Preliminary Forecasts
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FXUS02 KWNH 280642
PREEPD

PRELIMINARY EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
241 AM EDT TUE MAR 28 2017

VALID 12Z FRI MAR 31 2017 - 12Z TUE APR 04 2017

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

MOST OF THE LATEST MULTI-DAY MEANS AGREE FAIRLY WELL WITH A
PERSISTENT SPLIT FLOW REGIME FEATURING A WRN CANADA/ALASKA RIDGE
AND INTERIOR WRN U.S. TROUGH.  THIS PATTERN IS CONSISTENT WITH
TELECONNECTIONS RELATIVE TO THE TWO BEST DEFINED HGT ANOMALIES, A
NEGATIVE ONE OVER THE CNTRL PAC AND A POSITIVE ONE IN THE VICINITY
OF GREAT BRITAIN/FRANCE.  IN SPITE OF THE SIMILARITIES IN THE
LONGWAVE PATTERN THE OPERATIONAL MODELS AND ENSEMBLES STILL DIFFER
SIGNIFICANTLY FOR IMPORTANT EMBEDDED DETAILS WITH LITTLE APPARENT
DECREASE IN SPREAD OVER THE PAST DAY.

GUIDANCE IS STILL FLUCTUATING FROM RUN TO RUN FOR THE SYSTEM
AFFECTING THE EAST FRI INTO THE WEEKEND, LIKELY REFLECTING THE
SENSITIVE NATURE OF MID LVL EVOLUTION WITH SMALL DIFFS IN STREAM
PHASING AFFECTING THE TRACK OF THE SFC LOW.  IN GENERAL THE PAST
COUPLE MODEL RUNS HAVE TENDED TO LEAN TOWARD A LESS PHASED
SCENARIO ALOFT LEADING TO A MODEST SWD NUDGE IN THE CONSENSUS SFC
LOW TRACK.  SOME GFS RUNS HAVE APPEARED A LITTLE OVERDONE WITH THE
SWD TRACK.  HOWEVER RECENT ECMWF RUNS HAVE BEEN FARTHER REMOVED
FROM LATEST CONSENSUS, ON THE MORE PHASED SIDE LEADING TO A
FASTER/NWD SYSTEM.  THE 00Z GFS HAS ACTUALLY TRENDED FARTHER SWD
VS 12Z/18Z RUNS INTO EARLY SAT BEFORE CONVERGING TOWARD CONSENSUS
OFFSHORE.

UPSTREAM FOCUS WILL BE ON A DEEP CLOSED LOW EXPECTED TO TRACK INTO
THE SRN ROCKIES BY EARLY DAY 4 SAT WITH INCREASING TRACK/TIMING
SPREAD THEREAFTER IN RESPONSE TO SIGNIFICANT DIFFS IN PAC FLOW
REACHING WRN N AMERICA.  CONFIDENCE CONTINUES TO BE LOW IN
DIAGNOSING THE PAC DIFFS AS THE SHRTWV DIFFS ARISE FROM INITIALLY
LOW AMPLITUDE FLOW 2-3 DAYS OUT IN TIME.  IT MAY BE DIFFICULT TO
VOUCH FOR ANY INDIVIDUAL GFS/CMC/UKMET OR GEFS/CMC ENSEMBLE RUN
BUT THEY DO SHARE THE COMMON THEME OF DELAYING THE ARRIVAL OF HGT
FALLS TO THE WEST COAST RELATIVE TO THE 12Z ECMWF/ECMWF MEAN.
THIS LEADS TO MAJORITY AGREEMENT IN PRINCIPLE THAT THE SRN ROCKIES
UPR LOW SHOULD IN SOME FASHION EJECT SLOWER THAN FCST BY THE
ECMWF/EC MEAN, WITH ONGOING SUPPORT FOR A DETERMINISTIC FCST THAT
LEANS ABOUT 70 PCT TOWARD THE GFS-BASED CLUSTER.  AS FOR THE
ENERGY ENTERING THE WEST, VERY HIGH SPREAD IN DETAILS WOULD FAVOR
A CONSERVATIVE FCST OF A MODERATE SHRTWV TROUGH UNTIL BETTER
CLUSTERING EMERGES.

BASED ON THE ABOVE CONSIDERATIONS FOR DATA THROUGH THE 12Z/18Z
CYCLES, THE UPDATED FCST WAS WEIGHTED SOMEWHAT MORE TOWARD THE
12Z-18Z GFS/18Z GEFS MEAN VS THE 12Z ECMWF/ECMWF MEAN DAYS 3-4
FRI-SAT AND THEN 70 PCT TOTAL GFS RUNS AND 18Z GEFS/12Z NAEFS
MEANS VS 30 PCT ECMWF MEAN FOR DAYS 5-7 SUN-TUE.  NOT SURPRISINGLY
NEW 00Z GFS/CMC/UKMET INTRODUCE NEW DETAIL UNCERTAINTIES RELATIVE
TO THIS SCENARIO FOR THE EJECTING WRN LOW AND UPSTREAM ENERGY BUT
AT LEAST MAINTAIN SLOWER TIMING THAN THE ECMWF/ECENS.


...SENSIBLE WEATHER HIGHLIGHTS/POTENTIAL HAZARDS...

SYSTEM AFFECTING THE EAST FRI INTO SAT WILL BRING A BROAD SHIELD
OF PRECIP ACROSS THE REGION WITH SOME AREAS OF MDT-HVY RNFL ALONG
WITH POTENTIAL FOR STRONG CONVECTION IN THE WARM SECTOR.  EXTREME
NRN AREAS MAY SEE SOME WINTRY WEATHER WITH THIS SYSTEM.  GUIDANCE
CONTINUES TO SHOW STRONG SIGNALS THAT THE UPR LOW FCST TO DROP
INTO THE SRN ROCKIES BY THE WEEKEND ALONG WITH A PERIOD OF LOW LVL
UPSLOPE FLOW SHOULD GENERATE A PERIOD OF HVY SNOW/LOWER ELEV RAIN
FOCUSED ALONG THE ERN SLOPES OF THE COLORADO ROCKIES.  AS HGT
FALLS PROGRESS EWD EXPECT LOW LVL GULF INFLOW TO STRENGTHEN AND
CONTRIBUTE TO A POTENTIALLY HVY/STRONG CONVECTIVE EVENT FROM THE
SRN PLAINS EWD/NEWD DURING SUN-TUE.  CURRENTLY THERE IS DECENT
PROBABILITY OF A HVY RAIN THREAT WHILE MAGNITUDE AND DURATION ARE
MUCH LESS CERTAIN DUE TO THE SPREAD OF SOLNS FOR TIMING/TRACK OF
THE UPR LOW.  UPSTREAM ENERGY FLOWING INTO THE WRN U.S. WILL
LIKELY PRODUCE PERIODS OF RAIN AND HIGH ELEV SNOW FROM THE PAC NW
EWD/SEWD, BUT WITH LOW CONFIDENCE IN TIMING AND INTENSITY/AREAL
COVERAGE.

COLDEST TEMPS RELATIVE TO NORMAL SHOULD BE FROM THE GREAT BASIN
THROUGH THE FOUR CORNERS STATES INTO THE HIGH PLAINS WITH THE UPR
LOW TRACKING INTO THE SRN ROCKIES DURING THE FIRST HALF OF THE
PERIOD, AND ALONG THE NRN HALF OF THE EAST COAST LATE THIS WEEK
WITH PRECIP/ONSHORE FLOW.  DURING THE OVERALL 5-DAY PERIOD THE
WARMEST ANOMALIES FOR MAX TEMPS SHOULD OCCUR OVER THE NRN PLAINS.
THE SRN PLAINS TO SOUTHEAST SHOULD SEE ABOVE NORMAL HIGHS THOUGH
WITH SOME DAY TO DAY VARIABILITY.  THE ACTIVE WEATHER PATTERN WILL
LIKELY PROMOTE WARMER THAN AVERAGE MIN TEMPS OVER MUCH OF THE
LOWER 48.

RAUSCH

$$





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