305  
FXUS02 KWBC 290659  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
158 AM EST SAT DEC 29 2018  
 
VALID 12Z TUE JAN 01 2019 - 12Z SAT JAN 05 2019  
   
..OVERVIEW
 
 
THE MEDIUM RANGE PERIOD (TUE-SAT) CONTINUES TO FEATURE A  
POSITIVELY TILTED TROUGH STRETCHING ACROSS THE CENTRAL U.S. AND  
INTO THE SOUTHWEST MOVING EASTWARD AS THE SUBTROPICAL RIDGE ACROSS  
THE SOUTHEAST RETREATS INTO THE ATLANTIC. TO THE WEST, UPPER  
RIDING WILL BUILD ACROSS THE PACIFIC NORTHWEST AND SHIFT INLAND  
LATER IN THE WEEK WITH A DOWNSTREAM UPPER TROUGH. THIS PATTERN  
WILL BRING RAINFALL ACROSS THE EASTERN U.S. WITH SOME  
PRECIPITATION INCREASING ACROSS THE PACIFIC NORTHWEST.  
 
   
..GUIDANCE EVALUATION/PREFERENCES
 
 
THERE WAS GOOD AGREEMENT BETWEEN THE 12Z GFS/18Z FV3-GFS AND 00Z  
ECMWF FOR THE FIRST TWO PERIODS OF MEDIUM RANGE (TUE/WED). THE 12Z  
GFS OVERALL WAS IN STEP WITH THE ECMWF WITH HAVING A CLOSED  
MID-LEVEL LOW ACROSS THE FOUR CORNERS AND MOVING INTO THE SOUTHERN  
PLAINS BY LATE WEDNESDAY INTO THURSDAY. THE 18Z GFS WAS VERY FAST  
WITH THIS TROUGH IN COMPARISON TO THE 12Z GFS, 18Z FV3-GFS AND THE  
00Z ECMWF. BY DAY 5/THURSDAY AND BEYOND, THE TIMING OF HOW QUICKLY  
THE TROUGH PROGRESSES EASTWARD/THE UPPER RIDGING IN THE WESTERN  
U.S. BECOMES AN ISSUE ESPECIALLY FOR THE OPERATIONAL MODELS. THE  
12Z GFS WAS FASTER THAN THE 00Z ECMWF WITH THE TROUGH IN THE  
EAST--BUT STILL SLOWER THAN THE 18Z FV3-GFS. BY FRIDAY AND INTO  
SATURDAY, AS THE TROUGH IN THE PACIFIC BEGINS TO APPROACH THE  
PACIFIC NORTHWEST, DIFFERENCES BETWEEN THE GFS AND ECMWF ARE  
NOTICEABLE SINCE THE ECMWF HAS THE UPPER RIDGE MORE AMPLIFIED AND  
PROGRESSING SLOWER THAN THE GFS. THE ENSEMBLE MEANS WERE MORE  
REASONABLE WITH THE DETAIL IN THE TIMING AND DETAILS. THUS, THE  
BLEND USED IN THIS PORTION OF THE MEDIUM RANGE WAS BASED ON THE  
18Z GEFS MEAN/12Z ECMWF ENSEMBLE MEAN/12Z NAEFS.  
 
   
..WEATHER HIGHLIGHTS/HAZARDS
 
 
WITH THE TROUGH DIGGING EASTWARD MIDWEEK, A COLD FRONT WILL MOVE  
ACROSS THE EASTERN U.S. ON TUESDAY AND RETREAT NORTHWARD AS A WARM  
FRONT BY WED-THU ALONG THE GULF COAST. DEEP SOUTHERLY FLOW AHEAD  
OF THE TROUGH COMBINED WITH THE BOUNDARY BEING A FOCAL POINT FOR  
CONVECTION WILL GIVE WAY TO AN ACTIVE PERIOD FOR THE  
SOUTHEAST--ESPECIALLY THROUGH THURSDAY. THE BEST SIGNAL FOR THE  
HIGHEST 5-DAY QPF TOTALS STRETCH FROM THE CENTRAL GULF COAST TO  
THE CAROLINAS. MOISTURE REACHING THE GREAT LAKES/NEW ENGLAND MAY  
BE IN THE FORM OF SNOW, ESPECIALLY WITH LAKE EFFECT ACTIVITY  
BEHIND LOW PRESSURE EXITING NEW ENGLAND MID-WEEK. IN THE  
SOUTHWEST, AN INITIAL UPPER LOW EARLY IN THE WEEK WILL LIKELY  
RESULT IN ANOTHER ROUND OF PRECIPITATION FOR PARTS OF EASTERN  
ARIZONA AND NEW MEXICO, WITH THE HIGHEST POTENTIAL FOR  
ACCUMULATING SNOWS CONFINED TO THE HIGHER MOUNTAINS. THE PACIFIC  
NORTHWEST CAN EXPECT AN INCREASE IN RAIN/HIGH ELEVATION SNOW BY  
THURSDAY AND CONTINUING INTO FRIDAY--WITH SOME MEANINGFUL TOTALS  
LIKELY IN THE OLYMPICS/WASHINGTON CASCADES AND THE NORTHERN  
PORTIONS OF THE NORTHERN ROCKIES.  
 
IN REGARDS TO TEMPERATURES, THE NORTHERN PLAINS CAN EXPECT VERY  
COLD TEMPERATURES ON TUESDAY--WITH HIGH TEMPERATURES REACHING  
15-25F BELOW NORMAL. BY WEDNESDAY, TEMPERATURES WILL REBOUND IN  
THIS REGION AND ARE EXPECTED TO BE UPWARDS OF 10-15F ABOVE NORMAL.  
THE CENTRAL AND SOUTHERN PLAINS WILL ALSO BE MUCH BELOW NORMAL ON  
TUESDAY AND WEDNESDAY--BECOMING NEAR NORMAL BY THURSDAY. THE  
EXCEPTION WILL BE ALONG THE WESTERN GULF COAST AS AN ACTIVE PERIOD  
FOR PRECIPITATION IS EXPECTED. MEANWHILE, THE EAST COAST WILL BE  
ANOMALOUSLY WARM ON TUESDAY--RETURNING TO NEAR NORMAL BY  
WEDNESDAY. LATER IN THE MEDIUM RANGE PERIOD (FRI-SAT), THE CENTRAL  
U.S. WILL BE ABOVE AVERAGE--WITH HIGH TEMPERATURES 15-20F ABOVE  
NORMAL. THE WEST COAST SHOULD BE NEAR NORMAL THROUGH THE MEDIUM  
RANGE PERIOD.  
 
REINHART/SANTORELLI  
 
WPC MEDIUM RANGE 500 MB HEIGHTS, SURFACE SYSTEMS, WEATHER GRIDS,  
QUANTITATIVE PRECIPITATION, WINTER WEATHER OUTLOOK PROBABILITIES  
AND HEAT INDEXES ARE FOUND AT:  
 
HTTPS://WWW.WPC.NCEP.NOAA.GOV/MEDR/5DAYFCST500_WBG.GIF  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/MEDR/5DAYFCST_WBG_CONUS.GIF  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/5KM_GRIDS/5KM_GRIDSBODY.HTML  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/QPF/DAY4-7.SHTML  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/WWD/PWPF_D47/PWPF_MEDR.PHP?DAY=4  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/HEAT_INDEX.SHTML  
 

 
 
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