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Extended Forecast Discussion
 
(Latest Discussion - Issued 1613Z Sep 19, 2017)
 
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Abbreviations and acronyms used in this product
 
Geographic Boundaries -  Map 1: Color  Black/White       Map 2: Color  Black/White


EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1213 PM EDT TUE SEP 19 2017

VALID 12Z FRI SEP 22 2017 - 12Z TUE SEP 26 2017

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

THE SYNOPTIC-SCALE PATTERN IS NOT EXPECTED TO CHANGE APPRECIABLY
DURING THE PERIOD AS AN ANOMALOUS UPPER TROUGH AFFECTS THE WESTERN
HALF OF THE COUNTRY WHILE POSITIVE HEIGHT ANOMALIES CENTER OVER
THE EASTERN U.S. THE 00Z GFS 500-MB HEIGHT FORECAST INDICATES
DEPARTURES FROM NORMAL ON THE ORDER OF 2.5 TO 3 STANDARD
DEVIATIONS BELOW CLIMATOLOGY ACROSS THE SOUTHWESTERN U.S. THIS
WEEKEND. EQUALLY IMPRESSIVE...A POWERFUL 588-DM RIDGE IS EXPECTED
TO SITUATE OVER THE GREAT LAKES/OH VALLEY AND INTO THE
MID-ATLANTIC WHICH WILL ULTIMATELY AFFECT THE PATH OF WHAT IS
CURRENTLY HURRICANE JOSE. THE EASTWARD EXTENSION OF THIS RIDGE
AXIS WILL THWART AN ESCAPE OUT INTO THE OPEN ATLANTIC. BASED ON
THE LATEST NHC FORECAST...THE CIRCULATION SHOULD BECOME
POST-TROPICAL WHICH IS EXPECTED TO LINGER IN THE OFFSHORE WATERS
OF THE MID-ATLANTIC POSSIBLY INTO EARLY NEXT WEEK. OF COURSE THIS
WILL ALSO HAVE RAMIFICATIONS ON THE TRACK OF HURRICANE MARIA GIVEN
JOSE PROVIDES SUFFICIENT WEAKNESS IN THE MID-LEVEL RIDGE TO
SUPPORT A POSSIBLE NORTHEASTWARD TURN AWAY FROM THE EASTERN
SEABOARD. FOR THE LATEST INFORMATION ON THESE TROPICAL
CYCLONES...PLEASE CHECK OUT THE NHC WEBSITE AT WWW.NHC.NOAA.GOV.

THROUGHOUT THE PERIOD...THE LONGWAVE TROUGH ACROSS THE WESTERN
U.S. WILL SLOWLY MOVE EASTWARD WITH A GENERAL WESTWARD TREND NOTED
IN THE PAST SEVERAL ENSEMBLE SPAGHETTI PLOTS. THIS IS PARTICULARLY
THE CASE WITH THE 00Z GEFS MEMBERS WHICH EVENTUALLY FAVORS
PINCHING OFF A CLOSED LOW SOMEWHERE IN THE VICINITY OF NORTHERN
BAJA CA BY EARLY NEXT WEEK. THIS IS ALSO SUPPORTED BY OTHER MODEL
SOLUTIONS AS A BROAD UPPER RIDGE BEGINS TO EXPAND OVER THE FAR
EASTERN PACIFIC. MEANWHILE...THE PRIMARY BAND OF HEIGHT FALLS
FINALLY EJECT EASTWARD TOWARD THE GREAT PLAINS WITH A MULTITUDE OF
SURFACE WAVES TRAVERSING NORTH-NORTHEASTWARD ALONG THE SLOW-MOVING
FRONTAL ZONE. TIMING DIFFERENCES ARE OF COURSE PREVALENT DEEPER
INTO THE FORECAST BUT SOLUTIONS ARE GENERALLY ALIGNED REASONABLY
WELL WITH ONE ANOTHER.

DURING THE FORECAST...TOOK AN APPROACH OF SPLITTING THE DIFFERENCE
BETWEEN THE 06Z/00Z GFS AND 00Z ECMWF SOLUTIONS WITH MORE LIMITED
USE OF THE 06Z GEFS MEAN AND 00Z ECMWF/NAEFS ENSEMBLE MEANS
THROUGH DAY 4/SATURDAY. BEYOND THIS PERIOD...STARTED TO DECREASE
OPERATIONAL INFLUENCES GIVEN GROWING UNCERTAINTY IN THE STATE OF
JOSE AS WELL DETAILS OF THE EVOLVING WESTERN U.S. TROUGH. BY DAY
7/SEPTEMBER 26...FAVORED ALMOST PURE USE OF THE MENTIONED ENSEMBLE
MEANS WITH 20 PERCENT OPERATIONAL GUIDANCE...06Z/00Z GFS AND 00Z
ECMWF TO GAIN SOME MODEST DETAILS.


...SENSIBLE WEATHER/HIGHLIGHTS...

ACROSS THE WESTERN U.S...WELL BELOW NORMAL TEMPERATURES WILL
PREVAIL GIVEN THE PERSISTENCE OF UPPER TROUGHING OVER THE REGION.
THROUGH THIS WEEKEND...CURRENT FORECAST DEPARTURES SHOULD BE ON
THE ORDER OF 20 TO 25 DEGREES BELOW CLIMATOLOGY ACROSS SECTIONS OF
THE UPPER INTERMOUNTAIN WEST/NORTHERN ROCKIES. THIS WILL SUPPORT
WIDESPREAD HIGH TEMPERATURES IN THE UPPER 40S TO LOWER 50S WITH
EVEN A POCKET OF MID 30S LIKELY ACROSS WESTERN WY. IN
ADDITION...THERE WILL BE A THREAT FOR SOME HIGHER ELEVATIONS SNOWS
ACROSS THE SECTIONS OF EASTERN ID/SOUTHWESTERN MT/WESTERN WY GIVEN
ACTIVE PRECIPITATION OVER THE REGION. FARTHER EAST...CONTINUED
RIDGING CENTERED OVER THE GREAT LAKES WILL KEEP CONDITIONS QUITE
MILD AS HIGHS SIT IN THE MID/UPPER 80S AS FAR NORTH AS WI/MI THIS
WEEKEND. SUCH FORECAST HIGH TEMPERATURES RATE ABOUT 15 TO 20
DEGREES ABOVE AVERAGE. SOME RECORD WARM MINIMA ARE POSSIBLE ON
SATURDAY MORNING WITH LOW 70S BEING FORECAST ACROSS THE UPPER
MIDWEST.

HEAVY RAINFALL IS LIKELY FROM THE CENTRAL PLAINS INTO THE UPPER
MIDWEST GIVEN FAVORABLE SOUTH-SOUTHWESTERLY FLOW ALOFT PERSISTING
THROUGH MONDAY. THERE MAY BE SOME HYDROLOGIC ISSUES WITH THIS SET
UP AS MULTIPLE DAYS OF PRECIPITATION ARE LIKELY TO AFFECT THIS
REGION. BY EARLY NEXT WEEK...THE UPPER TROUGH PROGRESSING EASTWARD
SHOULD HELP ADVANCE THE BAND OF RAINFALL TOWARD THE GREAT LAKES.
HOWEVER...WITH AN UPPER LOW SPINNING IN THE VICINITY OF BAJA
CA...SUFFICIENT UPPER DIFFLUENCE TOWARD THE SOUTHERN PLAINS MAY
SUPPORT ANOTHER REGION OF HEAVY RAINFALL ON MONDAY/TUESDAY.
DETAILS ARE A BIT MORE UNCERTAIN GIVEN IT IS A DAY 6/7 FORECAST.


RUBIN-OSTER