Prognostic Meteorological Discussion
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000
FXUS02 KWBC 251600
PMDEPD

EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1159 AM EDT MON SEP 25 2017

VALID 12Z THU SEP 28 2017 - 12Z MON OCT 02 2017

...OVERVIEW AND GUIDANCE EVALUATION/PREFERENCES...

THE EARLY/MIDDLE PART OF THE FORECAST CURRENTLY SHOWS DECENT
CLUSTERING OF SIGNIFICANT FEATURES BUT WITH TRENDS IN SOME
GUIDANCE OVER THE PAST 24-36 HOURS.  UNCERTAINTIES REGARDING
PACIFIC TROUGH EVOLUTION TOWARD 160-170W LONGITUDE AROUND DAYS 4-5
FRI-SAT LEAD TO A RAPID INCREASE OF MODEL/ENSEMBLE SPREAD AND RUN
TO RUN VARIABILITY FROM THE EASTERN PACIFIC ACROSS THE LOWER 48 BY
DAYS 6-7 SUN-MON.

FROM THU INTO THE WEEKEND EXPECT SOUTHERN CANADA ENERGY TO AMPLIFY
A TROUGH THAT NEARS THE EAST COAST WHILE AN EASTERN PACIFIC TROUGH
WILL APPROACH/REACH THE WEST COAST.  THE EASTERN TROUGH HAS
EXHIBITED A FASTER TREND IN SOME GUIDANCE, INCLUDING THE 06Z GFS
RELATIVE TO ITS 00Z RUN.  ECMWF RUNS HAVE BEEN GRADUALLY TRENDING
AWAY FROM EARLIER RUNS THAT HAD BEEN MUCH SLOWER AND MORE CLOSED.
BY LATE FRI-SAT THE 00Z GFS HOLDS ONTO A SLOWER/STRONGER SURFACE
LOW OVER THE NORTHEAST COMPARED TO MOST OTHER SOLUTIONS.
MEANWHILE THERE HAS BEEN A STRONGER TREND WITH THE UPPER TROUGH
REACHING THE WEST ALONG WITH SOME MODERATE DIFFERENCES IN
AMPLITUDE BY SAT.  THIS TROUGH WILL HELP TO EJECT UPPER LOW ENERGY
EXPECTED TO CLOSE OFF OVER THE SOUTHWEST DURING THE SHORT RANGE.
OVERALL PREFER AN INTERMEDIATE SOLUTION INCORPORATING THE MOST
AGREEABLE ASPECTS OF THE 00Z-06Z GFS/00Z ECMWF AND TO A MINOR
EXTENT 06Z GEFS/00Z ECMWF MEANS, AS THE BEST WAY TO NUDGE
CONTINUITY TOWARD LATEST GUIDANCE IDEAS.

AROUND FRI-SAT ENERGY DROPPING SOUTH FROM THE BERING SEA MAY
INTERACT WITH A MID-LATITUDE PACIFIC TROUGH.  BEYOND THE QUESTION
OF HOW MUCH INTERACTION OCCURS, THERE IS ALSO RAPIDLY INCREASING
MODEL/ENSEMBLE SPREAD WITH RESPECT TO HOW MUCH ENERGY MAY GET
PULLED OFF THE SOUTHERN PART OF THE OVERALL TROUGH.  THESE ISSUES
ULTIMATELY LEAD TO MAJOR DIFFERENCES/RUN TO RUN CHANGES FROM THE
EASTERN PACIFIC INTO THE LOWER 48.  FOR EXAMPLE BY DAY 7 MON 00Z
GFS/ECMWF RUNS FROM 24 HOURS AGO SUGGESTED DIFFERING DEGREES OF
EASTERN PACIFIC TROUGH/WESTERN U.S. RIDGE WHILE 12Z/24 RUNS
FLIPPED TO AN EASTERN PACIFIC RIDGE, AND ARE NOW BACK TO A STRONG
EASTERN PACIFIC TROUGH.  THE PREVAILING ENSEMBLE MEAN SCENARIO
LEANS TOWARD AN EASTERN PACIFIC TROUGH THAT SUPPORTS A LEADING
NORTHEAST PACIFIC SURFACE SYSTEM BUT ONE THAT IS SOMEWHAT NORTH OF
THE 06Z GFS.  THESE DIFFERENCES TRANSLATE DOWNSTREAM INTO A
WIDENING GUIDANCE SPREAD FOR WHAT HAPPENS WITH UPPER TROUGH ENERGY
CROSSING THE WEST DURING THE WEEKEND.  SELECTED OPERATIONAL MODEL
SOLUTIONS BY LATE DAY 7 MON RANGE FROM THE SOUTHERN HIGH PLAINS
(00Z GFS/CMC) TO THE MID MS VALLEY (00Z ECMWF) OR NORTHWESTERN
GREAT LAKES (06Z GFS).  D+8 MULTI-DAY MEANS AGREE FAIRLY WELL WITH
A CORE OF POSITIVE HEIGHT ANOMALIES CENTERED NEAR JAMES BAY, WITH
TELECONNECTIONS SEEMING TO RECOMMEND LEANING TOWARD SOLUTIONS THAT
HAVE HIGHER HEIGHTS OVER THE EAST.  THE 06Z GFS COULD BE
PROGRESSIVE ENOUGH WITH ITS LATE PERIOD CENTRAL U.S. ENERGY TO FIT
INTO THAT THEME FOR A MULTI-DAY AVERAGE AND THE SLOWER 00Z ECMWF
SEEMS MORE QUESTIONABLE, BUT EITHER DIFFERS TOO MUCH FROM THE
ENSEMBLE MEANS TO HAVE ANY CONFIDENCE LATE IN THE FORECAST.  GIVEN
THE INCREASING FORECAST UNCERTAINTY AND ERRATIC NATURE OF RECENT
OPERATIONAL RUNS, FORECAST PREFERENCE GOES FROM AN EVEN MODEL/MEAN
BLEND DAY 6 SUN TO THE ENSEMBLE MEANS EXCLUSIVELY BY DAY 7 MON.


...WEATHER HIGHLIGHTS...

SOME ENHANCED PRECIP MAY PERSIST INTO LATE WEEK OVER THE SOUTHERN
HALF OF THE ROCKIES/SOUTHERN PLAINS AS SOUTHWEST/GREAT BASIN
ENERGY ALOFT EJECTS NORTHEASTWARD AND HIGH PRESSURE OVER THE
CENTRAL HIGH PLAINS PROMOTES LOW LEVEL UPSLOPE FLOW AT SOME
LOCATIONS.  MOIST FLOW NEAR AHEAD OF A SLOW MOVING FRONT MAY BRING
PERIODS OF MODERATE-HEAVY RAINFALL TO THE FLORIDA PENINSULA.
MEANWHILE A WEAKENING WAVE WILL LIKELY BRING A PERIOD OF
LIGHT-MODERATE PRECIP TO THE GREAT LAKES/NORTHEAST AND POSSIBLY
NORTHERN MID ATLANTIC THU-SAT.  THE NORTHWEST WILL SEE SOME MODEST
ACTIVITY WITH THE UPPER TROUGH REACHING THE WESTERN STATES BY THE
WEEKEND.  THIS TROUGH AND EJECTING SOUTHWEST ENERGY AHEAD OF IT
SHOULD BRING SOME RAINFALL INTO THE PLAINS BY THE WEEKEND BUT
DECREASING PREDICTABILITY IN SPECIFICS OF FLOW ALOFT REDUCES
CONFIDENCE IN COVERAGE/INTENSITY/TIMING.  LIKEWISE CONFIDENCE IS
CURRENTLY BELOW AVERAGE REGARDING HOW MUCH MOISTURE REACHES THE
NORTHWEST SUN-MON.  THE MOST EXTREME TEMPS SHOULD BE FOR BELOW
NORMAL HIGHS WITH CLOUDS/RAIN OVER THE SOUTHERN HALF OF THE HIGH
PLAINS DURING THE FIRST HALF OF THE PERIOD.  THE NORTHWEST WILL BE
QUITE WARM ON THU BEFORE MODERATING TO NEAR NORMAL LEVELS
THEREAFTER.

RAUSCH

$$





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