Prognostic Meteorological Discussion
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000
FXUS02 KWBC 200647
PMDEPD

EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
246 AM EDT WED SEP 20 2017

VALID 12Z SAT SEP 23 2017 - 12Z WED SEP 27 2017

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

LATEST MODEL/ENSEMBLE MEAN RUNS CONTINUE TO SHOW FAIRLY GOOD
AGREEMENT REGARDING A PERSISTENT AND VERY AMPLIFIED WESTERN
TROUGH/EASTERN RIDGE PATTERN FOR MOST OF THE PERIOD AND THEN THE
BEGINNING OF A TRANSITION TOWARD A NEW FLOW REGIME BY THE MIDDLE
OF NEXT WEEK.  WITHIN THIS EVOLUTION THE PRIMARY UNCERTAINTIES
EVIDENT IN LATEST/RECENT GUIDANCE ARE MID-LATE PERIOD, INVOLVING
THE COMBINATION OF ENERGY ROUNDING THE SLOWLY ADVANCING EASTERN
PACIFIC RIDGE AND SOME WESTERN TROUGH ENERGY ULTIMATELY LEADING TO
A TROUGH THAT SHOULD ADVANCE TOWARD THE MS VALLEY BY THE END OF
THE PERIOD.  OVER THE PAST 1-2 DAYS THERE HAS BEEN SOME
CONVERGENCE OF GEFS/ECMWF MEANS, THE ECMWF MEAN TRENDING TOWARD
THE GEFS MEAN`S SLOWER PROGRESSION OF HEIGHT FALLS INTO THE UPPER
MIDWEST/GREAT LAKES AND THE GEFS MEAN TRENDING TOWARD THE ECMWF
MEAN IDEA OF MORE SEPARATION BETWEEN THE ADVANCING NORTHERN TIER
TROUGH AND WEAK TROUGH THAT LINGERS OVER THE SOUTHWEST
U.S./EXTREME NORTHWEST MEXICO.  RECENT ECMWF/CMC RUNS HAVE TENDED
TO BE ON THE STRONGER SIDE WITH THE CORE OF ENERGY WITHIN THE
NORTHERN TIER TROUGH AND THUS MORE DEVELOPED THAN MOST OTHER
SOLUTIONS WITH SURFACE LOW PRESSURE.  INDIVIDUAL MODEL RUNS HAVE
BEEN INCONSISTENT FOR WHAT HAPPENS WITHIN THE LATE PERIOD
SOUTHWEST TROUGH, RANGING BETWEEN AN OPEN TROUGH VERSUS SOME
ENERGY PINCHING OFF A CLOSED LOW NEAR BAJA CALIFORNIA.  THE FULL
ENSEMBLE SPREAD ENCOMPASSES THESE VARIED SOLUTIONS WITH THE
RESULTING MEANS PROVIDING A MORE STABLE WEAK OPEN TROUGH WITH
SIMILAR ALIGNMENT.

COMPLEXITY OF THE WESTERN ATLANTIC PATTERN CONTINUES TO POSE
FORECAST CHALLENGES FOR T.S. JOSE, WHICH SHOULD HAVE POST-TROPICAL
STATUS AS OF THE START OF THE PERIOD, AND HURRICANE MARIA THAT
WILL LIKELY TRACK OFFSHORE THE EAST COAST TO SOME DEGREE.  JOSE
WILL BE STUCK BETWEEN EASTERN U.S. AND WESTERN ATLANTIC RIDGES
ALOFT WITH SUBTLE DETAIL DIFFERENCES AFFECTING THE EVENTUAL TRACK.
 ECMWF/UKMET RUNS HAVE TENDED TO BE THE MOST EAGER TO PULL THE
WEAKENING SYSTEM WESTWARD INTO THE MID ATLANTIC-- A SCENARIO THAT
HAS BEEN SHOWING UP PRIMARILY WITH SOME ECMWF ENSEMBLE MEMBERS
OVER RECENT DAYS, THOUGH A SMALL NUMBER OF GEFS MEMBERS HAVE NOW
STARTED TO HINT AT IT TO A LESSER DEGREE.  HOWEVER AN EARLY LOOK
AT THE NEW 00Z ECMWF SHOWS A REVERSAL TO A MORE OFFSHORE SOLUTION
BUT NOT TO THE EXTENT OF THE 00Z/19 RUN.  MEANWHILE THE 18Z/00Z
GFS RUNS AND LATER IN THE PERIOD 12Z ECMWF STRAY TO THE WEST OF
THE CENTROID OF GUIDANCE FOR HURRICANE MARIA, ALBEIT STILL WITHIN
THE ENSEMBLE SPREAD THAT HAS TENDED TO DIVERGE FAIRLY RAPIDLY BY
THE DAYS 5-7 TIME FRAME.  OPERATIONAL RUNS HAVE NOT BEEN VERY
CONSISTENT ON TRACK LONGITUDE THUS FAR.  IN ORDER TO MAINTAIN
CONTINUITY WITH A CONSENSUS EXTRAPOLATION OF THE NHC ADVISORY, THE
18Z GFS WAS EXCLUDED FROM THE FORECAST BLEND MON ONWARD AND THE
12Z ECMWF WED ONWARD.  CHECK NHC PRODUCTS FOR LATEST INFORMATION
ON JOSE AND MARIA.

REASONABLE AGREEMENT IN MOST RESPECTS EARLY IN THE PERIOD FAVORED
PRIMARY EMPHASIS ON THE OPERATIONAL 12Z/18Z MODELS, WITH A
TRANSITION TOWARD THE 18Z GEFS/12Z ECMWF MEANS LATER IN THE PERIOD
AS OPERATIONAL RUNS DIVERGED.  BY DAY 7 WED THE 12Z GFS WAS THE
ONE SOLUTION SIMILAR ENOUGH TO THE MEANS TO ALLOW FOR MINORITY
INCLUSION.


...SENSIBLE WEATHER/HIGHLIGHTS...

GUIDANCE SIGNALS THAT HAVE PERSISTED OVER MULTIPLE DAYS ARE
STEADILY INCREASING THE CONFIDENCE IN HEAVY RAIN POTENTIAL WITHIN
AN AREA APPROXIMATELY FROM MN THROUGH THE CENTRAL PLAINS AND
SOUTHERN HIGH PLAINS. THIS IS IN ASSOCIATION WITH THE SLOW MOVING
WAVY FRONT CROSSING THE CENTRAL U.S. DURING THE PERIOD AND A
MULTI-DAY PERIOD OF SOUTH-SOUTHWESTERLY FLOW ALOFT.
NORTHERN-CENTRAL AREAS SHOULD SEE HEAVIEST RAINFALL DURING THE
FIRST HALF OF THE PERIOD, WITH GRADUAL PROGRESSION OF THE FRONT
AND ITS UPPER SUPPORT HELPING TO TAPER OFF AMOUNTS THEREAFTER.
SOUTHERN AREAS WILL LIKELY SEE PERIODS OF RAIN THROUGH THE PERIOD,
INITIALLY IN THE WARM SECTOR, AND THEN POSSIBLY ENHANCED BY LOW
LEVEL UPSLOPE FLOW AFTER FRONTAL PASSAGE.  A LINGERING UPPER
TROUGH OVER THE SOUTHWEST U.S./NORTHWEST MEXICO MAY AID IN
PERSISTENCE OF SOUTHERN PLAINS ACTIVITY AS WELL.  REPEAT
ACTIVITY/TRAINING MAY PRODUCE LOCALIZED 5-DAY TOTALS OF SEVERAL
INCHES WITHIN THE THREAT AREA.

LOCATIONS NEAR THE EAST COAST SHOULD CONTINUE TO MONITOR FORECASTS
FOR JOSE AND MARIA.  SOME PRECIP COULD STRAY INTO PARTS OF THE MID
ATLANTIC/NEW ENGLAND DEPENDING ON THE TRACK OF JOSE AS IT WEAKENS.
 A TRACK FOR MARIA IN THE WESTERN PART OF THE FULL GUIDANCE SPREAD
WOULD BRING SOME RAIN/WIND TO SOME EAST COAST LOCATIONS NEXT WEEK.
 OPERATIONAL MODELS HAVE BEEN INCONSISTENT ON TRACK LONGITUDE THUS
FAR, LIMITING CONFIDENCE IN ANY SPECIFIC SOLUTION.

CHILLY TEMPS WILL EXTEND FROM MUCH OF THE WEST INTO THE
NORTHERN-CENTRAL PLAINS AND EVENTUALLY SOUTHERN HIGH PLAINS.
WESTERN STATES WILL SEE A GRADUAL MODERATING TREND AS THE WESTERN
TROUGH ALOFT SLOWLY WEAKENS BUT THE UPSTREAM PACIFIC RIDGE SHOULD
ADVANCE ONLY FAR ENOUGH TO BRING ABOVE NORMAL TEMPS TO THE WEST
COAST STATES BY MIDWEEK.  EXPECT BROAD COVERAGE OF MINUS 10F AND
GREATER ANOMALIES WITH SOME HIGHS 20F OR MORE BELOW NORMAL ONE OR
MORE DAYS EARLY IN THE PERIOD FROM THE GREAT BASIN INTO
NORTH-CENTRAL ROCKIES/HIGH PLAINS.  FARTHER EAST EXPECT WARMEST
TEMPS RELATIVE TO NORMAL FROM THE MIDWEST THROUGH NEW ENGLAND WITH
PLUS 10-20F ANOMALIES FOR HIGHS.  SOME RECORD WARM MINS EXCEEDING
20F ABOVE NORMAL ARE POSSIBLE EARLY IN THE PERIOD OVER THE UPPER
MIDWEST.  THIS AREA OF WARMTH WILL GRADUALLY ERODE FROM THE WEST
AS THE WAVY PLAINS FRONT PUSHES EASTWARD.

RAUSCH

$$




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