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Extended Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 1559Z Feb 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
1059 AM EST TUE FEB 20 2018

VALID 12Z FRI FEB 23 2018 - 12Z TUE FEB 27 2018

...OVERVIEW...

EXPECT A FAIRLY PERSISTENT LARGE SCALE PATTERN FEATURING A MEAN
TROUGH OVER THE WEST AND A RIDGE OFF THE GULF/SOUTHEAST COASTS. 
MOST GUIDANCE EXPECTS THE RIDGE TO FLATTEN GRADUALLY WITH ITS
CENTER DRIFTING FROM JUST NORTH OF THE BAHAMAS INTO THE GULF OF
MEXICO.  THIS FLOW REGIME WILL CORRESPOND TO BELOW NORMAL
TEMPERATURES OVER MUCH OF THE WESTERN U.S. INTO THE NORTHERN
PLAINS AND VERY WARM CONDITIONS--PERHAPS SETTING DAILY RECORDS ONE
OR MORE DAYS--OVER THE THE EAST.  FOCUSED BY INDIVIDUAL
SHORTWAVES, THERE WILL BE PERIODS OF UNSETTLED WEATHER WITH LOW
SNOW LEVELS OVER PORTIONS OF THE WEST WHILE A HEAVY RAINFALL
THREAT WILL PERSIST APPROXIMATELY FROM THE SOUTHERN PLAINS
NORTHEASTWARD THROUGH THE OHIO VALLEY.


...MODEL GUIDANCE/UNCERTAINTY ASSESSMENT/PREFERENCES...

AS HAS GENERALLY BEEN THE CASE IN THIS PATTERN RECENTLY, THERE IS
REASONABLE AGREEMENT FROM THE MULTI-DAY MEAN PERSPECTIVE BUT
GREATER UNCERTAINTY FOR EMBEDDED DETAILS.  THE MOST AGREEABLE
FEATURE IN PRINCIPLE IS THE SHORTWAVE EJECTING FROM THE WEST FRI
ONWARD, SUPPORTING SURFACE LOW PRESSURE THAT SHOULD REACH THE
GREAT LAKES BY AROUND EARLY DAY 5 SUN.  THE PARENT LOW SHOULD
CONTINUE INTO CANADA THEREAFTER WHILE ANOTHER WAVE TRACKS OFF THE
NEW ENGLAND COAST.  THERE IS STILL A FAIR AMOUNT OF MODEL/ENSEMBLE
SPREAD FOR DEPTH AND TRACK THOUGH.  00Z/06Z GFS RUNS ARE
SIGNIFICANTLY MORE WRAPPED UP AT THE SURFACE AND ALOFT VERSUS
OTHER GUIDANCE THOUGH THERE ARE SOME INCONSISTENCIES AMONG OTHER
SOLUTIONS IN TERMS OF HEIGHTS ALOFT AND CORRESPONDING SURFACE LOW
DEPTH.  ECMWF CONTINUITY AND SOMEWHAT GREATER TENDENCY OF GFS RUNS
TO DEEPEN MIDWEST SYSTEMS A LITTLE PREMATURELY SEEM TO FAVOR
LEANING CLOSEST TO THE 00Z ECMWF DEPTH AT THIS TIME.

GUIDANCE SUGGESTS THERE WILL BE A MORE DIFFUSE SHORTWAVE
APPROACHING THE NORTHWEST ON DAY 4 SAT AND PROGRESSING THROUGH THE
WEST AND BEYOND FROM SUN ONWARD.  00Z GEFS MEMBERS DIFFERED FROM
MOST OTHER SOLUTIONS IN HOLDING BACK TROUGH ENERGY FARTHER
WESTWARD BUT THE 06Z GEFS MEAN HAS TRENDED FASTER TO COMPARE MORE
FAVORABLY TO OTHER GUIDANCE.

BY EARLY NEXT WEEK MODELS/ENSEMBLES DIVERGE MORE WITH RESPECT TO
EAST PACIFIC/WESTERN U.S. DETAILS ALOFT, AS SOLUTIONS RANGE
BETWEEN AN INLAND OPEN TROUGH TO SEPARATED FLOW WITH AN UPPER LOW
DRIFTING SOUTHWARD JUST OFF THE WEST COAST.  RECENT ECMWF/GFS RUNS
DO NOT APPEAR TO PROVIDE ADDED CLUES AS THEY HAVE WAFFLED BETWEEN
THESE DIFFERING SOLUTIONS IN RECENT DAYS.  ENSEMBLE MEANS HAVE
BEEN FAIRLY STABLE IN DEPICTING AN OPEN TROUGH ALONG OR JUST
INLAND FROM THE WEST COAST BY DAY 7 TUE, WITH THE 06Z GFS/00Z
ECMWF SIMILAR TO THE MEANS.  THUS PREFER THIS SCENARIO AS THE MOST
LIKELY TO OCCUR FOR THE PURPOSES OF A SINGLE DETERMINISTIC
FORECAST.  HOWEVER NOTE THAT THE LAST COUPLE DAYS OF THE PERIOD
FEATURE CONSIDERABLE ENSEMBLE SPREAD BOTH FOR THE WESTERN U.S.
EVOLUTION AS WELL AS THE RESULTING SHORTWAVE DETAILS DOWNSTREAM.

FOR DAY 3 FRI INTO THE WEEKEND THE FORECAST STARTED WITH AN
OPERATIONAL MODEL BLEND INCORPORATING THE 00Z ECMWF/06Z GFS/00Z
UKMET/00Z CMC IN ORDER FROM MOST TO LEAST WEIGHT, TO REPRESENT
PREFERENCES FOR THE SYSTEM TRACKING INTO THE GREAT LAKES AND
ACCOUNT FOR CURRENT GUIDANCE SPREAD.  THE FORECAST BEGAN TO
INCORPORATE A LITTLE 06Z GEFS/00Z ECMWF MEAN INPUT BY DAY 5 SUN
WITH THE MEANS BECOMING 70 PERCENT OF THE FORECAST BY DAY 7 TUE
WHILE HOLDING ONTO MINORITY WEIGHT OF 06Z GFS/00Z ECMWF RUNS.


...WEATHER HIGHLIGHTS/THREATS...

THE HEAVY RAINFALL THREAT FROM THE ARKLATEX REGION THROUGH THE OH
VALLEY AND POSSIBLY INTO THE CENTRAL APPALACHIANS WILL BE A MAJOR
ISSUE DURING THE LATE WEEK/WEEKEND TIME FRAME.  WITH SIGNIFICANT
RAINFALL LIKELY TO OCCUR ALREADY BEFORE THE START OF THIS PERIOD,
ADDITIONAL HEAVY RAIN MAY EXACERBATE FLOODING CONCERNS WITH RIVERS
LIKELY TO BE AT VARIOUS FLOOD STAGES.  SYSTEM EXPECTED TO TRACK
THROUGH THE GREAT LAKES BY MID-LATE WEEKEND WILL PUSH A COLD FRONT
THROUGH THE EAST, WHICH SHOULD SUPPORT A DRIER TREND BY EARLY NEXT
WEEK.  HOWEVER THE FRONT MAY STALL NEAR THE GULF COAST AND PROMOTE
LINGERING RAINFALL ACROSS PARTS OF THE SOUTHERN TIER WITH SOME
POCKETS OF MODERATE TO HEAVY ACTIVITY POSSIBLE.  NORTHWEST/NORTH
OF THE HEAVY RAINFALL AREA, LOCATIONS FROM THE NORTH-CENTRAL
PLAINS THROUGH THE UPPER MIDWEST INTO INTERIOR NEW ENGLAND WILL
SEE THE BEST POTENTIAL FOR WINTRY WEATHER. OVER THE WEST, EXPECT
PERIODS OF COASTAL/LOW ELEVATION RAIN AND SNOW OTHERWISE OVER THE
PACIFIC NORTHWEST INTO THE UPPER INTERMOUNTAIN WEST.  SNOW LEVELS
SHOULD CONTINUE TO FALL WITH THE LOWERING THICKNESS VALUES. 
UNCERTAINTY OVER THE DETAILS OF FLOW ALOFT DURING THE LATTER HALF
OF THE PERIOD KEEPS CONFIDENCE FAIRLY LOW WITH RESPECT TO HOW MUCH
PRECIP EXTENDS SOUTHWARD INTO CALIFORNIA AND CORRESPONDING SNOW
LEVELS.

SPRING-LIKE MOISTURE RETURN LIFTING UP FROM THE LOWER MS VALLEY
INTO THE EASTERN U.S. WILL SUPPORT UNSEASONABLY MILD OVERNIGHT
TEMPERATURES WITH FORECAST ANOMALIES AS HIGH AS 20-30 DEGREES
ABOVE CLIMATOLOGY.  SUCH ANOMALIES ARE MOST LIKELY OVER THE
MID-SOUTH/TN VALLEY INTO PARTS OF THE MID ATLANTIC.  MANY DAILY
TEMPERATURE RECORDS COULD BE BROKEN GIVEN SUCH WARM OVERNIGHT LOWS
EXPECTED.  CONDITIONS WILL REMAIN MILD DURING THE DAYS AS WELL
ALTHOUGH RECORDS SHOULD BE BROKEN ON A MORE ISOLATED BASIS.  ON
THE CONTRARY, COLD WEATHER WILL PERSIST OVER THE WESTERN STATES
INTO THE NORTHERN PLAINS.  EXPECT THE SHARPEST DEPARTURES FROM
AVERAGE TO BE OVER THE NORTHERN ROCKIES/HIGH PLAINS, GENERALLY IN
THE 20 TO 30 DEGREES BELOW NORMAL RANGE, BUT THE ENTIRE REGION
WILL BE QUITE CHILLY.  NEAR FREEZING TEMPERATURES SHOULD CONTINUE
TO BE A DAILY OCCURRENCE EACH MORNING OVER THE PACIFIC NORTHWEST
AND EXTENDING INTO VAST SECTIONS OF CA.  WHILE THE MAJOR CITY
CENTERS SHOULD STAY ABOVE FREEZING, MANY OF THE COOLER VALLEY
LOCATIONS CAN EXPECT A FREEZE.

RAUSCH/RUBIN-OSTER