Prognostic Meteorological Discussion
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000
FXUS02 KWBC 121600
PMDEPD

EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1059 AM EST TUE DEC 12 2017

VALID 12Z FRI DEC 15 2017 - 12Z TUE DEC 19 2017

...OVERVIEW AND GUIDANCE/PREDICTABILITY ASSESSMENT...

LATEST GUIDANCE CONTINUES TO SHOW A TRANSITION FROM AN AMPLIFIED
WESTERN RIDGE/EASTERN TROUGH PATTERN DURING THE SHORT RANGE TIME
FRAME TO A MORE PROGRESSIVE REGIME FEATURING MORE PACIFIC
INFLUENCE.  MULTI-DAY MEANS OF MOST OPERATIONAL AND ENSEMBLE
SOLUTIONS AGREE FAIRLY WELL WITH THE OVERALL EVOLUTION ALOFT BUT
THERE ARE SOME SIGNIFICANT EMBEDDED DETAIL UNCERTAINTIES THAT
SIGNIFICANTLY LOWER CONFIDENCE IN SENSIBLE WEATHER EFFECTS OVER
SOME AREAS.

THE MOST PROMINENT UNCERTAINTY INVOLVES THE SHORTWAVE FORECAST TO
REACH WESTERN NORTH AMERICA AROUND FRI.  FROM THIS TIME ONWARD
MODEL/ENSEMBLE SOLUTIONS INCREASINGLY DIVERGE REGARDING HOW MUCH
ENERGY MAY SEPARATE OVER THE SOUTHWEST OR FOUR CORNERS REGION
VERSUS CONTINUING ONWARD IN PROGRESSIVE FLOW.  THE 00Z CMC WAS
SOMEWHAT ON THE EXTREME SIDE OF THE ENVELOPE BUT THE FULL ARRAY OF
00Z/06Z GUIDANCE DOES SUGGEST AN INCREASING POTENTIAL FOR MORE
ENERGY TO CROSS THE SOUTHWEST THAN IN PREVIOUS CYCLES.  THIS TREND
IS ALSO EVIDENT WHEN COMPARING 06Z GFS/GEFS MEAN RUNS TO THEIR 00Z
COUNTERPARTS.  THROUGH DAY 4 SAT THE 06Z GFS/00Z ECMWF COMPARE
MOST CLOSELY TO THE ENSEMBLE MEANS.  AFTER THAT TIME THE
FASTER/DEEPER 00Z ECMWF BECOMES AN OUTLIER TO THE FULL ENSEMBLE
ENVELOPE AT SOME HEIGHT VALUES.  GIVEN THE CONTINUED GUIDANCE
SPREAD AND RELATIVELY NEW NATURE OF THE TREND TOWARD GREATER
SEPARATION, PREFER A FAIRLY CONSERVATIVE ADJUSTMENT BY WAY OF
GREATER ENSEMBLE MEAN EMPHASIS BY THE LATTER HALF OF THE PERIOD.
AHEAD OF THIS FEATURE IS ALSO ANOTHER SIGNIFICANT UNCERTAINTY,
LEADING SHORTWAVE ENERGY THAT MAY CLOSE OFF A COMPACT LOW ANYWHERE
BETWEEN NORTHERN MEXICO AND SOUTHWEST OF SOUTHERN BAJA CALIF.
THIS DIFFERENCE EARLY IN THE PERIOD LEADS TO CONSIDERABLE SPREAD
IN WHETHER AND HOW QUICKLY NORTHEASTWARD EJECTION OCCURS.  THE 00Z
UKMET WAS THE NORTHEAST EXTREME WHILE THE NEXT-FASTEST 00Z ECMWF
WAS MUCH FASTER THAN PREVIOUS RUNS.  SOME DEGREE OF COMPROMISE
APPEARS BEST AT THIS TIME.

UPSTREAM EXPECT NORTHEAST PACIFIC TROUGHING TO BEGIN APPROACHING
THE NORTHWEST TOWARD THE END OF THE PERIOD.  THE 00Z ECMWF IS
SOMEWHAT MORE AGGRESSIVE WITH THE HEIGHT FALLS THAN THE MAJORITY
OF OTHER SOLUTIONS, BUT WITHIN THE ENSEMBLE SPREAD AND CLOSER TO
RUNS FROM 2-3 CYCLES AGO THAN THE 12Z/11 RUN.  PREFER A STARTING
BLEND BETWEEN THE 00Z ECMWF MEAN AND SLIGHTLY SLOWER 06Z GEFS/00Z
NAEFS MEANS WHILE AWAITING ANY FURTHER SUPPORT FOR THE ECMWF
SCENARIO.

ALONG THE EAST COAST EARLY IN THE PERIOD, NORTHEAST GULF/SOUTHEAST
COAST WAVINESS SHOULD STRENGTHEN/CONSOLIDATE OFF THE NORTHEAST
COAST AND CANADIAN MARITIMES IN RESPONSE TO THE UPPER TROUGH
CROSSING THE EAST.  THERE ARE STILL SOME NOTICEABLE DETAIL
DIFFERENCES ALOFT OVER THE NORTHEAST BY DAY 4 SAT BUT
MODEL/ENSEMBLE CLUSTERING IS MUCH BETTER THAN IN PREVIOUS DAYS.
SOLUTIONS ON THE NORTHWEST SIDE OF THE ENVELOPE BY LATE FRI TENDED
TO BE FROM SOME OF THE CMC ENSEMBLES.

BEHIND A WEAK GREAT LAKES SYSTEM ON FRI, ECMWF-BASED GUIDANCE HAS
TRENDED WEAKER WITH A CLIPPER SYSTEM THAT HAS BEEN FORECAST TO
TRACK INTO THE GREAT LAKES REGION BY EARLY DAY 5 SUN.  THE
GFS/GEFS MEAN HAVE TRENDED SOMEWHAT WEAKER IN THEIR 06Z RUNS BUT
THERE IS STILL DECENT CONSENSUS FOR A BETTER DEFINED SYSTEM THAN
IN THE 00Z ECMWF/ECMWF MEAN.

ADDING TOGETHER THE ABOVE CONSIDERATIONS, THE STARTING BLEND
CONSISTED OF THE 06Z GFS/GEFS MEAN AND 00Z ECMWF/ECMWF MEAN FOR
DAYS 3-4 FRI-SAT.  00Z ECMWF INPUT WAS DECREASED BY DAY 5 SUN AND
THE 06Z GFS DAY 6 MON, LEADING TO A HALF 00Z ECMWF MEAN AND 25 PCT
EACH 06Z GEFS MEAN/00Z NAEFS BLEND BY DAY 7 TUE.


...WEATHER HIGHLIGHTS/THREATS...

OVER THE WEST EXPECT THE MAJORITY OF PRECIP COVERAGE TO EXTEND
FROM THE PACIFIC NORTHWEST SOUTHEAST ACROSS THE ROCKIES, WITH
HIGHEST TOTALS OF RAIN/HIGHER ELEVATION SNOW LIKELY TO BE OVER
EXTREME NORTHERN AREAS.  CONFIDENCE IN COVERAGE/INTENSITY OF
ACTIVITY FARTHER SOUTH IS LOWER THAN DESIRED DUE TO GUIDANCE
SPREAD WITH THE EVOLUTION OF THE UPPER TROUGH ARRIVING LATE THIS
WEEK.  FROM THE WEEKEND INTO EARLY NEXT WEEK, ISSUES WITH THIS
TROUGH AFFECT PRECIP POTENTIAL FROM THE SOUTHEAST PLAINS/LOWER MS
VALLEY NORTHEASTWARD.  DEPENDING ON HOW THIS TROUGH EVOLVES AND
HOW MUCH LEADING ENERGY EJECTS ACROSS NORTHERN MEXICO, SOME
PRECIP-- THE MAJORITY LIKELY TO BE RAIN-- MAY BE ON THE HEAVIER
SIDE.  LATE THIS WEEK EXPECT A PERIOD OF MOSTLY LIGHT SNOW OVER
THE GREAT LAKES WHILE CONSOLIDATING LOW PRESSURE OFF THE EAST
COAST WILL BRING A BAND OF RAIN OVER/NEAR NORTHERN FLORIDA AND
POSSIBLY A LITTLE SNOW TO AREAS ALONG THE NORTHERN HALF OF THE
EAST COAST.  CURRENTLY ANTICIPATE ANY EAST COAST PRECIP TO BE
FAIRLY LIGHT BUT IT WOULD NOT TAKE MUCH OF A WESTWARD SHIFT IN
SURFACE LOW TRACK TO INCREASE AMOUNTS.  ANOTHER NORTHERN TIER
SYSTEM SHOULD BRING A BAND OF LIGHT PRECIP, MOSTLY SNOW, INTO THE
GREAT LAKES/NORTHEAST DURING THE WEEKEND.  SOME POCKETS OF SNOW
ASSOCIATED WITH UPSTREAM ENERGY MAY ARRIVE OVER THE NORTHERN TIER
EARLY NEXT WEEK.

THE MOST EXTREME AND PERSISTENT ANOMALIES FOR TEMPS WILL BE OVER
THE NORTHERN PLAINS, WHICH SHOULD SEE MIN AND/OR MAX READINGS
10-20F ABOVE NORMAL MOST DAYS WITH SOME LOCALLY HIGHER ANOMALIES
POSSIBLE.  SOME OF THIS WARMTH MAY EXTEND FARTHER SOUTH/SOUTHEAST
AT TIMES.  SOUTHWESTERN STATES/GREAT BASIN WILL ALSO BE ON THE
WARM SIDE WITH HIGHS TENDING TO BE 5-15F ABOVE NORMAL.  THE
CURRENT COOL SPELL OVER THE EAST WILL LINGER INTO THE END OF THE
WEEK WITH SOME TEMPS AS LOW AS 10-15F BELOW NORMAL OVER THE GREAT
LAKES/NORTHEAST ON FRI.  LESS EXTREME COOL READINGS WILL LINGER
INTO SAT FOLLOWED BY NEAR TO ABOVE NORMAL TEMPS.

RAUSCH

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