST.
EVOLUTION OF FLOW ALOFT OVER THE WEST SHOULD SUPPORT A SIGNIFICANT
COOLING TREND IN THE NORTHWEST AFTER SOME LOCATIONS SEE HIGHS
10-20F ABOVE NORMAL EARLY IN THE FORECAST.

KOCIN

$$




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