849  
FXUS02 KWBC 170647  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
146 AM EST SAT FEB 17 2018  
 
VALID 12Z TUE FEB 20 2018 - 12Z SAT FEB 24 2018  
   
..HEAVY RAINFALL THREAT FROM THE ARKLATEX TO THE TN/OH VALLEYS
 
 
...FRIGID CONDITIONS FOR THE NORTHERN/CENTRAL ROCKIES AND GREAT  
PLAINS...   
..RECORD WARMTH FROM THE SOUTH-CENTRAL TO EASTERN U.S
 
 
   
..PATTERN OVERVIEW
 
 
LITTLE CHANGE IN THE SYNOPTIC-SCALE PATTERN IS EXPECTED DURING THE  
PERIOD 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  
TOWARD THE TN/OH VALLEYS WHERE A HEAVY RAINFALL THREAT SHOULD  
REMAIN IN PLAY.  
 
   
..MODEL GUIDANCE/UNCERTAINTY ASSESSMENT/PREFERENCES
 
 
THE MOST CONFIDENT PART OF THE UPCOMING FORECAST IS REGARDING THE  
POSITIVE HEIGHT ANOMALY HOLDING STRONG OVER THE WESTERN ATLANTIC  
DOWN INTO THE CENTRAL/EASTERN CARIBBEAN. TOWARD THE END OF THE  
UPCOMING WORK WEEK...A NORTHERN EROSION OF THIS RIDGE IS EXPECTED  
AS NORTHERN STREAM ENERGY HELPS SUFFICIENTLY LOWER HEIGHTS. THIS  
WILL ALLOW A BACKDOOR FRONT TO SWEEP SOUTHWARD ACROSS THE  
NORTHEASTERN U.S....THUS AFFORDING A COOL DOWN FROM THE RECORD  
WARMTH. WHERE THE MODELS SEEM TO BE STRUGGLING THE MOST IS WITH  
THE ACTIVE LONGWAVE TROUGH EXTENDING OVER THE WESTERN/CENTRAL  
STATES.  
 
CONSIDERING EACH EJECTING ELEMENT WITHIN THIS LONGWAVE PATTERN...A  
SHARP BAND OF HEIGHT FALLS WILL BE IN THE PROCESS OF EJECTING OUT  
OF THE SOUTHERN/CENTRAL ROCKIES WITH OPERATIONAL MODEL DIFFERENCES  
EVIDENT BY AS EARLY AS DAY 3/TUESDAY. AS HAS BEEN COMMON IN RECENT  
DAYS...THE 00Z GFS AND PRECEDING RUNS FAVOR A QUICKER DOWNSTREAM  
PROGRESSION WHICH ALLOWS FOR SPATIAL DIFFERENCES IN FRONTAL  
PLACEMENTS. WHILE THE GFS SOLUTIONS WOULD SUPPORT A COLD FRONT  
PASSING THROUGH THE I-95 CORRIDOR ON WEDNESDAY...OTHER GLOBAL  
MODELS KEEP THIS BOUNDARY BACK TOWARD THE APPALACHIANS AND MOVE  
THE FRONT OFF THE EASTERN SEABOARD BY THE FOLLOWING DAY. THE  
SOUTHWESTERN FLANK OF THIS BAROCLINIC ZONE 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 THIS MENTIONED BOUNDARY.  
 
THE UPSTREAM PATTERN OVER THE EASTERN PACIFIC/WESTERN U.S. REMAINS  
SOMEWHAT COMPLEX AS THERE IS A SEPARATION IN THE JETS. THE PRIOR  
THREE RUNS OF THE GFS FAVOR A WESTWARD RETROGRESSION OF A  
SHORTWAVE BACK TOWARD 130W WHICH IS NOT SUPPORTED BY OTHER GLOBAL  
MODELS. OTHER SOLUTIONS FAVOR FORWARD PROGRESSION INTO THE DESERT  
SOUTHWEST BY MID-WEEK ALTHOUGH ENSEMBLE SPAGHETTI PLOTS SHOW QUITE  
A BIT OF SCATTER AS WAS EXPECTED. ADDITIONAL SHORTWAVE ENERGY  
SWEEPS THROUGH THE PACIFIC NORTHWEST WITH EVENTUAL PLACEMENT  
UNKNOWN GIVEN VAST MODEL SPREAD. THE 00Z/18Z GFS FAVOR ADJOINING  
THIS SYSTEM WITH THE NORTHERN STREAM WHILE THE OTHER GUIDANCE TAKE  
A MORE SOUTHWARD TRACK INTO THE CENTRAL GREAT BASIN/SOUTHWESTERN  
U.S. ALL AND ALL...WHILE THE GUIDANCE AGREE ON A NEGATIVE HEIGHT  
ANOMALY HOLDING ON OVER THE WESTERN THIRD OF THE NATION...THE  
DETAILS VARY GREATLY.  
 
WHILE THE GFS SOLUTIONS APPEARED ON THEIR OWN THROUGHOUT THE  
PERIOD...DID NOT WANT TO DISCOUNT THEM JUST YET. THUS...FAVORED A  
50/50 BLEND OF THE 12Z GFS/ECMWF THROUGH DAY 4/WEDNESDAY WHILE  
QUICKLY ADDING ENSEMBLE MEANS TO THE PICTURE FOR THE REMAINING FEW  
DAYS. GIVEN THE COMPLEXITY OF THE FLOW DEEPER INTO THE  
PERIOD...LOWERED OPERATIONAL CONTRIBUTIONS TO AROUND 20 PERCENT BY  
DAY 6/FRIDAY BEFORE GOING TO ALL ENSEMBLE MEANS THEREAFTER. THIS  
WAS LED BY THE 12Z ECMWF ENSEMBLE MEAN WHICH RECEIVED HALF OF THE  
WEIGHTING WHILE SPLITTING THE REMAINDER BETWEEN THE 12Z GEFS/NAEFS  
ENSEMBLE MEANS.  
 
   
..WEATHER HIGHLIGHTS/THREATS
 
 
THE PERSISTENT HIGH-AMPLITUDE PATTERN WILL MAKE FOR AN ACTIVE  
PERIOD IN TERMS OF PRECIPITATION ALONG WITH SOME VERY IMPRESSIVE  
TEMPERATURE ANOMALIES. IN SOME CASES...DEPARTURES FROM CLIMATOLOGY  
WILL BE ON THE ORDER OF 30 TO 40 DEGREES...PARTICULARLY WITHIN THE  
DOME OF ARCTIC AIR. ON TUESDAY/WEDNESDAY...FRIGID TEMPERATURES  
SHOULD FOCUS OVER THE NORTHERN/CENTRAL ROCKIES INTO THE GREAT  
PLAINS. THE COLDEST AIR IS EXPECTED ACROSS THE NORTHERN ROCKIES AS  
HIGHS BARELY MOVE ABOVE 0 DEGREES WHILE OVERNIGHT TEMPERATURES  
DESCEND INTO THE NEGATIVE TEENS OR EVEN SLIGHTLY LOWER. SOME  
MODERATION IS EXPECTED BEYOND WEDNESDAY ALTHOUGH COOLER THAN  
AVERAGE TEMPERATURES WILL PREVAIL. EVEN THE PACIFIC NORTHWEST WILL  
GET IN ON THE ACTION AS ARCTIC AIR CROSSES THE CONTINENTAL DIVIDE  
AS OVERNIGHT LOWS PLUNGE INTO THE LOWER 20S ALL THE WAY TO SEATTLE  
WA AND PORTLAND OR.  
 
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.  
 
RUBIN-OSTER  
 

 
 
CLICK HERE TO GO TO PREVIOUS BULLETINS.

The Nexlab HPC Page Main Text Page