Preliminary Forecasts
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FXUS02 KWNH 220649
PREEPD

PRELIMINARY EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
249 AM EDT SUN OCT 22 2017

VALID 12Z WED OCT 25 2017 - 12Z SUN OCT 29 2017

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

GUIDANCE PERSISTS WITH THE AGREEABLE IDEA OF AMPLIFIED TROUGHING
OVER THE LOWER 48 BETWEEN STRONG RIDGES OVER THE EASTERN
PACIFIC/WEST COAST AND WEST-CENTRAL ATLANTIC.  TWO UNCERTAINTIES
WITHIN THIS PATTERN WILL INVOLVE A LEADING EASTERN TROUGH LIFTING
NORTHEASTWARD MID-LATE WEEK, FOLLOWED BY UPSTREAM TROUGH ENERGY
AMPLIFYING INTO THE CENTRAL U.S. AND ASSOC SURFACE EVOLUTION.
THEN BY LATER IN THE PERIOD FURTHER UNCERTAINTIES ORIGINATING OVER
THE PACIFIC HAVE THE POTENTIAL TO DISRUPT MEDIUM-SCALE DETAILS
EVEN IF THE RIDGE-TROUGH-RIDGE LONGWAVE PATTERN ENDS UP PREVAILING
FROM THE MULTI-DAY MEAN PERSPECTIVE.

THERE IS REASONABLE AGREEMENT THAT THE INITIAL EASTERN TROUGH WILL
TAKE ON A NEGATIVE TILT AS IT PROGRESSES NORTHEASTWARD, POSSIBLY
CONTAINING AN EMBEDDED CLOSED LOW FOR A TIME, LEADING TO POTENTIAL
SURFACE WAVE DEVELOPMENT THAT WOULD AFFECT NEW ENGLAND PRIMARILY
WED-THU.  THE MOST SIGNIFICANT DIFFERENCES WITH THIS FEATURE
CURRENTLY INVOLVE TIMING AS THE 12Z ECMWF AND RECENT CMC RUNS
STRAY FASTER THAN THE MAJORITY CLUSTER CONSISTING OF THE GFS/UKMET
RUNS AND GEFS/ECMWF MEANS.  THE 12Z ECMWF IS QUESTIONABLE ALOFT AS
IT PULLS OFF ENERGY FROM THE SOUTHERN PART OF THE TROUGH TO FORM A
WESTERN ATLANTIC SYSTEM THAT ENDS UP WHERE ALL OTHER GUIDANCE HAS
HIGH PRESSURE BY FRI.  IN ADDITION TO REPRESENTING CONSENSUS, THE
GFS CLUSTER MAINTAINS REASONABLE CONTINUITY FROM PRIOR CYCLE.

ADJUSTMENTS IN THE 12Z ECMWF FROM THE MORE EXTREME 00Z/21 RUN HAVE
YIELDED SOMEWHAT LESS SPREAD IN GUIDANCE FOR THE TRAILING CENTRAL
U.S. TROUGH AMPLIFICATION.  CURRENTLY THERE IS A DECENT SIGNAL FOR
THE CORE OF THE TROUGH TO TRACK OVER THE NORTHERN PLAINS/UPPER MS
VALLEY REGION, WITH SOUTHWESTERN CANADA LOW PRESSURE GENERALLY
EXPECTED TO TRACK OVER OR NEAR A NORTHERN TIER-UPPER GREAT LAKES
PATH WHILE THE TRAILING COLD FRONT DROPS THROUGH THE WEST/PLAINS
AND EVENTUALLY PUSHES INTO THE EAST.  WITH THE 18Z GFS (AS WELL AS
NEW 00Z RUN) LEANING ON THE SOUTHERN SIDE OF THE SPREAD FOR THE
SURFACE LOW, AN AVERAGE OF THE 12Z GFS/ECMWF AND 18Z GEFS/ECMWF
MEANS PROVIDE A GOOD INTERMEDIATE SOLUTION MID-LATE WEEK.

WELL AHEAD OF THE FRONT THERE CONTINUES TO BE A LOT OF SPREAD
REGARDING POTENTIAL MOISTURE FEED OR EVEN TROPICAL ACTIVITY (IN
THE CASE OF THE CMC RUNS AND 00Z UKMET) EMANATING FROM THE
NORTHWEST CARIBBEAN AND AFFECTING PARTS OF FLORIDA.  THE ENVELOPE
REMAINS FAIRLY SIMILAR TO 24 HOURS AGO WITH GFS RUNS SHOWING THE
MOST PRONOUNCED INCREASE OF MOISTURE OVER FL WHILE ECMWF-BASED
GUIDANCE THE LEAST.  GEFS MEANS ARE STILL BETWEEN THE TWO
EXTREMES.  SOME DEGREE OF COMPROMISE LOOKS BEST GIVEN THAT
EVENTUAL APPROACH OF THE SURFACE FRONT MAY HELP TO PULL UP
SOMEWHAT MORE MOISTURE THAN FORECAST BY ECMWF RUNS.

OVER THE EASTERN PACIFIC/WEST COAST, GFS/ECMWF RUNS AND THEIR
MEANS HAVE STABILIZED OR SLIGHTLY FURTHERED PRIOR TRENDS TOWARD A
VERY STRONG EAST PACIFIC RIDGE BUILDING BY THU-FRI AND DRIFTING
TOWARD THE WEST COAST, DRIFTING INLAND TO ABOUT 120W BY THE END OF
THE PERIOD.  THIS IS A REASONABLE SCENARIO TO BE CONSIDERED AS
MOST LIKELY, HOWEVER CMC RUNS/CMC ENSEMBLES ALONG WITH THE UKMET
SUGGEST POTENTIAL FOR ANOTHER DAMPENING/REBUILDING CYCLE FOR THE
OVERALL RIDGE IN RESPONSE TO THE NEXT NORTH PACIFIC SHORTWAVE.
THE LESS LIKELY CMC/UKMET SCENARIO WOULD LEAD TO A SIGNIFICANT
RESHUFFLING OF SHORTWAVE DETAILS OVER THE LOWER 48 BY NEXT
WEEKEND.  ALSO A FAIR AMOUNT OF SPREAD DEVELOPS FOR AN EASTERN
PACIFIC TROUGH WHICH THE MEANS BRING TO ABOUT 140W BY DAY 7 SUN.
SOME INDIVIDUAL ENSEMBLE MEMBERS BRING THE FEATURE CLOSER TO THE
WEST COAST.  SUCH TIMING MAY BE A LITTLE TOO PROGRESSIVE GIVEN THE
STRENGTH OF THE WEST COAST RIDGE.

IN ORDER TO ACCOMMODATE CONSENSUS/CONTINUITY OR INTERMEDIATE
SOLUTION DEPENDING ON THE REGION, THE UPDATED FORECAST
INCORPORATED COMPONENTS OF THE 12Z GFS/ECMWF, 18Z GEFS/12Z ECMWF
MEAN, AND 12Z UKMET FOR DAYS 3-5 WED-FRI.  DUE TO ISSUES OVER THE
WESTERN ATLANTIC 12Z ECMWF WEIGHT WAS MINIMIZED FRI ONWARD WHILE
ENSEMBLE MEANS GAINED MAJORITY WEIGHT AFTER EARLY FRI TO DOWNPLAY
OTHER OPERATIONAL MODEL DETAIL UNCERTAINTIES.


...SENSIBLE WEATHER HIGHLIGHTS/THREATS...

THE INITIAL SYSTEM AFFECTING THE EAST SHOULD BRING THE HEAVIEST
RAINFALL POTENTIAL TO NEW ENGLAND AROUND WED-THU AS A SLOW MOVING
WAVY FRONT ENHANCES ATLANTIC INFLOW OVER THE REGION.  THERE MAY BE
SOME LIGHTER PRECIP LINGERING OVER THE GREAT LAKES AND VICINITY ON
WED.  THE NEXT SYSTEM DROPPING INTO THE CENTRAL U.S. WILL GENERATE
ONE AREA OF RAIN/SNOW EXTENDING SOUTHEAST ACROSS THE ROCKIES
MID-LATE WEEK, WITH FURTHER AREAS OF PRECIP NEAR NORTHERN TIER LOW
PRESSURE AND COLD FRONT EMERGING FROM THE PLAINS AND PUSHING INTO
THE EAST.  TEMPS OVER THE EXTREME NORTH MAY BE COLD ENOUGH FOR
SOME PRECIP TO FALL IN THE FORM OF SNOW.  DEPENDING ON EXACT
DETAILS/TIMING OF THE OVERALL SYSTEM THERE MAY BE POCKETS OF
MODERATE/HEAVY RAINFALL ALONG THE COLD FRONT OVER SOME LOCATIONS
FROM THE MS VALLEY EASTWARD FRI-SUN.  FARTHER SOUTHEAST THERE IS
STILL BELOW AVERAGE CONFIDENCE REGARDING HOW MUCH MOISTURE MAY
REACH FLORIDA LATE IN THE PERIOD.

VERY WARM TEMPS OF AT LEAST 10-15F ABOVE NORMAL OVER THE
SOUTHWEST/CALIFORNIA AND CONDITIONS THAT MAY FAVOR WILDFIRE RISK
SHOULD MODERATE AFTER MIDWEEK.  AN AREA OF HIGH TEMPS 10-20F ABOVE
NORMAL OVER THE INTERIOR WEST/ROCKIES INTO THE NORTHERN-CENTRAL
PLAINS ON WED WILL BE QUICKLY REPLACED BY NEGATIVE ANOMALIES OF
SIMILAR MAGNITUDE WHICH WILL SPREAD FROM THE HIGH PLAINS INTO THE
EAST-CENTRAL U.S..  EXCEPT FOR NEW ENGLAND WHERE THERE MAY BE SOME
RECORD WARM LOWS ON WED, MUCH OF THE EAST WILL BE ON THE CHILLY
SIDE TO START THE PERIOD WITH A WARMING TREND LATE IN THE WEEK.

RAUSCH

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