Preliminary Forecasts
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FXUS02 KWNH 230653
PREEPD

PRELIMINARY EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
252 AM EDT MON OCT 23 2017

VALID 12Z THU OCT 26 2017 - 12Z MON OCT 30 2017

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

A GOOD CONSENSUS OF GUIDANCE CONTINUES TO SHOW AMPLIFIED EASTERN
U.S. TROUGHING ALOFT IN THE MEAN, AS SUPPORTED BY STRONG POSITIVE
HEIGHT ANOMALY CENTERS OFF THE BC COAST AND NORTHERN ATLANTIC PER
MULTI-DAY MEAN CHARTS.  WITHIN THIS AGREEABLE PATTERN THERE HAS
BEEN MEANINGFUL SPREAD AND VARIABILITY AMONG INDIVIDUAL MODELS AND
ENSEMBLE MEMBERS FOR SOME OF THE IMPORTANT DETAILS.

ONE SUCH DETAIL INVOLVES THE SHORT RANGE UPPER TROUGH LIFTING INTO
THE NORTHEAST BY THU.  AS IN PAST DAYS THERE IS STILL A DECENT
AMOUNT OF TIMING SPREAD, WHICH IMPACTS THE DURATION OF HEAVY
RAINFALL OVER NEW ENGLAND.  TRENDS OVER THE PAST 24 HOURS HAVE
GENERALLY BEEN SOMEWHAT FASTER WHILE THE UPPER SHORTWAVE TAKES
LONGER TO CLOSE OFF A LOW IF IT OCCURS.  AN AVERAGE OF THE 12Z
GFS/ECMWF AND 18Z GEFS/12Z ECMWF ENSEMBLE MEANS PROVIDED A
REASONABLE STARTING POINT TO REFLECT THE TRENDS WHILE WAITING FOR
FURTHER CONFIRMATION BEFORE ADJUSTING EVEN FASTER SUCH AS
SUGGESTED BY THE CMC/UKMET/18Z GFS.  THUS FAR NEW 00Z RUNS STILL
FAVOR FASTER PROGRESSION.

UPSTREAM THE GUIDANCE HAS DIFFERED WITH THE TIMING/ORIENTATION OF
TROUGH ENERGY AMPLIFYING INTO THE CENTRAL-EASTERN U.S., IN
PARTICULAR WITH SOME ECMWF RUNS TENDING TO HOLD ENERGY BACK
FARTHER WEST THAN THE MAJORITY OF MODELS/MEANS.  PRIOR 00Z CYCLE
HAD NUDGED SOMEWHAT SLOWER VERSUS CONTINUITY ONLY TO REVERT BACK
TO TIMING AT LEAST AS FAST OR EVEN FASTER IN THE NEWER 12Z/18Z
CYCLE.  THIS TREND CORRESPONDS TO A SLIGHT EASTWARD SHIFT OF THE
STRONG RIDGE THAT BUILDS OFF THE WEST COAST BY THU.  IN
ASSOCIATION WITH THIS TROUGH EXPECT A FAIRLY STRONG STORM SYSTEM
TO TRACK ACROSS THE NORTHERN TIER/UPPER GREAT LAKES AND THEN
PROGRESS/ELONGATE INTO CANADA WHILE THE TRAILING COLD FRONT PUSHES
INTO THE EAST FROM THE PLAINS THU-SAT.  AS WITH THE PRECEDING
TROUGH THERE IS A DECENT SIGNAL THAT THIS TROUGH MAY TAKE ON A
NEGATIVE TILT LATE IN THE PERIOD AND SUPPORT EAST COAST/NEW
ENGLAND INTO EASTERN CANADA WAVE DEVELOPMENT.  OVERALL A
MODEL/ENSEMBLE MEAN BLEND (SAME COMPONENTS AS FOR THE LEADING
TROUGH), WITH MORE ENSEMBLE MEAN WEIGHT LATER IN THE FORECAST,
REPRESENTS THE IDEAS OF LATEST GUIDANCE WELL.  AHEAD OF THE
SOUTHERN PART OF THE COLD FRONT THAT APPROACHES FLORIDA, GUIDANCE
SEEMS TO BE NAVIGATING TOWARD A COMPROMISE BETWEEN PRIOR WET
GFS/DRY ECMWF RUNS IN THE LATE WEEK/WEEKEND TIME FRAME.

BY NEXT WEEKEND CMC/UKMET RUNS AND CMC ENSEMBLES THROUGH THE 12Z
CYCLE HAVE MAINTAINED THEIR IDEA OF DAMPENING THE STRONG WESTERN
RIDGE MORE AGGRESSIVELY WITH INCOMING NORTHERN PACIFIC ENERGY
VERSUS THE GFS/ECMWF AND THEIR MEANS.  THE AMPLIFIED PATTERN
LEADING UP TO THAT POINT IN TIME HAS FAVORED THE GFS/ECMWF
CLUSTER.  THE NEW 00Z RUNS ARE FINALLY ADDING MORE SUPPORT FOR
THAT SCENARIO WITH THE UKMET NOW MUCH CLOSER TO THE GFS/ECMWF AND
THE CMC NUDGING ITS FORECAST A LITTLE IN THAT DIRECTION.  WHAT
SHORTWAVE ENERGY DOES ROUND THE RIDGE WILL LIKELY SUPPORT A SYSTEM
DROPPING INTO THE NORTHERN TIER LATE IN THE PERIOD.  HOWEVER THERE
IS MINIMAL CLUSTERING OR CONTINUITY AMONG MODELS/ENSEMBLE MEMBERS
AT THIS TIME SO CONFIDENCE IN THE SPECIFICS IS RATHER LOW.

THE FORECAST IS STILL FAIRLY AMBIGUOUS REGARDING WHAT MAY HAPPEN
WITH EASTERN PACIFIC TROUGH ENERGY REACHING NEAR 140W BY SAT, WITH
SOLUTIONS RANGING FROM KEEPING IT ENTIRELY OFFSHORE TO HAVING AT
LEAST SOME OF THE ENERGY REACH THE COAST.  THIS FEATURE WILL BE
NEAR OR WITHIN THE NEXT WEAKENING/REBUILDING CYCLE OF THE OVERALL
WESTERN U.S./EASTERN PACIFIC RIDGE SO IT MAY TAKE A WHILE FOR THIS
ASPECT OF THE FORECAST TO BE RESOLVED.


...SENSIBLE WEATHER HIGHLIGHTS/THREATS...

POTENTIALLY HEAVY RAINFALL OVER NEW ENGLAND SHOULD TREND LIGHTER
EARLY IN THE PERIOD AS DYNAMICS ALOFT/WAVY FRONT LIFT INTO CANADA.
 UPSTREAM SYSTEM TRACKING ACROSS THE UPPER MS VALLEY/UPPER GREAT
LAKES LATE THIS WEEK MAY PRODUCE FOCUSED PRECIP AND STRONG WINDS
IN ITS VICINITY, WITH TEMPS AT SOME LOCATIONS POTENTIALLY COLD
ENOUGH TO PRODUCE SNOW.  BEHIND THE TRAILING FRONT PORTIONS OF THE
ROCKIES/HIGH PLAINS MAY SEE A PERIOD OF RAIN/SNOW DEPENDING ON
ELEVATION.  FROM LATE THIS WEEK ONWARD THE COLD FRONT SHOULD BE
ACCOMPANIED BY SOME AREAS OF RAIN FROM THE MS VALLEY INTO THE
EAST.  RECENT FASTER TRENDS IN GUIDANCE ARE REDUCING THE
PROBABILITY OF HEAVY AMOUNTS WEST OF THE APPALACHIANS BUT SOME
LOCALIZED HEAVY ACTIVITY MAY STILL BE POSSIBLE.  AS THE SUPPORTING
UPPER TROUGH TAKES ON A NEGATIVE TILT LATE IN THE PERIOD,
POTENTIAL REMAINS FOR ATLANTIC INFLOW TO GENERATE ENHANCED
ACTIVITY NEAR THE EAST COAST BY SUN-MON.  THERE MAY ALSO BE A
WINDOW OF TIME IN WHICH THE FRONT COULD TAP SOME OF THE NORTHWEST
CARIBBEAN MOISTURE THUS FURTHER INCREASING RAINFALL.

UNDER THE AMPLIFYING CENTRAL-EASTERN U.S. UPPER TROUGH EXPECT
HIGHS 10-20F BELOW NORMAL TO SPREAD FROM THE NORTHERN PLAINS THU
INTO MUCH OF THE PLAINS/EAST-CENTRAL U.S. BY FRI-SAT WITH SOME
POCKETS OF SUCH ANOMALIES PERSISTING OVER THE EAST INTO NEXT MON.
WESTERN STATES WILL GENERALLY BE WARMER THAN NORMAL, BY 5-15F IN
MOST CASES, THROUGH THE PERIOD WITH GRADUAL MODERATION OVER THE
SOUTHWEST/CALIFORNIA.

RAUSCH

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