Preliminary Forecasts
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FXUS02 KWNH 230647
PREEPD

PRELIMINARY EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
247 AM EDT TUE MAY 23 2017

VALID 12Z FRI MAY 26 2017 - 12Z TUE MAY 30 2017

PATTERN OVERVIEW AND MODEL PREFERENCES
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

A VIGOROUS UPPER LEVEL TROUGH ALONG WITH A CLOSED LOW OVER
SOUTHERN CANADA AT THE BEGINNING OF THE PERIOD SHOULD GRADUALLY
BECOME BROADER AND MORE DIFFUSE AS IT DRIFTS EASTWARD WHILE THE
AXIS OF AN UPSTREAM RIDGE APPROACHES THE WEST COAST, AND THE RIDGE
REMAINS IN PLACE THERE.  AT THE SAME TIME AN AMPLIFIED UPPER
TROUGH INITIALLY OVER THE NORTHEAST U.S. WILL LIKELY LIFT AWAY
FROM THE EASTERN STATES BY THE WEEKEND WITH HEIGHTS ALOFT RISING
IN ITS WAKE.  ENSEMBLE MEMBERS CONTINUE TO SHOW REASONABLE
CLUSTERING WITH AN ASSOCIATED SURFACE LOW EXPECTED TO TRACK
NORTHEASTWARD ALONG THE NEW ENGLAND COAST FRIDAY INTO SATURDAY.
BY THE END OF THE FORECAST PERIOD NEXT TUESDAY, THE OPERATIONAL
GFS AND ECMWF ARE NOW BOTH HINTING AT THE IDEA OF AN UPPER LOW
CLOSING OFF IN THE SOUTHERN STREAM BY DAY 7 JUST WEST OF THE
CALIFORNIA COAST THAT MAY TRY TO PASS UNDERNEATH THE UPPER RIDGE
TO ITS NORTH.

IN TERMS OF MODEL PERFORMANCE, THE 12Z UKMET DIFFERED
SIGNIFICANTLY FROM THE MODEL CONSENSUS REGARDING THE ORIENTATION
OF THE UPPER LOW OVER SOUTHERN CANADA FOR THE FIRST HALF OF THE
FORECAST PERIOD, AND EXTENDED THE TROUGH AXIS WESTWARD WHERE THE
OTHER MODELS ARE INDICATING UPPER LEVEL RIDGING BUILDING IN ACROSS
THE NORTHWEST.  AS A RESULT, IT IS SLOWER WITH THE PROGRESSION OF
THE SURFACE COLD FRONT ACROSS THE PLAINS AND UPPER MIDWEST.  THE
CMC HAS TRENDED MORE IN LINE WITH THE CONSENSUS COMPARED TO
PREVIOUS DAYS THROUGH DAY 5, AFTER WHICH IT BECOMES A FASTER
SOLUTION WITH THE TROUGH AXIS ACROSS THE OHIO VALLEY AND
NORTHEAST.  THE 18Z GFS CONTINUES TO BE MORE AMPLIFIED WITH THE
TROUGH BUILDING IN ACROSS THE CENTRAL U.S. EARLY NEXT WEEK AND IS
THUS SLOWER THAN THE MODEL CONSENSUS, BUT NOT TO THE DEGREE SEEN
IN THE 6Z AND 12Z GFS RUNS.  FORECAST CONFIDENCE IS HIGHEST FOR
THE BUILDING RIDGE ALONG THE WEST COAST, AND THE DEPARTING UPPER
LOW ACROSS NEW ENGLAND FOLLOWED BY RIDGING ALOFT.

THROUGH SATURDAY AN OPERATIONAL MODEL BLEND INCLUDING THE 12Z
ECMWF/CMC AND A LESSER AMOUNT OF THE 18Z GFS REFLECTED THE MOST
AGREEABLE ASPECTS OF GUIDANCE WHILE DOWNPLAYING LESS CONFIDENT
MEDIUM TO SMALLER SCALE DETAILS PRESENT IN EACH THOSE MODEL RUNS.
BY THE LATTER HALF OF THE FORECAST PERIOD, AN INCREASING
PERCENTAGE OF THE 18Z GEFS MEAN AND 12Z EC MEAN WERE INCORPORATED
TO ACCOUNT FOR THE INCREASING MODEL SPREAD.


WEATHER HIGHLIGHTS
~~~~~~~~~~~~~~~~~~

THE BEGINNING OF THE MEDIUM RANGE PERIOD ON FRIDAY WILL FEATURE
HOT AND DRY CONDITIONS ACROSS THE SOUTHERN PLAINS, WITH HIGHS 5 TO
10 DEGREES ABOVE NORMAL.  AFTER A FEW DAYS OF COOLER TEMPERATURES,
THE HEAT RETURNS TO THE INTERIOR PACIFIC NORTHWEST BY NEXT WEEKEND
WITH HIGHS IN THE 80S TO LOW 90S.  THE HEAT CONTINUES TO BUILD
ACROSS THE GREAT BASIN AND ALSO FOR THE DESERT SOUTHWEST BY THE
BEGINNING OF NEXT WEEK, WITH WIDESPREAD 100+ DEGREE READINGS FOR
THE LOWER ELEVATIONS OF CALIFORNIA, NEVADA, AND ARIZONA AS THE
UPPER LEVEL RIDGE BUILDS IN.  UNSEASONABLY COOL CONDITIONS ARE
EXPECTED ACROSS THE NORTH-CENTRAL U.S. AND THE NORTHERN ROCKIES
FOR FRIDAY WITH THE UPPER LEVEL TROUGH IN PLACE, WITH DEPARTURES
OF 5 TO 15 DEGREES BELOW CLIMATOLOGICAL AVERAGES.  BY EARLY NEXT
WEEK, BELOW NORMAL TEMPERATURES ARE LIKELY ACROSS MUCH OF THE
WESTERN HIGH PLAINS.

CONCERNING PRECIPITATION PROSPECTS DURING THIS TIME PERIOD, THE
HEAVIEST RAINFALL AND GREATEST THUNDERSTORM ACTIVITY IS LIKELY
FROM SOUTHERN TEXAS TO THE CENTRAL GULF COAST REGION AND ALSO FOR
THE TENNESSEE RIVER VALLEY AS DEEP MOISTURE FROM THE GULF OF
MEXICO ADVECTS NORTHWARD AND INTERACTS WITH A FRONTAL BOUNDARY.
THERE MAY BE SOME INSTANCES OF RAINFALL HEAVY ENOUGH TO CAUSE
FLASH FLOODING OVER THIS REGION, AND FUTURE EXCESSIVE RAINFALL
OUTLOOKS WILL PROVIDE ADDITIONAL DETAILS AS WE GO FORWARD IN TIME.
 ANOTHER AREA OF ENHANCED RAINFALL IS EXPECTED ACROSS THE WESTERN
HIGH PLAINS THIS WEEKEND IN ASSOCIATION WITH THE UPPER LEVEL
TROUGH.  WITH THE FRONT APPROACHING THE EAST COAST EARLY NEXT
WEEK, AN AXIS OF ORGANIZED RAINFALL SEEMS MORE LIKELY FROM FROM
THE CENTRAL APPALACHIANS TO NEW YORK STATE.

HAMRICK

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