Prognostic Meteorological Discussion
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FXUS02 KWBC 301444
PMDEPD

EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1043 AM EDT THU MAR 30 2017

VALID 12Z SUN APR 02 2017 - 12Z THU APR 06 2017

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

INTO NEXT WEEK GUIDANCE CONTINUES THE ESTABLISHED PATTERN OF SPLIT
FLOW WITH PROGRESSIVE SRN CANADA/NRN TIER CONUS FEATURES AND
PERIODIC WRN U.S. TROUGH AMPLIFICATION/EMBEDDED CLOSED LOWS
SUPPORTING A SERIES OF STRONG SYSTEMS ACROSS THE LOWER 48.  AN
ADVERTISED CHANGE IN THE PATTERN TOWARD A LARGER SCALE AND
HOPEFULLY MORE PREDICTABLE REGIME LATE IN THE PERIOD APPEARS TO BE
ON TRACK, WITH A DEEPENING TROUGH BECOMING ESTABLISHED OVER THE
NERN PACIFIC WHILE DOWNSTREAM RIDGING BUILDS OVER THE WEST AND
MEAN TROUGHING HEADS INTO THE EAST.

FOR THE MOST PART THE PAST 24-36 HRS OF MODEL/ENSEMBLE RUNS HAVE
BEEN IN THE PROCESS OF SETTLING INTO BETTER AGREEMENT FOR THE TWO
PRIMARY SYSTEMS OF INTEREST.  THE FIRST WILL EMERGE FROM THE SRN
PLAINS ON SUN AND AFFECT THE EAST INTO THE FIRST HALF OF NEXT WEEK
WITH A SFC LOW TRACK LIKELY TO BE FARTHER S THAN ITS PREDECESSOR
LATE THIS WEEK.  THE SECOND WILL BE SUPPORTED BY SHRTWV ENERGY
ENTERING THE WEST SUN-SUN NIGHT AND SHOULD REACH THE EAST BY DAY 7
THU.  SFC LOW TRACK SHOULD BE FARTHER N THAN THE FIRST.  THE MOST
SIGNIFICANT DIFFS THAT PERSIST INVOLVE SYSTEM TIMING AND PSBL NRN
STREAM INTERACTION.  GIVEN THE RELATIVE AGREEMENT IN PRINCIPLE,
THE UPDATED FCST BASED ON DATA THROUGH 18Z CONTINUED TO REFLECT A
CONSENSUS AMONG GFS/ECMWF RUNS AND THEIR MEANS ALONG WITH SOME
NAEFS MEAN INCLUDED LATE.  OPERATIONAL RUNS DOMINATED THE BLEND
EARLY-MID PERIOD WHILE THE MEANS TOOK OVER MAJORITY WEIGHT WITH
TYPICALLY DECLINING CONFIDENCE IN MODEL SPECIFICS TOWARD DAYS 6-7
WED-THU.

FOR THE LEADING SYSTEM TRACKING OUT OF THE SRN PLAINS, TWO MAIN
CAMPS OF LOW TRACKS EXIST, WITH THE 00Z GFS AND 00Z GEFS MEMBERS
CONSIDERABLY FASTER AND FARTHER EAST COMPARED TO THE 00Z ECMWF AND
ECMWF ENSEMBLE MEMBERS, WITH DIFFERENCES GENERALLY RELATED TO
HANDLING OF THE NORTHERN STREAM FLOW. THE GFS IS MORE AMPLIFIED
THAN THE ECMWF, THUS HELPING TO EJECT THE SOUTHERN STREAM LOW. THE
LARGE SEPARATION OF STREAMS LEADING UP TO THE LOW FORMING
GENERALLY SUPPORTS LEANING IN THE ECMWF DIRECTION, WHICH IS ALSO
SUPPORTED BY ITS HIGHER SPATIAL RESOLUTION, WITH THE FINAL
PLACEMENT ABOUT 70 PERCENT TOWARD THE ECMWF.

MEANWHILE MODELS ARE CONVERGING DECENTLY WITH THE AMPLIFYING WRN
TROUGH THAT SHOULD REACH NEAR THE ROCKIES BY EARLY TUE.  FOR THE
PAST DAY BOTH GFS AND ECMWF RUNS HAVE BEEN SUGGESTING AN EMBEDDED
CLOSED LOW MAY FORM FOR A TIME OVER THE ROCKIES/PLAINS.  BEYOND
THE PLAINS THE SPECIFICS OF THE UPR LOW/OVERALL TROUGH WILL ONCE
AGAIN BE SENSITIVE TO NRN STREAM INVOLVEMENT. GIVEN THAT THIS LOW
IS PROJECTED TO FORM FARTHER NORTH AND CLOSER TO THE NORTHERN
STREAM, THERE APPEARS A HIGHER CHANCE THAT THE LOW WILL EJECT MORE
QUICKLY THAN THE PREVIOUS LOW. HOWEVER, THE MODEL SPREAD REMAINS
VERY HIGH. THUS, HAVE ADJUSTED THE OVERNIGHT LOW POSITION AND
DEPTH ABOUT 35% TOWARD THE 06Z GFS FOR THE DAY 7 POSITION, WITH
THE OTHER 65% FROM AN OVERNIGHT MODEL CONSENSUS.

THE MAIN CHALLENGE ALONG THE WEST COAST LATE IN THE PERIOD WILL BE
WITH SHRTWV IMPULSES STREAMING AHEAD OF THE DEEPENING NERN PAC
TROUGH.  SCALE AND TIME FRAME OF THESE FEATURES LEAD TO LOW
CONFIDENCE IN THE SPECIFICS AT THIS TIME.  TELECONNECTIONS
RELATIVE TO THE TROUGH SUPPORT THE MEAN RIDGE BUILDING INTO THE
WRN PART OF THE CONTINENT AND A TENDENCY TOWARD MORE PERSISTENT
PRECIP FROM THE PAC NW INTO NRN CA.

...SENSIBLE WEATHER HIGHLIGHTS AND POTENTIAL HAZARDS...

THE SYSTEM TRACKING NEWD FROM THE SRN PLAINS SUN ONWARD WILL
SPREAD A BROAD AREA OF PRECIP, MOSTLY RAIN, AND OVER SOME REGIONS
POTENTIALLY HAZARDOUS WEATHER ACROSS THE ERN HALF OF THE COUNTRY.
HEAVIEST RAIN IS LIKELY TO FALL OVER THE GULF COAST STATES WHERE A
PERIOD OF LOW LVL GULF INFLOW MAY ENHANCE AMOUNTS AND FLASH
FLOODING THREATS MAY NEED TO BE MONITORED AS THE EVENT DRAWS
CLOSER.  THE REMAINDER OF THE EAST MAY SEE AREAS OF MDT-HVY RAIN
AS WELL.  SPC IS ALSO MONITORING THE THREAT FOR SEVERE WEATHER
ACROSS THE SRN TIER.  EXPECT THE NEXT SYSTEM TO SPREAD AREAS OF
RAIN AND HIGH ELEV SNOW FROM THE PAC NW INTO THE NRN-CNTRL ROCKIES
EARLY IN THE WEEK.  DEPENDING ON IF/WHERE AN UPR LOW CLOSES OFF
THERE MAY BE AN AXIS OF HEAVIER PRECIP FROM THE NRN OR CNTRL HIGH
PLAINS EWD.  EXTENT OF ANY WINTRY WEATHER E OF THE ROCKIES WILL BE
SENSITIVE TO HOW STRONGLY THE STORM DEVELOPS.  THE FARTHER NWD
TRACK OF THE OVERALL SYSTEM AND LESS PRONOUNCED GULF INFLOW
RELATIVE TO THE LEADING SYSTEM SHOULD BRING LIGHTER RNFL OVER SRN
LATITUDES.  EXPECT THE NRN-CNTRL WEST COAST TO SEE INCREASING
PRECIP COVERAGE/INTENSITY WED-THU.

TEMPS WILL VARY WITH SYSTEM PROGRESSION.  FOR THE OVERALL PERIOD
LOCATIONS FROM CNTRL-SRN TX INTO THE SOUTHEAST AS WELL AS THE NRN
PLAINS AND VICINITY SHOULD SEE WARMEST TEMPS RELATIVE TO NORMAL.
THE UPR RIDGE BUILDING INTO THE WEST LATE IN THE PERIOD SHOULD
LEAD TO A SIGNIFICANT WARMING TREND OVER THAT REGION BY WED-THU.
ON THE OTHER HAND THE SRN HALF OF THE ROCKIES/HIGH PLAINS WILL SEE
BELOW NORMAL TEMPS EARLY IN THE PERIOD WITH THE FIRST SYSTEM.  THE
NEXT SYSTEM WILL LIKELY BRING A DAY OR TWO OF COOL TEMPS ACROSS
THE WEST AND THEN INTO THE CNTRL PLAINS.

RAUSCH/JAMES

$$




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