Preliminary Forecasts
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177
FXUS02 KWNH 190652
PREEPD

PRELIMINARY EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
251 AM EDT TUE SEP 19 2017

VALID 12Z FRI SEP 22 2017 - 12Z TUE SEP 26 2017

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

THE AMPLIFIED WESTERN TROUGH/EASTERN RIDGE PATTERN WILL BE SLOW TO
EVOLVE DURING THE PERIOD, WITH ONLY GRADUAL WEAKENING OF THE CORE
OF THE WESTERN TROUGH AND EVENTUAL EROSION OF 588+DM HEIGHTS OVER
THE EAST.  GUIDANCE REMAINS AGREEABLE IN PRINCIPLE BUT STILL
DISAGREES ON SOME LOCALLY IMPORTANT DETAILS.  THIS DETAIL SPREAD
DEVELOPS ALREADY IN THE LATTER PART OF THE SHORT RANGE PERIOD.  AS
OF THE START OF DAY 3 FRI RECENT GFS/GEFS MEAN RUNS HAVE DEPICTED
STRONGER THAN CONSENSUS LOW PRESSURE OVER CANADA LEADING TO A MORE
SUPPRESSED WAVE NEAR THE U.S.-CANADIAN BORDER THE NEXT DAY.  ON
THE OTHER HAND THE 12Z ECMWF/ECMWF MEAN WERE ON THE
WESTERN/NORTHWESTERN SIDE OF THE GUIDANCE SPREAD WITH THE LATTER
WAVE THAT TRACKS FROM THE NORTHERN PLAINS INTO CANADA.  BY DAYS
6-7 MON-TUE THE PRIMARY QUESTION MARKS INVOLVE WHETHER/HOW SOME OF
THE WESTERN TROUGH ENERGY MAY BEGIN TO EJECT TO THE
EAST/NORTHEAST, ALONG WITH INFLUENCE OF TROUGH ENERGY OVER
SOUTHERN CANADA AND BY THE END OF THE PERIOD ENERGY ROUNDING THE
UPSTREAM RIDGE.  RECENT TRENDS CONTINUE TO FAVOR A MORE PERSISTENT
TROUGH OVER MUCH OF THE WEST BUT DETAILS ACROSS THE EXTREME
NORTHERN TIER ARE LESS CONCLUSIVE.  THROUGH THE 12Z/18Z CYCLES THE
12Z ECMWF MEAN BECOME SOMEWHAT FASTER THAN MOST OTHER GUIDANCE TO
BRING HEIGHT FALLS INTO THE UPPER MIDWEST/GREAT LAKES AND THUS IS
FASTER WITH THE LEADING WAVY FRONT.  EARLY-MID PERIOD STRENGTH OF
THE EASTERN RIDGE MAY FAVOR A SOLUTION SOMEWHAT SLOWER THAN THE
ECMWF MEAN.

OVER THE WESTERN ATLANTIC, JOSE SHOULD BE A POST-TROPICAL CYCLONE
TO THE SOUTHEAST OF NEW ENGLAND AS OF DAY 3 FRI.  FROM THERE
MODEL/ENSEMBLE SPREAD DIVERGES CONSIDERABLY IN LONGITUDE, RANGING
BETWEEN LINGERING TO THE SOUTH OF OR NEAR NEW ENGLAND (12Z
ECMWF/00Z UKMET) OR CONTINUED EASTWARD PROGRESSION AS IN THE 00Z
GFS AND RECENT CMC RUNS.  AMONG ENSEMBLE MEMBERS, THE WESTERN EDGE
OF THE ENVELOPE THAT HAS INCLUDED RETROGRESSION INTO THE MID
ATLANTIC HAS BEEN OCCUPIED NEARLY EXCLUSIVELY BY SOME ECMWF
MEMBERS.  OPERATIONAL ECMWF RUNS HAVE BEEN TRENDING AWAY FROM THAT
IDEA.  THE ULTIMATE TRACK STILL APPEARS TO DEPEND ON SUBTLE
DETAILS THAT HAVE LOWER PREDICTABILITY MULTIPLE DAYS OUT IN TIME.
MEANWHILE TO THE SOUTH, GUIDANCE SPREAD FOR THE TRACK AND TIMING
OF HURRICANE MARIA RAPIDLY WIDENS AFTER MID-PERIOD.  MONITOR NHC
PRODUCTS FOR THE LATEST INFO ON JOSE AND MARIA.

IN ORDER TO ARRIVE AT AN INTERMEDIATE SOLUTION WHERE MEANINGFUL
DETAIL DIFFERENCES EXIST DURING THE FIRST HALF OF THE PERIOD, THE
UPDATED BLEND INCORPORATED VARIOUS IDEAS FROM THE 12Z-18Z GFS AND
12Z ECMWF/UKMET/CMC.  ENSEMBLE MEAN WEIGHT INCREASED BY THE LATTER
HALF OF THE PERIOD WITH SOMEWHAT MORE 18Z GEFS MEAN THAN ECMWF
MEAN BY DAY 7 TUE GIVEN SLOWER PREFERENCES OVER THE CENTRAL
U.S./GREAT LAKES REGION.  THE BLEND MAINTAINED SOME INPUT FROM THE
OPERATIONAL GFS/ECMWF RUNS THROUGH THE END OF THE FORECAST AND THE
CMC THROUGH DAY 6 MON.  THE BLEND WAS ADJUSTED TO REFLECT NHC
FORECASTS OR EXTRAPOLATION FOR ATLANTIC TROPICAL SYSTEMS.


...SENSIBLE WEATHER/HIGHLIGHTS...

THE ROCKIES AND CENTRAL U.S. WILL BE THE DOMINANT FOCUS FOR
SIGNIFICANT PRECIP DURING THE PERIOD, WITH THE NORTHERN ROCKIES
TRENDING GRADUALLY DRIER DURING THE WEEKEND/EARLY NEXT WEEK.
HIGHER ELEVATIONS OF THE NORTHERN ROCKIES SHOULD SEE SOME SNOW.
FROM ABOUT FRI NIGHT ONWARD, GUIDANCE HAS BEEN SIGNALING FOR
MULTIPLE DAYS THE POTENTIAL FOR A HEAVY RAINFALL EVENT WITHIN AN
AREA EXTENDING FROM THE NORTHERN PLAINS/UPPER MS VALLEY INTO THE
SOUTHERN HIGH PLAINS.  PERSISTENCE OF MOIST SOUTH-SOUTHWESTERLY
FLOW AHEAD OF THE WESTERN TROUGH ALONG WITH VERY SLOW MOVEMENT OF
THE LEADING WAVY SURFACE FRONT WILL INCREASE THE LIKELIHOOD OF
REPEAT ACTIVITY/TRAINING OVER SOME LOCATIONS.  LATE IN THE PERIOD
LOW LEVEL FLOW COULD HAVE SOME DEGREE OF UPSLOPE COMPONENT NEAR
THE SOUTHERN HIGH PLAINS.  WITHIN THE THREAT AREA LOCALIZED TOTALS
OF SEVERAL INCHES FOR THE 5-DAY PERIOD MAY BE POSSIBLE.  FARTHER
EAST, DEPENDING ON ITS TRACK JOSE MAY CONTINUE TO GENERATE A
PERIOD OF STRONG WINDS OVER WATERS NEAR NEW ENGLAND ALONG WITH
PERHAPS SOME PERIPHERAL RAINFALL NEAR SOUTHERN NEW ENGLAND.  A
WEAKNESS ALOFT MAY PROMOTE AREAS OF RAIN OVER THE SOUTH PRIMARILY
IN THE FIRST HALF OF THE PERIOD AND ONSHORE FLOW COULD BRING SOME
LIGHT RAIN TO FLORIDA.

THE TEMPERATURE FORECAST IS CONSISTENT IN SHOWING THE CORE OF
COOLEST TEMPS OVER THE WEST EXTENDING FROM THE GREAT BASIN INTO
NORTHERN HIGH PLAINS WITH BROAD COVERAGE OF MINUS 10-20F ANOMALIES
AND SOME POCKETS EXCEEDING MINUS 20F ANOMALIES.  THE MIDWEST/GREAT
LAKES/NORTHEAST WILL BE THE FOCUS FOR WARMEST TEMPS, 10-20F ABOVE
NORMAL, THOUGH WITH SOME EROSION OF WARMTH OVER THE WESTERN PART
OF THIS AREA WITH TIME.  CONTINUE TO EXPECT TEMPS TO MODERATE OVER
THE WEST COAST STATES AS THE EASTERN PACIFIC RIDGE ALOFT
APPROACHES.

RAUSCH

$$





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