Preliminary Forecasts
Issued by NWS

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FXUS02 KWNH 240652
PREEPD

PRELIMINARY EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
251 AM EDT WED MAY 24 2017

VALID 12Z SAT MAY 27 2017 - 12Z WED MAY 31 2017

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

THE MEDIUM RANGE PERIOD THROUGH THE MIDDLE OF NEXT WEEK WILL
GENERALLY BE FEATURED WITH A LARGE SCALE UPPER LEVEL TROUGH OVER
THE CENTRAL U.S. AND A PERSISTENT UPPER LEVEL RIDGE ALONG THE WEST
COAST.  WEAK RIDGING ALOFT ACROSS THE EASTERN U.S. SHOULD
GRADUALLY GIVE WAY TO LOWERING HEIGHTS AS THE CENTRAL U.S. TROUGH
MOVES EASTWARD, WITH THE PARENT UPPER LOW TRACKING JUST NORTH OF
THE GREAT LAKES.  THIS WILL LEAD TO A NORTHWEST FLOW PATTERN ALOFT
ACROSS THE EASTERN HALF OF THE NATION THAT WILL ALLOW FOR TWO
SEPARATE FRONTAL BOUNDARIES TO PASS THROUGH AND MAINTAIN THE
NEGATIVE TEMPERATURE ANOMALIES OVER THE NORTH-CENTRAL U.S.

IN TERMS OF MODEL PERFORMANCE, THE 12Z UKMET HAS TRENDED CLOSER TO
THE MODEL CONSENSUS REGARDING THE ORIENTATION OF THE UPPER LOW
OVER SOUTHERN CANADA FOR THIS WEEKEND, ALTHOUGH IT IS STILL A
LITTLE SLOWER THAN THE OTHER MODELS.  THE 12Z CMC BECOMES QUICKER
WITH THE LEAD SHORTWAVE TROUGH PASSING THROUGH THE MIDWEST STATES,
AND THUS THE FASTER PROGRESSION OF ITS COLD FRONT DEPICTION ACROSS
THE DEEP SOUTH EARLY NEXT WEEK.  THE 18Z GFS INDICATES A SECONDARY
REINFORCING SHORTWAVE TROUGH WITHIN THE BROADER CYCLONIC FLOW THAT
IS MORE AMPLIFIED THAN THE OTHER GUIDANCE FOR TUESDAY AND
WEDNESDAY, WITH THE OPERATIONAL ECMWF AND CMC SHOWING LESS OF THIS
SHORTWAVE SIGNAL.  THERE IS A SLIGHT HINT IN THE MODEL GUIDANCE
FOR A WEAK CUT-OFF LOW DEVELOPING NEAR SOUTHERN CALIFORNIA BY THE
MIDDLE OF NEXT WEEK, BUT GIVEN POOR MODEL CONTINUITY FROM
YESTERDAY`S FORECAST AND HARDLY ANY SIGNAL IN THE ENSEMBLE MEANS,
CONFIDENCE IN THIS MATERIALIZING REMAINS LIMITED FOR NOW.
HOWEVER, FORECAST CONFIDENCE IS HIGH FOR THE BUILDING RIDGE OVER
THE WESTERN U.S. THIS WEEKEND AND INTO EARLY NEXT WEEK.

THROUGH THE UPCOMING WEEKEND, AN OPERATIONAL MODEL BLEND INCLUDING
THE 12Z ECMWF/CMC AND 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.  GIVEN
THAT THE OPERATIONAL ECMWF HAS SHOWN BETTER RUN-TO-RUN CONTINUITY
WITH THE ORIENTATION OF THE UPPER LEVEL TROUGH OVER THE
NORTH-CENTRAL U.S., A SLIGHTLY HIGHER PERCENTAGE OF THE ECMWF WAS
USED.  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 SATURDAY WILL FEATURE
HOT AND DRY CONDITIONS ACROSS THE PACIFIC NORTHWEST.  THE HEAT
CONTINUES TO BUILD ACROSS THE GREAT BASIN AND 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.  TEMPERATURES ARE
EXPECTED TO BE BELOW NORMAL FROM THE THE CENTRAL PLAINS TO THE
UPPER MIDWEST COURTESY OF THE UPPER LEVEL TROUGH IN PLACE.

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.
 THE POTENTIAL EXISTS FOR SEVERAL INCHES OF RAIN OVER PARTS OF
TEXAS NEXT WEEK AS A STATIONARY FRONTAL BOUNDARY PERSISTS ACROSS
THE REGION.  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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