584  
FXUS02 KWBC 171600  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1059 AM EST SAT FEB 17 2018  
 
VALID 12Z TUE FEB 20 2018 - 12Z SAT FEB 24 2018  
 
...ARCTIC AIR DROPS THROUGH THE WEST-CENTRAL US AND RECORD WARMTH  
FOR THE EAST...  
...CLASHING AIRMASSES SET STAGE FOR ARKLATEX TO TN/OH VALLEYS  
HEAVY RAINFALL THREAT...  
 
   
..PATTERN OVERVIEW
 
 
EXPECT LITTLE CHANGE IN THE SYNOPTIC-SCALE PATTERN OVER THE NEXT  
WEEK AS A SUBTROPICAL RIDGE HOLDS STRONG OVER THE SOUTHWESTERN  
ATLANTIC. A MID-LEVEL RIDGE WITH AN INTENSITY POSSIBLY REACHING  
594-DM MAY APPROACH OR EVEN EXCEED 2.5 STANDARD DEVIATIONS ABOVE  
CLIMATOLOGY FOR MID-FEBRUARY. THE RESULTANT UPSTREAM PATTERN WILL  
FAVOR BELOW AVERAGE HEIGHTS WITH AN UPPER TROUGH THAT CONTINUALLY  
RE-LOADS ACROSS THE SOUTHWESTERN STATES. A STEADY PERIOD OF  
SOUTHWARD PROGRESSING SYSTEMS ARE NOTED ALONG THE EASTERN PACIFIC  
GIVEN THE STOUT UPPER RIDGE REMAINING IN PLACE FARTHER TO THE  
WEST. THIS OVERALL SETUP OVER NORTH AMERICA WILL FAVOR A RATHER  
SHARP AXIS OF MID-LEVEL CONFLUENCE EXTENDING FROM THE ARKLATEX UP  
TO THE TN/OH VALLEYS WHERE A HEAVY RAINFALL THREAT REMAINS IN PLAY.  
 
   
..GUIDANCE/UNCERTAINTY ASSESSMENT
 
 
OVER THE WEST...MODELS ARE STRUGGLING MOST WITH AN ACTIVE LONGWAVE  
TROUGH EXTENDING OVER THE WRN TO CENTRAL STATES. UPSTREAM FLOW  
OVER THE ERN PACIFIC/WRN U.S. REMAINS SOMEWHAT COMPLEX WITH  
UNCERTAIN JET SEPARATION. A COMPOSITE GUIDANCE SOLUTION IN LINE  
WITH ENSEMBLE MEANS SEEMS REASONABLE INTO MIDWEEK THAT MITIGATES  
SMALL SCALE INDIVIDUAL MODEL VARIANCE. LATER...AMPLITUDE OF THE  
UPSTREAM ERN PACIFIC MID-UPPER LEVEL RIDGE MAY SUGGEST PREFERENCE  
FOR MORE AMPLIFIED WRN US TROUGHING DAYS 5-7 MORE IN LINE WITH  
ECMWF ENSEMBLES.  
 
DOWNSTREAM...AMPLE HEIGHT FALLS EJECT FROM THE SRN/CENTRAL ROCKIES  
WITH MODEL DIFFERENCES STARTING AS EARLY AS TUE. RECENT GFS/GEFS  
MEMBERS TEND TO FAVOR QUICKER PROGRESSION MID-LATE WEEK AND  
SPATIAL DIFFERENCES IN FRONTAL PLACEMENTS MAINLY OCCUR FROM MORE  
RAPID EXPANSION OF LOW LEVEL HIGH PRESSURE OUT FROM THE PLAINS.  
THIS SEEMS MORE LIKELY CONSIDERING EARLY PERIOD HIGH STRENGTH.  
THESE SOLUTIONS DOWNSTREAM SUPPORT A COLD FRONT THROUGH THE NERN  
US/MID-ATLANTIC WED AS OTHER MODELS KEEP THE FRONT BACK TOWARD THE  
APPALACHIANS WED THEN OFF THE ERN SEABOARD THU. THIS AFFORDS A  
COOL DOWN FROM RECORD WARMTH. THE SWRN FLANK OF THIS BAROCLINIC  
ZONE THOUGH SHOULD BE SLOWER MOVING ACCOMPANIED BY A SERIES OF  
WEAK PERTURBATIONS ALONG THE FRONT. THIS SUPPORTS AN EXTENDED  
PERIOD OF MODERATE/HEAVY RAINFALL FROM THE ARKLATEX INTO THE  
OZARKS/LOWER MS VALLEY AS THE MID-LEVEL FLOW GENERALLY ALIGNS WITH  
THE BOUNDARY.  
 
   
..WEATHER HIGHLIGHTS/THREATS
 
 
PERSISTENT HIGH-AMPLITUDE FLOW MAKES AN ACTIVE PERIOD FOR  
PRECIPITATION ALONG WITH SOME VERY IMPRESSIVE TEMPERATURE  
ANOMALIES. IN SOME CASES...DEPARTURES FROM CLIMATOLOGY MAY BE NEAR  
30 TO 40 DEGREES...PARTICULARLY IN THE ARCTIC AIR DOME. FRIGID  
TEMPERATURES FOCUS FROM THE NORTHERN/CENTRAL ROCKIES TO THE GREAT  
PLAINS TUE/WED. COLDEST AIR IS EXPECTED ACROSS THE NORTHERN  
ROCKIES WITH SINGLE DIGIT HIGHS AND NEGATIVE TEENS LOWS. SOME  
MODERATION IS EXPECTED STARTING MIDWEEK ALTHOUGH COOLER THAN  
AVERAGE TEMPERATURES WILL PREVAIL. ARCTIC AIR EVEN SPILLS ACROSS  
THE CONTINENTAL DIVIDE INTO THE PACIFIC NORTHWEST.  
 
THE EXPANSIVE WARM SECTOR STRETCHING FROM THE SOUTH-CENTRAL U.S.  
UP TOWARD NEW ENGLAND WILL FEATURE SOME EXTREME ANOMALIES AS  
WELL...MAXIMIZING IN THE 30 TO 35 DEGREE RANGE FOR OVERNIGHT  
MINIMA. WIDESPREAD RECORDS MAY BE BROKEN ON TUESDAY/WEDNESDAY  
MORNINGS AS MORNING LOWS STAY IN THE 50S AS FAR NORTH AS THE OH  
VALLEY/LOWER GREAT LAKES. CONSIDERING FORECAST HIGHS...70S ARE  
EXPECTED UP INTO THE OH VALLEY...CENTRAL APPALACHIANS...AND  
MID-ATLANTIC. A FEW 80 DEGREE READINGS COULD POP UP IN THIS REGION  
THROUGH MID-WEEK. SOME COOLING IS EXPECTED LATER IN THE WEEK  
ALTHOUGH THIS COULD BE BRIEF AS A WARM FRONT LIFTS NORTHWARD UP  
TOWARD 40N LONGITUDE BY EARLY NEXT WEEKEND.  
 
WITH UNIDIRECTIONAL FLOW LIKELY SETTING UP ACROSS THE ARKLATEX UP  
THROUGH THE OZARKS AND INTO THE OH VALLEY...MULTIPLE ROUNDS OF  
HEAVY RAINFALL IS LIKELY...PARTICULARLY IN THE TUESDAY THROUGH  
THURSDAY TIME SPAN. THEREAFTER...UNCERTAINTIES IN THE PATTERN MAKE  
HEAVY RAINFALL PLACEMENT DIFFICULT. HOWEVER...WITH THIS BOUNDARY  
STILL LOOMING OVER THE CENTRAL/EASTERN U.S...ADDITIONAL  
PRECIPITATION IS LIKELY GIVEN LITTLE CHANGE IN THE SYNOPTIC SETUP.  
GENERALLY FLOODING CONCERNS MAY BE AN ISSUE WITH THIS MULTI-DAY  
HEAVY RAINFALL THREAT IN PLACE. THE DURATION OF THIS EVENT COULD  
LEAD TO GENERAL FLOODING CONCERNS BUT SPRINGTIME WARMTH/MOISTURE  
RETURN COULD SUPPORT SOME FLASH FLOODING ISSUES GIVEN SUFFICIENT  
INSTABILITY AND FAVORABLE BACKBUILDING ACTIVITY.  
ELSEWHERE...UNSETTLED CONDITIONS ARE TO RETURN TO THE PACIFIC  
NORTHWEST GIVEN THE SERIES OF SHORTWAVES MOVING THROUGH. MODIFIED  
ARCTIC AIR WILL LOWER SNOW LEVELS APPRECIABLY TO BRING SOME  
ACCUMULATING SNOWS TO VALLEY LOCALES IN ADDITION TO THE USUAL  
HIGHER TERRAIN.  
 
SCHICHTEL  
 

 
 
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