Preliminary Forecasts
Issued by NWS

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FXUS02 KWNH 240553
PREEPD

PRELIMINARY EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
152 AM EDT MON APR 24 2017

VALID 12Z THU APR 27 2017 - 12Z MON MAY 01 2017

...LATE SEASON WINTER STORM LIKELY FOR THE FRONT RANGE OF WY/CO
AND POSSIBLE FOR THE HIGH PLAINS THIS COMING WEEKEND...

...HEAVY RAIN PROBABLE FOR THE MIDDLE TO LOWER MISSISSIPPI VALLEY
OVER THE WEEKEND...

...OVERVIEW AND GUIDANCE EVALUATION ...

THE UPPER PATTERN ACROSS THE LOWER 48 WILL BE DOMINATED BY A
PERSISTENT TROUGH EXTENDING FROM THE ROCKIES ACROSS THE CENTRAL
PLAINS AND MS VALLEY AND UPPER GREAT LAKES.  THE TROUGH IS FLANKED
ON BOTH SIDES IN THE EASTERN PACIFIC AND WESTERN ATLANTIC/EAST
COAST WITH HIGH AMPLITUDE RIDGES.

WITHIN THE CENTRAL US TROUGH...THE MODELS/ENSEMBLES THIS PERIOD
FOCUS ON TWO  CYCLONES:  THE FIRST MOVES ON DAY 3 THU FROM THE MS
VALLEY ACROSS THE UPPER LAKES INTO CANADA.  THE TRAILING COLD
FRONT CROSSES THE LOWER LAKES INTO THE NORTHEAST FRI.  THE
ECMWF/GFS MODELS AND THEIR ENSEMBLE MEANS HAVE SHIFTED SLIGHTLY
WEST OF PRIOR CYCLES WITH THE STORM TRACK ACROSS THE UPPER MS
VALLEY/UPPER LAKES.  GIVEN GOOD OVERALL AGREEMENT A BLEND OF THESE
FORECASTS WAS USED.

THE SECOND CYCLONE DEVELOPS LATER FRI 28 APR IN THE SOUTHERN HIGH
PLAINS AND MOVES NORTHEAST ACROSS THE SOUTHERN PLAINS TO THE MID
MS VALLEY SUN AND THEN EITHER THE GREAT LAKES OR UPPER MS VALLEY
MON BEFORE EXITING INTO CANADA. THE 18Z GFS/18Z GEFS MEAN/12Z
ECMWF/00Z SUN ECMWF CLUSTER WELL...WITH THE 12Z SUN OPERATIONAL
ECMWF FURTHER WEST WITH ITS CYCLONE TRACK. THE 00Z GFS HAS SLOWED
ITS FORWARD PROGRESSION SEVERAL HOURS...SO IT IS FURTHER SOUTH
THAN THE PREDOMINANT CLUSTER. A CONSENSUS OF THE MAJORITY CLUSTER
WAS USED FOR THE DAYS 5-7 PROGS...CLOSEST TO THE 12Z ECMWF
ENSEMBLE MEAN/18Z GEFS MEAN/18Z SUN GFS.

A CLOSED LOW OFF THE MID-ATLANTIC COAST WILL WEAKEN AS IT EXITS
INTO THE NORTHWESTERN ATLANTIC ON DAY 3/THU 27 APR. THE MODELS NOW
CLUSTER WELL WITH THIS EVOLUTION...LENDING THE SITUATION TO A
MULTI-MODEL/ENSEMBLE MEAN BLEND.


...WEATHER HIGHLIGHTS...

BOUTS OF SHOWERS AND WILL CONTINUE IN THE PACIFIC NORTHWEST
THROUGH THE REST OF THE WORK WEEK BEFORE SUBSIDING OVER THE
WEEKEND. SNOW IS EXPECTED ACROSS THE RANGES OF WA/OR AND FARTHER
INLAND ACROSS THE MOUNTAINS OF ID/UT/MT/WY/CO. BY FRI INTO THE
WEEKEND, THE TREND TOWARD A DEEPER/COLDER SYSTEM IN THE ROCKIES
SUPPORTS WIDESPREAD MODERATE TO HEAVY SNOW FROM THE WY/CO FRONT
RANGE INTO THE FOOTHILLS. THE SNOW MAY EXPAND INTO THE HIGH PLAINS
NEXT WEEKEND AS COLD AIR IS ADVECTED SOUTH DOWN THE PLAINS ON THE
WESTERN SIDE OF THE CIRCULATION.  SNOW MAY ALSO OCCUR WED INTO
EARLY THU IN THE ARROWHEAD OF MN INTO THE WESTERN UP OF MI.

RAINFALL BECOMES WIDESPREAD ACROSS THE CENTRAL PLAINS THU AND THU
NIGHT WITH A SHOT OF WARM ADVECTION AND AGAIN FRI AS THE SOUTHERN
PLAINS STORMS DEVELOPS AND MOISTURE SURGES NORTH...CONTINUING INTO
THE MID MS VALLEY AND GREAT LAKES OVER THE WEEKEND. SEE THE STORM
PREDICTION CENTER CONVECTIVE OUTLOOKS THAT ADDRESS SEVERE WEATHER
POTENTIAL. LOCALLY HEAVY RAIN IS POSSIBLE IN THE MID TO LOWER MS
VALLEY TO THE SOUTHERN PLAINS LATE IN THE WEEK THROUGH NEXT
WEEKEND AS GEFS M-CLIMATE QPF VALUES INCREASE TO THE 97 TO 99TH
PERCENTILE FROM EASTERN CO AND AGAIN FROM EASTERN OK AND
NORTHEASTERN TEXAS THROUGH MISSOURI TO CENTRAL/SOUTHERN IL.

MUCH OF THE EASTERN THIRD OF THE CONUS WILL SEE INCREASING
TEMPERATURES LATER THIS WEEK AFTER THE COASTAL LOW DEPARTS. AREAS
OF RECORD WARM TEMPERATURES ARE EXPECTED FROM FRIDAY 28 APR
THROUGH SUNDAY 30 APR FROM THE LOWER RIO GRANDE VALLEY OF TEXAS
EASTWARD THROUGH THE SOUTHEAST INTO THE MID-ATLANTIC. BELOW
AVERAGE TEMPERATURES WILL REMAIN ANCHORED NEAR THE UPPER TROUGH
AXIS ACROSS THE CENTRAL ROCKIES AND PLAINS TO THE UPPER MS VALLEY
AND UPPER GREAT LAKES. THE PACIFIC NORTHWEST SHOULD REMAIN COOLER
THAN AVERAGE THROUGH THE WEEK BEFORE A RETURN TOWARD MORE TYPICAL
TEMPERATURES. MUCH OF THE SOUTHWEST (SOUTHERN CA INTO NV/AZ)
SHOULD TREND TO ABOVE NORMAL TEMPERATURES AT THE END OF THE
WEEKEND INTO EARLY NEXT WEEK.

PETERSEN/FRACASSO


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