Preliminary Forecasts
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FXUS02 KWNH 170640
PREEPD

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

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

...MODELS SUGGESTING THE POTENTIAL FOR AN OUTBREAK OF ARCTIC AIR
ACROSS THE NORTHERN PLAINS AND UPPER MIDWEST BY NEXT WEEKEND...


...OVERVIEW...

MODELS/ENSEMBLES CONTINUE TO SHOW A STRONG SIGNAL TOWARD
AMPLIFICATION OF THE LARGE SCALE FLOW OVER NORTH AMERICA OVER THE
NEXT WEEK. MEAN TROUGHING EXTENDING FROM THE BERING SEA TO HAWAI`I
WILL HELP TO AMPLIFY A RIDGE JUST OFF THE WEST COAST, EXTENDING
WELL NORTH TO THE ARCTIC. MEANWHILE, POSITIVELY TILTED MEAN
TROUGHING WILL BECOME ESTABLISHED FROM NEAR HUDSON BAY THROUGH THE
CENTRAL U.S. THIS PATTERN SHOWS SIMILARITIES TO PREVIOUS FLOW
AMPLIFICATION 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 SOUTH CENTRAL U.S. OR NORTHERN GULF OF MEXICO INTO THE
EASTERN U.S. AS ARCTIC AIR IS ABLE TO PLUNGE SOUTH INTO THE NORTH
CENTRAL CONUS AND WARM, MOIST AIR ON THE WESTERN SIDE OF THE
SUBTROPICAL RIDGE ATTEMPTS TO SPREAD NORTH INTO THE SOUTHEASTERN
STATES. SEVERAL MODEL SOLUTIONS SUGGEST THE POTENTIAL FOR AN
OUTBREAK OF EXTREMELY COLD TEMPERATURES ACROSS THE NORTHERN PLAINS
AND UPPER MIDWEST NEXT WEEKEND. THE LARGE SCALE FLOW PATTERN WOULD
CERTAINTY SUPPORT SUCH AN EVENT, WITH 500 HPA FLOW ESSENTIALLY
EXTENDING FROM THE NORTH POLE SOUTHWARD TOWARD THE NORTH CENTRAL
CONUS. THE DEGREE TO WHICH THIS IS A POSSIBILITY WILL DEPEND ON
ADDITIONAL DETAILS WHICH ARE DISCUSSED BELOW.


...GUIDANCE/PREDICTABILITY ASSESSMENT...

MODELS HAVE COME INTO BETTER AGREEMENT SURROUNDING THE SYSTEM
INITIALLY ACROSS THE LOWER MS VALLEY ON DAY 3 (WED). THE ECMWF HAD
THE BEST HANDLE ON THIS FEATURE LAST NIGHT AND MOST GUIDANCE HAS
TRENDED TO THAT SOLUTION. THUS, EXPECT SHORTWAVE INITIALLY OVER
THE SOUTHERN PLAINS EARLY WED TO MOVE EAST OFF THE SOUTHEAST COAST
BY THU. THE ECMWF REMAINS VERY WELL CENTERED WITHIN THE ENSEMBLE
SPREAD IN TERMS OF THE SURFACE LOW TRACK, AND THE 12Z CMC WAS VERY
CLOSE TO THE ECMWF. THE GFS CONTINUES TO DEPICT A SURFACE LOW ON
THE SOUTHEAST EDGE OF THE CONSENSUS. ADDITIONAL UPPER TROUGH
AMPLIFIES ACROSS THE WEST ON DAY 3. BY DAYS 4-5 (THU-FRI), SOME
SPREAD REMAINS AS TO WHETHER UPPER-LEVEL ENERGY CUTS OFF ACROSS
THE SOUTHWEST OR EVEN NORTHERN MEXICO, OR MOVES MORE QUICKLY EAST
AS AN OPEN WAVE. WHILE DETERMINISTIC GUIDANCE YESTERDAY SEEMED TO
FAVOR A CUTOFF SOLUTION, A SIGNIFICANT TREND WAS NOTED OVER THE
PAST DAY TOWARD KEEPING A MORE OPEN WAVE. THIS TREND WAS THE
STRONGEST IN THE ECMWF, WHICH IS NOW THE QUICKEST SOLUTION TO MOVE
THE WAVE EAST INTO THE SOUTHERN PLAINS ON FRI. THE 00Z GFS HAS
TRENDED TOWARD THE ECMWF WITH RESPECT TO THIS SYSTEM, NOW MOVING
THE WAVE EAST A BIT FASTER THAN ITS 18Z RUN. GIVEN THE DESCRIBED
CONSIDERATIONS, A HEAVILY ECMWF-BASED BLEND WAS FAVORED DURING
DAYS 3-4, WITH SMALLER COMPONENTS OF THE CMC/GFS ALSO INCLUDED.

FROM DAY 5 ONWARD, GENERAL CONSENSUS AMONG THE GUIDANCE IS FOR A
BROAD AND DEEP TROUGH TO DIG ACROSS THE NORTH CENTRAL U.S.,
DOWNSTREAM OF THE VERY STRONG RIDGE OFF THE WEST COAST. A
SHORTWAVE ENTERING THE NORTHERN ROCKIES FRI NIGHT SHOULD BE THE
BEGINNING OF THIS TROUGH AMPLIFICATION, WITH ADDITIONAL SHORTWAVE
ENERGY REACHING THE NORTHERN PLAINS ON SUN FURTHER DEEPENING THE
TROUGH. THE POTENTIAL EXISTS WITH SUCH AN AMPLIFIED PATTERN FOR
THE TRANSPORT OF AIR FROM THE ARCTIC HIGH LATITUDES SOUTHWARD INTO
THE NORTH CENTRAL U.S. THIS DEGREE TO WHICH THIS CAN OCCUR WILL
LARGELY DEPEND ON THE EVOLUTION OF AN UPPER LOW INITIALLY NORTH OF
HUDSON BAY ON DAY 3, WHICH THEN DRIFTS SOUTHWARD BY THE WEEKEND -
ALTHOUGH MODELS DIFFER ON THE EXTENT. THE 12Z ECMWF AND 00Z GFS
ARE MORE AGGRESSIVE WITH MOVING THE UPPER LOW SOUTHWARD, WHICH
WOULD BRING THE POTENTIAL FOR AN EXTREMELY COLD AIR MASS INTO THE
NORTHERN PLAINS AND UPPER MIDWEST BY NEXT WEEKEND. ENSEMBLES HAVE
SHOWN A MODEST SHIFT IN SUPPORT TOWARD THIS IDEA AS WELL, WITH
ECENS/GEFS MEAN 500 HPA HEIGHTS ACROSS THE UPPER MIDWEST
DECREASING FOR SEVERAL CONSECUTIVE RUNS. GIVEN THE GRADUALLY
INCREASING UNCERTAINTY, THE ECENS/GEFS MEANS WERE GIVEN INCREASING
WEIGHT THROUGH TIME IN THE FORECAST AFTER DAY 5.


...WEATHER HIGHLIGHTS/THREATS...

THE NORTHERN/CENTRAL ROCKIES WILL BE UNSETTLED EARLY IN THE PERIOD
AS A STRONG LOW PRESSURE SYSTEM CROSSES THE REGION ON WED. FARTHER
EAST, HEAVY RAIN ONGOING AT THE START OF DAY 3 ALONG A
STATIONARY/WARM FRONT ACROSS THE LOWER MS VALLEY/SOUTHEAST IS
EXPECTED TO SPREAD EAST IN ASSOCIATION WITH AN AREA OF LOW
PRESSURE, MOVING OFF THE SOUTHEAST U.S. COAST BY THU. ANOTHER
EPISODE OF SHOWERS AND THUNDERSTORMS APPEARS POSSIBLE FRI-SAT AS
ANOTHER COLD FRONT CROSSES THE MS/OH VALLEYS AND THE SOUTHEAST.
THE FRONT MAY STALL IN THE GULF OF MEXICO WITH OVERRUNNING
MOISTURE MAINTAINING PERIODS OF PRECIPITATION OVER THE SOUTHERN
STATES INTO NEXT WEEKEND. 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. WED-THU BEFORE AN AREA OF POLAR HIGH PRESSURE
BEGINS TO NOSE SOUTHWARD EAST OF THE ROCKIES THU INTO FRI,
BRINGING BELOW AVERAGE TEMPERATURES TO THE NORTH CENTRAL U.S. AN
ARCTIC FRONT IS EXPECTED TO USHER EVEN COLDER AIR INTO THE
NORTHERN PLAINS AND UPPER MIDWEST BY NEXT WEEKEND, WITH
TEMPERATURES (CONSERVATIVELY) 15 DEGREES TO PERHAPS EVEN EXCEEDING
20 DEGREES BELOW AVERAGE FOR SOME AREAS. THE SUBTROPICAL RIDGE
WILL KEEP ABOVE AVERAGE TEMPS IN PLACE ACROSS THE SOUTHEAST AND
MUCH OF THE EASTERN SEABOARD INTO NEXT WEEKEND.

RYAN

$$





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