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Extended Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 1559Z Jan 21, 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 SUN JAN 21 2018

VALID 12Z WED JAN 24 2018 - 12Z SUN JAN 28 2018

...OVERVIEW...

EXPECT FLOW ALOFT TO MAINTAIN MODERATE PROGRESSION BUT BECOME
AMPLIFIED WITH TIME.  A VIGOROUS TROUGH OVER THE EASTERN PACIFIC
AS OF EARLY WED WILL PROCEED INTO THE CENTRAL U.S. BY NEXT WEEKEND
WHILE THE DOWNSTREAM RIDGE WILL REACH THE WESTERN ATLANTIC AND
JOIN WITH RIDGING ALREADY PRESENT EAST OF THE BAHAMAS.  BEHIND THE
UPPER TROUGH, A STRONG RIDGE WILL BUILD INTO THE WEST WITH SOME
AID FROM A DEEP TROUGH/CLOSED LOW BECOMING ANCHORED OVER THE
NORTHEAST PACIFIC/GULF OF ALASKA BY THE LATTER HALF OF THE PERIOD.
 THE TROUGH CROSSING THE LOWER 48 WILL SPREAD A BROAD AREA OF
ACTIVE WEATHER ACROSS THE LOWER 48 AND SUPPORT A WAVY FRONTAL
SYSTEM SEPARATING VERY CONTRASTING AIR MASSES, WHILE EVOLUTION OF
EASTERN PACIFIC FLOW MAY BRING INCREASING MOISTURE INTO PARTS OF
THE PACIFIC NORTHWEST NEXT WEEKEND.


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

BROADLY SPEAKING GUIDANCE CONTINUES TO SHOW BETTER THAN AVERAGE
AGREEMENT WITH FAIRLY GOOD CONTINUITY IN MOST RESPECTS, WITH SOME
EMBEDDED DETAIL/TIMING QUESTIONS EARLY-MID PERIOD AND LINGERING
UNCERTAINTIES ON A SOMEWHAT LARGER SCALE BY NEXT WEEKEND.  OF NOTE
OFF THE PACIFIC NORTHWEST ON DAY 3 WED, THE 00Z ECMWF IS ON THE
SOUTHERN FRINGE OF GUIDANCE INCLUDING NEARLY ALL 00Z
GEFS/ECMWF/CMC ENSEMBLE MEMBERS FOR SURFACE LOW PRESSURE NEARING
THE COAST.  FARTHER EAST DIFFERENCES/ADJUSTMENTS ARE GENERALLY DUE
TO SMALLER SCALE SHORTWAVE DETAILS THAT ARE TYPICALLY NOT WELL
RESOLVED BEYOND THE SHORT RANGE TIME FRAME.  THERE IS GOOD
CONTINUITY WITH THE CONCEPT OF LOW PRESSURE TRACKING ACROSS THE
EXTREME NORTHERN TIER BUT SOME ENSEMBLES AND THE UKMET ARE HINTING
AT THE POTENTIAL FOR SLIGHTLY FASTER TIMING.  THE FULL CONSENSUS
OF MODELS/MEANS IS STILL FAIRLY CLOSE TO CONTINUITY BUT THE
TRAILING FRONT HAS ADJUSTED A BIT FASTER DUE TO THE LOW BEING
SOMEWHAT STRONGER THAN FORECAST YESTERDAY.  AS THE WAVY FRONT
HEADS INTO THE EAST DURING THE WEEKEND THERE IS DECENT AGREEMENT
THAT EVOLUTION OF UPPER TROUGH ENERGY SHOULD SUPPORT CONSOLIDATION
OF LOW PRESSURE BETWEEN THE EASTERN GREAT LAKES AND CENTRAL
APPALACHIANS BY THE END OF DAY 7 SUN.

BY NEXT WEEKEND SOME UNCERTAINTY ARISES WITH RESPECT TO THE
PRECISE AMPLITUDE OF EASTERN PACIFIC/WESTERN U.S. FLOW ALOFT. 
GEFS/ECMWF MEANS HAVE BEEN FAIRLY STABLE OVER THE PAST DAY BUT
INDIVIDUAL ENSEMBLE MEMBERS AND GFS/ECMWF RUNS DISPLAY SUFFICIENT
DIFFERENCES TO HAVE AN IMPACT ON DETAILS OF THE STRONG FLOW OF
MOISTURE (WITH SOME CONNECTION TO LOWER LATITUDES) EXPECTED TO
REACH THE PACIFIC NORTHWEST/VANCOUVER ISLAND AREA.  DETAILS HERE
WILL ULTIMATELY AFFECT THE CHARACTER OF THE UPPER TROUGH REACHING
THE PLAINS/MS VALLEY LATE IN THE PERIOD.  MANY CANADIAN ENSEMBLES
ARE STILL FLATTER THAN MOST OTHER GUIDANCE BUT NOT TO THE DEGREE
SEEN YESTERDAY, WHILE THE OPERATIONAL 00Z CMC HAS ADJUSTED TOWARD
ITS MEAN VERSUS YESTERDAY'S 00Z RUN THAT WAS MORE SIMILAR TO THE
GFS/ECMWF AND THEIR MEANS.

FOR DAYS 3-5 WED-FRI A BLEND OF THE 00Z/06Z GFS AND 00Z ECMWF
ALONG WITH LESSER WEIGHT OF THE 00Z CMC AND UKMET REPRESENTED
CONSENSUS WELL FOR THAT PART OF THE FORECAST, WITH INPUT FROM
REMAINING MODELS HELPING TO DOWNPLAY THE LESS CONFIDENT 00Z ECMWF
DETAILS OFF THE PACIFIC NORTHWEST ON WED.  DAYS 6-7 SAT-SUN BEGAN
TO INCLUDE 06Z GEFS/00Z ECMWF ENSEMBLE MEANS ALONG WITH THE
GFS/ECMWF RUNS (WHILE PHASING OUT THE CMC) TO RESOLVE UNCERTAINTY
OF SPECIFICS WITHIN THE FAIRLY AGREEABLE LARGE SCALE PATTERN.


...SENSIBLE WEATHER HIGHLIGHTS/THREATS...

THE STRONG SYSTEM HEADING INTO THE WEST WILL INITIALLY FOCUS
HEAVIEST RAIN/HIGHER ELEVATION SNOW OVER/NEAR NORTHERN CALIFORNIA
ON WED WITH LESSER AMOUNTS TO THE NORTH/NORTHEAST.  PRECIP SHOULD
LESSEN IN INTENSITY SOMEWHAT, BUT STILL WITH POTENTIAL FOR
LOCALIZED ENHANCEMENT, AS MOISTURE EXTENDING SOUTH AND EAST
EVENTUALLY CROSSES MUCH OF THE NORTHERN/CENTRAL WESTERN STATES
THROUGH LATE WEEK.  EMERGENCE OF THE LEADING COLD FRONT OVER THE
PLAINS BY FRI AND FLOW AROUND EASTERN U.S. INTO WESTERN ATLANTIC
HIGH PRESSURE WILL BRING GULF OF MEXICO MOISTURE NORTHWARD,
EVENTUALLY GENERATING A LARGE AREA OF PRECIP OVER THE EASTERN HALF
OF THE COUNTRY FROM FRI NIGHT THROUGH THE WEEKEND.  THE FULL ARRAY
OF MODEL/ENSEMBLE GUIDANCE CURRENTLY SIGNALS TWO AREAS THAT SHOULD
BE MOST FAVORED FOR HEAVY RAINFALL POTENTIAL, ONE FROM THE
SOUTH-CENTRAL MS VALLEY INTO THE OH VALLEY/LOWER GREAT LAKES AND A
SECOND FROM THE CENTRAL/EASTERN GULF COAST INTO SOUTHERN
APPALACHIANS/MID ATLANTIC.  EXTREME NORTHERN TIER LOCATIONS MAY
SEE SNOW IN THE VICINITY OF ONE OR MORE SURFACE WAVES.

A MAJORITY OF THE WEST WILL SEE A DRIER TREND BY NEXT WEEKEND AS
THE ABOVE SYSTEM DEPARTS.  THE ONE EXCEPTION WILL BE OVER PORTIONS
OF THE PACIFIC NORTHWEST INTO VANCOUVER ISLAND AND POSSIBLY TO
SOME DEGREE INTO THE EXTREME NORTHERN ROCKIES AS EVOLUTION OF FLOW
ALOFT SUPPORTS A POTENTIAL ATMOSPHERIC RIVER EVENT.  CURRENTLY THE
FAVORED TERRAIN OVER WA AND VANCOUVER ISLAND HAVE THE HIGHEST
PROBABILITY OF SEEING HEAVY PRECIP NEXT WEEKEND BUT THERE IS
ENOUGH SPREAD IN GUIDANCE TO ALLOW FOR THE POTENTIAL THAT SOME OF
THIS MOISTURE COULD EXTEND SOMEWHAT FARTHER SOUTH.

AROUND MIDWEEK EXPECT MOSTLY LIGHT LAKE EFFECT SNOW IN THE WAKE OF
THE STRONG/WINDY SHORT RANGE SYSTEM TRACKING AWAY FROM NEW
ENGLAND, WHILE A FRONTAL PASSAGE MAY BRING A PERIOD OF RAIN TO
SOUTHERN FLORIDA.

WARM FLOW AHEAD OF THE SYSTEM PROGRESSING INLAND FROM THE WEST
COAST WILL INITIALLY BRING MODERATELY ABOVE NORMAL READINGS TO THE
WESTERN STATES AND THEN MUCH ABOVE NORMAL TEMPS TO THE
CENTRAL-EASTERN STATES THU ONWARD, WITH SOME PLUS 15-30F
ANOMALIES.  TRAILING COLDER AIR CROSSING THE WEST LATE WEEK AND
REACHING THE CENTRAL U.S. NEXT WEEKEND SHOULD BE LESS EXTREME,
ONLY 5-15F BELOW NORMAL AT MOST LOCATIONS.

RAUSCH