Preliminary Forecasts
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000
FXUS02 KWNH 240644
PREEPD

PRELIMINARY EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
243 AM EDT TUE OCT 24 2017

VALID 12Z FRI OCT 27 2017 - 12Z TUE OCT 31 2017

...GUIDANCE/UNCERTAINTY ASSESSMENT AND WEATHER/THREATS
HIGHLIGHTS...

THE DOMINANT FOCUS OF THE FORECAST WILL BE ON THE EVOLUTION OF AN
AMPLIFYING TROUGH ALOFT REACHING THE CENTRAL U.S. AS OF THE START
OF THE PERIOD FRI.  MODELS AND ENSEMBLES HAVE BEEN PROVIDING
SIMILAR MESSAGES IN RECENT RUNS BUT WITH SOME SPREAD AND RUN TO
RUN VARIABILITY FOR DETAILS/TIMING.  THE MOST NOTABLE TREND
REPRESENTED BY THE FAVORED 18Z GFS/GEFS MEAN AND 12Z ECMWF/ECMWF
MEAN (PLUS THE 12Z CMC) IS AN INCREASING SIGNAL THAT THE CORE OF
THE UPPER TROUGH MAY CLOSE OFF A LOW, LEADING TO A MORE PERSISTENT
INITIAL GREAT LAKES SURFACE LOW AND SLOWER PROGRESSION OF THE
TRAILING COLD FRONT.  WHILE INCORPORATING THIS SLOWER TREND
GUIDANCE AGREES FAIRLY WELL WITH THE IDEA OF A FRONTAL WAVE
DEVELOPING NEAR THE EAST COAST AROUND DAY 5 SUN AS THE UPPER
TROUGH TAKES ON A NEGATIVE TILT, AND THEN TRACKING NORTH-NORTHWEST
INTO CANADA.  00Z GUIDANCE THUS FAR SEEMS TO BE CONSISTENT WITH
THE ABOVE MAJORITY CLUSTER OF SOLUTIONS.  THE RELATIVELY LARGE
SCALE NATURE OF FLOW YIELDS DECENT CONFIDENCE IN THE OVERALL
PATTERN BUT THE FINER DETAILS WILL LIKELY TAKE MULTIPLE DAYS TO
RESOLVE.  ONE SUCH DETAIL WILL BE THE EXTENT TO WHICH THIS SYSTEM
INCORPORATES MOISTURE AND POTENTIALLY A TROPICAL SYSTEM TRACKING
OUT OF THE NORTHWEST CARIBBEAN.  CURRENT FORECAST REFLECTS
YESTERDAY`S NHC/WPC COORDINATION FOR THE POTENTIAL TROPICAL
FEATURE.  OVERALL THE SLOWER TIMING OF THE UPPER TROUGH INCREASES
THE POSSIBILITY FOR INCLUSION OF TROPICAL MOISTURE IN THIS SYSTEM
NEAR THE EAST COAST.

EARLY IN THE PERIOD THE INITIAL GREAT LAKES SURFACE LOW SHOULD
SUPPORT A PERIOD OF FAIRLY STRONG WINDS ALONG WITH A CONCENTRATED
AREA OF PRECIP WITH SOME IN THE FORM OF SNOW.  RAIN ALONG THE
TRAILING FRONT SHOULD DEVELOP NEAR THE MS VALLEY AROUND FRI AND
THEN CONTINUE INTO THE EASTERN STATES WITH TIME.  SOME POCKETS OF
LOCALLY MODERATE-HEAVY RAIN MAY EXIST FROM THE MS VALLEY TO WEST
OF THE APPALACHIANS.  FROM NEAR THE APPALACHIANS TO THE EAST
COAST, THE POTENTIAL FOR INCORPORATION OF TROPICAL MOISTURE ALONG
WITH STRENGTHENING ATLANTIC INFLOW RAISE THE THREAT FOR A PERIOD
OF MORE ORGANIZED HEAVY TO EXCESSIVE RAINFALL.  ENSEMBLE GUIDANCE
CURRENTLY SUGGESTS THE BEST PROBABILITY FOR HIGHEST TOTALS DURING
THE PERIOD WILL EXTEND ACROSS THE NORTHERN MID ATLANTIC/EASTERN
GREAT LAKES/NORTHEAST.  EXPECT MOST PRECIP TO BE IN THE FORM OF
RAIN THOUGH A LITTLE SNOW MAY NOT BE OUT OF THE QUESTION IF THE
UPPER LOW CENTER TRACKS OVER HIGHER ELEVATIONS.

AT LEAST TO ABOUT MID-PERIOD THERE IS GRADUALLY IMPROVING UPSTREAM
SIMILARITY AMONG GUIDANCE REGARDING ANOTHER NORTHERN PACIFIC
SHORTWAVE ROUNDING THE WESTERN RIDGE AND DESCENDING INTO THE LOWER
48 BY EARLY NEXT WEEK.  BY THAT TIME THE QUESTION POSED BY MODELS
AND ENSEMBLE MEMBERS IS TO WHAT EXTENT THE ENERGY AMPLIFIES OVER
THE WEST.  TELECONNECTIONS RELATIVE TO THE CORE OF STRONG POSITIVE
HEIGHT ANOMALIES OVER THE NORTHEAST PACIFIC IN LATEST D+8
MULTI-DAY MEAN CHARTS OFFER POTENTIAL FOR SOME TROUGH ELONGATION
OVER THE WEST SUCH AS IN THE 12Z ECMWF AND A MINORITY OF
GEFS/ECMWF/CMC ENSEMBLE MEMBERS.  HOWEVER THE RELATIVE LACK OF
AGREEMENT/CONTINUITY THUS FAR RECOMMENDS LEANING CLOSER TO THE
ENSEMBLE MEANS LATE IN THE PERIOD.  THUS THE 18Z GFS/GEFS MEAN AND
12Z ECMWF/ECMWF MEAN BLEND USED FOR THE FORECAST ADJUSTS FROM MORE
OPERATIONAL WEIGHT EARLY TO MOSTLY ENSEMBLE MEAN WEIGHT LATE.
THIS FEATURE AND ASSOCIATED FRONT MAY PRODUCE AN AREA OF MOSTLY
LIGHT RAIN/SNOW OVER AND EAST OF THE ROCKIES.

A BROAD AREA OF HIGHS 10-20F BELOW NORMAL OVER THE CENTRAL U.S. ON
FRI WILL MAKE STEADY PROGRESS INTO THE EAST DURING THE
WEEKEND/EARLY NEXT WEEK WITH MODERATION LIKELY BY TUE.  GREATEST
NEGATIVE ANOMALIES FOR MIN TEMPS SHOULD PROGRESS FROM THE SOUTHERN
PLAINS INTO THE SOUTHEAST CORRESPONDING TO THE PATH OF SURFACE
HIGH PRESSURE.  ON THE OTHER HAND THE WEST SHOULD SEE ABOVE NORMAL
TEMPS DURING THE PERIOD WITH HIGHEST ANOMALIES OF PLUS 10-20F FOR
HIGHS LIKELY DURING FRI-SUN.

RAUSCH

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