912  
FXUS02 KWBC 260649  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
149 AM EST SUN NOV 26 2017  
 
VALID 12Z WED NOV 29 2017 - 12Z SUN DEC 03 2017  
   
..OVERVIEW AND GUIDANCE EVALUATION/PREFERENCES
 
 
THE INITIALLY PROGRESSIVE MEAN PATTERN SHOWS SIGNS OF DECELERATING  
OVER THE COURSE OF THE PERIOD, WITH DOMINANT FEATURES BECOMING  
LARGER IN SCALE. THE TWO PRIMARY SYSTEMS OF INTEREST WILL BE A  
SHORTWAVE TROUGH REACHING THE INTERIOR WEST BY EARLY DAY 3 WED AND  
THE EASTERN STATES BY DAY 5 FRI FOLLOWED BY AN AMPLIFYING EASTERN  
PACIFIC TROUGH WHOSE AXIS WILL LIKELY REACH THE WEST COAST/120W  
LONGITUDE BY NEXT WEEKEND. AHEAD OF THESE SYSTEMS WILL BE A  
SMALLER SCALE SHORTWAVE/POSSIBLE CLOSED LOW WHICH SHOULD BE OVER  
THE SOUTH-CENTRAL PLAINS/MS VALLEY AS OF EARLY WED AND THEN  
WEAKEN/OPEN UP AS IT PROGRESSES EASTWARD. GUIDANCE HAS BEEN DOING  
WELL OVER THE PAST COUPLE DAYS IN IDENTIFYING EXISTENCE OF THESE  
FEATURES BUT WITH CONSIDERABLE DIFFICULTY IN RESOLVING IMPORTANT  
DETAILS THAT WILL AFFECT ASSOCIATED SENSIBLE WEATHER.  
 
GUIDANCE IS STILL HAVING TROUBLE LATCHING ONTO CONSISTENT TIMING  
FOR THE LEADING SOUTHERN STREAM IMPULSE THAT WEAKENS AFTER EARLY  
WED. AN AVERAGE AMONG THE 12Z-18Z GFS/12Z ECMWF SEEMED TO PROVIDE  
THE BEST INTERMEDIATE SOLUTION WITHIN A STILL BROAD ENSEMBLE  
SPREAD. THE NEW 00Z GFS/GEFS MEAN HAVE TRENDED SOMEWHAT FASTER  
BUT THE 00Z UKMET HAS ADJUSTED BACK TO THE AFOREMENTIONED CLUSTER  
AND THE CMC IS MAINTAINING CONTINUITY THAT IS AT LEAST AS SLOW.  
 
THE TROUGH REACHING THE WEST BY THE START OF THE PERIOD STILL HAS  
A HIGH DEGREE OF UNCERTAINTY FOR SPECIFICS, ESPECIALLY AS IT  
REACHES THE EASTERN STATES. IN VARYING WAYS GUIDANCE IS STILL  
PULLING OFF SOME OF THE SOUTHERN ENERGY WITHIN THE TROUGH WHILE  
STILL OFFSHORE DURING THE SHORT RANGE, COLLECTING THIS ENERGY OFF  
CALIFORNIA AND BAJA CALIF. EXACTLY HOW MUCH ENERGY GETS SEPARATED  
MAY PLAY SOME ROLE IN DOWNSTREAM TROUGH EVOLUTION. STARTING WITH  
THE 12Z/25 CYCLE, GFS/ECMWF RUNS HAVE MADE A DRASTIC CHANGE FROM  
MOST PRIOR RUNS IN CLOSING OFF A DEEP UPPER LOW BEFORE THE TROUGH  
REACHES THE EAST COAST-- IN CONTRAST TO EARLIER SIGNALS THAT SUCH  
AN EVOLUTION IF OCCURRING WOULD BE MORE LIKELY OVER THE WESTERN  
ATLANTIC. THE NEW 00Z GFS HAS TONED DOWN THE STRENGTH OF ITS  
SYSTEM BUT IS DEEPER/SLOWER WITH THE SOUTHERN HALF OF THE TROUGH  
VERSUS CONSENSUS ALREADY OVER THE HIGH PLAINS EARLY THU. ON THE  
OTHER HAND THE CMC/UKMET SHOW A MORE OPEN TROUGH MORE SIMILAR TO  
WHAT THE ENSEMBLE MEANS HAVE BEEN SHOWING. IT IS NOTABLE HOWEVER  
THAT THE 12Z/18Z MEANS HAVE TRENDED SOMEWHAT WESTWARD WITH SURFACE  
LOW PRESSURE DEVELOPING OFF THE EAST COAST. THIS TREND ALONG WITH  
THE FACT THE CLOSED LOW SCENARIO DOES FIT WITHIN THE OVERALL  
ENSEMBLE MEAN TROUGH (ALBEIT WITH RESULTING TROUGH DEPTH IN THE  
MINORITY AMONG INDIVIDUAL MEMBERS), ULTIMATELY SEEMS TO FAVOR AN  
INITIAL ADJUSTMENT TOWARD INCLUDING JUST ENOUGH OPERATIONAL MODEL  
GUIDANCE WITH THE MEANS TO SHOW A LITTLE MORE SEPARATION ALOFT  
(BUT NOT TO THE POINT OF A CLOSED LOW) AND SLIGHTLY SLOWER LOW  
PRESSURE THAN THE ENSEMBLE MEANS BY DAYS 6-7 SAT-SUN.  
 
ENSEMBLE MEANS AND THE ECMWF MEAN IN PARTICULAR REMAIN THE MOST  
STABLE SOLUTIONS FOR THE UPPER TROUGH REACHING THE WEST COAST  
LATER IN THE PERIOD. INDIVIDUAL MODEL RUNS SHOW A VARIETY OF WAYS  
THAT SHORTWAVE DETAILS COULD BECOME MORE REFINED WITHIN THE  
OVERALL TROUGH. THUS FAR TELECONNECTIONS RELATIVE TO THE UPSTREAM  
RIDGE/POSITIVE HEIGHT ANOMALY CENTER HAVE TENDED TO SUPPORT THE  
BROAD AND SLIGHTLY POSITIVELY TILTED TROUGH DEPICTED BY THE MEANS,  
SO PREFERENCE LEANS MUCH CLOSER TO THE MEANS UNTIL OPERATIONAL  
RUNS SHOW BETTER AGREEMENT. INTERESTINGLY, OVER RECENT DAYS CMC  
RUNS HAVE GENERALLY TENDED TO BE CLOSEST TO THE MEANS IN PRINCIPLE  
WITH RESPECT TO BROADNESS OF THE TROUGH. ON THE OTHER HAND 18Z  
AND NEW 00Z GFS RUNS APPEAR SOMEWHAT QUESTIONABLE IN HOW MUCH  
ENERGY GETS PULLED OFF THE SOUTHWEST SIDE OF THE TROUGH WELL  
OFFSHORE. MEANWHILE MOST GUIDANCE EJECTS LINGERING ENERGY  
OFFSHORE CA/BAJA CALIF NORTHEASTWARD AHEAD OF THE LARGER SCALE  
TROUGH. SO FAR THERE IS POOR AGREEMENT ON TIMING/TRACK, LEADING  
TO BELOW AVERAGE CONFIDENCE ON EXTENT OF EFFECTS ACROSS THE  
SOUTHERN HALF OF THE WEST.  
 
GIVEN ABOVE CONSIDERATIONS AND FAIRLY RAPID DECLINE IN AGREEMENT  
AMONG MODELS/ENSEMBLE MEANS AFTER THE START OF THE PERIOD,  
MAJORITY EMPHASIS ON THE 12Z-18Z GFS/12Z ECMWF EXTENDED ONLY INTO  
DAY 4 THU FOLLOWED BY 70-80 PCT RELIANCE ON THE 18Z GEFS/00Z ECMWF  
MEANS BY DAYS 5-7 FRI-SUN. THE MINORITY OPERATIONAL INPUT BY THAT  
TIME CAME FROM THE 12Z GFS/ECMWF AS THE 18Z GFS BECAME FLATTER  
THAN CONSENSUS WITH THE WEST COAST UPPER TROUGH.  
 
   
..WEATHER HIGHLIGHTS
 
 
LEADING IMPULSE OVER THE SOUTH-CENTRAL PLAINS/MS VALLEY MAY  
PRODUCE A LOCALIZED AREA OF ENHANCED RAINFALL AROUND MIDWEEK  
BEFORE WEAKENING. TRAILING WESTERN U.S. SHORTWAVE SHOULD PRODUCE  
SOME LIGHT RAIN/HIGH ELEVATION SNOW OVER/NEAR THE NORTHERN/CENTRAL  
ROCKIES WED AND THEN PROMOTE A GENERAL INCREASE IN COVERAGE OF  
PRECIP OVER THE EAST BY THU-FRI. HOWEVER WIDELY DIFFERING  
GUIDANCE DETAILS FOR UPPER FLOW AND SURFACE PATTERN BY THIS TIME  
FRAME LEAD TO LOW CONFIDENCE IN PRECIP INTENSITY AND DURATION, AND  
IN SOME CASES EVEN PRECIP TYPE DEPENDING ON EXISTENCE OF A CLOSED  
LOW. IF ANY HEAVIER PRECIP OCCURS IT IS MOST LIKELY FROM THE MID  
ATLANTIC INTO THE NORTHEAST, BUT NOTE THAT A FAIRLY  
PROGRESSIVE/LIGHT EPISODE OF PRECIP IS ALSO A POSSIBLE SCENARIO.  
ALSO THERE IS POTENTIAL FOR A PERIOD OF BREEZY TO WINDY  
CONDITIONS. MEANWHILE THE WEST WILL SEE A GRADUAL INCREASE IN  
MOISTURE WITH TIME AS UPPER TROUGHING APPROACHES/ARRIVES.  
MOISTURE WILL REACH THE NORTHWEST AROUND LATE WEEK WITH OTHER  
PARTS OF THE WEST STARTING TO SEE AREAS OF PRECIP DURING THE  
WEEKEND. THE PACIFIC NORTHWEST/NORTHERN ROCKIES WILL BE ONE  
FAVORED AREA OF FOCUS. THE AREA ENCOMPASSING CENTRAL-SOUTHERN  
CALIFORNIA NORTHEASTWARD INTO THE ROCKIES MAY BE ANOTHER REGION  
WITH SOME LOCALLY ENHANCED TOTALS DEPENDING ON DETAILS OF THE  
UPPER TROUGH AND LINGERING SOUTHERN STREAM ENERGY EJECTING AHEAD  
OF THE TROUGH.  
 
EXPECT THE NORTHERN PLAINS AND VICINITY TO SEE THE MOST CONSISTENT  
WARMTH DURING THE PERIOD WITH TEMPS AVERAGING 10-15F ABOVE NORMAL.  
SOME OF THIS WARMTH WILL EXTEND INTO THE CENTRAL HIGH PLAINS DAY  
3 WED, AND PATTERN EVOLUTION BY NEXT WEEKEND SHOULD SUPPORT A  
BROADER AREA OF PLUS 10F OR GREATER ANOMALIES OVER THE CENTRAL  
U.S.. THE SYSTEM CROSSING THE EAST WILL BRING A COOLING TREND TO  
THAT PART OF THE COUNTRY FOR THE LATE WEEK/WEEKEND TIME FRAME AND  
UPPER TROUGHING WILL SUPPORT A TREND TOWARD BELOW NORMAL TEMPS  
OVER AN INCREASING PORTION OF THE WEST NEXT WEEKEND.  
 
RAUSCH  
 

 
 
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