Preliminary Forecasts
Issued by NWS

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000
FXUS02 KWNH 230700
PREEPD

PRELIMINARY EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
259 AM EDT FRI JUN 23 2017

VALID 12Z MON JUN 26 2017 - 12Z FRI JUN 30 2017

***PATTERN OVERVIEW***

THE MEDIUM RANGE PERIOD WILL BEGIN ON MONDAY WITH A SYNOPTIC SCALE
UPPER LEVEL TROUGH OVER THE EASTERN U.S. AND AN UPPER LEVEL RIDGE
OVER THE INTERMOUNTAIN WEST AND DESERT SOUTHWEST.  AN UPPER LOW
OVER BRITISH COLUMBIA AND SHORTWAVE ENERGY PIVOTING AROUND IT WILL
BEGIN TO WEAKEN THE WESTERN U.S. RIDGE BY TUESDAY WITH LOWERING
HEIGHTS ACROSS THE NORTHWEST, AND THE CORE OF THE UPPER RIDGE
SINKING SOUTHWARD.  THE TROUGH ACROSS THE EAST IS FORECAST TO LIFT
OUT AND LEAVE MORE OF A ZONAL FLOW PATTERN BY THE MIDDLE TO END OF
NEXT WEEK.


***MODEL EVALUATION AND PREFERENCES***

THE LAST FEW RUNS OF THE GFS, INCLUDING THE 00Z RUN, ARE
INDICATING A MORE AMPLIFIED TROUGH ACROSS SOUTHWEST CANADA AND THE
NORTHERN ROCKIES, WHEREAS THE 18Z GEFS MEAN IS FARTHER NORTH WITH
THE UPPER LOW PLACEMENT AND CLOSER TO THE OPERATIONAL ECMWF AND
CMC.  OVER THE PAST SEVERAL MODEL CYCLES, THE MEANS HAVE TRENDED
SOMEWHAT FASTER AND BETTER DEFINED WITH THIS FEATURE AND ITS
ASSOCIATED FRONTAL BOUNDARY.  THE EVOLVING FLATTER FLOW PATTERN
SUPPORTS THE IDEA OF A FASTER TREND IN THE GUIDANCE.  WITH RESPECT
TO THE EASTERN U.S. TROUGH, THE GFS IS THE QUICKEST WITH THE
PASSAGE OF THE TROUGH AXIS OVER THE EAST COAST.  THE GUIDANCE
OVERALL IS INDICATING MAINLY WEAK SURFACE FEATURES IN ASSOCIATION
WITH THIS TROUGH BUT CONTINUES TO STRUGGLE ON HOW TO CORRECTLY
HANDLE THE EMBEDDED SHORTWAVE ENERGY.

IN TERMS OF MODEL PREFERENCES, A BLEND OF THE OPERATIONAL CMC,
UKMET, GFS, AND ECMWF WAS INCORPORATED, WITH GREATER WEIGHTING
PLACED ON THE ECMWF GIVEN ITS HANDLING OF THE FLOW PATTERN OVER
SOUTHERN CANADA, AND ALSO ITS GREATER CONSISTENCY OVER THE PAST
FEW MODEL CYCLES.  INCREASING PERCENTAGES OF THE GEFS AND EC MEANS
WERE USED FOR NEXT THURSDAY AND FRIDAY WHEN UNCERTAINTIES RELATED
TO MESOSCALE DETAILS BECOME SIGNIFICANT.  CONFIDENCE BY THE END OF
THE FORECAST PERIOD IS BELOW AVERAGE.


***SENSIBLE WEATHER***

A SLOW MOVING FRONTAL BOUNDARY NEAR THE GULF COAST AND OVER
FLORIDA WILL PROVIDE A FOCUS FOR PERIODS OF SHOWERS AND
THUNDERSTORMS, MAINLY DURING THE AFTERNOON HOURS.  THE SURFACE
HIGH PRESSURE NORTH OF THE FRONT WILL PROMOTE A PERIOD OF LOW
LEVEL UPSLOPE FLOW AND POTENTIAL CONVECTIVE ENHANCEMENT ALONG THE
EASTERN SLOPES OF THE SOUTHERN ROCKIES AND ADJACENT WESTERN HIGH
PLAINS.  THE UPPER LEVEL LOW MOVING INLAND ACROSS BRITISH COLUMBIA
MAY PROVIDE A BRIEF PERIOD OF CONVECTIVE POTENTIAL OVER THE
PACIFIC NORTHWEST EARLY IN THE WEEK.  EXPECT GREATER COVERAGE AND
INTENSITY OF CONVECTION TO SPREAD ACROSS THE PLAINS AND UPPER
MIDWEST BY MIDWEEK AS THIS DISTURBANCE APPROACHES FROM THE WEST.
THE HEAT WAVE ACROSS THE DESERT SOUTHWEST SHOULD NOT BE AS EXTREME
HEADING INTO NEXT WEEK, BUT READINGS WILL STILL LIKELY BE ABOVE
AVERAGE.  A HIGH QUALITY AIRMASS WITH PLEASANT TEMPERATURES IS
EXPECTED ACROSS THE EASTERN U.S. FOR THE BEGINNING TO MIDDLE OF
THE WEEK.


D. HAMRICK

$$




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