064  
FXUS02 KWBC 181558  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1157 AM EDT WED OCT 18 2017  
 
VALID 12Z SAT OCT 21 2017 - 12Z WED OCT 25 2017  
 
...HEAVY RAINFALL POTENTIAL IN THE EASTERN U.S. WITH RECORD HEAT  
IN THE SOUTHWESTERN STATES NEXT WEEK...  
 
   
..PATTERN OVERVIEW/MODEL GUIDANCE/FORECAST PREFERENCES
 
 
DURING THE UPCOMING FORECAST PERIOD...A HIGHLY AMPLIFIED SYNOPTIC  
PATTERN WILL FEATURE SOME IMPRESSIVE 500-MB HEIGHT ANOMALIES WITH  
A PAIR OF NOTABLE FEATURES. A CLOSED LOW POTENTIALLY PEELING OFF  
FROM THE PREVAILING WESTERLIES MAY AFFECT THE GULF COAST REGION  
EARLY NEXT WEEK. GIVEN POTENTIAL PHASING WITH A NORTHERN STREAM  
TROUGH...A RESULTANT NEGATIVELY-TILTED TROUGH COULD LEAD TO 500-MB  
HEIGHT DEPARTURES ON THE ORDER OF 3 TO 4 STANDARD DEVIATIONS BELOW  
AVERAGE BY THE MIDDLE OF NEXT WEEK ACROSS THE EAST COAST. WHILE A  
SHARP NEGATIVE HEIGHT ANOMALY ENCOMPASSES THE EASTERN THIRD OF THE  
NATION...PRONOUNCED RIDGING OVER THE WEST WILL BRING WARM/DRY  
CONDITIONS TO THE REGION. EQUALLY IMPRESSIVE...THE 00Z GFS FAVORS  
500-MB HEIGHT ANOMALIES IN THE 1.5 TO 2 STANDARD DEVIATION RANGE.  
 
CONSIDERING THE EVOLUTION OF THE LONGWAVE PATTERN...THE INITIAL  
BAND OF HEIGHT FALLS WILL BE TRAVERSING THROUGH THE HIGH PLAINS  
EARLY SATURDAY WHILE SWINGING EASTWARD IN TIME. THE CORE OF LOWER  
HEIGHTS ARE FORECAST TO LIFT TOWARD SOUTHERN MANITOBA INTO WESTERN  
ONTARIO OVER THE WEEKEND WHILE THERE ARE INCREASING INDICATIONS OF  
A SPLIT IN THE JET STREAM TAKING SHAPE. BY SUNDAY...A MAJORITY OF  
THE AVAILABLE SOLUTIONS FAVOR A SOUTHERN STREAM SYSTEM UNFOLDING  
OVER THE SOUTHERN PLAINS WITH GRADUAL DOWNSTREAM PROGRESSION. THE  
LARGEST UNCERTAINTY IS HOW QUICKLY THE SHORTWAVE ENERGY WILL TRACK  
TOWARD THE GULF COAST AS OPERATIONAL MODELS CONTINUE TO WAVER  
BETWEEN RUNS AND AMONGST THEMSELVES. THERE HAS BEEN PLENTY OF  
ENSEMBLE SUPPORT VIA SPAGHETTI PLOTS TO SUPPORT A GENERAL SLOWING  
AND CLOSING OFF OF AN UPPER LOW. THE OVERALL PATTERN IS CONDUCIVE  
TO SUCH A FORECAST GIVEN AMPLE SEPARATION BETWEEN THE NORTHERN AND  
SOUTHERN STREAMS. HOWEVER...THE POOR RUN-TO-RUN MODEL CONTINUITY  
LEAVES A GREAT DEAL OF UNCERTAINTY MOVING FORWARD. MOST  
NOTABLY...THE 00Z CMC/UKMET/GFS DEPICT A POWERFUL UPPER LOW IN THE  
VICINITY OF THE UPPER TX/LA COAST ON MONDAY EVENING WHILE THE 06Z  
GFS AND 00Z/12Z ECMWF ARE MORE PROGRESSIVE. THE 16/1200Z CYCLE OF  
THE ECMWF WAS THE MOST RECENT IN THE SUITE TO SUPPORT A CLOSED LOW  
AS SLOW AS THE 00Z CMC/UKMET/GFS ARE SUGGESTING. WHILE WITHIN THE  
REALM OF POSSIBILITIES...IT HAS BEEN A FEW RUNS SINCE THE ECMWF  
INDICATED SUCH A FORECAST. EVENTUALLY THIS SOUTHERN STREAM FEATURE  
WILL BECOME ABSORBED BY A LARGER-SCALE TROUGH ACROSS THE GREAT  
LAKES WITH A RESULTANT NEGATIVE TILT EXPECTED. AN ABUNDANT SURGE  
OF TROPICAL MOISTURE IS LIKELY TO ADVECT POLEWARD BRINGING A  
THREAT FOR HEAVY RAINFALL ALONG AND AHEAD OF THE COLD FRONT.  
EAST/WEST TIMING DIFFERENCES ARE NOTED WITH THE WEAKER SOUTHERN  
STREAM SOLUTIONS FAVORING A QUICKER EASTWARD PROGRESSION.  
ELSEWHERE...THERE IS NOT MUCH TO COMMENT ON ACROSS THE WESTERN  
U.S. WITH BROAD RIDGING ESTABLISHING ITS PRESENCE OVER THE REGION.  
THE CENTER OF THE UPPER RIDGE SHOULD BE FOCUSED OVER THE  
SOUTHWESTERN STATES LEADING TO A POTENTIAL HEAT WAVE.  
 
DECIDED TO MAINTAIN CONTINUITY FAVORING A MORE PROGRESSIVE  
SOUTHERN STREAM SOLUTION. THE 06Z GFS AND PREVIOUS 12Z ECMWF  
SUPPORT THIS NOTION THROUGHOUT THE PERIOD. UTILIZED 20 PERCENT OF  
THE OVERNIGHT SHIFT FORECAST BLEND IN COMBINATION WITH THESE TWO  
OPERATIONAL MODELS. GRADUALLY INCORPORATED THEIR ENSEMBLE  
COUNTERPARTS FROM DAY 5/MONDAY ONWARD GIVEN INCREASING UNCERTAINTY  
ACROSS THE EASTERN U.S. GIVEN THE HIGH AMPLITUDE PATTERN IN  
PLACE...THE FORECAST CONFIDENCE IS AROUND AVERAGE BUT MUCH OF THIS  
FOCUSES ON THE EVOLVING UPPER LOW.  
 
   
..SENSIBLE WEATHER HIGHLIGHTS/THREATS
 
 
MILD AIR PUMPING NORTHWARD AHEAD OF THE SLOW MOVING UPPER TROUGH  
WILL POTENTIALLY LEAD TO A NUMBER OF DAILY TEMPERATURE RECORDS  
BEING BROKEN. WHILE THIS IS NOT EXPECTED FOR THE AFTERNOON  
HIGHS...THE OVERNIGHT WARMTH MAY LEAD TO SOME RECORDS ANYWHERE  
FROM THE SOUTHERN/CENTRAL PLAINS INTO THE UPPER MIDWEST ON  
SATURDAY MORNING. READINGS AROUND 20 TO 25 DEGREES ABOVE NORMAL  
WILL BE COMMONPLACE FROM EASTERN KS UP INTO IA/MN/WI. WHILE THE  
COLD FRONTAL PASSAGE WILL BRING COOLER WEATHER WITH IT...THE LACK  
OF ARCTIC AIR SHOULD ONLY LEAD TO TEMPERATURES RETURNING CLOSER TO  
CLIMATOLOGY. HOWEVER...SOME BELOW NORMAL TEMPERATURES ARE LIKELY  
GIVEN ACTIVE PRECIPITATION ACROSS THE SOUTHERN TIER STATES.  
ELSEWHERE...A STRONG UPPER RIDGE BUILDING ACROSS THE SOUTHWESTERN  
U.S. SHOULD BRING SOME RECORD WARMTH TO THE REGION EARLY NEXT  
WEEK. CURRENT FORECAST HIGHS ABOVE THE 100 DEGREE MARK OVER  
SOUTHERN CA MAY BREAK SOME DAILY RECORDS. OFFSHORE GRADIENTS WILL  
LEAD TO HOT/DRY/BREEZY CONDITIONS POTENTIALLY AIDING IN A FIRE  
WEATHER RISK DURING THE PERIOD.  
 
OVER THE WEEKEND...ZONAL FLOW ACROSS THE PACIFIC NORTHWEST WILL  
LEAD TO HEAVY PRECIPITATION OVER THE OROGRAPHICALLY FAVORED  
LOCATIONS OF THE OLYMPICS AND CASCADES. THIS WESTERLY FLOW REGIME  
SHOULD BE DEVOID OF ANY APPRECIABLE COLD AIR WHICH MAINTAINS SNOW  
OVER THE HIGHER ELEVATIONS. WITH HEIGHTS BUILDING IN TIME...THE  
ACTIVE STORM TRACK WILL LIFT UP INTO BRITISH COLUMBIA NEXT WEEK.  
FARTHER EAST...WIDESPREAD RAINFALL SHOULD BE ONGOING ALONG AND  
AHEAD OF THE COLD FRONT MOVING THROUGH THE CENTRAL U.S. ON  
SATURDAY. THE ACTIVITY WILL LIKELY PICK UP IN COVERAGE AND  
INTENSITY AS THE SOUTHERN STREAM SHORTWAVE DEVELOPS WITH HEAVIER  
AMOUNTS POSSIBLE IF THE SYSTEM SLOWS DOWN. A HEAVY RAINFALL SIGNAL  
EXISTS OVER THE ENTIRE EASTERN SEABOARD FROM OCTOBER 23-25 ONCE  
THE UPPER TROUGH STRENGTHENS AND A SERIES OF SURFACE LOWS FORM.  
SOME OPERATIONAL MODELS PRODUCE 2 TO 4 INCHES IN A 24-HOUR PERIOD  
SUGGESTING THE POTENTIAL FOR FLASH FLOODING IN CERTAIN LOCATIONS.  
IT STILL IS TOO FAR OUT IN TIME TO DETERMINE ANY SPECIFIC THREATS.  
 
RUBIN-OSTER  
 

 
 
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