Preliminary Forecasts
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000
FXUS02 KWNH 190638
PREEPD

PRELIMINARY EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
137 AM EST FRI JAN 19 2018

VALID 12Z MON JAN 22 2018 - 12Z FRI JAN 26 2018

...OVERVIEW...

A FAIRLY PROGRESSIVE FLOW REGIME WILL BE IN PLACE ACROSS THE CONUS
THROUGH THE MEDIUM RANGE. A BUILDING UPPER RIDGE EXTENDING FROM
THE NORTH CENTRAL PACIFIC OCEAN TO THE BERING SEA THROUGH THE
PERIOD WILL PROMOTE CONTINUED TROUGHING IN THE GULF OF ALASKA AND
OFF THE NORTH AMERICAN WEST COAST. THIS PERSISTENT TROUGH WILL
SERVE TO CONTINUALLY EJECT UPPER LEVEL SHORTWAVES EASTWARD ACROSS
NORTH AMERICA WHICH WILL AMPLIFY AS THEY ENCOUNTER INCREASING
BAROCLINICITY ACROSS THE CONUS. THE END RESULT WILL BE A PATTERN
MORE REMINISCENT OF EARLY SPRING THAN ONE OF MID-WINTER, WITH
FREQUENT AND PERHAPS RELATIVELY INTENSE SURFACE CYCLONES ACROSS
THE CENTRAL U.S., CHARACTERIZED BY STRONG THERMAL GRADIENTS ACROSS
THEM.


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

WHILE MODELS/ENSEMBLES SHOW GOOD AGREEMENT WITH RESPECT TO THE
LARGE SCALE PATTERN DESCRIBED ABOVE, SOME SIGNIFICANT DIFFERENCES
EXIST WITH RESPECT TO THE DETAILS OF THE FORECAST. DEEP UPPER LOW
AND ASSOCIATED SURFACE CYCLONE EXPECTED TO BE LOCATED ACROSS THE
CENTRAL PLAINS/UPPER MS VALLEY ON MON HAS SHOWN SOME IMPROVEMENT
AMONG THE GUIDANCE, WITH THE 00Z GFS MOVING TOWARD THE CONSENSUS
(A SOMEWHAT SLOWER SOLUTION THAT WAS SHOWN BY THE 12Z/18Z RUNS).
THE GUIDANCE CONSENSUS SUGGESTS A LOW TRACK ACROSS THE GREAT LAKES
EARLY TUE BEFORE LIFTING NORTH INTO EASTERN QUEBEC AND ONTARIO.
THE OTHER AREA OF CONTENTION EARLY IN THE PERIOD IS A
SHORTWAVE/LOW PRESSURE SYSTEM APPROACHING THE WEST COAST. THE
ECMWF REMAINS FASTER/STRONGER WITH THIS SHORTWAVE ENERGY RELATIVE
TO THE GFS/CMC, AND NEITHER MODEL SEEMS TO BE TRENDING
SIGNIFICANTLY TOWARD THE OTHER. THE ECMWF WAS GENERALLY WELL
CENTERED WITH RESPECT TO ITS ENSEMBLE MEMBERS, AND HAS BEEN
CONSISTENT, THUS WAS NOT COMFORTABLE THROWING THE SOLUTION OUT AT
THIS TIME. GIVEN THESE CONSIDERATIONS, A BLEND OF THE
ECMWF/GFS/CMC ALONG WITH THE ECMWF ENSEMBLE MEAN WAS USED A
STARTING POINT FOR THE FORECAST DURING DAYS 3-4 (SUN-MON).

DIFFERENCES SURROUNDING THE WAVE REACHING THE WEST COAST ON DAY 4
TRANSLATE DOWNSTREAM INTO THE ROCKIES/CENTRAL U.S. DAYS 5-7
(TUE-THU). DUE TO ITS FASTER TIMING INITIALLY, THE ECMWF EJECTS
LEADING SHORTWAVE ENERGY INTO THE SOUTHERN/CENTRAL PLAINS QUICKER
THAN THE GFS AS WELL. GUIDANCE ARE GENERALLY IN AGREEMENT ON THIS
BEING A RATHER ENERGETIC WAVE, FURTHER AMPLIFYING AS IT MOVES FROM
THE FOUR CORNERS REGION INTO THE CENTRAL U.S. INTO A REGION OF
STRONGER LOW-LEVEL THERMAL GRADIENT. INTERESTINGLY, THE 00Z GFS IS
QUITE A BIT WEAKER WITH THIS WAVE ACROSS THE ROCKIES ON DAY 7
RELATIVE TO ANY OF ITS RECENT RUNS OR TO THE ECMWF. ENSEMBLE MEANS
SUGGEST A CONTINUED COMPROMISE MAY BE IN ORDER, AND THIS WAS
GENERALLY THE APPROACH TAKEN. THE WPC FORECAST FOR DAYS 5-7 SHOWED
GRADUALLY INCREASING ENSEMBLE (ECENS/GEFS) MEAN WEIGHTING THROUGH
TIME, WITH MINORITY COMPONENTS OF THE ECMWF/GFS STILL INCLUDED.


...SENSIBLE WEATHER HIGHLIGHTS/THREATS...

POTENTIALLY SIGNIFICANT SNOWFALL ALONG WITH GUSTY WINDS WILL BE
POSSIBLE ACROSS PORTIONS OF THE UPPER MIDWEST/GREAT LAKES MON/MON
NIGHT ON THE NORTH SIDE OF THE SURFACE LOW PRESSURE SYSTEM.
FARTHER SOUTH, RAIN AND PERHAPS SOME THUNDERSTORMS WILL BE
POSSIBLE ALONG/HEAD OF THE SURFACE COLD FRONT FROM THE GULF COAST
TO THE OH VALLEY. AS THE SYSTEM REACHES THE NORTHEAST LATE MON
INTO TUE, AREAS OF HEAVY RAIN WILL BE POSSIBLE ACROSS SOUTHERN NEW
ENGLAND WHILE A WINTRY MIX OF PRECIPITATION IS POSSIBLE FOR
NORTHERN NEW ENGLAND. MEANWHILE, THE WESTERN U.S. WILL SEE A
PROLONGED PERIOD OF ONSHORE FLOW WITH SEVERAL UPPER-LEVEL IMPULSES
AND SURFACE FRONTAL SYSTEMS ENHANCING PRECIPITATION AT TIMES.
LOW-ELEVATION RAIN AND MOUNTAIN SNOWS ARE EXPECTED THROUGH MUCH OF
THE PERIOD. SNOW LEVELS SHOULD DECREASE BY MID-WEEK AS HEIGHTS
FALL IN RESPONSE TO THE AMPLIFYING UPPER TROUGH. THE SOUTH CENTRAL
U.S. WILL SEE INCREASING POTENTIAL FOR SHOWERS AND THUNDERSTORMS
BY LATE NEXT WEEK AS THE NEXT UPPER TROUGH/FRONTAL SYSTEM REACH
THE PLAINS. THE DESCRIBED DIFFERENCES IN TIMING BETWEEN VARIOUS
MODEL SOLUTIONS WILL HAVE AN IMPACT ON HOW QUICKLY LOW-LEVEL
MOISTURE RETURN COMMENCES. THUS, CONFIDENCE IN THE SPECIFICS OF
WHERE/WHEN ANY POTENTIAL PRECIPITATION WOULD OCCUR IS RELATIVELY
LOW AT THIS TIME.

MUCH OF THE EASTERN THIRD OF THE CONUS WILL SEE ABOVE AVERAGE
TEMPS MON-TUE IN THE WARM SECTOR AHEAD OF THE INTENSIFYING MS
VALLEY/GREAT LAKES CYCLONE. HIGH TEMPERATURES MAY REACH 10-20 DEG
F ABOVE AVERAGE. GIVEN THE PROGRESSIVE PATTERN, ANY COOL DOWN
BEHIND THE FRONT WILL BE RATHER BRIEF, WITH WARM AIR ADVECTION
AHEAD OF THE NEXT DEVELOPING CYCLONE EXPECTED TO BOOST
TEMPERATURES ABOVE AVERAGE ONCE AGAIN ACROSS MUCH OF THE CENTRAL
U.S. BY THE MIDDLE OF NEXT WEEK.

RYAN

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