Preliminary Forecasts
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FXUS02 KWNH 090631
PREEPD

PRELIMINARY EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
130 AM EST FRI DEC 09 2016

VALID 12Z MON DEC 12 2016 - 12Z FRI DEC 16 2016

...OVERVIEW...

A DEEP UPR LOW FCST TO BE JUST W OF HUDSON BAY AS OF EARLY MON
SHOULD TRACK E-SE INTO MIDWEEK AND CONTINUE EWD INTO ERN CANADA
THOUGH WITH INCREASING UNCERTAINTY AS TO TIMING.  UPSTREAM THE
MEAN RIDGING OVER THE NERN PAC/ALASKA SHOULD ENCOURAGE SOME DEGREE
OF ERN PACIFIC TROUGHING WHOSE AXIS MAY REACH THE WEST TOWARD LATE
NEXT WEEK.  EXPECT THE LOWER 48 TO REMAIN UNDER FAST AND LOW
AMPLITUDE FLOW.  PSBL INTERACTIONS AMONG NRN STREAM FLOW AND
INCOMING PAC IMPULSE/S WILL ADD TO THE UNCERTAINTIES THAT ARE
ALREADY TYPICALLY GREATER THAN AVG DUE TO THE PROGRESSIVE NATURE
OF THE FLOW.  THE LARGE SCALE PATTERN WILL LIKELY MAINTAIN VERY
COLD TEMPS ACROSS THE NRN TIER STATES WHILE ANOTHER COLD SURGE
WILL REACH THE ERN STATES BY THE LATTER HALF OF THE WEEK.
HEAVIEST PRECIP SHOULD BE ALONG CENTRAL LATITUDES OF THE WEST.


...GUIDANCE EVALUATION/PREFERENCES...

OVERALL FCST PREFS START WITH A COMPROMISE AMONG THE 12Z ECMWF
MEAN/18Z GEFS MEAN/12Z ECMWF IN ORDER OF HIGH TO LOW WEIGHTING FOR
DAYS 3-4 MON-TUE BASED ON DISTRIBUTION OF LATEST GUIDANCE FOR THE
SYSTEM CROSSING THE GRTLKS/NORTHEAST.  THE FCST THEN TRANSITIONS
TO AN EVENLY WEIGHTED GEFS/ECMWF MEAN BLEND FOR DAY 5 WED AS
OPERATIONAL SOLNS SHOW FURTHER DIVERGENCE AND REMAINING TIMING
DIFFS ARE WITHIN TYPICAL ERROR RANGE FOR THE VALID TIME.  TOWARD
LATE WEEK TELECONNECTIONS RELATIVE TO NERN PAC/ALASKA RIDGING
RECOMMEND GOING AT LEAST 2/3 TOWARD THE GEFS MEAN VS THE ECMWF
MEAN FOR TROUGHING NEAR THE WEST COAST.

UP TO THIS POINT GUIDANCE HAS BEEN ERRATIC FOR THE EARLY PERIOD
GRTLKS/NORTHEAST SYSTEM.  WHILE CLUSTERING SEEMS TO BE IMPROVING
THERE IS STILL A FAIR DEGREE OF SPREAD FOR LATITUDE/DEPTH OF THE
SFC LOW.  FLAT NATURE OF THE MEAN FLOW ALOFT SUGGESTS THE 12Z/18Z
GFS RUNS MAY BE TOO STRONG.  GEFS MEANS ARE LESS EXTREME IN THAT
RESPECT BUT THEIR TRACK IS STILL ON THE NRN SIDE OF THE SPREAD.
ON THE OTHER HAND THE 12Z ECMWF COULD BE A LITTLE TOO SUPPRESSED.
AS THE SYSTEM DEPARTS FROM NEW ENGLAND DAY 4 TUE THE GFS/GEFS MEAN
RUNS ARE ON THE SLOWER SIDE.

THE NEXT UNCERTAINTY FOR THE EAST ACCOMPANIES THE ARCTIC FRONT
REACHING THE EAST COAST AROUND MIDWEEK... WITH SOME SOLNS SHOWING
POTENTIAL FOR SFC DEVELOPMENT WITHIN AN AREA COVERING FROM OFF THE
MID ATLC COAST THROUGH NEW ENGLAND INTO THE CANADIAN MARITIMES.
VERY WIDE SPREAD AMONG INDIVIDUAL MODEL RUNS/ENSEMBLE MEMBERS
FAVORS A CONSERVATIVE APPROACH CLOSER TO THE MEANS AT THIS TIME.

OVER THE WEST THE PRIMARY QUESTION MARKS INVOLVE ONE OR MORE WAVES
THAT MAY COME IN FROM THE PAC CORRESPONDING TO SHRTWV ENERGY
WITHIN COMPLEX MEAN TROUGHING.  THE ELONGATED NATURE OF ENERGY
INITIALLY OFF THE PAC NW COAST LEADS TO REDUCED PREDICTABILITY FOR
HOW IT EVOLVES WITH TIME.  THEN GUIDANCE SUGGESTS POTENTIAL FOR
THIS ENERGY TO INTERACT WITH AN UPR TROUGH THAT WILL BE PASSING
ACROSS/N OF HAWAI`I EARLY IN THE WEEK.  FINALLY BY NEXT THU-FRI
GUIDANCE BEGINS TO DIVERGE ON THE AMPLITUDE/POSN OF TROUGHING NEAR
THE WEST COAST.  12Z/18Z GFS RUNS APPEAR TO BE ON THE DEEPER SIDE.
 HOWEVER TELECONNECTIONS RELATIVE TO NERN PAC/ALASKA RIDGING
SUPPORT A MORE WWD AND MODERATELY AMPLIFIED TROUGH AS PER 12Z/18Z
GEFS MEANS VS THE FLATTER/EWD 12Z ECMWF MEAN.  IN ADDITION TRENDS
FOR THE UPR TROUGH IN THE PAST COUPLE 12-HRLY CYCLES OF GUIDANCE
ARE IN THE WWD DIRECTION.  PRECISE DETAILS OF THIS OVERALL
EVOLUTION WILL BE CRITICAL FOR DETERMINING HOW MUCH LOW LATITUDE
MSTR MAY GET DRAWN NEWD TOWARD THE WEST COAST AND WHERE THIS MSTR
IS ORIENTED.


...WEATHER HIGHLIGHTS/THREATS...

ACROSS THE WEST... CURRENTLY EXPECT BEST POTENTIAL FOR ONE OR MORE
EPISODES OF ENHANCED COASTAL RAIN AND INLAND/HIGHER ELEV SNOW FROM
NRN-CNTRL CA INTO THE CNTRL ROCKIES.  COMPLEXITY OF ERN PAC
EVOLUTION LOWERS CONFIDENCE IN SPECIFICS BUT THERE WILL BE
POTENTIAL FOR SIGNIFICANT LOCALIZED TOTALS OVER PARTS OF NRN-CNTRL
CA DEPENDING ON HOW MUCH TROPICAL MSTR MAY GET INCORPORATED INTO
THE FLOW.  ARCTIC BNDRY DRAPED FROM THE NORTHWEST INTO THE ROCKIES
MAY PROVIDE ANOTHER FOCUS FOR PCPN.  EXPECT TEMPS TO RANGE FROM
10-20F BELOW NORMAL MOST DAYS EXTREME NORTH TO 5-15F ABOVE NORMAL
OVER AZ/NM.

OVER THE ERN 2/3 OF THE COUNTRY... THE NRN PLAINS SHOULD BE MOST
CONSISTENTLY VERY COLD WITH HIGHS LIKELY TO AVG 20-30F BELOW
NORMAL FOR THE 5-DAY PERIOD.  IN MODIFIED FORM SOME OF THIS COLD
AIR WILL PUSH SEWD FROM THE NRN PLAINS MON ONWARD... BRINGING SOME
MINUS 10-20F OR SO ANOMALIES TO THE NRN-CNTRL PARTS OF THE EAST
FOR THE LATTER HALF OF THE WEEK.  DAILY RECORDS ARE QUITE EXTREME
BY THIS POINT IN THE SEASON SO CURRENT FCSTS WOULD YIELD AT BEST A
FEW ISOLATED RECORD VALUES.  GRTLKS/NERN SYSTEM ON MON WILL BRING
AN AREA OF SNOW TO LOCATIONS N OF THE LOW TRACK AND RAIN FARTHER
SWD.  PASSAGE OF THIS SYSTEM AND UPSTREAM ARCTIC FRONT SHOULD
PROMOTE A MULTI-DAY PERIOD OF LAKE EFFECT SNOW.  TOWARD THE LATTER
HALF OF THE WEEK THERE MAY BE ONE OR MORE AREAS OF MOSTLY LGT PCPN
OVER PARTS OF THE CNTRL-ERN STATES BUT WITH LOW CONFIDENCE ON
LOCATION/TIMING DUE TO UNCERTAINTIES WITH SHRTWV DETAILS ALOFT.

RAUSCH

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