Prognostic Meteorological Discussion
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FXUS02 KWBC 161558
PMDEPD

EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1057 AM EST SAT DEC 16 2017

VALID 12Z TUE DEC 19 2017 - 12Z SAT DEC 23 2017

...OVERVIEW...

GUIDANCE CONTINUES TO SHOW A STRONG SIGNAL TOWARD AMPLIFICATION OF
THE LARGE SCALE FLOW OVER NORTH AMERICA DURING THE NEXT WEEK.
MEAN TROUGHING EXTENDING FROM THE BERING SEA TO HAWAI`I WILL HELP
TO AMPLIFY A RIDGE JUST OFF THE WEST COAST AND EXTENDING WELL
NORTHWARD THROUGH AND BEYOND ALASKA, WHILE POSITIVELY TILTED MEAN
TROUGHING WILL BECOME ESTABLISHED FROM NEAR HUDSON BAY THROUGH THE
CENTRAL U.S. AND LIKELY INTO NORTHERN MEXICO.  THERE ARE SOME
SIMILARITIES TO OTHER RECENT AMPLIFIED EPISODES OVER THE CONUS IN
RECENT WEEKS, BUT A STARK DIFFERENCE THIS TIME SEEMS TO BE THE
WESTERN ATLANTIC SUBTROPICAL RIDGE ATTEMPTING TO BUILD INTO THE
SOUTHEASTERN STATES.  THIS PATTERN WOULD EVENTUALLY FAVOR A
PERSISTENT, SHARP, AND WAVY BAROCLINIC ZONE FROM NEAR THE SOUTHERN
PLAINS NORTHEASTWARD AS ARCTIC AIR IS ABLE TO PLUNGE SOUTH INTO
THE NORTH CENTRAL CONUS WHILE WARM AND MOIST AIR FROM THE GULF OF
MEXICO SPREADS NORTH INTO THE LOWER MS VALLEY AND SOUTHEAST.


...GUIDANCE/PREDICTABILITY ASSESSMENT...

CURRENT SATELLITE IMAGERY SEEMS TO BE SUPPORTING LATEST GUIDANCE
TRENDS THROUGH THE 00Z/06Z CYCLES THAT UPPER TROUGH ENERGY NOW
OVER THE WEST SHOULD FAIRLY SOON FORM A CLOSED LOW SUFFICIENTLY
SEPARATED FROM THE WESTERLIES THAT IT WILL REACH NO FARTHER EAST
THAN THE SOUTHERN ROCKIES/NORTHERN MEXICO BY THE START OF THE
MEDIUM RANGE PERIOD.  IMPROVED AGREEMENT IN THE SHORT RANGE
EVOLUTION LEADS TO BETTER CLUSTERING THAN SEEN PREVIOUSLY AS THE
REMAINING SHORTWAVE EJECTS ACROSS THE CENTRAL/EASTERN STATES.
WIDE SPREAD/RUN TO RUN VARIABILITY IN PRIOR CYCLES STILL PROVIDE
LINGERING UNCERTAINTY FOR THE FORECAST OF THIS FEATURE.  THE
CURRENT ARRAY OF GUIDANCE SUGGESTS THE 00Z-06Z GFS/GEFS MEAN RUNS
MAY BE SOMEWHAT TOO SUPPRESSED WITH THE SURFACE REFLECTION
CROSSING THE SOUTHERN TIER DURING THE FIRST HALF OF THE PERIOD.

SIMILAR TO YESTERDAY, THROUGH ABOUT NEXT THU THE MAJORITY OF
GUIDANCE APPEARS FAIRLY WELL CLUSTERED FOR THE UPPER TROUGH
AMPLIFYING INTO THE WEST WITH AN EMBEDDED LOW CENTER DROPPING
SOUTHEAST FROM NEAR HAIDA GWAII TO UTAH.  THEN MODELS/ENSEMBLES
DIVERGE AND VARY FROM RUN TO RUN REGARDING THE STRENGTH OF HEIGHT
FALLS ACROSS THE NORTHERN TIER/GREAT LAKES AND HOW MUCH TROUGHING
LINGERS OVER THE SOUTHWEST.  DIFFERENCES IN TELECONNECTION FAVORED
FLOW FOR THE SOUTHWEST U.S. RELATIVE TO NEGATIVE HEIGHT ANOMALIES
NORTH OF HAWAI`I (HIGHER HEIGHTS) AND POSITIVE ANOMALIES NEAR THE
ALASKA PANHANDLE (MORE TROUGHING) IN RECENT D+8 MEAN CHARTS
SUPPORT THE IDEA THAT THE SOUTHWEST U.S. IS THE AREA OF GREATEST
UNCERTAINTY.  THIS FAVORS AN INTERMEDIATE SOLUTION FOR SOUTHWEST
TROUGHING.  LATE IN THE PERIOD THE 00Z/06Z GFS, 00Z CMC, AND 00Z
ECMWF MEAN ARE CLOSEST TO A MIDDLE GROUND WHILE THE 00Z ECMWF
STRAYS EASTWARD AND THE 06Z GEFS MEAN LAGS.  FARTHER
NORTHEASTWARD, ENSEMBLE CLUSTERING CONTINUES TO INDICATE THAT LOW
PRESSURE EMERGING FROM THE WEST IS MOST LIKELY TO REACH THE GREAT
LAKES BY EARLY DAY 6 FRI AND NORTHEAST OF MAINE BY SAT.  OVERALL
THE OPERATIONAL SOLUTIONS FROM THE PAST DAY HAVE TENDED TO
GRAVITATE TOWARD THE WEAKER HALF OF THE SPREAD FOR LOW PRESSURE
DEPTH.  SIMILAR STRENGTH IN THE ENSEMBLE MEANS WOULD SUGGEST THAT
THERE ARE STILL SOME MEMBERS THAT KEEP A DEEPER SCENARIO IN PLAY.

FINALLY THERE HAS BEEN DECENT AGREEMENT/CONTINUITY FOR A WEAK
CLIPPER SYSTEM THAT MAY REACH THE UPPER GREAT LAKES BY LATE IN THE
PERIOD.  SUCH FEATURES TEND TO BE IN THE LOWER HALF OF THE
PREDICTABILITY SPECTRUM SO IT WILL NOT BE SURPRISING IF SOME
ASPECTS OF THIS SYSTEM CHANGE IN SUBSEQUENT DAYS.

FOR THE MOST PART THE UPDATED FORECAST REFLECTS ONLY MINOR
REFINEMENTS TO CONTINUITY.  DAYS 3-5 EMPHASIZED THE OPERATIONAL
MODELS WITH GFS/ECMWF/UKMET/CMC WEIGHTS SET TO REFLECT CONSENSUS
DETAILS.  DAYS 6-7 STARTED WITH A MORE EVEN WEIGHT OF
MODEL/ENSEMBLE GUIDANCE WITH THE ECMWF MEAN GIVEN HIGHER PRIORITY
THAN THE 06Z GEFS MEAN GIVEN BETTER DEFINITION OF THE GREAT LAKES
SYSTEM AND INTERMEDIATE UPPER TROUGH POSITION OVER THE SOUTHWEST.


...WEATHER HIGHLIGHTS/THREATS...

THE PACIFIC NORTHWEST AND NORTHERN/CENTRAL ROCKIES WILL BE
UNSETTLED EARLY IN THE PERIOD AS A STRONG LOW PRESSURE SYSTEM
MOVES INLAND TUE-WED.  HEAVY RAINS ALONG THE COAST AND MOUNTAIN
SNOWS WILL BE POSSIBLE.  FARTHER EAST, CONFIDENCE IS INCREASING
WITH RESPECT TO HEAVY RAIN POTENTIAL FROM THE SOUTHERN PLAINS TO
THE SOUTHEAST/SOUTHERN APPALACHIANS.  EXPECT ONE ROUND OF ENHANCED
ACTIVITY TUE-WED IN ASSOCIATION WITH THE EJECTING SOUTHERN ROCKIES
SHORTWAVE AND ASSOCIATED FRONTAL WAVE, FOLLOWED BY AN EPISODE OF
SHOWERS AND THUNDERSTORMS FRI-SAT AS AN UPSTREAM COLD FRONT
CROSSES THE REGION.  THE FRONT MAY STALL IN THE GULF OF MEXICO
WITH OVERRUNNING MOISTURE MAINTAINING PERIODS OF RAINFALL OVER THE
SOUTHERN STATES.  A COUPLE ROUNDS OF SNOW ARE POSSIBLE ACROSS THE
NORTHERN PLAINS/UPPER MIDWEST THU-SAT, THE FIRST WITH A SURFACE
LOW MOVING FROM THE CENTRAL PLAINS ON THU TO THE GREAT LAKES ON
FRI WITH POTENTIAL FOR IT TO BE ACCOMPANIED BY FAIRLY BRISK WINDS
DEPENDING ON SYSTEM STRENGTH.  A CLIPPER SYSTEM QUICKLY CROSSING
THE NORTHERN PLAINS/UPPER MIDWEST LATE FRI INTO SAT COULD BRING
ANOTHER ROUND OF SNOW ALONG WITH A MUCH COLDER AIR MASS.

TEMPERATURES WILL INITIALLY BE ABOVE AVERAGE ACROSS MUCH OF THE
CENTRAL/EASTERN U.S. TUE-THU BEFORE AN AREA OF ARCTIC HIGH
PRESSURE BEGINS TO NOSE SOUTHWARD EAST OF THE ROCKIES THU INTO
FRI, BRINGING BELOW AVERAGE TEMPERATURES TO THE NORTH CENTRAL U.S.
 THE AFOREMENTIONED SUBTROPICAL RIDGE WILL KEEP ABOVE AVERAGE
TEMPS IN PLACE ACROSS THE SOUTHEAST INTO NEXT WEEKEND.

RAUSCH/RYAN

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