Prognostic Meteorological Discussion
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FXUS02 KWBC 171600
PMDEPD

EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1059 AM EST SUN DEC 17 2017

VALID 12Z WED DEC 20 2017 - 12Z SUN DEC 24 2017

...ABOVE AVERAGE CONFIDENCE IN THE POTENTIAL FOR AN OUTBREAK OF
ARCTIC AIR ACROSS INTO THE CENTRAL U.S. BY NEXT WEEKEND...


...OVERVIEW...

GUIDANCE CONTINUES TO SHOW A DRAMATIC AMPLIFICATION/SHARPENING OF
EASTERN PACIFIC RIDGING THROUGH ALASKA/NORTHWEST CANADA INTO THE
ARCTIC, SUPPORTING THE ESTABLISHMENT OF A DEEP MEAN TROUGH FROM
NEAR HUDSON BAY SOUTHWEST INTO THE LOWER 48 AND A FLOW OF
INCREASINGLY COLD AIR SOUTHWARD THROUGH CANADA AND MOST LIKELY
DIRECTED INTO THE ROCKIES/PLAINS.  TELECONNECTIONS RELATIVE TO THE
POSITIVE HEIGHT ANOMALY CENTER ASSOCIATED WITH THE RIDGE, EVEN
WITH SOME DISAGREEMENT IN EXACT LOCATION OF THE CENTER IN LATEST
D+8 CHARTS, AGREE WITH MOST GUIDANCE THAT HIGHLIGHTS THE NORTHERN
INTO CENTRAL PLAINS AS BEING THE PRIME FOCUS FOR COLDEST
TEMPERATURE ANOMALIES BY NEXT WEEKEND.  BETWEEN THE MEAN TROUGH
AND A PERSISTENT RIDGE OFF TO THE SOUTHEAST, A FRONTAL ZONE SHOULD
SET UP FROM THE EXTREME SOUTHERN PLAINS/NORTHERN MEXICO
EAST-NORTHEAST TO NEAR THE SOUTHEAST COAST.  THE COMBINATION OF
ADVANCING COLD AIR AND MOISTURE FLOW BETWEEN THE UPPER TROUGH AND
RIDGE SHOULD ULTIMATELY PROMOTE INCREASED POTENTIAL FOR PRECIP OF
VARIOUS TYPES WHETHER LATE IN THE FORECAST PERIOD OR JUST BEYOND.


...GUIDANCE/PREDICTABILITY ASSESSMENT...

GUIDANCE CONTINUES TO SHOW SOME SPREAD/VARIABILITY WITH THE
WEAKENING SHORTWAVE/SURFACE REFLECTION CROSSING THE SOUTHERN TIER
AROUND MIDWEEK.  AFTER BEING EARLY TO SIGNAL A TREND TOWARD SLOWER
PROGRESSION, THE PAST COUPLE ECMWF RUNS HAVE BEGUN TO ACCELERATE A
BIT AGAIN.  AS OF THE 00Z/06Z CYCLES GUIDANCE IS FAIRLY EVENLY
DISTRIBUTED WITHIN THE TIMING SPECTRUM WHILE THE 00Z ECMWF IS ON
THE NORTHERN SIDE OF THE SPREAD FOR THE SURFACE WAVE BY LATE WED.
AN AVERAGE OF LATEST OPERATIONAL GUIDANCE APPEARS BEST GIVEN
LINGERING UNCERTAINTY FOR THIS RELATIVELY SMALL SCALE FEATURE.

LOOKING AT THE DETAILS OF THE AMPLIFYING TROUGH OVER THE LOWER 48
AND ASSOCIATED SYSTEMS, IT HAS BECOME EVIDENT OVER THE PAST DAY
THAT ECMWF-BASED GUIDANCE SEEMS TO BE LEADING THE TRENDS.  THE
CONSENSUS SOLUTION IS FAIRLY STABLE WITH THE INITIAL
SHORTWAVE/EMBEDDED UPPER LOW MOVING INTO THE WEST WED-THU WITH THE
TRENDS BECOMING INCREASINGLY NOTICEABLE AFTER THAT TIME.
SHORTWAVE ENERGY ROUNDING THE UPSTREAM RIDGE AROUND MIDWEEK AND
DROPPING SOUTHWARD THEREAFTER HAS DEVELOPED MORE SOUTHWESTWARD
AMPLITUDE SINCE YESTERDAY, ULTIMATELY LEADING TO FASTER EJECTION
OF THE INITIAL WED-THU SHORTWAVE OVER THE WEST.  AT THE SAME TIME
THIS EVOLUTION SEEMS TO BE HOLDING BACK NORTHERN TIER HEIGHT FALLS
THUS SLOWING THE SURFACE SYSTEM THAT IS STILL ADVERTISED TO TRACK
FROM THE CENTRAL PLAINS THROUGH THE GREAT LAKES INTO EASTERN
CANADA.  RECENT GFS/GEFS-BASED GUIDANCE HAS BEEN LAGGING
ECMWF/ECMWF MEAN TRENDS FOR THESE ASPECTS OF THE FORECAST BY AT
LEAST ONE 12-HOURLY CYCLE.  THE 00Z CMC/UKMET ARE ON THE EXTREME
SIDE OF THE ENSEMBLE SPREAD FOR HOW AMPLIFIED THE UPSTREAM
SHORTWAVE ENERGY BECOMES AND APPEAR LOW IN PROBABILITY GIVEN
TELECONNECTION-FAVORED FLOW BUT WILL HAVE TO BE MONITORED GIVEN
RECENT TRENDS.

CONTINUITY LOOKS BETTER THAN AVERAGE FOR THE WEAK CLIPPER SYSTEM
EXPECTED TO BECOME ILL-DEFINED AS IT TRACKS TOWARD THE UPPER GREAT
LAKES LATE IN THE WEEK.  TRENDS FOR FLOW ALOFT OVER THE WEST LEAD
TO A SLIGHTLY SLOWER SOUTHWARD PUSH OF THE TRAILING COLD FRONT
OVER THAT PART OF THE COUNTRY.  THEN THERE IS DECENT AGREEMENT
ASIDE FROM TYPICAL DETAIL SPREAD THAT NORTHERN CANADA UPPER LOW
ENERGY SHOULD STRETCH SOUTHWARD TOWARD THE NORTHERN TIER STATES,
BRINGING A FRONT WITH EVEN COLDER AIR TO THE NORTHERN PLAINS BY
EARLY DAY 7 SUN.

BASED ON THE ABOVE CONSIDERATIONS, THE UPDATED FORECAST BLEND
INCORPORATED MOSTLY OPERATIONAL GUIDANCE EARLY-MID PERIOD FOLLOWED
BY A MORE EVEN MODEL/ENSEMBLE MEAN WEIGHTING FOR NEXT WEEKEND,
WITH HIGHER PERCENTAGES GIVEN TO THE ECMWF/ECMWF MEAN GUIDANCE
THAN GFS/GEFS MEAN.


...WEATHER HIGHLIGHTS/THREATS...

THE SYSTEM INITIALLY CROSSING THE WEST WILL BRING A PERIOD OF
SNOW/LOW ELEVATION RAIN AND WINDY CONDITIONS WED-THU.  THEN EXPECT
A BAND OF SNOW TO EXTEND TO THE NORTHWEST OF LOW PRESSURE TRACKING
FROM THE CENTRAL PLAINS NORTHEASTWARD THU ONWARD.  CURRENTLY THE
BEST POTENTIAL FOR MEANINGFUL SNOW SHOULD BE ACROSS THE UPPER MS
VALLEY/UPPER GREAT LAKES.  SOME SNOW MAY EXTEND BACK INTO THE
PLAINS, AS WELL AS OVER NEW ENGLAND WHERE WARM ADVECTION WILL BE
THE MORE LIKELY MECHANISM.  THERE IS STILL A FAIR AMOUNT OF
UNCERTAINTY OVER EXACTLY HOW STRONG THIS SYSTEM WILL BE, AND THUS
FOR PRECISE INTENSITY OF SNOW/WINDS.

FOR THE 5-DAY PERIOD THE AREA EXTENDING FROM FROM EASTERN
TEXAS/LOWER MS VALLEY INTO THE SOUTHERN APPALACHIANS/CAROLINAS HAS
THE HIGHEST PROBABILITY OF SEEING THE HEAVIEST RAINFALL.  THIS
WILL BE FROM A COMBINATION OF THE SYSTEM CROSSING THE SOUTHERN
TIER AROUND WED, THE COLD FRONT CROSSING THE REGION LATER IN THE
WEEK, AND THEN WITH OVERRUNNING PRECIP AS FLOW ALOFT BECOMES
NEARLY PARALLEL TO THE FRONT THAT SHOULD SETTLE NEAR THE
GULF/CAROLINA COASTS.

THE ROCKIES/PLAINS WILL SEE A DRAMATIC REVERSAL FOR TEMPS FROM WED
INTO THE WEEKEND, AS READINGS AS WARM AS 10-20F ABOVE NORMAL EARLY
IN THE PERIOD ARE DISPLACED BY HIGHS AS MUCH AS 20-30F BELOW
NORMAL BY NEXT WEEKEND.  ANOMALIES FOR MIN TEMPS SHOULD BE
SOMEWHAT LESS EXTREME.  RELATIVE WARMTH OVER THE EAST WITH SOME
ANOMALIES EXCEEDING PLUS 10F SHOULD EXTEND THROUGH FRI AND
POSSIBLY SAT BEFORE FADING BY SUN WITH FRONTAL PASSAGE.

RAUSCH

$$





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