Preliminary Forecasts
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172
FXUS02 KWNH 060613
PREEPD

PRELIMINARY EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
113 AM EST TUE DEC 06 2016

VALID 12Z FRI DEC 09 2016 - 12Z TUE DEC 13 2016

...OVERVIEW...

LATEST GUIDANCE MAINTAINS GOOD AGREEMENT AND CONTINUITY WITH THE
IDEA OF MEAN FLOW ACROSS THE LOWER 48 SETTLING INTO A NEARLY ZONAL
TO BROADLY CYCLONIC PATTERN WHILE UPR RIDGING PERSISTS OVER/NEAR
ALASKA AND AN UPR LOW DESCENDS FROM NRN CANADA INTO CNTRL/S-CNTRL
CANADA.  DETERMINING THE SPECIFICS OF IMPULSES EMBEDDED WITHIN THE
MEAN FLOW AND ASSOC SFC EVOLUTION REMAINS PROBLEMATIC IN SUCH A
PROGRESSIVE AND LOW AMPLITUDE PATTERN.  THE MOST COMMON THEMES ARE
PERSISTENT TERRAIN-FOCUSED PCPN OVER THE NRN 2/3 OF THE WEST...
VERY COLD TEMPS OVER THE NRN PLAINS... AND BY MID-LATE PERIOD A
RETURN OF MSTR INTO THE ERN HALF OF THE COUNTRY IN ASSOC WITH ONE
OR MORE WAVES/FRONTS.


...GUIDANCE EVALUATION/PREFERENCES...

FOR DAYS 3-5 FRI-SUN THE FCST INCORPORATES 70 PCT TOTAL WEIGHTING
OF 18Z GEFS MEAN AND 12Z/18Z GFS RUNS WITH THE OTHER 30 PCT
DEVOTED TO THE 12Z ECMWF MEAN.  ONCE THE GFS RUNS STRAY FROM WHAT
CLUSTERING EXISTS MID-PERIOD THEN THE 12Z NAEFS MEAN TAKES THE
PLACE OF THE GFS CONTRIBUTION FOR DAYS 6-7 MON-TUE.

EVEN BEFORE THE 12Z FRI THERE REMAINS SIGNIFICANT SPREAD FOR THE
SYSTEM OFF THE PAC NW COAST.  CONSULT THE MODEL DIAGNOSTIC
DISCUSSION PMDHMD FOR UPDATED PREFS BASED ON NEW 00Z GUIDANCE
THROUGH 12Z FRI.  THE INITIAL BLEND BASED ON 12Z/18Z SOLNS WAS
ADJUSTED A BIT TO A COMPROMISE BTWN THE 18Z GEFS/12Z ECMWF MEANS.
CLUSTERING IMPROVES SOMEWHAT BY SAT AS THE SFC LOW GRAVITATES
TOWARD VANCOUVER ISLAND OR NRN WA.

FROM FRI INTO SAT THE 12Z ECMWF/ECMWF MEAN BECOME NOTICEABLY
SLOWER THAN MOST OTHER SOLNS WITH ERN PAC ENERGY FLOWING INTO THE
WEST.  THIS LEADS TO SOMEWHAT SLOWER UPR TROUGH PROGRESSION OVER
THE CONUS FOR THE REMAINDER OF THE PERIOD.  ANOTHER POTENTIAL
SLOWING INFLUENCE ARISES IN THE ECMWF MEAN BY DAYS 6-7 AS IT
BUILDS A MODEST RIDGE OVER THE EXTREME NERN PAC/GULF OF ALASKA.
THIS IS IN CONTRAST TO MOST OTHER GUIDANCE THAT MAINTAINS SOME
DEGREE OF ELONGATED TROUGHING.  FLOW OVER THIS AREA HAS
SENSITIVITY TO EXACT POSN OF THE UPSTREAM UPR RIDGE... BUT
TELECONNECTIONS RELATIVE TO WHERE MOST MULTI-DAY MEANS ARE PLACING
THE ASSOC POSITIVE HGT ANOMALY WOULD SUPPORT A PATTERN CLOSER TO
THE GFS/CMC AND THEIR ENSEMBLE MEANS.

PSBL WAVINESS EMANATING FROM COMPLEX E-CNTRL PAC EVOLUTION
PROVIDES ANOTHER SOURCE OF UNCERTAINTY TO THE WEST COAST FCST BY
THE LATTER HALF OF THE PERIOD.  THE DEGREE OF SPREAD LEADS TO LOW
CONFIDENCE IN ANY PARTICULAR SOLN... FAVORING MINIMAL DEFINITION
OF ANY WAVE UNTIL THERE IS BETTER AGREEMENT IN THE GUIDANCE.

AS FOR EVOLUTION OVER THE ERN HALF OF THE COUNTRY DAYS 5-7
SUN-TUE... THERE IS FAIR AGREEMENT THAT AN AREA OF LOW PRES SHOULD
HEAD INTO MS VLY/GRTLKS BUT AS OF SUN THERE IS ALREADY SOME
DIVERGENCE IN THE GUIDANCE FOR STRENGTH AND TIMING.  AFTER THAT
TIME IT BECOMES INCREASINGLY DIFFICULT TO RESOLVE FCST SPECIFICS.
ALBEIT WITH LOW CONFIDENCE IN ABSOLUTE TERMS THE MOST LIKELY
SCENARIO APPEARS TO BE CONTINUED PROGRESSION OF THIS LEADING
SYSTEM WHILE ENERGY IMMEDIATELY UPSTREAM MAY SUPPORT A TRAILING
WAVE.  STRONGER DEPICTION OF MID LVL ENERGY SUPPORTING THE LEADING
WAVE AS DEPICTED IN SOME OPERATIONAL RUNS WOULD BRING THE TRAILING
COLD FRONT FARTHER SEWD THAN CURRENTLY DEPICTED IN THE MANUAL FCST.


...WEATHER HIGHLIGHTS/THREATS...

OVER THE WEST THE LARGE SCALE PATTERN WILL LIKELY PROMOTE
PERSISTENT RAIN AND INLAND/HIGHER ELEV SNOW ACROSS THE NRN 2/3 OF
THE WEST WITH A PRONOUNCED COASTAL/TERRAIN FOCUS FOR HIGHEST AMTS.
 CURRENT SPREAD FOR FINER DETAILS OF THE FCST PRECLUDE PINPOINTING
LOCATION/TIME FRAME OF HEAVIEST ACTIVITY.  HOWEVER IN GENERAL THE
POTENTIAL EXISTS FOR ONE OR MORE EPISODES OF LOCALLY HVY PCPN...
MOST LIKELY OVER NRN CALIFORNIA AND/OR SWRN OREGON.  NEAR TO
SLIGHTLY ABOVE NORMAL TEMPS ON FRI SHOULD TREND NEAR OR BELOW
NORMAL FOR THE WEEKEND AND EARLY NEXT WEEK.

AWAY FROM THE GRTLKS EXPECT TEMPS TO BE 10-20F BELOW NORMAL OVER
THE ERN HALF OF THE COUNTRY DURING THE WEEKEND WITH SOME WARMING
TO THE W OF THE APLCHNS ON SUN.  ERN STATES SHOULD SEE READINGS
WITHIN A FEW DEGS OF NORMAL BY EARLY NEXT WEEK.  FCST FOR
CONTINUED VERY COLD TEMPS OVER THE NRN PLAINS INTO NEXT WEEK...
PERHAPS MORE THAN 20F BELOW NORMAL... APPEARS CONSISTENT WITH THE
EXPECTED LARGE SCALE PATTERN.  LAKE EFFECT SNOW ON FRI SHOULD
BEGIN TAPERING OFF THEREAFTER.  FROM THE WEEKEND INTO EARLY NEXT
WEEK MSTR WILL SPREAD ACROSS THE NRN TIER AND EXPAND OVER THE ERN
HALF OF THE COUNTRY.  THERE MAY SOME AREAS OF LOCALLY ENHANCED
PCPN BUT CURRENTLY WITH CONSIDERABLE UNCERTAINTY OVER
LOCATION/TIMING.  BEST OPPORTUNITY FOR WINTRY WEATHER SHOULD BE
OVER THE NRN THIRD OF THE CNTRL-ERN STATES.

RAUSCH

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