431  
FXUS02 KWBC 041554  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1154 AM EDT THU OCT 04 2018  
 
VALID 12Z SUN OCT 07 2018 - 12Z THU OCT 11 2018  
 
...STORMY PATTERN FOR THE WEST/ROCKIES AND A CENTRAL U.S.  
EXCESSIVE RAINFALL THREAT...  
   
..PATTERN OVERVIEW AND GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
FOR THE MEDIUM RANGE PERIOD (SUN - THURS), THE GUIDANCE REMAINS  
OVERALL WELL CLUSTERED WITH THE LARGER SCALE DEVELOPMENT OF A  
HIGHLY AMPLIFIED PATTERN SHOWING A REINFORCING WESTERN U.S. TROUGH  
(WITH 2+ STANDARD DEVIATION HEIGHT ANOMALIES) SANDWICHED BETWEEN 2  
RIDGES OVER THE EASTERN PACIFIC AND THE EAST COAST/ATLANTIC OCEAN.  
WITHIN THE WESTERN TROUGH, A DEEPENING/CLOSED LOW SETS UP OVER THE  
GREAT BASIN WITH MULTIPLE WAVES LIFTING FROM THE GREAT  
BASIN/ROCKIES INTO THE PLAINS. A DEEP SOUTHERLY FUNNEL OF  
ANOMALOUS GULF OF MEXICO MOISTURE POOLS ALONG A SLOW MOVING  
FRONTAL BOUNDARY, LEADING TO MULTIPLE DAYS OF HEAVY TO EXCESSIVE  
RAINFALL.  
 
AFTER ABOUT DAYS 4-5, THE DETERMINISTIC MODELS CONTINUE TO SHOW  
FAIRLY SIGNIFICANT RUN-TO-RUN VARIANCE WITH TIMING AND AMPLITUDE  
OF SMALLER SCALE SYSTEMS ENTERING AND REINFORCING THE LARGE SCALE  
TROUGH, WHICH LEADS TO THE SPIN-UP OF NUMEROUS SURFACE FRONTAL  
LOWS. ACCORDINGLY, THIS CYCLE OF THE WPC MEDIUM RANGE SURFACE  
PROGS WERE BASED ON A MAINLY DETERMINISTIC BLEND FOR DAYS 3 AND 4  
(BETWEEN THE 06Z GFS/00Z ECMWF), TRANSITIONING TO A MAJORITY 06Z  
GEFS/00Z ECENS BLEND BY THE END OF THE PERIOD IN ORDER TO MITIGATE  
AND SMOOTH OUT THE MORE UNPREDICTABLE SMALL SCALE FEATURES. THIS  
BLEND MAINTAINS GOOD WPC CONTINUITY AS WELL.  
   
..WEATHER HIGHLIGHTS/THREATS
 
 
HIGHLY UNSETTLED/WET WEATHER DEVELOPS AS A PERSISTENT AND HIGHLY  
AMPLIFIED TROUGH ALOFT SETS UP AND IS REINFORCED OVER THE WEST  
FROM THE GREAT BASIN/SOUTHWEST TO THE ROCKIES. TERRAIN/UPSLOPE  
FLOW SNOWS SPREAD DOWN TO FOCUS OVER THE NORTHERN GREAT  
BASIN/ROCKIES/PLAINS, ENHANCED BY POST-FRONTAL COLD HIGH PRESSURE  
SURGES. WITH THE STRONG WARMING/BLOCKING RIDGE OVER THE EAST,  
WAVES ALONG/OVER THE CENTRAL U.S. FRONTS SHOW LITTLE EASTWARD  
PROGRESSION UNTIL LATE IN THE PERIOD. THIS SETS THE STAGE FOR A  
PROLONGED PERIOD FAVORING MULTIPLE ROUNDS OF HEAVY  
RAIN/CONVECTION, AMPLE TRAINING, AND AN EXTENDED EXCESSIVE  
RAINFALL THREAT. WARM SECTOR TEMPERATURES SOUTH OF THE FRONT WILL  
BE SUMMER-LIKE, SO A MAIN FRONTAL ZONE WILL REMAIN QUITE DEFINED.  
 
REGARDING TEMPERATURES, WELL BELOW AVERAGE NUMBERS ARE LIKELY FROM  
THE UPPER MIDWEST BACK TO THE WEST COAST. THE COLDEST AIR WILL  
RESIDE ACROSS THE NORTHERN/CENTRAL ROCKIES AND PLAINS WITH SOME  
DEPARTURES FROM CLIMATOLOGY UPWARDS TO 20 TO 30 DEGREES. HIGHEST  
ELEVATIONS WILL STRUGGLE TO GET OUT OF THE 30S FOR HIGHS WHILE  
OVERNIGHT LOWS PLUNGE INTO THE 20S. IN STARK CONTRAST, IT WILL  
FEEL LIKE SUMMER IN THE VAST WARM SECTOR WITH HIGHS AROUND 10 TO  
15 DEGREES ABOVE AVERAGE, ESPECIALLY NEAR THE GREAT LAKES/OHIO  
VALLEY. SOME DAILY RECORDS MAY BE BROKEN GIVEN THE VAST OVERNIGHT  
WARMTH.  
 
ELSEWHERE, A DEEP LAYERED EASTERLY FETCH OFF THE ATLANTIC  
FORTIFIES AND PERSISTS INTO NEXT WEEK ACROSS THE SOUTHEAST,  
SPREADING SHOWERS ACROSS FLORIDA AND THE GULF COAST/ GULF OF  
MEXICO UNDERNEATH A NORTHWARD SHIFTING EASTERN U.S. CLOSED RIDGE  
ALOFT. MODELS SHOW A POSSIBLE TROPICAL DISTURBANCE IN THE GULF OF  
MEXICO, WITH A WIDE ENVELOPE OF POTENTIAL DEVELOPMENTS THAT IS  
BEING MONITORED BY WPC/NHC.  
 
SANTORELLI/SCHICHTEL  
 
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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