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Extended Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 1541Z Jan 20, 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
1041 AM EST SAT JAN 20 2018

VALID 12Z TUE JAN 23 2018 - 12Z SAT JAN 27 2018

...OVERVIEW...

A PROGRESSIVE PATTERN CONTAINING FEATURES OF VARYING SCALES EARLY
IN THE PERIOD-- THE MOST PROMINENT BEING THE STRONG STORM TRACKING
NORTHEAST FROM THE GREAT LAKES-- WILL GAIN AMPLITUDE WITH TIME AND
BECOME DOMINATED BY INCREASINGLY LARGE SCALE FEATURES THAT TEND TO
HAVE GREATER PREDICTABILITY.  AFTER DEPARTURE OF THE NORTHEASTERN
SYSTEM, THE PRIMARY FOCUS WILL BE ON AN EASTERN PACIFIC UPPER
TROUGH THAT WILL LIKELY COME ASHORE INTO THE WEST AROUND THU AND
REACH THE ROCKIES/PLAINS BY FRI-SAT.  THIS TROUGH WILL BE
ACCOMPANIED BY LEADING SURFACE LOW PRESSURE/WAVY FRONTAL SYSTEM. 
ON EITHER SIDE OF THE UPPER TROUGH, EXPECT UPPER RIDGES TO PREVAIL
NEAR THE EAST AND WEST COASTS BY NEXT WEEKEND.


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

RECENT TRENDS IN THE GUIDANCE TOWARD BETTER AGREEMENT FOR
SIGNIFICANT FEATURES OVER THE PAST DAY HAVE YIELDED, AT LEAST FOR
TIME BEING, PRACTICALLY THE BEST POSSIBLE DEGREE OF CONSENSUS THAT
COULD BE DESIRED THROUGH THE DAY 7 TIME FRAME.  THE ONE EXCEPTION
COMES FROM THE 00Z CMC ENSEMBLES WHICH BECOME CURIOUSLY OUT OF
PHASE RELATIVE TO OTHER SOLUTIONS (INCLUDING OPERATIONAL CMC RUNS)
FOR PACIFIC FLOW AND FLAT WITH THE PATTERN OVER THE LOWER 48 BY
THE LATTER HALF OF THE PERIOD.  THE MERE EXISTENCE OF THIS
SEEMINGLY UNLIKELY SCENARIO AND TYPICAL GUIDANCE ERROR IN THE
EXTENDED TIME FRAME PROVIDE A REMINDER THAT SOME CHANGES IN
CONSENSUS ARE VERY POSSIBLE IN UPCOMING DAYS.  HOWEVER THE LARGE
SCALE OF FEATURES BY THE END OF PERIOD MAY HELP TO TEMPER FUTURE
SPREAD/VARIABILITY TO SOME DEGREE.

FOR THE SYSTEM TRACKING OUT OF THE GREAT LAKES TUE ONWARD,
ENSEMBLE LOW PLOTS FROM RECENT DAYS INDICATE THAT A SLOWER TREND
OF GFS RUNS/GEFS MEMBERS CONTRIBUTED THE MOST TO CURRENT
CONSENSUS.  TOWARD NEW ENGLAND AND THE CANADIAN MARITIMES THE
LATEST STORM TRACK LIES BETWEEN PRIOR SOUTHERN ECMWF/NORTHERN GFS
RUNS.

ON THE OTHER HAND FOR EASTERN PACIFIC HEIGHT FALLS MOVING INTO THE
WEST, ECMWF AND ESPECIALLY ECMWF ENSEMBLE GUIDANCE FROM RECENT
DAYS HAS SLOWED CONSIDERABLY TOWARD THE GFS/GEFS MEAN SCENARIO. 
IN LIGHT OF THIS TREND AND CURRENT GUIDANCE COMPARISONS, THE 00Z
ECMWF MEAN MAY STILL BE A BIT FAST WITH LEADING HEIGHT FALLS FROM
LATE WED ONWARD.  AN AVERAGE OF OPERATIONAL GUIDANCE PROVIDES A
REASONABLE STARTING POINT FOR LOW PRESSURE OFF THE PACIFIC
NORTHWEST/BC COAST TUE INTO THU AS WELL AS THE WAVE FORECAST TO
REACH THE NORTHERN TIER STATES BY FRI-SAT ALONG WITH THE TRAILING
FRONT.

GUIDANCE IS STILL SOMEWHAT IN FLUX FOR DETAILS OF SHORTWAVE ENERGY
BETWEEN THE ABOVE TWO SYSTEMS.  THE PRIMARY TREND IS TOWARD A MORE
PROGRESSIVE AND/OR SHEARED DEPICTION WHICH LEADS TO MUCH LOWER
RAINFALL POTENTIAL ACROSS FLORIDA AND EXTREME SOUTHEAST COMPARED
TO WHAT SOME GFS RUNS HAD BEEN ANTICIPATING IN RECENT DAYS.

IN LIGHT OF THE REMARKABLY GOOD AGREEMENT IN MOST ASPECTS OF
GUIDANCE THROUGH THE 06Z CYCLE, THE DAYS 3-5 TUE-THU FORECAST USED
EXCLUSIVELY OPERATIONAL MODEL INPUT FROM THE 00Z ECMWF/06Z GFS AND
00Z UKMET/CMC WHILE DAYS 6-7 FRI-SAT MAINTAINED MAJORITY WEIGHT OF
THE ECMWF/GFS WITH LESSER CONTRIBUTION FROM THE 06Z GEFS/00Z ECMWF
MEANS AND 00Z CMC.


...SENSIBLE WEATHER HIGHLIGHTS/THREATS...

EXPECT SNOW AND SOME GUSTY WINDS OVER PARTS OF THE GREAT LAKES
REGION ON TUE AS STRONG LOW PRESSURE TRACKS NORTHEASTWARD. 
TRAILING COLD CYCLONIC FLOW MAY PRODUCE AREAS OF LAKE EFFECT SNOW
INTO WED.  RAIN AND PERHAPS A FEW THUNDERSTORMS WILL BE POSSIBLE
ALONG/AHEAD OF THE SURFACE COLD FRONT ON TUE ALONG MOST OF THE
EAST COAST.  STRONG DYNAMICS ALOFT AND LOW LEVEL ATLANTIC INFLOW
MAY GENERATE AREAS OF HEAVY RAIN OVER SOUTHERN NEW ENGLAND WHILE A
WINTRY MIX OF PRECIPITATION IS POSSIBLE FOR NORTHERN NEW ENGLAND.

THE EASTERN PACIFIC UPPER TROUGH EVENTUALLY MOVING INTO THE
WESTERN/CENTRAL U.S. WILL SPREAD A BROAD AREA OF PRECIP FROM THE
WEST COAST INTO THE EAST-CENTRAL STATES OVER THE COURSE OF THE
PERIOD.  LOCALLY HEAVY RAIN AND HIGHER ELEVATION SNOW SHOULD BE
CONFINED TO THE CENTRAL/NORTHERN WEST COAST TUE-WED.  THEN EXPECT
SOME OF THIS MOISTURE TO EXTEND SOUTH INTO AT LEAST CENTRAL
CALIFORNIA AND EASTWARD ACROSS THE NORTHERN 2/3 TO 3/4 OF THE WEST
THROUGH THE ROCKIES BY THU-FRI.  SNOW LEVELS SHOULD TREND LOWER BY
MID-LATE WEEK AS THE UPPER TROUGH MOVES INTO THE WEST.  LEADING
LOW PRESSURE REACHING THE NORTHERN TIER BY FRI-SAT MAY BE
ACCOMPANIED BY AREAS OF COLD SECTOR SNOW.  MEANWHILE AHEAD OF THE
TRAILING COLD FRONT, SOUTHERLY FLOW WILL BEGIN TO SPREAD MOISTURE
AND INSTABILITY FROM THE GULF OF MEXICO NORTHWARD TOWARD THE END
OF THE WEEK INTO THE WEEKEND.  SPECIFICS OF RAINFALL INITIATION
AND COVERAGE/INTENSITY WILL BE SENSITIVE TO DETAILS THAT WILL TAKE
TIME TO RESOLVE, BUT THE CONSENSUS UPPER PATTERN BY NEXT SAT
(CENTRAL U.S. TROUGH, EAST COAST/WESTERN ATLANTIC RIDGE) SUGGESTS
A HIGH PROBABILITY OF SIGNIFICANT RAINFALL OVER AT LEAST A PORTION
OF THE EASTERN HALF OF THE COUNTRY BY NEXT WEEKEND.  CURRENT
GUIDANCE PROVIDES THE BEST SIGNAL FOR ENHANCED RAINFALL FROM THE
LOWER HALF OF THE MS VALLEY INTO THE SOUTHEAST.

THE PERIOD WILL START WITH VERY WARM TEMPS ALONG THE EAST COAST
AHEAD OF THE APPROACHING COLD FRONT ON TUE, WITH SOME PLUS 10-20F
ANOMALIES FOR HIGHS AFTER EVEN MORE EXTREME DEPARTURES FROM NORMAL
FOR THE MORNING LOWS.  BEHIND THE FRONT A MAJORITY OF THE LOWER 48
SHOULD SEE HIGHS WITHIN 10F EITHER SIDE OF NORMAL FOR TUE-WED. 
AMPLIFICATION AND MODERATE PROGRESSION OF THE UPPER PATTERN WILL
BRING ANOTHER SURGE OF WARMTH INTO THE CENTRAL/EASTERN STATES
THU-SAT WITH TEMPS AT SOME LOCATIONS REACHING 15-30F ABOVE NORMAL.
 AT THE SAME TIME COLDER AIR WILL SPREAD ACROSS THE WESTERN HALF
OF THE COUNTRY WITH MANY AREAS SEEING AT LEAST A DAY OR TWO WITH
TEMPS 5-15F BELOW NORMAL.

RAUSCH