Prognostic Meteorological Discussion
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FXUS02 KWBC 270543
PMDEPD

EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
142 AM EDT THU APR 27 2017

VALID 12Z SUN APR 30 2017 - 12Z THU MAY 04 2017

...LATE SEASON WINTER STORM EXPECTED TO BRING SNOW TO THE
SOUTHERN/CENTRAL ROCKIES AND PORTIONS OF THE ADJACENT HIGH PLAINS
THIS WEEKEND...

...THREAT FOR HEAVY AND EXCESSIVE RAINFALL OVER THE MID TO LOWER
MISSISSIPPI VALLEY THIS WEEKEND...

...PATTERN OVERVIEW AND GUIDANCE EVALUATION...

MODEL GUIDANCE REMAINED IN GOOD AGREEMENT WITH THE LARGE SCALE
FLOW PATTERN OVER THE LOWER 48 THIS WEEKEND AND INTO THE MIDDLE OF
OF NEXT WEEK WHICH WILL FEATURE A MEAN CENTRAL U.S. TROUGH
SANDWICHED BETWEEN UPPER RIDGING OVER THE EASTERN PACIFIC AND
WESTERN ATLANTIC.  A CLOSED UPPER LOW OVER THE CENTRAL PLAINS SUN
IS FORECAST TO MOVE NORTHEAST INTO THE UPPER MS VALLEY MONDAY 01
MAY AND ACROSS THE UPPER GREAT LAKES AND INTO CANADA BY TUE
MORNING 02 MAY.

AS A HIGH AMPLITUDE EAST PACIFIC RIDGE APPROACHES THE WEST COAST
TUE AND MOVES ONSHORE ON WED 03 MAY...DOWNSTREAM TROUGH
DEVELOPMENT OCCURS IN THE PLAINS STATES WED AND INTO THE MS VALLEY
THU. SURFACE LOW DEVELOPMENT OCCURS IN THE MS VALLEY AND MOVES
INTO THE OH VALLEY THU 04 MAY.

THE 18Z GFS/12Z ECMWF AND THEIR ENSEMBLE MEANS ARE IN GOOD
AGREEMENT WITH THE CENTRAL PLAINS/UPPER MS VALLEY/UPPER GREAT
LAKES LOW...SO THE FORECAST WAS BASED ON A BLEND BETWEEN THE 12Z
ECMWF/12Z ECMWF ENSEMBLE MEAN AND 18Z GFS/GFS ENSEMBLE MEAN.   FOR
THE WED-THU SYSTEM IN THE MID MS VALLEY TO THE OH VALLEY...
A SIMILAR MODEL/ENSEMBLE BLEND WAS USED...WITH BETTER THAN AVERAGE
CLUSTERING FOR A DAYS 6-7 FORECAST.  TIMING DIFFERENCES EXIST PLUS
THE POSSIBILITY IS THERE THAT A WAVY FRONT FROM THE OH VALLEY TO
THE SOUTHEAST AND NEARBY APPALACHIANS COULD HAVE MORE THAN ONE LOW
PRESSURE CENTER.

IN PACIFIC NORTHWEST...THE GEFS MEAN LAGGED THE CANADIAN/ECMWF
MODELS AND ENSEMBLE MEANS IN MOVING THE UPPER RIDGE TO THE COAST
WED 03 MAY AND THU 04 MAY...SO THE 12Z NAEFS AND CANADIAN ENSEMBLE
MEAN/ECMWF OPERATIONAL AND ENSEMBLE MEAN SOLUTIONS WERE PREFERRED
DUE TO EXCELLENT CLUSTERING.

...WEATHER HIGHLIGHTS...

THE MODELS STILL FORECAST A HEAVY RAINFALL EVENT THIS WEEKEND
ACROSS THE MID TO LOWER MS VALLEY. SEE THE EXCESSIVE RAINFALL
OUTLOOKS AND QPF FOR DETAILS.
KANSAS AND MISSOURI.  LATE SEASON SNOWFALL IS EXPECTED OVER THE
CENTRAL/SOUTHERN ROCKIES AND INTO PORTIONS OF THE ADJACENT HIGH
PLAINS WHILE THE UPPER LOW CLOSES OFF OVER THE FOUR CORNERS REGION
EARLY SATURDAY...SO SEE THE WINTER WEATHER PROBABILITY FORECASTS.
THERE IS MORE UNCERTAINTY AS TO WHETHER IS IT COLD ENOUGH TO
SUPPORT MODERATE TO HEAVY SNOW OVER THE UPPER MISSISSIPPI VALLEY
AS A DEEPENING SURFACE LOW LIFTS THROUGH THE REGION SUNDAY NIGHT
INTO MONDAY.  PLEASE REFER TO THE STORM PREDICTION CENTER
(WWW.SPC.NCEP.NOAA.GOV) FOR INFORMATION ON THE SEVERE WEATHER
THREAT WITH THIS SYSTEM.

A HIGH AMPLITUDE UPPER RIDGE OVER THE SOUTHEASTERN U.S. COULD
BRING WIDESPREAD RECORD HIGH OR NEAR RECORD HIGH TEMPERATURES TO
THE SOUTHEAST AND MID-ATLANTIC STATES THIS WEEKEND...BUT THEN
TEMPERATURES SHOULD DECLINE TO NEAR NORMAL AFTER A COLD FRONT
CROSSES THE REGION EARLY NEXT WEEK.  WELL ABOVE NORMAL
TEMPERATURES ARE FORECAST ACROSS SOUTHERN CA INTO THE DESERT
SOUTHWEST EARLY TO MIDDLE PORTIONS OF NEXT WEEK AS WARM AIR ALOFT
PROGRESSES DOWNSTREAM FROM THE ANTICYCLONE IN THE EASTERN PACIFIC.
 THE WARM AIR BUILDS NORTH LATER IN THE WEEK ACROSS NV/UT/ID/OR AS
AN UPPER RIDGE APPROACHES THE REGION.

BELOW NORMAL TEMPERATURES PERSIST OVER THE UPPER MS VALLEY AND
UPPER GREAT LAKES AS BELOW NORMAL HEIGHTS CONTINUE.  THE INITIAL
SURGE OF COLD AIR INTO THE CENTRAL AND SOUTHERN PLAINS THIS
WEEKEND GRADUALLY MODIFIES WITH TIME TO GRADUALLY RETURN TOWARDS
NORMAL VALUES NEXT WEEK.

PETERSEN



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