Preliminary Forecasts
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000
FXUS02 KWNH 290652
PREEPD

PRELIMINARY EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
252 AM EDT SAT APR 29 2017

VALID 12Z TUE MAY 02 2017 - 12Z SAT MAY 06 2017

...PATTERN OVERVIEW AND GUIDANCE EVALUATION...

THE MAJORITY OF GUIDANCE CONTINUES TO SHOW AN EWD SHIFT OF THE
LARGE SCALE PATTERN, SETTLING INTO A SLOWER MOVING WEST COAST
TROUGH/CNTRL U.S. RIDGE/ERN TROUGH CONFIGURATION BY THE LATTER
PART OF THE PERIOD.  THUS FAR GUIDANCE HAS NOT MADE MUCH PROGRESS
IN RESOLVING IMPORTANT DETAILS OF AMPLIFYING CNTRL-ERN U.S. UPR
TROUGH ENERGY LEADING TO SOME VERY DIFFERENT SFC EVOLUTIONS LATE
WEEK INTO THE WEEKEND.  TROUGHING LIKELY TO REACH THE WEST COAST
LATE IN THE PERIOD SHOULD BE LESS EXTREME THAN THAT FCST OVER THE
EAST BUT THERE ARE ALSO SIGNIFICANT DIFFS AMONG THE GUIDANCE FOR
THAT TROUGH.

THE FIRST CONSIDERATION OF NOTE IS THAT 6-HRLY GFS RUNS FROM AT
LEAST AS FAR BACK AS 12Z HAVE BEEN FASTER THAN CONSENSUS,
INCLUDING GEFS MEANS, FOR FLOW ACROSS THE ALEUTIANS/NRN PAC BY DAY
4 WED.  WITHIN ABOUT A DAY THIS LEADS TO ACCELERATION OF NERN PAC
TROUGH ENERGY ACROSS SRN CANADA BEYOND WHAT IS FCST BY NEARLY ALL
OTHER GUIDANCE THAT HOLDS ONTO A MORE ROBUST RIDGE INSTEAD.
MAJORITY GUIDANCE TREND OVER THE PAST DAY HAS BEEN STRONGER FOR
THE U.S./SRN CANADA RIDGE, SO THERE ARE SIGNIFICANT QUESTION MARKS
WITH GFS SPECIFICS OVER/NEAR THE LOWER 48 AFTER ABOUT EARLY THU.

TELECONNECTIONS RELATIVE TO STRONG POSITIVE HGT ANOMALIES OVER
ICELAND/SRN GREENLAND PROVIDE SUPPORT FOR THE UPR TROUGH FCST BY
MOST SOLNS TO SETTLE OVER THE WEST COAST, AND IN FACT LOOSELY
SUPPORT THE POTENTIAL FOR SOME SEPARATION AS SUGGESTED IN THE 12Z
ECMWF/ECMWF MEAN AND NOW THE 00Z CMC.  THE 12Z CMC WAS JUST A
LITTLE DELAYED IN ITS UPR LOW FORMATION.  RECENT GEFS MEANS HAVE
BEEN A LITTLE FARTHER WWD WITH THE TROUGH BUT WITHIN TYPICAL DAYS
6-7 ERROR RANGES AND MUCH CLOSER TO CONSENSUS THAN OPERATIONAL GFS
RUNS.

SOLNS FOR THE AMPLIFYING CNTRL INTO ERN U.S. TROUGH WERE
REASONABLY SIMILAR INTO ABOUT EARLY DAY 5 THU WITH INCREASING
DIVERGENCE THEREAFTER.  THE PAST COUPLE DAYS HAVE FEATURED 12-HRLY
ECMWF RUNS ALTERNATING BTWN A CONSOLIDATED MID ATLC/NORTHEAST
SYSTEM (12Z RUNS) AND FLOW SEPARATION THAT RESULTS IN A SERN
SYSTEM AND MORE PROGRESSIVE NRN WAVE.  RECENT GFS/CMC RUNS HAVE
ALSO BEEN WAFFLING BTWN THESE IDEAS, AND NOT SURPRISINGLY THE FULL
ENSEMBLE ENVELOPE FOR SFC LOWS REMAINS EXTREMELY BROAD.  EVEN WITH
SUCH SPREAD THE ENSEMBLE MEANS CONTINUE TO BE FAIRLY CONSISTENT
WITH A CONSOLIDATED MID ATLC/NORTHEAST SYSTEM BY FRI-SAT.  THE 12Z
GEFS MEAN WHOSE SFC REFLECTION WAS BRIEFLY WEAKER/N-S ELONGATED
AROUND FRI WAS ONE EXCEPTION BUT THE 18Z RUN RETURNED BACK TO THE
GENERAL MULTI-RUN ENSEMBLE MEAN CONSENSUS.  THE HIGH UNCERTAINTY
IN SPECIFICS GREATLY FAVORS MAINTAINING THE ENSEMBLE MEAN SCENARIO
FOR THE DETERMINISTIC FCST.

FOR THE DAYS 3-5 TUE-THU PART OF THE FCST, AGREEMENT WAS
SUFFICIENTLY GOOD TO SUPPORT A BLEND CONSISTING OF MOSTLY
OPERATIONAL 12Z-18Z GFS/12Z ECMWF RUNS.  THEN AS GUIDANCE DIVERGED
THE 18Z GEFS/12Z ECMWF MEANS BECOME THE DOMINANT COMPONENTS OF THE
FCST FOR DAYS 6-7 FRI-SAT WITH THE 12Z ECMWF STILL CLOSE ENOUGH TO
THE MEANS TO ALLOW A MINORITY WEIGHTING OF ITS FCST.


...WEATHER HIGHLIGHTS...

AFTER TUE EXPECT CONDITIONS OVER THE MIDWEST/GRTLKS TO MODERATE AS
A COLD/WINDY SHORT RANGE SYSTEM CONTINUES ON ITS WAY ACROSS SERN
CANADA.  EXPECT SOME HIGHS OF 10-20F BELOW NORMAL TUE FOLLOWED BY
TEMPS THAT MAY REMAIN BELOW NORMAL FOR ONE OR MORE DAYS BUT WITH
LESS EXTREME ANOMALIES.

ENERGY ALOFT SUPPORTING THE EVENTUAL CNTRL-ERN U.S. SYSTEM WILL
FIRST BRING A BAND OF RAIN/HIGH ELEV SNOW SEWD ACROSS THE ROCKIES
AND THEN EXTEND RAIN/CONVECTION INTO THE PLAINS AS A SRN PLAINS
SFC WAVE DEVELOPS TUE-TUE NIGHT.  THEN EXPECT AREAS OF HEAVY RAIN
TOWARD THE GULF COAST/LWR MS VLY BY WED WITH AN EWD-NEWD
EXPANSION/PROGRESSION OF THE MSTR SHIELD THEREAFTER.  WIDE SPREAD
IN POTENTIAL EVOLUTION SFC/ALOFT MAINTAINS LOW CONFIDENCE IN
COVERAGE/LOCATION OF HEAVIEST RNFL OVER THE EAST BY LATE
WEEK/WEEKEND.  CURRENTLY THE BEST POTENTIAL FOR HIGHEST TOTALS
DURING THE PERIOD EXTENDS FROM THE LWR MS VLY/CNTRL GULF COAST
THROUGH THE MID ATLC INTO THE NORTHEAST.  DEPENDING ON SYSTEM
DETAILS THERE MAY BE ALSO ONE OR MORE BANDS OF ENHANCED RNFL TO
THE NW OF THIS AXIS.  SOME CONVECTION MAY BE STRONG TO SEVERE BUT
CURRENT GUIDANCE DIFFS MAKE IT DIFFICULT TO PINPOINT FAVORED AREAS
FOR SUCH THREATS AT THIS TIME.

STRONG RIDGE ALOFT BUILDING INTO/ACROSS THE WEST WILL BRING VERY
WARM TO HOT TEMPS TO THE REGION, WITH AN EVENTUAL COOLING TREND
OVER THE WEST COAST STATES BY SAT AS UPR TROUGHING APPROACHES
WHILE THE WARMTH SPREADS EWD INTO THE ROCKIES/HIGH PLAINS.  THE
BROADEST EXPANSE OF PLUS 10-20F ANOMALIES FOR HIGHS SHOULD BE
AROUND THU-FRI.  CURRENTLY EXPECT SOMEWHAT HIGHER POTENTIAL FOR
SCATTERED DAILY RECORD WARM LOWS VS RECORD HIGHS.  ON THE OTHER
HAND THE STRENGTHENING CNTRL-ERN U.S. UPR TROUGH AND ASSOC SFC
SYSTEM WILL BRING AN EXPANDING AREA OF BELOW NORMAL TEMPS FROM
PARTS OF THE ROCKIES/HIGH PLAINS TOWARD THE EAST COAST, LIKELY
INCLUDING SOME POCKETS OF AT LEAST MINUS 10F ANOMALIES.

RAUSCH

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