Prognostic Meteorological Discussion
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FXUS02 KWBC 180641
PMDEPD

EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
141 AM EST MON DEC 18 2017

VALID 12Z THU DEC 21 2017 - 12Z MON DEC 25 2017

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


...OVERVIEW...

CONFIDENCE REMAINS ABOVE AVERAGE FOR A MAJOR PATTERN CHANGE ACROSS
NORTH AMERICA DURING THE MEDIUM RANGE. A SPLIT FLOW PATTERN EARLY
IN THE PERIOD (THU) WILL QUICKLY UNDERGO A SIGNIFICANT
AMPLIFICATION, WITH AN EASTERN PACIFIC RIDGE AXIS EXTENDING FROM
JUST OFF THE U.S. WEST COAST NORTH TO THE ARCTIC, AND A DOWNSTREAM
MEAN TROUGH FROM NEAR HUDSON BAY SOUTHWEST INTO THE CONUS. THIS
SETUP WILL RESULT IN A FLOW OF INCREASINGLY COLD AIR SOUTHWARD
THROUGH CANADA, MOST LIKELY DIRECTED INTO THE ROCKIES/PLAINS/UPPER
MIDWEST. TELECONNECTIONS ASSOCIATED WITH THE AREA OF MAXIMUM
POSITIVE HEIGHT ANOMALIES ACROSS NORTHWEST CANADA (+2 TO +3
STANDARD DEVIATIONS ON THE GEFS MEAN) SUPPORT THE IDEA SHOWN BY
MOST GUIDANCE, WITH THE NORTHERN/CENTRAL PLAINS AND UPPER MIDWEST
THE FAVORED AREAS FOR THE COLDEST TEMPERATURES BY THE WEEKEND.
FARTHER EAST, THE SUBTROPICAL RIDGE IS FORECAST TO STRENGTHEN OVER
THE NEXT WEEK, CENTERED OVER THE BAHAMAS OR SOUTH FLORIDA (ALSO
SUPPORTED BY TELECONNECTIONS FROM THE NORTH PACIFIC RIDGE). THE
CONTRAST BETWEEN THE FLOW OF ARCTIC AIR SOUTHWARD INTO THE CENTRAL
U.S. AND WARM, MOIST SUBTROPICAL AIR TRANSPORTED NORTHWARD INTO
THE SOUTHERN/SOUTHEASTERN U.S. WILL SUPPORT A PERSISTENT FRONTAL
ZONE FROM THE EXTREME SOUTHERN PLAINS/NORTHERN MEXICO
EAST-NORTHEAST TO NEAR THE SOUTHEAST COAST. EXPECT THIS FRONTAL
ZONE TO SERVE AS A FOCUS FOR POTENTIAL ROUNDS OF PRECIPITATION AS
LONG AS THIS PATTERN PERSISTS, WITH SOME POTENTIAL FOR OCCASIONAL
WINTER WEATHER ON THE NORTHERN FRINGES OF THIS AREA GIVEN A RATHER
SHARP THERMAL GRADIENT.


...GUIDANCE/PREDICTABILITY ASSESSMENT...

MODELS SHOWED SUFFICIENT CONSENSUS EARLY IN THE MEDIUM RANGE TO
SUPPORT THE USE OF A MAJORITY DETERMINISTIC BLEND INCLUDING THE
12Z ECMWF/18Z GFS. THESE SOLUTIONS SHOWED MODEST TIMING AND
STRENGTH DIFFERENCES DURING DAYS 3-5 (THU-SAT) IN ASSOCIATION WITH
THE SHORTWAVE/UPPER LOW EXITING FROM THE FOUR CORNERS REGION INTO
THE PLAINS, AND WERE GENERALLY WELL-CLUSTERED WITH THE ENSEMBLE
MEANS. THE 12Z UKMET WAS INCLUDED IN THE BLEND ON DAY 3, BUT WAS
EXCLUDED AFTER THAT TIME DUE TO DIFFERENCES AT THE SURFACE ACROSS
THE PLAINS. AS THE SURFACE CYCLONE DEEPENS ACROSS THE GREAT LAKES
LATE FRI INTO SAT, THE GFS AND ECMWF REMAINED VERY WELL CENTERED
WITHIN THE SPREAD, BUT SHOWED SOME FAIRLY SIGNIFICANT DIFFERENCES
IN TERMS OF INTENSITY (WITH THE ECMWF 10-12 HPA WEAKER THAN THE
GFS).

FROM LATE DAY 5 ONWARD, THE LARGE SCALE TROUGH BEGINS TO AMPLIFY
ACROSS THE CENTRAL U.S. A SHORTWAVE MOVING FROM THE PAC NW ON DAY
5 INTO THE ROCKIES ON DAY 6 (SUN) WILL LOWER HEIGHTS SUFFICIENTLY
TO BEGIN CARVING OUT THE TROUGH, ALLOWING A DEEP UPPER LOW NEAR
HUDSON BAY TO DRIFT SOUTHWARD TOWARD THE NORTH CENTRAL U.S. MODELS
STILL SHOW A MODERATE DEGREE OF SPREAD AS TO THE EXACT DETAILS OF
THIS EVOLUTION, WITH RUN-TO-RUN VARIABILITY IN HOW QUICKLY THE
UPPER LOW MOVES SOUTHWARD. REGARDLESS, THE FLOW PATTERN WILL
SUPPORT TRANSPORT OF VERY COLD AIR FROM THE HIGH LATITUDES OF
CANADA SOUTHWARD INTO THE NORTH CENTRAL U.S. BY DAYS 6-7,
DETERMINISTIC SOLUTIONS BEGIN TO DIFFER EVEN FURTHER WITH THE
EVOLUTION OF THE SHORTWAVE ENERGY AS IT MOVES EAST FROM THE
ROCKIES. THE 12Z ECMWF/18Z GFS, ALONG WITH THEIR RESPECTIVE
ENSEMBLE MEANS WERE MUCH SLOWER THAN THE 00Z GFS TO MOVE THIS
ENERGY EAST. LOOKING BACK OVER THE PAST SEVERAL RUNS, MODELS HAVE
VARIED WITH RESPECT TO THIS FEATURE, AND ANY TRENDS REMAIN
DIFFICULT TO DISCERN. THUS, ENSEMBLE (12Z ECENS/18Z GEFS) WEIGHTED
WAS BOOSTED TO A MAJORITY OF THE FORECAST BLEND BY SUN-MON.


...WEATHER HIGHLIGHTS/THREATS...

EXPECT A BAND OF SNOW TO THE NORTHWEST OF LOW PRESSURE TRACKING
FROM THE CENTRAL PLAINS NORTHEASTWARD THU ONWARD. CURRENTLY THE
BEST POTENTIAL FOR MEANINGFUL SNOW APPEARS TO BE ACROSS THE UPPER
MS VALLEY/UPPER GREAT LAKES. SNOW WILL ALSO BE POSSIBLE OVER NEW
ENGLAND FRI/SAT WHERE WARM ADVECTION WILL BE THE LIKELY MECHANISM.
THERE REMAINS SOME SPREAD OVER EXACTLY HOW STRONG THIS SYSTEM WILL
BE, AND THUS FOR PRECISE INTENSITY OF SNOW/WINDS.

THE AREA EXTENDING FROM FROM LOWER MS/TN VALLEYS INTO THE SOUTHERN
APPALACHIANS HAS THE HIGHEST PROBABILITY OF SEEING THE HEAVIEST
RAINFALL DURING THE MEDIUM RANGE. THIS WILL BE FROM A COMBINATION
OF THE COLD FRONT CROSSING THE REGION LATER IN THE WEEK, AND THEN
WITH OVERRUNNING PRECIP NORTH OF THE FRONT THAT SHOULD SETTLE NEAR
THE GULF/CAROLINA COASTS BY OVER WEEKEND.

THE CENTRAL U.S. WILL SEE A DRAMATIC REVERSAL FOR TEMPS FROM THU
INTO THE WEEKEND, AS READINGS AS WARM AS 10-20 DEG F ABOVE NORMAL
EARLY IN THE PERIOD ARE DISPLACED BY HIGHS AS MUCH AS 20-30 DEG F
BELOW NORMAL BY NEXT WEEKEND. RELATIVE WARMTH OVER THE EAST WITH
SOME ANOMALIES EXCEEDING +10 DEG F SHOULD EXTEND THROUGH SAT
BEFORE BEGINNING TO FADE BY SUN-MON WITH FRONTAL PASSAGE.

RYAN

$$





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