Preliminary Forecasts
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360
FXUS02 KWNH 220605
PREEPD

PRELIMINARY EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
205 AM EDT THU JUN 22 2017

VALID 12Z SUN JUN 25 2017 - 12Z THU JUN 29 2017

...PATTERN OVERVIEW/MODEL EVALUATION...

THE FCST REMAINS ON TRACK WITH AN AMPLIFIED TROUGH FORECAST TO
PERSIST DAYS 3-5 AND THEN GRADUALLY DEAMPLIFY DAYS 6-7 WED 28 JUN
THU 29 JUN.  THE ERN PACIFIC RIDGE MOVES ACROSS THE WEST AND
DEAMPLIFIES AS WELL IN RESPONSE TO HEIGHT FALLS FROM AN
APPROACHING TROUGH FROM WESTERN CANADA.
THE HEIGHT FALLS IN TURN FORCE THE CLOSED HIGH OVER THE DESERT
SOUTHWEST INTO NORTHERN MEXICO...CAUSING MODERATION IN THE EXTREME
HEAT OF LATE.

THE PRIMARY UNCERTAINTIES INVOLVE THE PATH OF A NERN PAC UPR LOW
MOVING INTO WRN CANADA/AMPLITUDE OF THE TROUGH INTO THE PACIFIC
NORTHWEST AND NORTHERN ROCKIES...AND THEN DOWNSTREAM INFLUENCES OF
THE CANADIAN PATTERN.  THE MODELS CONTINUE TO HAVE DIFFERENCES
RESOLVING INDIVIDUAL SHRTWVS MOVING THROUGH THE ERN TROUGH.

REGARDING THE ERN PAC-WRN U.S./CANADA PATTERN, TELECONNECTIONS
RELATIVE TO THE BEST DEFINED ANOMALY CENTERS OVER THE NORTHERN
PACIFIC SUPPORT A TROUGH DEVELOPING IN SOUTHWEST CANADA TO THE
PACIFIC NORTHWEST. THIS SUGGESTS A LITTLE MORE WEST COAST
TROUGHING THAN DEPICTED BY THE 12Z ECMWF MEAN IN THE MIDDLE OF
NEXT WEEK.   THE 18Z-GFS AND 18Z GEFS MEAN RUNS MOVE THE LOW
FURTHER SOUTH THAN THE ECMWF/ECMWF ENSEMBLE MEMBERS WITH THE 12Z
CANADIAN MEMBERS AND RESULTANT CANADIAN MEAN IN THE MIDDLE.
OVERALL PREFER TO STAY FAIRLY CLOSE TO CONTINUITY BY WAY OF AN AVG
AMONG THE GUIDANCE...STAYING CLOSER TO THE INTERMEDIATE CANADIAN
CLUSTER UNTIL IT BECOMES CLEARER WHICH WAY THE SITUATION WILL
BREAK.

CONFIDENCE REMAINS LOW WITH SPECIFIC DETAILS FOR THE FEATURES
EMBEDDED WITHIN THE ERN U.S. MEAN TROUGH.  THERE WILL LIKELY BE
MULTIPLE SHRTWVS REFLECTING AT THE SFC AS FRONTS OR TROUGHS, WITH
THEIR SCALE AND HIGH LATITUDE ORIGIN LEADING TO LOW
PREDICTABILITY.  DURING THE FIRST HALF OF THE PERIOD A MODEL BLEND
INCORPORATE THE MOST AGREEABLE IDEAS OF GUIDANCE WHILE DOWNPLAYING
THE MORE UNCERTAIN SMALLER SCALE DETAILS.  THEN INDIVIDUAL RUNS
DIVERGE AMONG EACH OTHER AND FROM RUN TO RUN.

CONSEQUENTLY THE FCST STARTED WITH A BLEND OF 18Z GFS/12Z
ECMWF/12Z CANADIAN GLOBAL/18Z GEFS MEAN/12Z ECMWF ENSEMBLE MEAN
DAYS 3-5.  ON DAYS 6-7...GREATER WEIGHTING WAS GIVEN TO THE MEANS
AND 12Z ECMWF/12Z CANADIAN GLOBAL WHILE THE 18Z GFS WAS
DISCONTINUED...AS THE GFS WAS AN OUTLIER ON THE DEPTH OF THE
PACIFIC NORTHWEST TROUGH AND MID ATLANTIC TROUGH NEXT TUE-WED 28
JUN.

...WEATHER HIGHLIGHTS...

A WAVY FRONT STALLING NEAR THE GULF COAST AND NORTHERN FLORIDA IS
EXPECT TO PROVIDE A OF FOCUS FOR SHWRS/TSTMS EARLY NEXT WEEK.
MOIST FLOW FROM THE CARIBBEAN/GULF OF MEXICO COULD COMBINE WITH
COASTAL CONVERGENCE TO PRODUCE SHOWERS ALONG THE WRN GULF COAST.

LOW LVL UPSLOPE FLOW AND MOISTURE RETURN MAY ENHANCE CONVECTIVE
DEVELOPMENT ONE OR MORE DAYS OVER PARTS OF THE HIGH PLAINS/ERN
SLOPES OF THE ROCKIES.  ONCE THE FRONT MOVES OUT ON THE PLAINS DAY
7...LOW LEVEL CONVERGENCE AND POOLING OF INCREASING MOISTURE
SUPPORTS SHOWERS/STORMS BREAKING OUT THU 29 JUN FROM THE CENTRAL
PLAINS TO THE UPPER MS VALLEY.
ONE OR MORE IMPULSES ALOFT WITHIN THE ERN U.S. MEAN TROUGH ALONG
WITH ASSOC SFC FEATURES MAY GENERATE EPISODES OF CONVECTION FROM
THE UPPER MS VALLEY THROUGH THE GREAT LAKES AND NORTHEAST.

TEMPERATURE ANOMALIES ACROSS THE CONUS START WITH TEMPS 10-20
DEGREES ABOVE NORMAL IN INTERIOR WA/OR/ID/NORTHEAST CA/NORTHERN NV
ON DAYS 3-4.  THE ANOMALIES SHOULD MODERATE MON-WED
AS THE RIDGE GRADUALLY DEAMPLIFIES WITH TIME...WITH MOST AREAS OF
THE GREAT BASIN TO THE SOUTHWEST 5 TO 10 DEGREES ABOVE NORMAL BY
THU 20 JUN.  CONVERSELY...HIGHS FORECAST TO BE 10-20F BELOW NORMAL
OVER THE PLAINS SAT INTO MON BUT DEPARTURES FROM NORMAL MODERATE
AS THE TROUGH DEPARTS AND THE WESTERN RIDGE MOVES INTO THE
REGION...SO THE AREA WILL ACTUALLY FLIP TO ABOVE NORMAL VALUES
NEXT WED-THU.

TEMPERATURES ARE FORECAST TO BE BELOW NORMAL DAYS 3-5 IN THE UPPER
MS VALLEY/GREAT LAKES/NORTHEAST AND THEN MODERATE TO CLOSER TO
NORMAL BY NEXT THU 29 JUN.

PETERSEN

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