775  
FXUS02 KWBC 210420  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1119 PM EST TUE FEB 20 2018  
 
VALID 12Z SAT FEB 24 2018 - 12Z WED FEB 28 2018  
 
DUE TO TECHNICAL DIFFICULTIES...SOME PRODUCTS WILL NOT BE  
AVAILABLE OVERNIGHT. PLEASE CHECK ISSUANCE TIMES. SORRY FOR THE  
INCONVENIENCE...ANOTHER UPDATE WILL BE FORTHCOMING AS SOON AS WE  
GET MORE INFORMATION REGARDING THE STATUS OF THE OUTAGE.  
 
PREVIOUS DISCUSSION FROM FEB 20/16Z...  
 
   
..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  
 

 
 
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