Prognostic Meteorological Discussion
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000
FXUS02 KWBC 251559
PMDEPD

EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1159 AM EDT TUE JUL 25 2017

VALID 12Z FRI JUL 28 2017 - 12Z TUE AUG 01 2017

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

THE LATEST CYCLE OF GUIDANCE MAINTAINS GOOD CONTINUITY FOR THE
OVERALL MEAN PATTERN BUT WITH TYPICAL EMBEDDED DETAIL
UNCERTAINTIES.  WHILE A CLOSED LOW/TROUGH PERSISTS OVER THE NERN
PACIFIC, WRN U.S. RIDGING WILL STRENGTHEN DURING THE FIRST HALF OF
THE PERIOD AND LIKELY PERSIST THROUGH THE FIRST HALF OF NEXT WEEK.
 DOWNSTREAM EXPECT MEAN TROUGHING OVER THE EAST, WITH A LEADING
FEATURE/COMBINATION OF FEATURES LIFTING AWAY FROM THE EAST EARLY
NEXT WEEK AS CANADIAN FLOW AMPLIFIES INTO THE GREAT LAKES AND
VICINITY TOWARD MON-TUE.  MORE DIFFUSE ENERGY AMPLIFYING OVER THE
SOUTHEAST EARLY IN THE FCST SHOULD REMAIN OVER THE REGION
THEREAFTER, AND POTENTIALLY BEGIN TO RETROGRADE BY MON-TUE IN
RESPONSE TO A RIDGE BUILDING WWD OVER THE ATLANTIC.

THE MOST SIGNIFICANT UNCERTAINTIES IN TERMS OF GUIDANCE
SPREAD/CONTINUITY AND SENSIBLE WEATHER LIE WITHIN THE ERN U.S.
MEAN TROUGH.  THUS FAR GUIDANCE HAS BEEN HAVING DIFFICULTY IN
RESOLVING THE EVOLUTION OF A FRI-SAT GRTLKS/MID ATLC SHRTWV (WHOSE
SMALL SCALE LENDS ITSELF TO LOW PREDICTABILITY) AND ERN CANADA
CLOSED LOW ALONG WITH POTENTIAL FOR THEIR INTERACTION, ALONG WITH
ASSOC SFC REFLECTION.  ENSEMBLE SPAGHETTI PLOTS STILL SHOW A FAIR
AMOUNT OF SPREAD FOR BEING SO EARLY IN THE MEDIUM RANGE TIME
FRAME.  THE 00Z ECMWF TRENDED SOMEWHAT AWAY FROM ITS PRIOR TWO
RUNS THAT SPUN UP A RATHER CONCENTRATED SFC SYSTEM IN RESPONSE TO
PHASING OF THE TWO FEATURES BUT THE 00Z UKMET STILL HAS A STRONG
SYSTEM SFC/ALOFT.  IN CONTRAST TO THE ECMWF, LATEST GFS RUNS HAVE
TRENDED STRONGER.  AT THIS TIME PREFER TO MAINTAIN A CONSERVATIVE
BLENDED MODEL/ENSEMBLE MEAN/CONTINUITY APPROACH, GIVEN BELOW AVG
PREDICTABILITY AND OCCASIONAL TENDENCY FOR OPERATIONAL MODELS TO
OVER-DEVELOP SFC SYSTEMS IN THE WARM SEASON DUE TO FEEDBACK AND/OR
OTHER PROCESSES.  HOWEVER THIS IS WITH THE UNDERSTANDING THAT
THERE IS STILL POTENTIAL FOR DYNAMICS ALOFT TO EVOLVE IN SUCH A
WAY AS TO SUPPORT STRONGER DEVELOPMENT.  LATER IN THE PERIOD
ENSEMBLES/INDIVIDUAL MODELS SHOW FURTHER INCREASE IN SPREAD AS
UPSTREAM ENERGY BEINGS TO AMPLIFY INTO THE MEAN TROUGH POSN.  BY
THIS POINT IN TIME PREFER A GREATER EMPHASIS ON THE ENSEMBLE MEANS
TO YIELD BETTER CONTINUITY.

ACROSS THE PAC NW THE PRIMARY FCST ISSUE WILL BE THE SEWD EXTENT
OF ENERGY ON THE LEADING SIDE OF THE NERN PAC TROUGH.  DURING THE
WEEKEND SOME ECMWF ENSEMBLE MEMBERS AND 00Z UKMET RESIDE IN THE
SERN SIDE OF THE SPREAD FOR PARTICULAR HGT VALUES.  TENDENCIES FOR
ONE MODEL/ENSEMBLE SYSTEM BECOME LESS NOTICEABLE BY LATE IN THE
PERIOD.  MODELS/MEANS HAVE OSCILLATED OVER RECENT RUNS WITH NO
WELL DEFINED TREND TOWARD HIGHER OR LOWER HGTS OVER THE NORTHWEST,
FAVORING AN INTERMEDIATE SOLN.

THE PREFERRED BLEND APPROACH BASED ON ABOVE CONSIDERATIONS
INCORPORATED IDEAS FROM THE 00Z-06Z GFS/06Z GEFS MEAN AND 00Z
ECMWF/ECMWF MEAN AND CONTINUITY.  OPERATIONAL SOLNS WERE GIVEN
SOMEWHAT MORE WEIGHT FOR DAYS 3-5 FRI-SUN BEFORE EMPHASIS
TRANSITIONED MORE TO THE MEANS.


...WEATHER HIGHLIGHTS...

PERSISTENT MONSOONAL MOISTURE SHOULD SUPPORT PERIODS OF LOCALLY
HEAVY SHWRS/TSTMS OVER THE FOUR CORNERS STATES THROUGH THE PERIOD.
 POTENTIAL FOR SOME OF THIS ACTIVITY TO EXTEND INTO THE SRN HIGHS
PLAINS MAY INCREASE BY THE FIRST HALF OF NEXT WEEK.  ANY POSSIBLE
CONNECTIONS WITH MOISTURE FROM ERN PACIFIC TROPICAL SYSTEMS WILL
HAVE TO BE MONITORED.  THE OTHER AREA WITH GREATEST HEAVY PRECIP
POTENTIAL WILL EXTEND FROM THE EAST COAST TO THE GULF COAST WITH
WHAT COULD BE A FAIRLY VIGOROUS NRN MID ATLC/NERN U.S. WAVE LATE
THIS WEEK AND TRAILING FRONT SETTLING OVER THE SOUTHEAST/GULF
COASTS.  WITHIN THIS BROAD AREA, CURRENTLY EXPECT HIGHEST 5-DAY
RAINFALL TOTALS TO BE NEAR THE SOUTHEAST COAST WHERE THE
AFOREMENTIONED FRONT MAY SERVE AS A CONVECTIVE FOCUS FOR MULTIPLE
DAYS AND PRECIPITABLE WATER VALUES SHOULD REMAIN MOST CONSISTENTLY
HIGH.  A FRONT DROPPING INTO THE NRN TIER BY MID-PERIOD MAY
SUPPORT SOME SCATTERED CONVECTION AS IT CONTINUES SWD/SEWD.

AWAY FROM MONSOONAL CONVECTION OVER THE FOUR CORNERS STATES,
RIDGING ALOFT OVER THE WEST WILL SUPPORT ABOVE NORMAL TEMPS
THROUGH THE PERIOD ACROSS THE INTERIOR WEST/NRN ROCKIES WITH SOME
POCKETS OF PLUS 10-15F ANOMALIES FOR HIGHS OVER THE NRN
ROCKIES/INLAND PAC NW AND AT TIMES OVER NRN CA.  ON THE OTHER HAND
EXPECT NEAR TO BELOW NORMAL TEMPS TO PREVAIL FROM THE PLAINS
THROUGH THE EAST.  MOST ANOMALIES SHOULD BE IN THE SINGLE DIGITS
BUT SOME LOCALLY COOLER HIGH TEMPS ARE POSSIBLE WHERE CLOUDS/RAIN
HAPPEN TO PERSIST FOR MOST OF THE DAYTIME HOURS ON ANY PARTICULAR
DAY.

RAUSCH

$$





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