Preliminary Forecasts
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FXUS02 KWNH 270651
PREEPD

PRELIMINARY EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
251 AM EDT MON MAR 27 2017

VALID 12Z THU MAR 30 2017 - 12Z MON APR 03 2017

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

EXPECT A CONTINUATION OF THE SPLIT FLOW PATTERN FEATURING A
TENDENCY TOWARD AN AMPLIFIED MEAN TROUGH OVER THE WEST AND A MORE
PROGRESSIVE NRN STREAM THAT WILL CONTAIN SHRTWVS WHOSE CHARACTER,
AND DEGREE OF INTERACTION WITH SRN STREAM FLOW, WILL BE MORE
DIFFICULT TO RESOLVE SEVERAL DAYS IN ADVANCE.  A LEADING SYSTEM
EJECTING FROM THE WRN MEAN TROUGH IN THE SHORT RANGE WILL CONTINUE
ONWARD FROM THE PLAINS DURING THE LATTER HALF OF THE WEEK WHILE
SHRTWV ENERGY REACHING THE WEST COAST ON THU WILL QUICKLY AMPLIFY
INTO A FAIRLY DEEP CLOSED LOW DROPPING TOWARD AZ/NM.  AS GUIDANCE
SPREAD HAS DECREASED FOR THE LEADING SYSTEM, AN INCREASINGLY
SIGNIFICANT FOCUS OF FCST UNCERTAINTY WILL BE ON PACIFIC INTO WRN
U.S. FLOW AND ITS EFFECT ON THE WRN UPR LOW.

FOR THE LEADING SYSTEM CROSSING THE EAST THU ONWARD, GFS/ECMWF
RUNS AND THEIR ENSEMBLE MEANS APPEAR TO BE CONVERGING TOWARD A
COMMON SOLN BTWN PRIOR DAYS` CLUSTERS THAT HAD GFS-BASED SOLNS
TRACKING FARTHER SWD AND ECMWF SOLNS WELL NWD.  IN NEW 00Z
GUIDANCE THUS FAR THE UKMET IS THE ONE HOLDOUT FOR A MORE SRN
TRACK WHILE THE CMC IS HANGING ONTO MORE GRTLKS SFC LOW EMPHASIS
VS MOST OTHER GUIDANCE AFTER FRI.  OVERALL TIMING HAS TRENDED A
TAD FASTER OVER THE PAST 12-24 HRS WITH SOMEWHAT STRONGER/EWD
DEVELOPMENT JUST OFF THE NRN MID ATLC COAST BY SAT.

FOR THE UPR TROUGH/CLOSED LOW SETTLING INTO THE WEST THERE ARE
STILL SOME DETAIL/TRACK DIFFS AROUND WHAT HAS BEEN A FAIRLY STABLE
CONSENSUS SOLN INTO DAY 5 SAT.  EARLY IN THE EXTENDED PERIOD
MODELS/ENSEMBLES BEGIN TO DIVERGE REGARDING SHRTWV EVOLUTION
WITHIN INITIALLY LOW AMPLITUDE FLOW ACROSS THE WRN 2/3 OF THE
PACIFIC.  AS A RESULT SPAGHETTI PLOTS SHOW FEATURES BECOMING 180
DEGS OUT OF PHASE OVER THE CNTRL PAC AND THEN TOWARD THE WEST
COAST AS EARLY AS DAYS 4-5 FRI-SAT.  BROADLY SPEAKING 12Z ECMWF/EC
ENSEMBLES WANT TO BRING A SHRTWV TROUGH INTO THE NORTHWEST BY LATE
SAT WITH GFS-BASED SOLNS SHOWING A RIDGE AT THE SAME TIME.  THEN
BY SUN THE GFS/GEFS FINALLY BRING A SHRTWV TOWARD THE WEST COAST
WHILE THE EC/ECENS START TO BUILD IN A RIDGE.  12Z CMC ENSEMBLES
HAVE VARIATIONS OF EACH POSSIBILITY.  BY SUN-MON THE SPECIFICS OF
THIS PACIFIC FLOW WILL AFFECT THE ULTIMATE PATH/TIMING OF THE
CLOSED LOW FCST TO BE NEAR AZ/NM AS OF SAT, WITH THE 12Z ECMWF
SCENARIO RESULTING IN FASTER EJECTION OF THE UPR LOW VS THE
GFS/CMC AND THEIR MEANS.

CONFIDENCE IS VERY LOW IN ANY PARTICULAR SOLN OVER THE PAC GIVEN
THE INITIALLY LOW AMPLITUDE OF FLOW.  HOWEVER MULTI-DAY CONTINUITY
FOR THE WRN UPR LOW THUS FAR HAS SHOWN ECMWF RUNS TO BE QUITE
UNSTABLE BTWN SLOWER AND PROGRESSIVE SOLNS WHILE THE GFS/GEFS AND
CMC HAVE BEEN MORE CONSISTENT WITH SLOWER EJECTION.  SO EVEN WITH
THE MINIMAL CONFIDENCE IN DETAILS OF PAC FLOW REACHING THE WEST,
THERE STILL SEEMS TO BE A BETTER SIGNAL TOWARD MAINTAINING PRIOR
PREFERENCE ABOUT 2/3 TOWARD SLOWER EJECTION OF THE WRN UPR LOW AS
PER GFS/GEFS RUNS.  LOOKING AT THE NEW 00Z GFS/UKMET/CMC THERE ARE
STILL SIGNIFICANT DIFFS WITH PAC TO WEST COAST FLOW.  HOWEVER ALL
STILL ARGUE AGAINST THE 12Z ECMWF/ECMWF MEAN DETAILS AND CONTINUE
TO SUGGEST SLOWER EJECTION OF THE WRN UPR LOW.

BASED ON THE ABOVE CONSIDERATIONS THE UPDATED FCST REFLECTED A
CONSENSUS BLEND OF THE 18Z GFS/12Z ECMWF AND THEIR MEANS WITH MORE
OPERATIONAL MODEL WEIGHTING, QUICKLY TRANSITIONING AROUND
MID-PERIOD TOWARD ABOUT 2/3 TOTAL WEIGHTING OF THE 18Z
GEFS/12Z-18Z GFS/12Z NAEFS VERSUS 1/3 ECMWF MEAN.  THIS BLEND
YIELDED REASONABLY GOOD CONTINUITY FOR A FCST THAT BECOMES
INCREASINGLY UNCERTAIN DURING THE LATTER HALF OF THE PERIOD.


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

SYSTEM DEPARTING FROM THE PLAINS AND TRACKING THROUGH THE EAST
THU-SAT WILL BE SUFFICIENTLY ENERGETIC TO PRODUCE AREAS OF HVY
RAIN AND POTENTIALLY STRONG TO SEVERE CONVECTION, THE LATTER
ACROSS SRN AREAS.  CHECK SPC OUTLOOKS FOR LATEST INFO ON SEVERE
THREATS.  HIGHEST RNFL TOTALS SHOULD EXTEND FROM THE MID-LWR MS
VLY INTO THE MID ATLC/CAROLINAS.  THERE MAY BE SOME WINTRY WEATHER
IN THE EXTREME NRN FRINGE OF THE PRECIP SHIELD.  MEANWHILE A BAND
OF ENHANCED MSTR ASSOC WITH THE SHRTWV INITIALLY REACHING THE WEST
COAST AND THEN FORMING A CLOSED LOW TRACKING TOWARD AZ/NM SHOULD
SUPPORT SOME AREAS OF SIGNIFICANT RAIN/HIGH ELEV SNOW ACROSS THE
NRN 2/3 TO 3/4 OF THE WEST.  OVER THE PAST COUPLE DAYS GUIDANCE
HAS STABILIZED IN SUGGESTING THAT THE COMBINATION OF THE UPR LOW
AND A PERIOD OF EASTERLY LOW LVL UPSLOPE FLOW MAY PRODUCE A
CONCENTRATED AREA OF HVY SNOW AND PSBLY LOWER ELEV RAIN FOCUSED
PRIMARILY OVER COLORADO AROUND FRI NIGHT-SAT.  IN THE SAT-MON TIME
FRAME THERE MAY BE ANOTHER EPISODE OF HVY RAIN/CONVECTION
EXTENDING FROM THE SRN PLAINS AT LEAST INTO LWR MS VLY.  THIS PART
OF THE FCST WILL BE VERY SENSITIVE TO TIMING OF THE UPSTREAM UPR
LOW.  AT LEAST ONE PERIOD OF LGT-MDT PCPN IS PSBL OVER THE
NORTHWEST BUT WITH LOW CONFIDENCE GIVEN THE SPREAD FOR SHRTWV
DETAILS ALOFT.

THE ACTIVE WEATHER PATTERN WILL TEND TO KEEP MIN TEMPS ABOVE
AVERAGE OVER MOST OF THE LOWER 48 DURING THE PERIOD.  MAX TEMPS
WILL VARY RELATIVE TO INDIVIDUAL SYSTEMS/FLOW PATTERN.  THE NRN
PLAINS AND SOUTHEAST SHOULD BE MOST CONSISTENTLY ABOVE NORMAL WITH
THE NRN PLAINS SEEING THE HIGHEST MAX TEMP ANOMALIES OF PLUS
10-15F.  THE SYSTEM DROPPING INTO THE WEST WILL SUPPORT ONE OR
MORE COOL DAYS AND A LOCALIZED AREA OF MINUS 10-20F ANOMALIES FOR
HIGHS ARE PSBL AROUND SAT WITH THE POTENTIAL CNTRL HIGH PLAINS
UPSLOPE EVENT.  BEHIND THIS SYSTEM EXPECT MODERATELY WARM AIR TO
MOVE INTO THE WRN STATES.  PORTIONS OF THE EAST COAST MAY SEE ONE
OR MORE DAYS OF ONSHORE FLOW THAT WOULD KEEP HIGHS ON THE COOL
SIDE.

RAUSCH

$$





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