387  
FXUS02 KWBC 241548  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1148 AM EDT MON SEP 24 2018  
 
VALID 12Z THU SEP 27 2018 - 12Z MON OCT 01 2018  
 
...EARLY-SEASON COLD AIR EXPECTED TO GRADUALLY SPREAD INTO AREAS  
FROM THE NORTHERN ROCKIES TO THE UPPER MIDWEST LATER THIS WEEK...  
 
   
..PATTERN OVERVIEW AND GUIDANCE EVALUATION/PREFERENCES
 
 
EXPECT AN AMPLIFIED UPPER-LEVEL FLOW PATTERN TO PERSIST ACROSS  
NORTH AMERICA DURING THE MEDIUM RANGE PERIOD. AN  
ANOMALOUS/BLOCKING UPPER RIDGE WILL LIKELY SHIFT NORTHWARD FROM  
THE NORTH PACIFIC INTO ALASKA FROM LATE THIS WEEK INTO THE  
WEEKEND, WITH 500-HPA HEIGHTS REACHING +3.5 STANDARD DEVIATIONS  
ACROSS INTERIOR ALASKA BY LATE THIS WEEK. THE RIDGE ALONG WITH  
DOWNSTREAM PERSISTENT TROUGHING JUST WEST OF HUDSON BAY WILL  
PROMOTE SOUTHWARD TRANSPORT OF POLAR AIR THROUGH CENTRAL CANADA  
AND INTO THE NORTHERN PLAINS AND VICINITY. MEANWHILE UPPER RIDGING  
INITIALLY CENTERED OFF THE SOUTHEAST SHOULD GRADUALLY BUILD  
WESTWARD ACROSS THE SOUTHERN U.S. AND GULF OF MEXICO. THE  
COMBINATION OF THESE FEATURES WILL FAVOR THE SETUP OF A PERSISTENT  
AND RELATIVELY STRONG BAROCLINIC ZONE ACROSS THE CENTRAL PLAINS BY  
LATER THIS WEEK. FARTHER WEST OVER THE PACIFIC, UPPER-LEVEL ENERGY  
WILL BEGIN TO UNDERCUT THE STRENGTHENING ALASKA RIDGE, WITH A  
NUMBER OF MODEL SOLUTIONS SUGGESTING THE POTENTIAL FOR RATHER  
STRONG HEIGHT FALLS OFF/ALONG THE PACIFIC NORTHWEST COAST BY EARLY  
NEXT WEEK. SPECIFICS OF THIS UNDERCUTTING FLOW AND WESTERN CANADA  
FLOW DOWNSTREAM FROM THE RIDGE AMPLIFYING INTO ALASKA WILL PLAY A  
ROLE IN DETERMINING THE TIMING OF AN UPPER LOW THAT SHOULD BE  
APPROACHING THE WEST COAST AS OF EARLY SAT.  
 
THROUGH ABOUT DAYS 5-6 SAT-SUN LATEST MODEL/ENSEMBLE GUIDANCE  
MAINTAINS REASONABLE AGREEMENT AND CONTINUITY ASIDE FROM TYPICALLY  
UNAVOIDABLE SUBTLE ADJUSTMENTS. FEATURES OF INTEREST INCLUDE LOW  
PRESSURE THAT SHOULD TRACK NORTHEASTWARD FROM WESTERN ONTARIO  
EARLY THU ONWARD, BRINGING AN UPPER MIDWEST/PLAINS FRONT INTO THE  
EAST BY FRI-SAT, A TRAILING FRONT DROPPING INTO THE NORTHERN TIER  
STATES BY FRI AND SETTLING INTO THE CENTRAL PLAINS/MIDDLE MS  
VALLEY BY THE WEEKEND (SLIGHTLY FASTER TIMING THAN CONTINUITY),  
AND THE UPPER LOW THAT SHOULD BE NEARING NORTHERN CALIFORNIA BY  
EARLY SAT. IF ANYTHING, GUIDANCE CONSENSUS HAS ADJUSTED A TAD LESS  
AMPLIFIED WITH ELONGATED TROUGHING OVER/NEAR BRITISH COLUMBIA  
WHICH YIELDS A TRIMMING OF THE FAST SIDE OF PRIOR GUIDANCE SPREAD  
(SOME ECMWF RUNS AND THE 12Z/23 GFS) FOR THE AFOREMENTIONED UPPER  
LOW. ALSO WORTH NOTING, THE LATEST ECMWF/GEFS MEANS ARE FAIRLY  
SIMILAR OVER THE CONTIGUOUS U.S. DURING THE PERIOD BUT THE ECMWF  
MEAN HAS A LEADER AMONG THE MEANS (ALONG WITH A NUMBER OF  
OPERATIONAL MODEL SOLUTIONS) IN DEPICTING THE CLOSED UPPER HIGH  
OVER ALASKA.  
 
THE PRIMARY GUIDANCE SPREAD FROM LATE WEEKEND INTO EARLY NEXT WEEK  
INVOLVES THE SPECIFICS OF THE COMPLEX EASTERN PACIFIC/WESTERN  
NORTH AMERICA EVOLUTION. THE 00Z ECMWF PULLS THE ALASKA UPPER  
HIGH FARTHER NORTHWEST THAN MOST OTHER GUIDANCE INCLUDING ITS  
ENSEMBLE MEAN. TELECONNECTIONS RELATIVE TO THE ASSOCIATED POSITIVE  
HEIGHT ANOMALY INDICATE THE 00Z ECMWF IS INTERNALLY CONSISTENT IN  
BRINGING THE EASTERN PACIFIC/WEST COAST TROUGH FARTHER EASTWARD.  
HOWEVER ASSUMING THE MAJORITY SCENARIO OF KEEPING THE UPPER RIDGE  
OVER ALASKA FAVORS THE SLOWER TIMING OF THE MEAN TROUGH.  
OPERATIONAL RUNS SHOW A VARIETY OF IDEAS FOR EXACTLY WHERE AN  
EMBEDDED UPPER LOW MAY BE BY DAY 7 MON SO PREFERENCE LEANS  
INCREASINGLY TOWARD THE ENSEMBLE MEANS AT THAT TIME. FARTHER  
EASTWARD THERE IS BETTER THAN AVERAGE CLUSTERING/CONSISTENCY FOR  
SOME FRONTAL WAVINESS TO MIGRATE INTO THE GREAT LAKES EARLY NEXT  
WEEK IN RESPONSE TO THE SHORTWAVE ENERGY EMBEDDED WITHIN THE LOW  
AMPLITUDE MEAN FLOW ACROSS THE NORTHERN HALF OF THE LOWER 48.  
 
BASED ON THE LATEST ARRAY OF GUIDANCE, A BLEND OF OPERATIONAL  
GUIDANCE CONTINUES TO REPRESENT CONSENSUS WELL THROUGH DAY 5 SAT  
AND 70 PERCENT OPERATIONAL WEIGHT (00Z-06Z GFS/00Z ECMWF WITH A  
LITTLE 00Z CMC) LOOKS GOOD INTO DAY 6 SUN. CONSIDERATIONS BY DAY 7  
MON FAVOR A MORE PRONOUNCED SHIFT TO THE 06Z GEFS/00Z ECMWF MEANS  
(65-70 PERCENT) AT THAT TIME.  
 
   
..SENSIBLE WEATHER HIGHLIGHTS/THREATS
 
 
LOCALLY HEAVY CONVECTION WILL BE POSSIBLE FROM THE GULF COAST TO  
THE SOUTHERN MID-ATLANTIC THU-FRI, FOCUSED ALONG A COUPLE COLD  
FRONTS. THE SECOND OF THESE FRONTS MAY ALSO PRODUCE AREAS OF  
SHOWERS FROM THE NORTHERN ROCKIES TO THE UPPER MIDWEST. BY THE  
WEEKEND, AS THIS SECOND FRONT SETTLES ACROSS THE CENTRAL U.S. AND  
SHARPENS, WHILE ADDITIONAL UPPER SHORTWAVE ENERGY APPROACHES,  
EXPECT THE POTENTIAL FOR A ROUND OF MORE WIDESPREAD PRECIPITATION  
TO DEVELOP FROM THE NORTHERN ROCKIES INTO THE NORTHERN/CENTRAL  
PLAINS AND PORTIONS OF THE MIDWEST/GREAT LAKES. SUFFICIENT COLD  
AIR MAY BE IN PLACE IN THE POLAR AIR MASS NORTH OF THE FRONTAL  
BOUNDARY TO SUPPORT AREAS OF SNOW MIXING INTO THE RAIN AT TIMES  
ACROSS THE NORTHERN ROCKIES AND PERHAPS EVEN EASTWARD INTO THE  
NORTHERN HIGH PLAINS. WITHIN THIS PRECIP AREA GUIDANCE IS STILL  
SIGNALING THE BEST POTENTIAL FOR HEAVIEST RAINFALL WITHIN THE  
MIDWEST/GREAT LAKES REGION. FARTHER WEST EXPECT INCREASING  
CLOUDINESS AND GENERALLY LIGHT PRECIPITATION TO BEGIN SPREADING  
INTO NORTHERN CALIFORNIA AND PACIFIC NORTHWEST IN ADVANCE OF THE  
UPPER LOW HEADING INTO THE WEST DURING THE WEEKEND AND TRAILING  
SYSTEM THAT MAY APPROACH THE COAST EARLY NEXT WEEK.  
 
COOL TEMPERATURES WILL INCREASINGLY BECOME A STORY THROUGH THE  
WEEK AS A COUPLE COLD FRONTS SPREAD SUCCESSIVELY COOLER AIR INTO  
AREAS FROM THE NORTHERN ROCKIES TO THE UPPER MIDWEST. EXPECT HIGH  
TEMPERATURES THU-FRI TO BE 5 TO 15 DEG F BELOW AVERAGE ACROSS  
THESE AREAS, WITH COLDER TEMPERATURES, 15 TO 25 DEG BELOW AVERAGE,  
ARRIVING BY THE WEEKEND. HIGH TEMPERATURES SHOULD REMAIN IN THE  
40S BY NEXT WEEKEND ACROSS A WIDE AREA OF THE NORTHERN PLAINS AND  
UPPER MIDWEST WHILE LOWS DROP TO NEAR OR BELOW FREEZING OVER MANY  
OF THE SAME AREAS. MEANWHILE MUCH OF THE SOUTHEAST WILL REMAIN  
RELATIVELY WARM WITH HIGHS 5 TO 10 DEG F ABOVE AVERAGE. LOCATIONS  
TO THE SOUTH OF THE SHARP FRONT SETTLING OVER THE CENTRAL U.S.  
WILL SEE INCREASINGLY WARM MIN TEMPS IN PARTICULAR, 10 TO 15 F  
ABOVE NORMAL DURING THE WEEKEND/EARLY NEXT WEEK. HEIGHT FALLS  
REACHING THE WEST COAST DURING THE LATTER HALF OF THE PERIOD WILL  
LEAD TO A COOLER TREND OVER A MAJORITY OF THE WEST AFTER A WARM  
PERIOD LATE THIS WEEK.  
 
RAUSCH/RYAN  
 
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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