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Extended Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 0637Z Jan 22, 2018)
 
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Geographic Boundaries -  Map 1: Color  Black/White       Map 2: Color  Black/White


EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
137 AM EST MON JAN 22 2018

VALID 12Z THU JAN 25 2018 - 12Z MON JAN 29 2018

...OVERVIEW...

PROGRESSIVE FLOW ACROSS THE CONUS WILL GRADUALLY BECOME MORE
AMPLIFIED BY EARLY NEXT WEEK. A BLOCKING UPPER HIGH EXPECTED TO
ESTABLISH ITSELF FROM THE NORTH CENTRAL PACIFIC TO THE BERING SEA
WILL PROMOTE DOWNSTREAM TROUGHING ACROSS THE GULF OF AK, AND A
GRADUALLY AMPLIFYING FLOW ACROSS THE CONUS, INCLUDING A WESTERN
U.S. RIDGE AND CENTRAL/EASTERN U.S TROUGH BY EARLY NEXT WEEK. AN
UPPER TROUGH WILL MOVE ONSHORE IN THE WEST ON DAY 3 AND SHOULD
QUICKLY AMPLIFY AS IT CROSSES THE ROCKIES AND REACHES THE CENTRAL
U.S., WHERE SURFACE CYCLOGENESIS IS EXPECTED TO OCCUR. THE SURFACE
LOW WILL LIKELY DEEPEN QUICKLY AS IT MOVES NORTHEASTWARD, SIMILAR
TO TO THE SYSTEM CROSSING THE CENTRAL U.S. THIS EVENING, ALTHOUGH
FARTHER NORTH. UPPER TROUGH IS EXPECTED TO REACH THE EASTERN U.S.
BY NEXT WEEKEND, WITH AN ADDITIONAL WAVE OF LOW PRESSURE EXPECTED
TO DEVELOP SOMEWHERE NEAR THE MID-ATLANTIC REGION.


...GUIDANCE/UNCERTAINTY ASSESSMENT AND PREFERENCES...

MAIN POINT OF CONTENTION AMONG THE GUIDANCE IN THE MEDIUM RANGE IS
THE FORWARD SPEED OF THE UPPER TROUGH CROSSING THE CONUS. THE GFS
HAS CONSISTENTLY BEEN ON THE FAST SIDE OF THE GUIDANCE SPREAD,
WHILE THE ECMWF HAS ALSO TRENDED A BIT QUICKER AS WELL. THE
ECENS/GEFS MEANS ARE BOTH SUGGESTIVE OF A SLOWER TROUGH
PROGRESSION, AND ARE ACTUALLY QUITE SIMILAR TO EACH OTHER. THESE
DIFFERENCES TRANSLATE TO THE SURFACE AS WELL, IN TERMS OF TIMING
OF THE RATHER SHARP COLD FRONT MOVING FROM THE CENTRAL TO THE
EASTERN U.S. BY THE WEEKEND. OPTED TO MAKE A SOMEWHAT MODEST
ADJUSTMENT TOWARD A QUICKER SOLUTION IN THIS FORECAST CYCLE (GIVEN
THE DETERMINISTIC TRENDS), BUT MAINTAINED A FORECAST SOMEWHERE
BETWEEN THE QUICKER DETERMINISTIC SOLUTIONS AND THE SLOWER
ENSEMBLE MEANS. TO ARRIVE AT THIS SOLUTION, AN INITIAL BLEND
INCLUDING THE 12Z ECMWF/UKMET/18Z GFS EARLY IN THE PERIOD (DAY 3)
WAS QUICKLY TRENDED TOWARD HEAVIER ECMWF AND GEFS/ECENS WEIGHTING
FROM DAYS 4-5 ONWARD.

MODELS ARE IN BETTER AGREEMENT RELATIVE TO LAST NIGHT ON BUILDING
A STRONGER RIDGE ACROSS THE WEST BY THE WEEKEND. NEXT SIGNIFICANT
SHORTWAVE/FRONTAL SYSTEM WILL ATTEMPT TO APPROACH THE NORTHWEST BY
SUN-MON, BUT WILL STRUGGLE AS IT MOVES INTO THE RIDGE. MODEL
AGREEMENT WITH RESPECT TO THIS SHORTWAVE AND THE ASSOCIATED
SURFACE LOW WAS BETTER THAN AVERAGE GIVEN THE TIME SCALE, AND THE
AFOREMENTIONED ECMWF/ECENS/GEFS BLEND BY DAYS 5-7 SHOULD HANDLE
THIS WELL.


...SENSIBLE WEATHER HIGHLIGHTS/THREATS...

WHILE THE FRONTAL SYSTEM CROSSING THE ROCKIES ON THU WILL
INITIALLY BE RELATIVELY MOISTURE STARVED (PRODUCING MAINLY
ELEVATION SNOWS IN THE WEST), THE SYSTEM WILL EVENTUALLY BE ABLE
TO TAP INTO DEEP GULF OF MEXICO MOISTURE, PRODUCING WIDESPREAD
RAINS FROM THE LOWER MS VALLEY TO THE NORTHEAST FROM FRI NIGHT
THROUGH THE WEEKEND. MODEL SOLUTIONS VARY WITH RESPECT TO PRECIP
AMOUNTS GIVEN VARYING SOLUTIONS FOR MOISTURE RETURN AND SYSTEM
TIMING. AREAS ALONG THE IMMEDIATE GULF COAST APPEAR THE MOST
LIKELY TO RECEIVE POTENTIALLY HEAVY RAINFALL AT THIS TIME, GIVEN
THE LIKELIHOOD FOR CONVECTION AND A WEAK WEST-EAST ORIENTED
SURFACE BOUNDARY POTENTIALLY LINGERING OVERHEAD, PROVIDING FOCUS
FOR HEAVY RAINFALL. GIVEN THE IMPRESSIVE MOISTURE/INSTABILITY
ADVECTION SHOWN BY SOME GUIDANCE AHEAD OF THE SYSTEM, HOWEVER,
LOCALLY HEAVY RAINFALL WILL BE POSSIBLE ACROSS MUCH OF THE
SOUTHEAST, TN/OH VALLEYS, MID-ATLANTIC, AND NORTHEAST.
TEMPERATURES ARE EXPECTED TO BE WELL ABOVE AVERAGE ACROSS THE
CENTRAL AND EASTERN U.S. AHEAD OF THE COLD FRONT, WITH HIGHS AT
LEAST 10-2O DEG F ABOVE AVERAGE ACROSS A WIDE AREA.

MEANWHILE THE WEST COAST, PARTICULARLY THE NORTHWEST, WILL REMAIN
UNSETTLED THROUGH THE PERIOD. THE AMPLIFYING UPPER TROUGH ACROSS
THE WEST ON THU ALONG WITH ONSHORE FLOW WILL PRODUCE RAIN AND
MOUNTAIN SNOWS FROM CENTRAL CA NORTH ALONG THE COAST, UNTIL MUCH
OF THIS ACTIVITY IS CUT OFF BY THE BUILDING UPPER RIDGE. MORE
SIGNIFICANT PRECIPITATION WILL BE POSSIBLE ACROSS THE PAC NW WITH
APPROACH OF THE NEXT SYSTEM BY EARLY NEXT WEEK, WITH HEAVY RAIN
AND MOUNTAIN SNOWS.

RYAN