637  
FXUS02 KWBC 121553  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1052 AM EST TUE FEB 12 2019  
 
VALID 12Z FRI FEB 15 2019 - 12Z TUE FEB 19 2019  
 
16Z UPDATE... THE NEWEST CYCLE OF GUIDANCE CONTINUES TO SHOW  
FAIRLY GOOD AGREEMENT ON THE LARGE SCALE, WITH SMALLER SCALE  
DETAILS/SYSTEMS REMAINING LOW PREDICTABILITY AND CONFIDENCE. GIVEN  
THIS ASSESSMENT, OPTED TO USE ABOUT 25 PERCENT PREVIOUS SHIFT  
CONTINUITY IN THE BLEND, ALONG WITH THE LATEST RUNS OF THE  
GFS/ECMWF AND THEIR RESPECTIVE ENSEMBLE MEANS. THIS RESULTED IN A  
FORECAST VERY CLOSE TO THE PREVIOUS SHIFT WITH MINIMAL CHANGES  
NEEDED. SEE PREVIOUS DISCUSSION BELOW FOR ADDITIONAL DETAILS ON  
THE OVERALL PATTERN, GUIDANCE EVALUATION, AND SENSIBLE WEATHER  
IMPACTS.  
 
SANTORELLI  
 
PREVIOUS DISCUSSION...   
..OVERVIEW
 
 
MODELS/ENSEMBLES CONTINUE TO SHOW STRONG CONSENSUS THAT A REX  
BLOCK PATTERN ACROSS THE CENTRAL/NORTH PACIFIC, EXTENDING FROM  
NEAR HAWAII TO ALASKA, SHOULD PERSIST THROUGH THE NEXT WEEK. THIS  
WILL LEAVE THE FLOW REGIME ACROSS THE CONUS LARGELY QUASI-ZONAL.  
TELECONNECTIONS RELATIVE TO THE STRONGEST 500 HPA POSITIVE HEIGHT  
ANOMALY CENTER ACROSS THE NORTH PACIFIC CONTINUE TO FAVOR A  
POSITIVELY-TILTED MEAN TROUGH NEAR THE PACIFIC NORTHWEST WITH WEAK  
POSITIVE HEIGHT ANOMALIES FROM THE SOUTHERN PLAINS EAST INTO THE  
SOUTHEAST, AND A BROAD REGION OF MODESTLY CYCLONIC UPPER-LEVEL  
FLOW ACROSS MUCH OF THE CONUS. SHORTWAVE ENERGY EJECTING FROM THE  
NORTHEASTERN PACIFIC EVERY COUPLE DAYS WILL MOVE ACROSS THE CONUS,  
UNDERGOING GRADUAL AMPLIFICATION IN THE BROAD CYCLONIC FLOW. MODEL  
GUIDANCE SUGGESTS THAT THE DOMINANT STORM TRACK SHOULD GRADUALLY  
SHIFT SOUTHWARD OVER THE NEXT WEEK, WITH A MUCH MORE ACTIVE  
SOUTHERN STREAM TAKING SHAPE BY EARLY NEXT WEEK.  
 
   
..GUIDANCE/PREDICTABILITY ASSESSMENT AND PREFERENCES
 
 
DESPITE RELATIVELY GOOD AGREEMENT AMONG THE GUIDANCE ON THE LARGE  
SCALE FLOW REGIME, THE SMALLER SCALES REMAIN HIGHLY CHAOTIC WITH  
LOW PREDICTABILITY. THE SYSTEM CROSSING THE CENTRAL/EASTERN U.S.  
ON DAY 3 (FRI) HAS SHOWN CONTINUED VOLATILITY IN MODEL SOLUTIONS  
OVER THE PAST SEVERAL RUNS, WITH VARYING DEGREES OF INTERACTION  
BETWEEN NORTHERN/SOUTHERN STREAM SHORTWAVE ENERGY. THE TREND OVER  
THE PAST 12 HOURS HAS BEEN OVERWHELMINGLY TOWARD A MORE PHASED  
SYSTEM WITH LOW PRESSURE MOVING NORTHEASTWARD MUCH MORE QUICKLY,  
WITH A FRONTAL WAVE DEVELOPING ACROSS THE LOWER MISSISSIPPI VALLEY  
IN RESPONSE TO ADDITIONAL SHORTWAVE ENERGY. GIVEN THE TIME RANGE  
UNDER CONSIDERATION, ONE WOULD EXPECT THE DEGREE OF RUN-TO-RUN  
VARIABILITY TO BE QUICKLY DECREASING; BUT MODELS HAVE DEMONSTRATED  
DIFFICULTY FOR DAYS WITH THEIR HANDLING OF THIS SYSTEM, AND WOULD  
PREFER TO SEE ANOTHER RUN OF MORE STABLE SOLUTIONS BEFORE FORECAST  
CONFIDENCE INCREASES. FARTHER WEST, ADDITIONAL PACIFIC SHORTWAVE  
ENERGY IS EXPECTED TO CROSS THE SOUTHWEST AND APPROACH THE CENTRAL  
U.S. BY SAT. GIVEN THE INTENSE FRONTAL ZONE LINGERING NEAR THE  
GULF COAST, THIS WAVE SEEMS LIKELY TO GENERATE A FRONTAL WAVE BY  
DAY 5 (SUN) ALTHOUGH MODEL SOLUTIONS VARY WIDELY AS TO THE DETAILS  
OF ANY FRONTAL WAVE (OR WHETHER ONE OCCURS AT ALL). THE GFS  
CONTINUES TO DEVELOP THE STRONGEST WAVE, WITH THE ECMWF WEAKER.  
THE WPC FORECAST DURING DAYS 3-5 WAS BASED ON A BLEND OF THE 12Z  
ECMWF/18Z GFS, ALONG WITH THEIR RESPECTIVE ENSEMBLE MEANS, AND A  
SMALL COMPONENT OF FORECAST CONTINUITY TO TEMPER THE DRAMATIC  
ADJUSTMENT SLIGHTLY WITH THE NORTHERN STREAM SYSTEM DAYS 3-4.  
 
MEANWHILE, A MORE AMPLIFIED UPPER TROUGH APPEARS SET TO MOVE INTO  
THE SOUTHWEST ON SUN, CROSSING THE FOUR CORNERS REGION ON MON AND  
REACHING THE SOUTHERN/CENTRAL PLAINS BY TUE. WHILE MODEL/ENSEMBLE  
CONSENSUS IS INITIALLY GOOD WITH THIS SYSTEM, DIFFERENCES EMERGE  
BY THE TIME THE SHORTWAVE REACHES THE CENTRAL U.S., WITH DIFFERING  
LEVELS OF NORTHERN STREAM INFLUENCE RESULTING IN VARYING SURFACE  
EVOLUTIONS. ONE THING IN COMMON AMONG THE GUIDANCE THAT IS  
DIFFERENT FROM THE SYSTEM LATE THIS WEEK IS THAT THE SOUTHERN  
STREAM FEATURE APPEARS MORE DOMINANT. GIVEN THE PERSISTENT AND  
INTENSE BAROCLINIC ZONE ACROSS THE GULF OF MEXICO/GULF COAST BY  
THAT TIME WITH THE SUBTROPICAL RIDGE BUILDING OFF THE SOUTHEAST  
U.S. COAST, THIS AT LEAST OPENS THE DOOR TO THE DEVELOPMENT OF A  
SIGNIFICANT FRONTAL WAVE SOMEWHERE IN THE VICINITY OF THE GULF BY  
NEXT TUE. CONFIDENCE IN THE SPECIFICS OF ANY SUCH FEATURE IS QUITE  
LOW AT THIS POINT, HOWEVER. THE WPC FORECAST DURING DAYS 6-7 WAS  
BASED HEAVILY ON A BLEND OF THE ECENS/GEFS ENSEMBLE MEANS, WITH  
MINORITY COMPONENTS OF THEIR RESPECTIVE DETERMINISTIC SOLUTIONS.  
 
   
..WEATHER HIGHLIGHTS/HAZARDS
 
 
AREAS FROM CENTRAL/NORTHERN CALIFORNIA TO WESTERN OREGON AND  
WASHINGTON WILL SEE CONTINUED ROUNDS OF PRECIPITATION (RAIN AND  
SNOW VARYING WITH ELEVATION) THROUGH SAT BEFORE AT LEAST SOMEWHAT  
OF A LULL BY LATE IN THE WEEKEND/EARLY NEXT WEEK AS THE MAIN UPPER  
TROUGH SHIFTS INLAND. ANOTHER RELATIVELY WEAK SHORTWAVE/FRONTAL  
SYSTEM ARRIVING BY MON MAY PRODUCE ANOTHER ROUND OF PRECIPITATION  
FOR THE NORTHWEST. THE LOW PRESSURE SYSTEM EXPECTED TO CROSS  
ONTARIO/QUEBEC ON FRI MAY PRODUCE AREAS OF WINTRY WEATHER ACROSS  
THE INTERIOR NORTHEAST IN A REGION OF WARM-ADVECTION  
PRECIPITATION. FARTHER SOUTH, A FRONTAL WAVE CROSSING THE  
SOUTHEAST FRI-SAT WILL PRODUCE WIDESPREAD MODERATE RAINS AND  
PERHAPS A FEW THUNDERSTORMS ALONG THE GULF COAST. THE NEXT  
SOUTHERN STREAM FRONTAL WAVE CROSSING THE SOUTHEAST SUN-MON LOOKS  
TO PRODUCE A MORE SIGNIFICANT AREA OF PRECIPITATION FROM THE LOWER  
MISSISSIPPI VALLEY TO THE SOUTHERN MID-ATLANTIC, WITH AN AREA OF  
HEAVY RAINFALL POSSIBLE ACROSS THE TENNESSEE VALLEY AND THE  
SOUTHERN APPALACHIANS/FOOTHILLS. MODEL SOLUTIONS VARY AS TO THE  
DEGREE OF COLD AIR NORTH OF THE SYSTEM, BUT GIVEN THE RATHER TIGHT  
THERMAL GRADIENT DEPICTED IN MUCH OF THE MODEL GUIDANCE ACROSS THE  
REGION, IT SEEMS LIKELY THAT AT LEAST SOME AREAS ON THE NORTHERN  
PORTION OF THE PRECIPITATION AREA MAY SEE SOME POTENTIAL WINTER  
WEATHER.  
 
BELOW AVERAGE TEMPERATURES WILL PERSIST AND BECOME MORE WIDESPREAD  
ACROSS THE WESTERN/CENTRAL U.S. THROUGH THE MEDIUM RANGE. THE CORE  
OF THE COLD AIR LATE THIS WEEK WILL INITIALLY BE FROM THE NORTHERN  
PLAINS TO THE UPPER MIDWEST, WITH TEMPERATURES 15 TO 25 DEG F  
BELOW AVERAGE. BY SUN-MON AS THE UPPER TROUGH MOVE INTO THE WEST  
AND HEIGHTS FALL ACROSS MUCH OF THE WESTERN/CENTRAL U.S. EXPECT  
HIGH TEMPERATURES OF 15 TO 30 DEG BELOW AVERAGE ACROSS MUCH OF THE  
WESTERN AND CENTRAL U.S., CONTINUING THROUGH THE END OF THE  
FORECAST PERIOD (TUE).  
 
RYAN  
 
WPC MEDIUM RANGE 500MB HEIGHTS, SURFACE SYSTEMS, WEATHER GRIDS,  
QUANTITATIVE PRECIPITATION AND WINTER WEATHER OUTLOOK  
PROBABILITIES 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  
 

 
 
CLICK HERE TO GO TO PREVIOUS BULLETINS.

The Nexlab HPC Page Main Text Page