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Extended Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 0659Z Aug 16, 2014)
 
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EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
259 AM EDT SAT AUG 16 2014

VALID 12Z TUE AUG 19 2014 - 12Z SAT AUG 23 2014

...OVERVIEW...
UPPER-LEVEL JET INTENSIFICATION ACROSS THE NORTHERN PACIFIC LEADS
TO THE TRANSITION FROM HIGH-AMPLITUDE TROUGH OVER EASTERN CANADA
AND THE NORTHEAST US--TO A MORE ZONAL AND FLATTER LOW
AMPLITUDE/PROGRESSIVE PATTERN FOR THE THIS MEDIUM RANGE PERIOD.

IN THE WEST AND NORTH CENTRAL US...THE PATTERN WILL BE DOMINATED
BY A PROGRESSIVE LONGWAVE TROUGH TAKING SHAPE AS THE UPPER-LEVEL
JET BEGINS TO AMPLIFY AND CARVE A LONGWAVE TROUGH ACROSS MUCH OF
WESTERN CANADA AND THE NORTHWEST QUARTER OF THE LOWER 48 BEYOND
DAY 5.

...MODEL PREFERENCES/UNCERTAINTY ASSESSMENT...
AFTER SIFTING THROUGH THE POSSIBILITIES AND THE REALIZATION THAT A
BLOCKY PATTERN DOWNSTREAM OVER ATLANTIC CANADA AND GREENLAND WILL
CREATE A TEMPORARY SPLIT-FLOW PATTERN ACROSS THE LOWER 48 THIS
FORECAST PERIOD...THOUGHT THE 15/12Z ECENS MEAN HAS BEEN
DECIDEDLY--THE MORE CONSISTENT PIECES OF GUIDANCE. FOLLOWING
THIS--ALONG WITH THE 15/12Z NAEFS--SEEMS TO BEST FIT THE
DEVELOPING LOW-AMPLITUDE PATTERN AND WPC SFC/500MB GRAPHICS FROM
WEST TO EAST THROUGH THE ENTIRE DAY 3-7 PERIOD.

THOUGHT THIS FORECAST PACKAGE DISPLAYED A VERY SUBTLE AND
INTRIGUING PATTERN SHIFT FOR THE LOWER 48. BEST WAY TO DESCRIBE
THE CHANGES--THE PERSISTENT COOL PATTERN IN THE NORTHEASTERN US
BECOMES CLOUDIER WITH HIGHER TEMPERATURES AND HUMIDITY. AND THE
RELATIVELY DRY NORTHWEST STATES BECOME COOLER AND WETTER. A
LOCALLY-ACTIVE MONSOON FLOW FOR PARTS OF THE SOUTHWEST (AZ NM CO)
AND IN THE MID-SECTION OF THE COUNTRY...A PACIFIC COLD FRONT AND
APPRECIABLY MOIST AND COOLER WEATHER PATTERN MIGRATES FROM THE
NORTHERN DIVIDE EASTWARD INTO THE UPPER HALF OF THE MISSOURI RIVER
VALLEY FOR DAYS 6-7.

DOWNSTREAM OF THE ROCKIES...WHAT SIGNIFICANT DIFFERENCES EMERGING
FROM THIS PATTERN TRANSITION DURING THE MEDIUM RANGE
PERIOD...APPEARED TO FOLLOW THE INTENSITY AND LATITUDE OF THE
DEVELOPING SPLIT-FLOW PATTERN AND 'WEAKER' SOUTHERN BRANCH OF THE
JET. THE UNCERTAINTY AND EVENTUAL OUTCOME WILL DICTATE THE
DEPTH...SPEED AND DOWNSTREAM TRAJECTORIES OF THE INDIVIDUAL
LOW-AMPLITUDE WAVES MIGRATING ACROSS THE LOWER 48. WHAT SEEMS
INTRIGUING TO ME IS HOW MUCH RUN-TO-RUN VARIABILITY EXISTS AND
WHERE THIS NARROW 50KT-70KT 'SOUTHERN SPLIT' JET AXIS EXITS THE
CONTINENTAL DIVIDE. THIS LEADS TO A VARIETY OF PERPLEXING
RIDGE/TROUGH SOLUTIONS (LOW-AMPLITUDE WAVES MIND
YOU)...ORIGINATING ANYWHERE FROM NORTH CENTRAL WYOMING TO SOUTH
CENTRAL COLORADO. CONSEQUENTLY THIS LEADS TO A RATHER BROAD RANGE
OF DAY 4-5 AND DAY 5-6 QPF POSSIBILITIES ALONG AND EAST OF 98W
LONGITUDE BETWEEN SOUTHEAST DAKOTA AND NORTHEAST KANSAS STARTING
AROUND THE DAY 5 MID-POINT (21/12Z).

IN THE WEST...CAN LIVE WITH A BLEND OF THE 15/12Z DETERMINISTIC
CANADIAN...GFS AND ECMWF HEADING INTO DAY 4--20/12Z--WITH A
MID-LEVEL CYCLONIC CIRCULATION DRIFTING OFF THE CENTRAL CALIFORNIA
COAST BETWEEN CAPE MENDOCINO...PT REYES AND THE BAY AREA. THEN
TAKING A SOUTHWESTWARD TRACK AND HEADING WELL OUT TO SEA--WEST OF
POINT CONCEPCION. AFTER DAY 4 AND INTO DAY 5...THAT LOOKED
REASONABLE. WHAT DOESNT FIT THE ENSEMBLE MEANS--HAS TO BE THE
15/00Z AND 16/00Z DETERMINISTIC GFS RUNS AND THE 16/00Z CANADIAN.
THE CONSISTENT AND 'TRACK-ABLE' SYNOPTIC-SCALE FEATURE FOR DAYS
5-7 CONTINUES TO BE THE SIGNIFICANT HEIGHT FALL MIGRATION FROM THE
BC/WASHINGTON COAST (THE AMPLIFYING UPPER-LEVEL LONGWAVE TROUGH)
TO THE HIGH PLAINS OF EASTERN MONTANA AND WYOMING. THESE 'OUTLIER'
GFS/CANADIAN RUNS LEAVE ME WITH LITTLE CHOICE BUT TO RECOMMEND A
GENERAL COMPROMISE APPROACH USING THE 15/12Z NAEFS-ECENS AND IF
ITS TO YOUR LIKING...THE 18Z GEFS MEAN BLENDED EQUALLY TO COVER
THE DIFFERENCES ALONG THE WEST COAST.

...SENSIBLE WEATHER HIGHLIGHTS...
WET AND STORMY WEATHER WILL ACCOMPANY THE SURFACE WAVE TRACKING
THROUGH THE OHIO VALLEY/MID-ATLANTIC STATES EARLY NEXT WEEK...WITH
PERIODS OF HEAVY RAINS AND STRONG THUNDERSTORMS POSSIBLE.  SHOWERS
AND THUNDERSTORMS ARE ALSO EXPECTED WITH THE SHORTWAVE DROPPING
INTO THE UPPER MIDWEST/GREAT LAKES AND AT THE LEADING EDGE OF
HEIGHT FALLS DIGGING INTO THE NORTHWEST...WHICH SHOULD IMPACT THE
NORTHERN PLAINS BY LATE NEXT WEEK.  

THE RELATIVELY FLAT AND PROGRESSIVE FLOW OVER THE CONUS SHOULD
LIMIT ANY LARGE TEMPERATURE EXTREMES. IN GENERAL...TEMPERATURES
WILL BE ABOVE NORMAL SURROUNDING THE CENTRAL ROCKIES/HIGH
PLAINS...BENEATH MODEST RIDGING ALOFT.  ALSO...WITH MEAN TROUGHING
SETTING UP OVER THE WESTERN U.S. LATE NEXT WEEK...HIGHS AND LOWS
SHOULD BE BELOW NORMAL. 

VOJTESAK/GERHARDT