Preliminary Forecasts
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000
FXUS02 KWNH 160642
PREEPD

PRELIMINARY EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
241 AM EDT WED AUG 16 2017

VALID 12Z SAT AUG 19 2017 - 12Z WED AUG 23 2017

...PATTERN OVERVIEW/GUIDANCE EVALUATION/PREFERENCES...

GUIDANCE SHOWS A MODERATELY PROGRESSIVE PATTERN ACROSS NORTHERN
U.S. AND SOUTHERN CANADA.  A TROUGH CROSSING THE EAST WILL LIFT
AWAY AFTER THE WEEKEND WHILE AN UPSTREAM TROUGH WITH EMBEDDED
CLOSED LOW WILL CONTINUE STEADILY EASTWARD FROM WESTERN CANADA.
THIS TROUGH SHOULD AMPLIFY SOME AS IT REACHES EASTERN NORTH
AMERICA, LIKELY COVERING THE NORTHEASTERN QUADRANT OF THE LOWER 48
BY DAY 7 WED.  BEYOND THE RIDGE BUILDING INTO WESTERN-CENTRAL
CANADA AND NORTHERN ROCKIES/HIGH PLAINS, ANOTHER TROUGH WILL
APPROACH THE PACIFIC NORTHWEST AND WESTERN CANADA DURING THE
LATTER HALF OF THE PERIOD.  OVER MORE SOUTHERN LATITUDES ENERGY
ROUNDING THE EASTERN PACIFIC RIDGE WILL SETTLE INTO A TROUGH/UPPER
LOW NEAR THE CALIFORNIA COAST AND A RETROGRADING ATLANTIC RIDGE
WILL RAISE HEIGHTS FROM THE SOUTHEAST INTO SOUTHERN PLAINS.

BASED ON MODELS/ENSEMBLES THROUGH THE 12Z/18Z CYCLES THE MOST
CONTENTIOUS PART OF THE FORECAST INVOLVES THE NORTHEASTERN PACIFIC
TROUGH DURING THE LATTER HALF OF THE PERIOD.  THE PORTION OF THE
FEATURE REACHING THE GULF OF ALASKA REGION SHOULD INCLUDE A
CONTRIBUTION FROM RECURVING TYPHOON BANYAN PLUS INPUT FROM
UPSTREAM NORTHERN PACIFIC FLOW AND HIGHER LATITUDE ALASKA FLOW.
AMONG THE AVAILABLE SOLUTIONS THE FASTER 12Z ECMWF SEEMS TO BECOME
ONE OF THE LESS LIKELY BY DAY 7 WED AS THE GENERAL EVOLUTION
TOWARD MORE AMPLIFIED FLOW SHOULD HOLD BACK TROUGH TIMING
SOMEWHAT.  AT THE SAME TIME THE SLOWER GFS HAD BEEN SOMEWHAT
FASTER IN EARLIER RUNS.  THE 12Z CMC (PERHAPS WITH EXCESSIVE
AMPLITUDE) OFFERS A COMPROMISE.  THERE IS ENOUGH COMPLEXITY IN THE
OVERALL TROUGH EVOLUTION AND LACK OF CLEAR TRENDING IN THE ECMWF
MEAN AND SLOWER GEFS MEAN THUS FAR TO FAVOR A COMPROMISE AMONG THE
NON-12Z ECMWF SOLUTIONS.  WITH THE NEW 00Z GFS/GEFS MEAN/CMC IN,
THE LATTER TWO NUDGED TOWARD THE 12Z ECMWF MEAN WITH THE GFS
REMAINING SLOWER/MORE CLOSED.

AHEAD OF THE ADVANCING WESTERN CANADA TROUGH, THE COLD FRONT
CROSSING THE NORTHERN TIER SHOWS DECENT GUIDANCE CLUSTERING AT
MOST VALID TIMES.  RECENT TRENDS WITH THE FRONT HAVE BEEN FASTER.
NEW 00Z RUNS IN SO FAR SEEM TO CONTINUE THIS IDEA AND MAKE THE 12Z
ECMWF POTENTIALLY A LITTLE SLOW.

RECENT GFS/ECMWF RUNS AND THEIR MEANS REPRESENT THE BEST
INTERMEDIATE SOLUTION FOR THE SYSTEM INITIALLY OVER THE EAST.  THE
ONE EXCEPTION IS WITH THE 12Z/18Z GFS RUNS STILL BEING A LITTLE
STRONGER/SLOWER THAN MOST OTHER GUIDANCE FOR THE SHORTWAVE OVER
THE MS VALLEY AS OF EARLY SAT.  THE NEW 00Z GFS HAS TRENDED MORE
IN LINE WITH CONSENSUS.

BASED ON CONSIDERATIONS RELATIVE TO 12Z/18Z GUIDANCE, THIS
FORECAST UPDATE STARTED WITH A BLEND OF 18Z GFS/GEFS MEAN AND 12Z
ECMWF/ECMWF MEAN RUNS EARLY IN THE PERIOD.  HEADING INTO DAY 5 MON
A SMALL 12Z CMC WEIGHT COULD BE ADDED AS ITS SLOW NORTHEASTERN
SYSTEM EXITED THE PICTURE AND THE TIMING OF ITS NORTHEAST PACIFIC
TROUGH WAS NEAR THE MIDDLE OF THE SPREAD.  THEN THE REST OF THE
FORECAST INCREASED ENSEMBLE MEAN INPUT WHILE PHASING OUT THE 12Z
ECMWF, WITH MINORITY WEIGHTS OF THE 18Z GFS/12Z CMC THAT HAD
QUESTIONABLE SPECIFICS BUT WERE PART OF THE OVERALL PREFERRED
TIMING COMPROMISE OFF THE WEST COAST.


...WEATHER HIGHLIGHTS...

THE FRONT NEAR THE EAST COAST AS OF EARLY SAT WILL SETTLE OVER THE
SOUTHEAST AND POSSIBLY HELP TO ENHANCE DIURNALLY FAVORED
CONVECTION OVER THE REGION.  THE GULF COAST AND FLORIDA MAY ALSO
SEE SOME EPISODES OF LOCALLY HEAVY CONVECTION.  THE COLD FRONT
PROGRESSING SOUTHEAST FROM THE NORTHERN PLAINS FROM THE WEEKEND
ONWARD WILL LIKELY GENERATE SHOWERS/STORMS WITH BEST HEAVY RAIN
POTENTIAL EXPECTED TO BE OVER PARTS OF THE MIDWEST, WHICH MAY BE
ADDITIONALLY INFLUENCED BY THE TRAILING PART OF THE SOUTHEASTERN
FRONT THAT SHOULD EXTEND BACK INTO THE CENTRAL PLAINS FOR A PERIOD
OF TIME.  MONSOONAL CONVECTION OF VARYING INTENSITY SHOULD BE MOST
COMMON OVER THE SOUTHERN HALF OF THE ROCKIES.  LIGHTER AND MORE
SCATTERED ACTIVITY MAY OCCUR OVER THE REST OF THE FOUR CORNERS
REGION AND PARTS OF THE INTERIOR WEST AND NORTHERN ROCKIES.  THE
MOST PRONOUNCED AREA OF TEMPERATURE ANOMALIES SHOULD BE AHEAD OF
THE NORTHERN TIER FRONT WITH MAX/MIN READINGS TENDING TO BE 5-10F
ABOVE NORMAL AND PERHAPS LOCALLY A LITTLE WARMER.  EXPECT SOUTHERN
STATES TO SEE MIN TEMPS PERSISTENTLY ABOVE NORMAL BY A FEW DEGREES
F.

RAUSCH

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