Prognostic Meteorological Discussion
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000
FXUS02 KWBC 160655
PMDEPD

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

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

...OVERVIEW...

MODELS/ENSEMBLES ARE IN GENERAL AGREEMENT THAT THE LARGE SCALE
PATTERN ACROSS NORTH AMERICA WILL BECOME MORE AMPLIFIED OVER THE
NEXT WEEK. STRONGLY NEGATIVE HEIGHT ANOMALIES JUST NORTH OF HAWAII
WILL HAVE DOWNSTREAM EFFECTS OF ONCE AGAIN AMPLIFYING A RIDGE JUST
OFF THE NORTH AMERICAN WEST COAST (NORTH TO THE ARCTIC), WITH
TROUGHING PREVALENT ACROSS THE CENTRAL U.S. IN SOME WAYS THIS
SHOWS SIMILARITIES TO THE AMPLIFIED PATTERN EXPERIENCED ACROSS 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 FAVOR A PERSISTENT,
SHARP, AND WAVY BAROCLINIC ZONE FROM THE SOUTHERN PLAINS INTO THE
GREAT LAKES AS ARCTIC AIR IS ABLE TO PLUNGE SOUTH INTO THE NORTH
CENTRAL CONUS WHILE COINCIDENTALLY, WARM AND MOIST AIR FROM THE
GULF OF MEXICO SPREADS NORTH INTO THE LOWER MS VALLEY AND
SOUTHEAST.


...GUIDANCE/PREDICTABILITY ASSESSMENT...

A FAIRLY SIGNIFICANT DEGREE OF UNCERTAINTY EXISTS EVEN AT THE
BEGINNING OF THE MEDIUM RANGE PERIOD ON DAY 3 (TUE) WITH RESPECT
TO THE UPPER LOW ACROSS THE SOUTHWESTERN U.S. AND HOW QUICKLY IT
MOVES EASTWARD. THE 12Z ECMWF ALONG WITH THE ECENS MEAN HAD BEEN
SUGGESTING A SLOWER PROGRESSION, WHILE THE 18Z GFS WAS FASTER.
GIVEN A NOTED TREND AMONG ENSEMBLE GUIDANCE, THE WPC FORECAST
FAVORED THE SLOWER PROGRESSION SHOWN BY THE 12Z ECMWF/UKMET/ECENS.
WITH THE 00Z RUN THE GFS HAS ALSO TRENDED SUBSTANTIALLY IN THIS
DIRECTION AS WELL, ALBEIT KEEPING THE SHORTWAVE A BIT LESS
AMPLIFIED ON DAY 4 (WED) ACROSS THE SOUTHERN PLAINS. NEXT
AMPLIFYING TROUGH ENTERS THE NORTHWEST ON DAY 3, AND
MODELS/ENSEMBLE SHOW GOOD CONSENSUS THAT THE WAVE WILL RAPIDLY
AMPLIFY ACROSS THE GREAT BASIN BY DAY 5 (THU). THE GFS AND ECMWF
BOTH SUGGEST THAT ANOTHER CUTOFF LOW WILL FORM ACROSS THE
SOUTHWEST BY THU-SAT, WITH THE 12Z CMC THE ONLY DETERMINISTIC
SOLUTION TO KEEP AN OPEN WAVE THAT MOVES MORE QUICKLY EAST.
ENSEMBLE SUPPORT FAVORS SOMETHING MORE LIKE THE ECMWF/GFS HERE,
BUT THERE IS ENOUGH SPREAD THAT ENSEMBLE MEANS DO NOT YET SHOW A
CLOSED UPPER LOW. GIVEN THIS PATTERN, BY LATE NEXT WEEK EXPECT A
RAPID SUCCESSION OF LOW AMPLITUDE SHORTWAVES TO EJECT FROM THE
SOUTHWESTERN U.S. TROUGH/UPPER LOW INTO THE SOUTHERN PLAINS,
CREATING THE POTENTIAL FOR MULTIPLE WAVES ALONG THE FRONTAL
BOUNDARY ALONG THE GULF COAST. WHILE THE EXISTENCE OF THESE WAVES
IS LIKELY GIVEN THE PATTERN, CONFIDENCE IN THE SPECIFICS OF SUCH
FEATURES AND THE TIMING OF ANY FRONTAL WAVES AND ASSOCIATED PRECIP
IS LOW, GIVEN THE LOW PREDICTABILITY OF SUCH SMALL SCALE FEATURES
AT NEARLY A WEEK OUT.

FARTHER NORTH, A PIECE OF ENERGY IS FORECAST TO SPLIT OFF FROM THE
AMPLIFYING WESTERN TROUGH AND GET CARRIED EAST IN THE NORTHERN
STREAM, CROSSING THE MIDWEST ON DAY 6 (FRI). MODELS SUGGEST THIS
FEATURE WILL RESULT IN SURFACE FRONTAL WAVE DEVELOPMENT ACROSS THE
CENTRAL PLAINS ON DAY 5, WITH THE SURFACE LOW QUICKLY TRACKING
NORTHEASTWARD ACROSS THE GREAT LAKES ON DAY 6 AND INTO
SOUTHEASTERN CANADA ON DAY 7 (SAT). AT THE SAME TIME, GUIDANCE
SHOWS DECENT AGREEMENT THAT A RAPID SUCCESSION OF SHORTWAVE ENERGY
ORIGINATING IN THE ARCTIC WILL DIVE SOUTHWARD ALONG THE EASTERN
PERIPHERY OF THE RIDGE AND GRADUALLY AMPLIFY A BROAD TROUGH AXIS
ACROSS THE UPPER MIDWEST BY NEXT WEEKEND.

THE WPC FORECAST WAS HEAVILY BASED ON THE 12Z ECMWF/UKMET DURING
DAYS 3-4, WITH A GRADUAL TREND TOWARD HEAVIER 12Z ECENS/18Z GEFS
MEAN WEIGHTING FROM DAY 5 ONWARD.


...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, THE POTENTIAL APPEARS TO BE
GROWING FOR HEAVY RAINFALL FROM THE SOUTHERN PLAINS TO THE
SOUTHEAST/SOUTHERN APPALACHIANS TUE-WED IN ASSOCIATION WITH A
FRONTAL WAVE. A SECOND ROUND OF SHOWERS AND THUNDERSTORMS APPEARS
IN THE CARDS FOR FRI-SAT AS ANOTHER COLD FRONT MOVES THROUGH THE
SAME AREAS AND THEN POTENTIALLY STALLS IN THE GULF OF MEXICO WITH
MOISTURE OVERRUNNING THE FRONT INTO THE SOUTHERN STATES. A COUPLE
ROUNDS OF SNOW ARE POSSIBLE ACROSS THE NORTHERN PLAINS/UPPER
MIDWEST THU-SAT, THE FIRST IN ASSOCIATION WITH A SURFACE LOW
MOVING FROM THE CENTRAL PLAINS ON THU TO THE GREAT LAKES ON FRI. 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.

RYAN

$$





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