Prognostic Meteorological Discussion
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000
FXUS02 KWBC 191600
PMDEPD

EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1059 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
DURING MUCH OF THE MEDIUM RANGE PERIOD.  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 THROUGH MIDWEEK. THIS
PERSISTENT TROUGH WILL EJECT UPPER LEVEL SHORTWAVES EASTWARD
ACROSS NORTH AMERICA, SOME OF WHICH WILL AMPLIFY AS THEY ENCOUNTER
INCREASING BAROCLINICITY ACROSS THE CONUS.  LARGER SCALE EAST
PACIFIC TROUGHING WILL LIKELY MOVE INTO THE WEST LATER THIS WEEK.
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...

LATEST GUIDANCE IS SHOWING SOME SIMILAR TENDENCIES RELATIVE TO
EACH OTHER THOUGH PERHAPS WITH SOME SUBTLE HINTS AT CONVERGENCE.
MULTI-DAY TRENDS FOR THE STORM TRACKING NORTHEAST FROM THE
MIDWEST/GREAT LAKES HAVE BEEN SLOWER, CONTINUING TO FAVOR HEDGING
SOMEWHAT SLOWER THAN GFS/GEFS RUNS.  HOWEVER COMPARED TO YESTERDAY
THERE IS A BIT LESS OF A CONSENSUS OVER HOW MUCH SLOWER TIMING
THAN THE GFS IS OPTIMAL AND UPSTREAM FLOW IS STILL FLAT ENOUGH TO
KEEP THE DOOR OPEN TO FASTER PROGRESSION AT SOME POINT.  THERE ARE
ALSO SOME MODERATE BUT IMPORTANT TRACK DIFFERENCES WITH THE 00Z
ECMWF/ECMWF MEAN SOMEWHAT ON THE SOUTHERN SIDE OF THE SPREAD
VERSUS OTHER CURRENT SOLUTIONS AND CONTINUITY.  MEANWHILE THE
MAJORITY OF GUIDANCE HAS TRENDED A LITTLE MORE OFFSHORE FOR THE
EASTERN PACIFIC WAVE EXPECTED TO BE OFF THE PACIFIC NORTHWEST
COAST BY DAY 4 TUE.  ENSEMBLE SPREAD IS STILL CONSIDERABLE FOR
THIS FEATURE WITH ECMWF/CMC/UKMET RUNS STRONGER THAN THE 00Z/06Z
GFS.  A COMPROMISE APPROACH LOOKS BEST OVER THIS REGION.  FOR THE
FIRST PART OF THE FORECAST PERIOD, A BLEND OF THE 00Z ECMWF/06Z
GFS/00Z CMC IN ORDER OF DECREASING WEIGHT PROVIDED THE BEST
ACCOUNT FOR PREFERENCES WITH BOTH SYSTEMS OF INTEREST.

CONFIDENCE IS STILL LOW WITH LEADING PACIFIC ENERGY THAT CROSSES
THE LOWER 48 THROUGH DAY 6 THU.  GFS RUNS CONTINUE TO BE
SLOWER/SHARPER THAN ECMWF RUNS-- ALBEIT WITH THE 06Z GFS TRENDING
A LITTLE FASTER/WEAKER-- BUT 00Z UKMET/CMC RUNS DO ARGUE FOR A
MORE PRONOUNCED  FEATURE THAN THE ECMWF.

THE GFS/GEFS CONTINUE TO LEAN ON THE SLOWER SIDE OF THE GUIDANCE
SPREAD OVER THE LARGE SCALE TROUGH MOVING INTO THE WEST DURING THE
LATTER HALF OF THE WEEK.  HOWEVER THE SPREAD HAS NARROWED A BIT AS
THE ECMWF HAS SLOWED DOWN VERSUS PREVIOUS RUNS.  ALSO OF NOTE,
RECENT ECMWF MEANS HAVE BEEN SIMILAR BUT BY WED-THU AT LEAST THE
00Z RUN EXTENDS AN EAST-CENTRAL PACIFIC RIDGE FARTHER EAST INTO A
REGION OF NORTHWESTERLY FLOW DEPICTED BY CONSENSUS INCLUDING THE
OPERATIONAL ECMWF.  THIS CONSIDERATION RECOMMENDS KEEPING THE
ECMWF MEAN COMPONENT OF THE FORECAST SOMEWHAT LOWER THAN AVERAGE
MID-LATE PERIOD.  EVEN WITH THE DIFFERENCES THAT ARISE IN THE DAYS
4-6 TIME FRAME, THERE IS ACTUALLY BETTER CONSENSUS AMONG MOST
GUIDANCE (MINUS THE FLAT 00Z CMC MEAN) BY LATE DAY 7 FRI.
BALANCING CONSIDERATIONS FOR THIS TROUGH AND LEADING SHORTWAVE
ENERGY, THE MID-LATE PART OF THE FORECAST INCORPORATES A BLEND OF
THE 00Z ECMWF/06Z GFS/00Z CMC AND THE 06Z GEFS/00Z ECMWF MEANS.


...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 SYSTEM TRACKING OUT OF THE MIDWEST.
 FARTHER SOUTH, RAIN AND PERHAPS SOME THUNDERSTORMS WILL BE
POSSIBLE ALONG/AHEAD OF THE SURFACE COLD FRONT FROM THE GULF COAST
TO THE OH VALLEY. AS THE SYSTEM REACHES THE NORTHEAST LATE MON
INTO TUE, ATLANTIC INFLOW MAY PROMOTE AREAS OF HEAVY RAIN ACROSS
SOUTHERN NEW ENGLAND WHILE A WINTRY MIX OF PRECIPITATION IS
POSSIBLE FOR NORTHERN NEW ENGLAND.  THE PROPORTION OF PRECIP THAT
FALLS AS SNOW OVER NEW ENGLAND WILL BE SENSITIVE TO THE EXACT
SURFACE LOW TRACK, WITH CURRENT SPREAD WITHIN TYPICAL GUIDANCE
ERROR FOR 3-5 DAYS OUT IN TIME.  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.  EXPECT LOW-ELEVATION RAIN AND MOUNTAIN SNOWS THROUGH MUCH
OF THE PERIOD, WITH HIGHEST TOTALS FROM NORTHERN CALIFORNIA
THROUGH THE PACIFIC NORTHWEST.  SNOW LEVELS WILL LIKELY DECREASE
MID-LATE WEEK AS HEIGHTS ALOFT FALL IN RESPONSE TO THE AMPLIFYING
UPPER TROUGH MOVING INTO THE REGION. 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.  THE SAME IS TRUE IN ASSOCIATION WITH GULF MOISTURE THAT MAY
STREAM ACROSS THE EXTREME SOUTHEAST/FLORIDA AROUND WED-THU, WITH
PRECIP DEPENDENT ON LOW PREDICTABILITY SHORTWAVE SPECIFICS IN
PROGRESSIVE FLOW ALOFT.  THE OVERALL GUIDANCE AVERAGE HAS TRENDED
SOMEWHAT LIGHTER OVER THE PAST DAY BUT NOT YET TO THE POINT OF THE
DRIEST ECMWF-BASED GUIDANCE.

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 WITH SOME PLUS 20F OR GREATER ANOMALIES POSSIBLE
FOR MINS.  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 BY UP TO 10-20F ONCE AGAIN ACROSS MUCH OF THE
CENTRAL/EAST-CENTRAL U.S. BY THE MIDDLE OF NEXT WEEK, WHILE MUCH
OF THE WESTERN U.S. SHOULD SEE READINGS 5-15F BELOW NORMAL BY NEXT
FRI.

RAUSCH/RYAN

$$





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