467  
FXUS02 KWBC 181557  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1056 AM EST SUN NOV 18 2018  
 
VALID 12Z WED NOV 21 2018 - 12Z SUN NOV 25 2018  
 
...RECORD COLD TEMPERATURES ON THANKSGIVING FOR MUCH OF THE  
NORTHEAST...  
 
   
..OVERVIEW AND GUIDANCE/UNCERTAINTY ASSESSMENT
 
 
UPPER TROUGHING IN THE NORTHEAST WED/THU WILL GIVE WAY TO WEAK  
RIDGING AS LEAD AND THEN REINFORCING TROUGHING PUSH THROUGH THE  
WEST. THIS YIELDS A VERY COLD START TO THE PERIOD IN THE NORTHEAST  
AND A DECIDEDLY WET SEVERAL DAYS (AND WHITE AT HIGHER ELEVATIONS)  
WEST OF THE DIVIDE. FARTHER SOUTH, SOUTHERN STREAM SHORTWAVE  
ENERGY MAY ATTEMPT TO SPIN UP A LOW PRESSURE SYSTEM ALONG A LINGER  
FRONTAL BOUNDARY IN THE GULF OF MEXICO BY NEXT WEEKEND.  
 
MODEL CONSENSUS WAS GOOD DURING DAYS 3-4 (WED-THU), AND THE  
FORECAST WAS BASED ON A MULTI-MODEL BLEND INCLUDING THE  
ECMWF/GFS/FV3. DAY 5 (FRI) IS WHEN WE BEGIN TO SEE SOME  
DIFFERENCES EMERGE IN THE GUIDANCE THAT GROW THROUGH TIME. IN  
GENERAL, MODELS/ENSEMBLES HAVE SHOWN A TREND TOWARD GREATER  
AMPLIFICATION OF FEATURES BY LATE IN THE WEEK, WITH THE ECMWF  
SEEMING TO LEAD THE WAY. THIS TREND HOLDS TRUE FOR SHORTWAVE  
ENERGY CROSSING THE SOUTH CENTRAL U.S. ON FRI,AND PERHAPS  
AMPLIFYING FURTHER AS IT REACHES THE LOWER MISSISSIPPI VALLEY OR  
TENNESSEE VALLEY BY FRI NIGHT/SAT. LOOKING AT THE PAST SEVERAL  
RUNS, THE ECENS SEEMED TO SHOW THE FIRST SIGNS OF THIS TREND AND  
HAS BEEN THE MOST CONSISTENT, WHILE THE GFS/GEFS SEEMS TO BE  
PLAYING CATCH UP. THE 00Z DETERMINISTIC ECMWF BECAME SIGNIFICANTLY  
MORE AMPLIFIED WITH THIS SYSTEM BY SAT, WITH A CLOSED UPPER LOW  
DEVELOPING ACROSS THE LOWER/MID-MISSISSIPPI VALLEY BY SAT. IT IS  
POSSIBLE THE ECMWF MAY HAVE GONE TOO FAR WITH THIS, BUT GIVEN THE  
TRENDS IT CERTAINLY CANNOT BE RULED OUT. THESE TRENDS FAVOR THE  
DEVELOPMENT OF A MORE SIGNIFICANT WAVE OF LOW PRESSURE THAN  
PREVIOUSLY THOUGHT ALONG A LINGERING FRONTAL BOUNDARY IN THE GULF  
OF MEXICO BY FRI NIGHT/SAT, WITH PERHAPS A TRACK NEAR/ALONG THE  
EASTERN SEABOARD. THE SAME HOLDS TRUE FOR THE NEXT SHORTWAVE WHICH  
REACHES THE NORTHWEST LATE FRI AND THEN AMPLIFIES AS IT CROSSES  
THE ROCKIES AND REACHES THE PLAINS SAT-SUN. EACH CONSECUTIVE RUN  
OF BOTH THE ECMWF AND GFS HAS BECOME MORE AMPLIFIED WITH THIS  
FEATURE, WITH THE ECMWF/ECENS AGAIN LEADING THE WAY.  
 
HEMISPHERIC PATTERNS PROVIDE SOME SUPPORT FOR THE INCREASINGLY  
AMPLIFIED PATTERN, WITH D+8 ANOMALIES FROM THE ECENS AND GEFS  
SHOWING LIKELY DEVELOPMENT OF BLOCKING/STRONGLY NEGATIVE NAO/AO IN  
THE NORTH ATLANTIC WITH PERHAPS SOME DEGREE OF BLOCKING SETTING UP  
ACROSS THE NORTH CENTRAL PACIFIC IN THE FORM OF A BROAD AND  
PERSISTENT MEAN TROUGH SOUTH OF THE ALEUTIANS. THE COMBINATION OF  
THESE WOULD SUPPORT A TREND TOWARD FLOW AMPLIFICATION ACROSS THE  
CONUS, AND TELECONNECTIONS ASSOCIATED WITH THESE ANOMALY CENTERS  
FAVOR REBUILDING OF THE WESTERN RIDGE AND AMPLIFIED TROUGHING  
ACROSS THE EAST. GIVEN THESE CONSIDERATIONS, AND INCREASING  
UNCERTAINTY, WEIGHT WAS SHIFTED TOWARD ENSEMBLE MEANS DURING DAYS  
5-7, BUT WITH PARTICULARLY HEAVY WEIGHT PLACED TOWARD THE ECENS  
RELATIVE TO THE GEFS, AND SOME LIMITED INFLUENCE FROM THE  
DETERMINISTIC ECMWF CONTINUED THROUGH THE END OF THE PERIOD.  
 
   
..WEATHER HIGHLIGHTS/THREATS
 
 
THANKSGIVING SUMMARY (THU 11/22) - RECORD COLD WILL LIKELY BE AN  
UNWELCOME THANKSGIVING GUEST FOR MANY IN THE NORTHEAST, WITH HIGH  
TEMPERATURES 15 TO NEAR 30 DEG F BELOW AVERAGE AND MANY AREAS  
STAYING WELL BELOW FREEZING, WHILE LOW TEMPERATURES MAY REACH THE  
SINGLE DIGITS. WET ALONG THE WEST COAST INTO THE GREAT BASIN AND  
ROCKIES WITH NEAR AVERAGE TEMPERATURES AND HIGHER ELEVATION SNOW.  
MILDER AND DRY FOR THE HIGH PLAINS WITH 40S/50S COMMON. RAIN  
CHANCE FOR THE GULF COAST FROM TEXAS EASTWARD THROUGH MUCH OF  
FLORIDA TO THE NORTH OF A FRONT IN THE FLORIDA STRAITS.  
 
THE ENTIRE WEST COAST AND MUCH OF THE INTERIOR WILL SEE WIDESPREAD  
RAIN/ELEVATION SNOW TO START THE PERIOD WEDNESDAY. PRECIPITATION  
FOCUS STILL APPEARS TO BE AROUND NORTHERN CALIFORNIA AND THE  
NORTHERN SIERRAS INTO SOUTHWESTERN OREGON AS THE UPPER TROUGH  
SPLITS N-S. MULTI-DAY RAINFALL AMOUNTS MAY BE IN THE 3-6 INCH  
RANGE OVER FAVORED AREAS AND EVEN ABOUT AN INCH TO AN INCH AND A  
HALF IN THE CENTRAL VALLEY. SNOW LEVELS WILL FALL THROUGH THE  
PERIOD WITH SEVERAL FEET OF SNOW LIKELY AT THE HIGHEST ELEVATIONS.  
A POTENTIAL LOW PRESSURE SYSTEM MOVING FROM THE GULF AND ACROSS  
THE SOUTHEAST NEXT WEEKEND WOULD SUPPORT THE POTENTIAL FOR HEAVY  
PRECIPITATION ACROSS PORTIONS OF THE SOUTHEAST, MID-ATLANTIC, AND  
SOUTHERN/CENTRAL APPALACHIANS. CONFIDENCE IN THIS ASPECT OF THE  
FORECAST, HOWEVER, IS LOW AT THIS TIME, BUT THE TRENDS (AT THE  
MOMENT) SEEM TO BE IN THAT DIRECTION.  
 
MUCH BELOW AVERAGE TEMPERATURES IN THE NORTHEAST WILL BE SLOW TO  
EASE BUT WILL TREND BACK TOWARD TYPICAL LATE NOVEMBER VALUES BY  
NEXT WEEKEND. TEMPERATURES IN THE WEST WILL BE NEAR TO ABOVE  
AVERAGE FOR OVERNIGHT LOWS BUT NEAR TO BELOW AVERAGE FOR AFTERNOON  
HIGHS THANKS TO AN ABUNDANCE OF CLOUD COVER. THE PLAINS AND UPPER  
MIDWEST WILL SEE MILDER TEMPERATURES THURSDAY-SATURDAY, GENERALLY  
ABOVE FREEZING FOR AFTERNOON HIGHS. COLDER AIR WILL PROBABLY WORK  
BACK SOUTH ACROSS THE BORDER BY NEXT SUNDAY.  
 
RYAN/FRACASSO  
 
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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