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Extended Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 1953Z Apr 19, 2018)
 
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Geographic Boundaries -  Map 1: Color  Black/White       Map 2: Color  Black/White


EXTENDED FORECAST DISCUSSION...AMENDED
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
353 PM EDT THU APR 19 2018

VALID 12Z SUN APR 22 2018 - 12Z THU APR 26 2018

...OVERVIEW...

THERE WILL BE TWO MAIN FEATURES OF INTEREST IN THE MEDIUM RANGE
PERIOD.  THE FIRST WILL BE THE GULF COAST LOW THAT TRACKS TOWARDS
THE EAST COAST IN CONJUNCTION WITH AN UPPER LEVEL LOW OVER THE
INTERIOR SOUTHEAST, AND THE SECOND WILL BE A CLOSED LOW
APPROACHING CALIFORNIA AND ENSUING LOW PRESSURE OVER THE
INTERMOUNTAIN WEST. 
THE SYSTEM EXPECTED TO BRING THE THREAT OF HEAVY RAINFALL TO SOME
LOCATIONS ACROSS THE SOUTHEAST AND POSSIBLY INTO THE SOUTHERN MID
ATLANTIC SUN-TUE WILL MOST LIKELY LIFT NORTHWARD ALONG THE EAST
COAST THEREAFTER IN RESPONSE TO THE APPROACH OF SHORTWAVE ENERGY
THAT SHOULD BE REACHING WESTERN NORTH AMERICA AROUND THE START OF
THE PERIOD.  THIS ENERGY TRAILS A LEADING VIGOROUS SHORTWAVE THAT
INITIALLY PUSHES A COLD FRONT INTO THE NORTHERN
PLAINS/NORTH-CENTRAL WESTERN STATES.  THERE ARE SOME
LOW-PREDICTABILITY ASPECTS TO THE EASTERN U.S. EVOLUTION BY THE
LATTER HALF OF THE PERIOD BUT AGREEMENT IS STILL MUCH BETTER IN
PRINCIPLE THAN FOR THE EASTERN PACIFIC/WESTERN U.S. FORECAST BY
NEXT TUE-THU, AS GUIDANCE DIVERGES IN A WIDE VARIETY OF WAYS FOR
ENERGY WITHIN A TROUGH THAT SHOULD BE NEAR 150W LONGITUDE AS OF
EARLY SUN.


...GUIDANCE/PREDICTABILITY ASSESSMENT...

THE MODELS AND ENSEMBLE MEANS ARE INDICATING GOOD SYNOPTIC SCALE
AGREEMENT THROUGH MONDAY BOTH SURFACE AND ALOFT, SO A
DETERMINISTIC MODEL BLEND WAS INCORPORATED INTO THE FORECAST
PROCESS FOR THAT TIME.  BY TUESDAY, THE 6Z GFS AND 00Z CMC ARE
FASTER WITH THE UPPER TROUGH AND LOW OVER THE EASTERN PACIFIC
COMPARED TO THE SLOWER 00Z ECMWF AND UKMET, SO A POSITION CLOSE TO
THE EC MEAN WAS USED FOR THIS FORECAST.  THE CMC DEVELOPS MORE
UPPER LEVEL RIDGING OVER SOUTHERN CANADA BY THE END OF THE
FORECAST COMPARED TO THE REMAINING GUIDANCE, SO IT WAS NOT USED
AFTER DAY 6.   REGARDING THE EAST COAST SYSTEM NEXT WEEK, THE 6Z
GFS BECOMES MUCH FASTER WITH THE SOUTHEASTWARD PROGRESSION OF THE
NORTHERN STREAM TROUGH COMPARED TO BOTH ITS 00Z RUN AND THE MODEL
CONSENSUS BY WEDNESDAY MORNING, SO THE 00Z RUN WAS SUBSTITUTED IN
THE FORECAST BLEND FOR DAYS 6 AND 7 IN LIEU OF THE 6Z RUN.  THERE
HAS ALSO BEEN POOR MODEL CONTINUITY OVER THE PAST 36 HOURS WITH
BOTH THE ECMWF AND GFS CONCERNING THE ORIENTATION OF THE TROUGH
AXIS, SO THE GEFS AND EC MEAN PROVIDE A BETTER IDEA ON THE
LOCATION OF THE LOW AND THIS SUGGESTS A SURFACE LOW REACHING
SOUTHERN NEW ENGLAND BY THURSDAY MORNING, PERHAPS AS A NOR'EASTER.


...WEATHER/THREATS HIGHLIGHTS...

THE SYSTEM TRACKING ALONG THE SOUTHERN TIER/GULF COAST EARLY IN
THE WEEK WILL BRING AN EPISODE OF HEAVY RAINFALL TO LOCATIONS FROM
THE LOWER MS VALLEY INTO THE SOUTHEAST DURING THE LATE
WEEKEND/EARLY NEXT WEEK TIME FRAME.  THEN THE MOISTURE SHIELD WILL
LIKELY SPREAD NORTHWARD WITH TIME AS LOW PRESSURE BEGINS TRACKING
ALONG/NEAR THE EAST COAST.  RAINFALL AMOUNTS OVER THE
CENTRAL/NORTHERN PARTS OF THE EAST SHOULD TEND TO BE LOWER THAN
OVER THE SOUTH, BUT THERE ARE POSSIBLE SYSTEM EVOLUTION AT THE
SURFACE AND ALOFT THAT COULD LEAD TO SOME AREAS OF ENHANCED
RAINFALL.  THERE MAY ALSO BE A PERIOD OF STRONG WINDS ALONG THE
EAST COAST GIVEN THE GRADIENT BETWEEN THE SURFACE LOW AND STRONG
RIDGE TO THE NORTH.

THE COMBINATION OF SHORTWAVE ENERGY ALOFT AND WAVY FRONT PUSHING
SOUTHEAST FROM THE NORTHERN PLAINS/NORTH-CENTRAL WEST WILL BRING
AREAS OF PRECIP TO PORTIONS OF THE ROCKIES/PLAINS AND POSSIBLY
INTO THE MIDWEST/GREAT LAKES BEFORE POSSIBLY MERGING WITH THE
EASTERN SYSTEM.  THE NORTH-CENTRAL ROCKIES/HIGH PLAINS APPEAR TO
HAVE BEST POTENTIAL FOR HIGHEST AMOUNTS WITHIN THIS AREA OF
MOISTURE.  SOME SNOW IS POSSIBLE OVER HIGH ELEVATIONS.

UNCERTAINTY WITH THE LARGE SCALE EVOLUTION BY THE LATTER HALF OF
THE PERIOD LOWERS CONFIDENCE IN COVERAGE/INTENSITY OF ANY PRECIP
THAT MAY OCCUR OVER THE WEST BY MIDWEEK.  THE PREFERRED ENSEMBLE
MEAN SCENARIO ALOFT WOULD AT LEAST SUPPORT SOME LIGHT/SCATTERED
ACTIVITY.

ASIDE FROM SOME NEAR TO SLIGHTLY BELOW NORMAL READINGS OVER THE
NORTHWEST EARLY IN THE PERIOD, EXPECT MUCH OF THE WESTERN U.S. TO
SEE ABOVE NORMAL TEMPS DURING THE PERIOD WITH ANOMALIES REACHING
PLUS 10-15F AT SOME LOCATIONS.  ISOLATED DAILY RECORDS FOR
HIGHS/WARM MINIMA MAY BE POSSIBLE.  CLOUDS/PRECIP WILL KEEP HIGHS
WELL BELOW NORMAL OVER THE SOUTH AND THEN EAST AND THE FRONT
PROGRESSING SOUTHEAST FROM THE NORTHERN PLAINS WILL BRING
REINFORCING COOL AIR TO THE CENTRAL/EAST BY TUE-THU.


ASSOCIATED WPC MEDIUM RANGE FORECASTS OF 500 MB HEIGHTS...SURFACE
SYSTEMS...WEATHER GRIDS...QUANTITATIVE PRECIPITATION FORECASTS
(QPF)...AND WINTER WEATHER OUTLOOK SNOW/SLEET PROBABILITIES CAN BE
FOUND AT...

http://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif
http://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif
http://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html
http://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml
http://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4

HAMRICK/RAUSCH