Prognostic Meteorological Discussion
Issued by NWS

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000
FXUS02 KWBC 211443
PMDEPD

EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
942 AM EST WED FEB 21 2018

VALID 12Z SAT FEB 24 2018 - 12Z WED FEB 28 2018

DUE TO TECHNICAL DIFFICULTIES, SOME PRODUCTS MAY NOT BE AVAILABLE
OR UPDATED.  PLEASE CHECK ISSUANCE TIMES.  WE APOLOGIZE FOR ANY
INCONVENIENCE.


...PATTERN OVERVIEW...

MOST GUIDANCE MAINTAINS AN AMPLIFIED MEAN PATTERN CONSISTING OF
SHORTWAVES PERIODICALLY DIGGING INTO A WEST COAST/INTERIOR WEST
MEAN TROUGH AND THEN EJECTING AROUND A DOWNSTREAM RIDGE WHOSE
SHAPE WILL EVOLVE WITH TIME.  THE MOST COMMON SCENARIO AMONG
LARGEST GUIDANCE IS FOR A LEADING SHORTWAVE EJECTING FROM THE
ROCKIES/HIGH PLAINS AROUND THE START OF THE WEEKEND TO GENERATE A
POTENTIALLY MAJOR STORM REACHING THE GREAT LAKES BY SAT
NIGHT/EARLY SUN, FOLLOWED BY A FAIRLY WEAK/DIFFUSE AND PROGRESSIVE
SHORTWAVE, AND FINALLY DEEPER WESTERN TROUGH AMPLIFICATION NEXT
MON-WED.  THIS EVOLUTION/PROGRESSION WILL MOST LIKELY LEAD TO
STEADY EROSION OF INITIALLY STRONG RIDGING ALONG/OFFSHORE THE EAST
COAST FOLLOWED BY A SHIFT OF RIDGE EMPHASIS INTO THE GULF OF
MEXICO AND THEN SOME REBUILDING OF A RIDGE OVER THE EAST-CENTRAL
U.S. BY MIDWEEK AS WESTERN TROUGHING AMPLIFIES.


...MODEL GUIDANCE/UNCERTAINTY ASSESSMENT/PREFERENCES...

ONE SIGNIFICANT FOCUS OF THE FORECAST WILL BE ON THE STORM SYSTEM
ASSOCIATED WITH THE SHORTWAVE EMERGING FROM THE ROCKIES/HIGH
PLAINS AT THE START OF THE PERIOD EARLY SAT.  THERE IS STILL A
MODERATE AMOUNT OF SPREAD FOR EXACTLY HOW THIS SYSTEM WILL EVOLVE
BUT MOST MODEL/ENSEMBLE SOLUTIONS HAVE TRENDED NOTICEABLY DEEPER
VERSUS 24 HOURS AGO, WHILE THE GFS CONTINUES TO BE THE DEEPEST
MODEL.  THIS DEEPER TREND WOULD MERIT SOME INCORPORATION OF THE
00Z/06Z GFS SCENARIO INTO THE FORECAST AS PART OF A MODEL
COMPROMISE BUT OCCASIONALLY DEEP BIASES STILL LEAVE SOME LINGERING
DOUBT IN GOING AS DEEP AS THE GFS.  THERE ARE STILL INDICATIONS
THAT A WEAK WAVE WILL FORM OFF THE NORTHEAST COAST AS THE PARENT
LOW TRACKS OVER SOUTHEAST CANADA.  SPECIFICS OF THIS WAVE WILL
PLAY AN IMPORTANT ROLE IN HOW LONG COOL AIR REMAINS OVER NEW
ENGLAND.

THERE IS DECENT CONTINUITY FROM YESTERDAY THAT A TRAILING
SHORTWAVE REACHING THE NORTHWEST DURING THE WEEKEND SHOULD BE
FAIRLY WEAK AND PROGRESSIVE, WITH AT MOST A MODEST SURFACE
REFLECTION AS THE FEATURE CONTINUES ACROSS LOWER 48.

THE OTHER MEANINGFUL UNCERTAINTY IN TODAY`S FORECAST INVOLVES
EASTERN PACIFIC/WESTERN U.S. EVOLUTION TOWARD THE LATTER HALF OF
THE PERIOD, WITH MODELS AND TO A GREATER EXTENT INDIVIDUAL
ENSEMBLE MEMBERS DISPLAYING BROADENING SPREAD FOR COMBINED
EVOLUTION OF TROUGH ENERGY DIGGING INTO THE WEST AND
UPSTREAM/INFLUENCING ENERGY REACHING THE NORTHEAST PACIFIC.  BEST
MODEL/MEAN CONSENSUS (ECMWF/CMC AND GEFS/ECMWF/CMC MEANS) HAS A
FAIRLY SHARP AND DEEP UPPER TROUGH REACHING JUST INLAND FROM THE
WEST COAST BY DAYS 6-7 TUE-WED, WITH INDICATIONS THAT THERE COULD
BE AN EMBEDDED CLOSED LOW.  THIS MAJORITY SOLUTION IS A LITTLE
FASTER THAN THE 00Z GFS BUT SLOWER THAN THE 06Z RUN.


...WEATHER HIGHLIGHTS/THREATS...

THE DEEPENING STORM SYSTEM TRACKING FROM NEAR THE CENTRAL PLAINS
THROUGH THE GREAT LAKES INTO SOUTHEAST CANADA DURING THE WEEKEND
WILL BRING A BROAD AREA OF ACTIVE WEATHER TO THE CENTRAL AND
EASTERN U.S.  AHEAD OF THE FRONT TRAILING FROM THE SURFACE LOW,
EXPECT AREAS OF HEAVY RAINFALL INCLUDING OVER PORTIONS OF THE
MS/OH VALLEY REGION THAT HAS ALREADY EXPERIENCED
CONSIDERABLE/EXCESSIVE RAINFALL.  THUS FLOODING CONCERNS WILL
CONTINUE.  SOME CONVECTION MAY ALSO BE STRONG TO SEVERE WITH SPC
CURRENTLY HIGHLIGHTING SUCH POTENTIAL FROM EXTREME NORTHEAST TX
INTO WESTERN TN AND VICINITY.  CONSULT LATEST SPC OUTLOOKS FOR
LATEST INFO.  MEANWHILE BEST POTENTIAL FOR COLD SECTOR SNOW WILL
EXTEND FROM THE NORTH-CENTRAL PLAINS THROUGH THE UPPER MS
VALLEY/UPPER GREAT LAKES AND INTO NEW ENGLAND, WITH A TRANSITION
ZONE/CHANGEOVER POSSIBLE BETWEEN THE SNOW AND RAIN AREAS.  STRONG
WINDS MAY ALSO BECOME A CONCERN DEPENDING ON HOW THE SYSTEM
DEVELOPS.  AFTER SYSTEM PASSAGE RAINFALL OF VARYING INTENSITY MAY
LINGER FOR A TIME OVER THE SOUTHERN TIER/SOUTHERN MID ATLANTIC.

OVER THE WEST, INITIALLY EXPECT SOME COASTAL/LOW ELEVATION RAIN
AND OTHERWISE SNOW FROM THE PACIFIC NORTHWEST/NORTHERN ROCKIES
INTO THE GREAT BASIN/SOUTH-CENTRAL ROCKIES FOLLOWED BY ANOTHER
PERIOD OF PRECIP THAT SHOULD EVENTUALLY PUSH SOUTHWARD FROM
NORTHERN AREAS INTO THE GREAT BASIN AND CA.  GUIDANCE SPREAD FOR
DETAILS ALOFT IS STILL SUFFICIENTLY HIGH TO TEMPER CONFIDENCE IN
DETERMINING COVERAGE AND INTENSITY OF ACTIVITY OVER CA/GREAT
BASIN.  HOWEVER THERE IS ENOUGH AGREEMENT IN PRINCIPLE THAT UPPER
TROUGH DEPTH MAY SUPPORT FAIRLY LOW SNOW LEVELS OVER SOME AREAS BY
NEXT MON-WED.

THE AMPLIFIED PATTERN WILL MAINTAIN A SHARP CONTRAST IN
TEMPERATURE ANOMALIES OVER THE LOWER 48.  DURING THE WEEKEND SOME
AREAS IN THE EAST MAY SEE MINS OF 20-30F ABOVE NORMAL FOR
POTENTIAL DAILY RECORDS AND DAYTIME HIGHS 15-20F ABOVE NORMAL.
LESS EXTREME WARM ANOMALIES SHOULD PERSIST OVER THE EAST INTO NEXT
WEEK.  ON THE OTHER HAND MUCH OF THE WEST INTO THE NORTHERN PLAINS
SHOULD REMAIN WELL BELOW NORMAL DURING THE FORECAST PERIOD.  THE
NORTHERN HIGH PLAINS SHOULD BE COLDEST WITH MAX TEMPS AVERAGING
15-20F BELOW NORMAL BUT MANY OTHER LOCATIONS OVER THE
WEST/NORTHERN PLAINS SHOULD SEE ONE OR MORE DAYS WITH MIN AND/OR
MAX TEMPS AT LEAST 10F BELOW NORMAL AS WELL.

RAUSCH


$$




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