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Extended Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 1559Z Jan 18, 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
1059 AM EST THU JAN 18 2018

VALID 12Z SUN JAN 21 2018 - 12Z THU JAN 25 2018

...OVERVIEW...

WITHIN A MEAN PATTERN THAT AVERAGES OUT TO BROAD CYCLONIC FLOW
OVER THE LOWER 48, THERE WILL BE TWO PRIMARY AREAS OF SIGNIFICANT
WEATHER DURING THE PERIOD.  THE FIRST WILL BE WITH A STRENGTHENING
STORM SYSTEM TRACKING NORTHEAST FROM THE CENTRAL PLAINS SUN
ONWARD, BRINGING AN AXIS OF HEAVY SNOW AND STRONG WINDS MOST
LIKELY ALONG AN AXIS FROM THE NORTH-CENTRAL PLAINS INTO THE UPPER
GREAT LAKES WITH SOME SNOW POSSIBLY REACHING NORTHERN NEW ENGLAND
AS WELL.  BEHIND THIS SYSTEM AT LEAST A COUPLE OF FEATURES WILL
BRING EPISODES OF HEAVY RAIN/HIGHER ELEVATION SNOW TO LOCATIONS
ALONG THE CENTRAL/NORTHERN WEST COAST WITH SOME OF THIS ACTIVITY
EXTENDING INTO THE ROCKIES.  THUS FAR THE SIGNAL FOR THE LEADING
PLAINS STORM HAS BEEN MORE STABLE THAN FOR SOME DETAILS OF PACIFIC
FLOW REACHING THE WEST.


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

FOR THE DEEPENING STORM SYSTEM TRACKING NORTHEAST FROM THE CENTRAL
PLAINS, MULTI-DAY TRENDS AND THE OVERALL MAJORITY OF GUIDANCE
(INCLUDING THE 00Z CMC/CMC MEAN) CONTINUE TO FAVOR TILTING THE
FORECAST ABOUT 2/3 TO 3/4 TOWARD THE ECMWF/ECMWF MEAN VERSUS THE
FASTER GFS/GEFS MEAN.  THIS MAJORITY STILL INDICATES LESS PHASING
WITH CANADIAN FLOW THAN DEPICTED IN GFS/GEFS RUNS AS THE UPPER
TROUGH REACHES THE UPPER MS VALLEY/UPPER GREAT LAKES.  THERE
CONTINUES TO BE THE STRAY SOLUTION THAT IS EVEN A BIT SLOWER THAN
THE ECMWF, SUCH AS THE 12Z/17 CMC AND CURRENT 00Z UKMET.  HOWEVER
THE RELATIVELY LOW AMPLITUDE OF UPSTREAM PACIFIC FLOW AS IT MOVES
INTO THE WEST PROVIDES A LINGERING POTENTIAL THAT THE SYSTEM OF
INTEREST COULD END UP PROGRESSING A LITTLE FASTER THAN THE SLOW
SIDE OF THE SPREAD, HENCE PRECLUDING FULL EXCLUSION OF THE GFS
SCENARIO.  THERE IS STILL SOME MODEL/ENSEMBLE SPREAD FOR LATITUDE
OF THE SURFACE LOW TRACK BUT A MULTI-RUN CONSENSUS HAS PROVEN TO
BE FAIRLY STABLE OVER THE PAST COUPLE DAYS.

IN GENERAL GUIDANCE HAS BEEN MORE DIVERSE AND VARIABLE WITH
DETAILS OF UPSTREAM FLOW.  TO THE SOUTH OF AN UPPER LOW NEARING
THE BC COAST AS OF EARLY DAY 3 SUN, ENERGY STREAMING INTO THE WEST
HAS GENERALLY BEEN TRENDING FASTER AND LESS AMPLIFIED OVER RECENT
DAYS.  HOWEVER THERE IS STILL A LOT OF MODEL/ENSEMBLE SPREAD FOR
THE DETAILS OF INDIVIDUAL IMPULSES PROGRESSING ACROSS THE
CONTIGUOUS U.S. WITH THE BROAD CYCLONIC NATURE OF THE MEAN PATTERN
OFFERING LITTLE IN THE WAY OF A SIGNAL TO ASSIST IN DETERMINING
THE BEST SOLUTION.  THESE DIFFERENCES END UP PLAYING A SIGNIFICANT
ROLE IN DETERMINING HOW MUCH GULF MOISTURE MAY EXTEND TO THE
EAST/NORTHEAST ACROSS FLORIDA/EXTREME SOUTHEAST TOWARD DAYS 6-7
WED-THU.  FOR EXAMPLE THE SLOWER/SHARPER GFS DEPICTIONS OF ENERGY
ALOFT LEAD TO VERY WET SOLUTIONS IN CONTRAST TO THE FASTER 00Z
ECMWF LEADING TO A FAIRLY DRY FORECAST.  ON THE OTHER HAND THE
12Z/17 ECMWF RUN HAD LOOKED MORE LIKE THE GFS RUNS.  RELATIVELY
HIGH UNCERTAINTY ARGUES FOR A CONSERVATIVE BLENDED APPROACH AT
THIS TIME.

MEANWHILE GUIDANCE HAS BEEN WAFFLING WITH RESPECT TO SHORTWAVE
ENERGY NEARING THE PACIFIC NORTHWEST BY DAY 5 TUE AND TRAILING
LARGER SCALE TROUGH EXPECTED TO MOVE INTO THE WEST THEREAFTER.  AS
A WHOLE, OVER THE PAST DAY MOST OPERATIONAL MODEL RUNS HAVE
TRENDED BACK SOUTHWARD WITH A POTENTIAL WAVE TRACKING TOWARD THE
BC OR PACIFIC NORTHWEST COAST ON TUE.  DURING THE DAY TUE 00Z
ENSEMBLE SURFACE LOW PLOTS STILL SHOW A VERY BROAD NORTH-SOUTH
ENVELOPE WITH LITTLE INDICATION OF ANY TRUE CLUSTERING.  BY EARLY
WED THE BEST DENSITY OF ENSEMBLES EXTENDS FROM JUST OFF THE WA
COAST NORTHWESTWARD.  BY DAY 7 THU THE 00Z ECMWF/ECMWF MEAN HAVE
TRENDED NOTICEABLY MORE AMPLIFIED/FASTER THAN PREVIOUS RUNS WITH
OVERALL TROUGHING REACHING THE WEST.  TO VARYING DEGREES THE
CMC/CMC MEAN ALSO BRING SOMEWHAT MORE TROUGHING INTO THE WEST. 
FOR BOTH ASPECTS OF THE EASTERN PACIFIC/WESTERN U.S. FORECAST, AN
INTERMEDIATE SOLUTION THAT INCORPORATES TRAITS OF BOTH MODEL AND
ENSEMBLE GUIDANCE APPEARS BEST WHILE AWAITING BETTER GUIDANCE
AGREEMENT AND RUN TO RUN STABILITY.

COMBINING PREFERENCES FOR SYSTEMS OF INTEREST, THE UPDATED BLEND
STARTED WITH A 50/30/20 BLEND OF THE 00Z ECMWF/06Z GEFS/00Z CMC
INTO EARLY DAY 5 TUE.  AFTER THAT TIME THE BLEND BEGAN TO
INCORPORATE 06Z GEFS/00Z ECMWF MEAN INPUT WITH ENSEMBLE INPUT
ACCOUNTING FOR 60 PERCENT OF THE FORECAST BY DAY 7 THU.


...SENSIBLE WEATHER HIGHLIGHTS/THREATS...

IN SPITE OF LINGERING UNCERTAINTY ON TIMING AND TO SOME DEGREE
TRACK OF DEEPENING LOW PRESSURE EMERGING FROM THE CENTRAL PLAINS
ON SUN, THERE HAS BEEN A FAIRLY CONSISTENT SIGNAL FOR THE THREAT
OF A BAND OF HEAVY SNOW EXTENDING FROM THE NORTH-CENTRAL PLAINS
INTO THE UPPER GREAT LAKES.  TIGHT PRESSURE GRADIENT AROUND THE
STORM MAY PRODUCE WINDS STRONG ENOUGH TO PROMOTE A PERIOD OF
BLIZZARD CONDITIONS.  AHEAD OF THE SYSTEM, NORTHERN NEW ENGLAND
MAY BE COLD ENOUGH TO SEE SOME SNOW FROM THIS SYSTEM AS WELL.  IN
THE WARM SECTOR, EXPECT SOME AREAS OF LOCALLY MODERATE/HEAVY
RAINFALL OVER/NEAR THE LOWER HALF OF THE MS VALLEY LATE SUN INTO
MON.  AMOUNTS MAY TAPER OFF FOR A TIME FARTHER EAST BUT ATLANTIC
INFLOW AHEAD OF THE COLD FRONT MAY BRING SOMEWHAT HEAVIER ACTIVITY
INTO PARTS OF THE NORTHEAST BY LATE MON-TUE.

THE PRECIP FORECAST FOR AREAS ALONG THE GULF COAST AND
FLORIDA/EXTREME SOUTHEAST BECOMES VERY UNCERTAIN TOWARD MIDWEEK,
WITH SOLUTIONS SPANNING THE FULL RANGE OF DRY TO WET SCENARIOS. 
THE RELATIVELY SMALLER SCALE OF IMPORTANT DETAILS ALOFT WITHIN A
MORE AGREEABLE MEAN PATTERN LEADS TO LOW
PREDICTABILITY/CONFIDENCE, THUS FAVORING A COMPROMISE FOR ANY
DETERMINISTIC FORECAST.

IN SPITE OF SOME DETAIL DIFFERENCES AT THE SURFACE AND ALOFT FROM
DAY TO DAY, THERE HAS BEEN A CONSISTENT SIGNAL IN PRINCIPLE FOR
HEAVY RAIN/HIGHER ELEVATION SNOW FROM THE PACIFIC NORTHWEST INTO
NORTHERN CALIFORNIA WITH LESSER TOTALS EXTENDING INTO THE NORTHERN
ROCKIES.  DETAIL UNCERTAINTIES LATE IN THE PERIOD AFFECT THE
SOUTHWARD EXTENT OF PRECIP.  THERE SHOULD BE ONE PRONOUNCED WAVE
OF PRECIP AROUND SUN AND THEN ANOTHER AROUND TUE-WED.  PERIODS OF
STRONGER WINDS ARE POSSIBLE AS WELL.

IN THE WARM SECTOR AHEAD OF THE SYSTEM TRACKING OUT OF THE PLAINS,
EXPECT THE EASTERN HALF OF THE COUNTRY TO SEE HIGHS UP TO 10-20F
ABOVE NORMAL AND MINS AS WARM AS 15-25F ABOVE NORMAL INTO EARLY
NEXT WEEK.  TRAILING COLDER AIR OVER THE WEST SHOULD BE 5-15F
BELOW NORMAL.  TEMPERATURE ANOMALIES SHOULD BE MODEST OVER MOST OF
THE LOWER 48 FOR A DAY OR TWO AROUND MIDWEEK, FOLLOWED BY A
COOLING TREND OVER THE WEST AND WARMING OVER THE PLAINS/MS VALLEY
AS THE UPPER PATTERN BEGINS TO AMPLIFY.

RAUSCH