Preliminary Forecasts
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FXUS02 KWNH 210456
PREEPD

PRELIMINARY EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1256 AM EDT MON AUG 21 2017

VALID 12Z THU AUG 24 2017 - 12Z MON AUG 28 2017


...GUIDANCE EVALUATION/PREFERENCES...

MODELS/ENSEMBLES SHOW GENERALLY GOOD AGREEMENT WITH RESPECT TO THE
LARGE-SCALE PATTERN.  THE INITIAL UPPER TROUGH OVER THE LOWER
GREAT LAKES AND NORTHEAST IS SLOW TO MOVE OFFSHORE LATER THIS WEEK
AND WEEKEND.

THE UPSTREAM TROUGH COMING ONSHORE IN THE PACIFIC NORTHWEST THU IS
MORE PROGRESSIVE...CROSSING THE NORTHERN ROCKIES BY FRI MORNING.
THE TROUGH MOVES INTO THE CANADIAN PRAIRIES BY 12Z SAT WHEN A
SPLIT DEVELOPS. THE 12Z ECMWF SEPARATES FROM OTHER MODELS IN
DEVELOPING A 500 MB LOW IN SOUTHERN MANITOBA THAT THEN SLOWS DOWN
AND PERSISTS NEAR THE MN BORDER WITH CANADA SUN NIGHT TO 12Z MON
28 AUG.

MOST ECMWF ENSEMBLES HAVE AN UPPER WAVE DEVELOPING WITHOUT CLOSING
OFF A LOW.  THE ECMWF AND GEFS MEAN ARE IN AGREEMENT IN THE UPPER
TROUGH TRANSLATING OUT OF SOUTH CENTRAL CANADA INTO THE UPPER MS
VALLEY NEXT WEEKEND.
WITH GOOD ENSEMBLE MEAN AGREEMENT AND THE GFS SHOWING A FASTER
MOTION IN THE 12-18Z RUNS AND HISTORIC FAST GFS BIAS...MANUAL
PROGS WEIGHTED HEAVILY TOWARDS THE ENSEMBLE MEANS NEXT WEEKEND
INTO MON AUG 28.

THE MODELS ARE IN BETTER THAN AVERAGE AGREEMENT ON THE DEVELOPMENT
OF A LARGE SPRAWLING ANTICYCLONE OVER THE SOUTHERN GREAT BASIN TO
FOUR CORNERS REGION NEXT WEEKEND INTO EARLY NEXT WEEK. THE
WEAKNESS BETWEEN THIS ANTICYCLONE AND THE MEAN RIDGE OVER THE
SOUTHWEST ATLANTIC PERSISTS OVER THE LOWER MS VALLEY AND
SOUTHEAST/GULF COAST.

THE MODELS/ENSEMBLES CONTINUE TO SHOW GENERALLY GOOD AGREEMENT
THAT THE REMNANTS OF TROPICAL STORM HARVEY WILL CROSS THE BAY OF
CAMPECHE AND MOVE GENERALLY WEST-NORTHWESTWARD INTO MEXICO ON DAYS
4-6 (FRI-SUN).

...WEATHER HIGHLIGHTS...

SHOWERS AND STORMS FOCUS ACROSS THE SOUTHEAST AND GULF COAST.
FLORIDA SHOULD ALSO SEE AN INCREASE IN RAINFALL AS MOISTURE POOLS
AHEAD OF THE COLD FRONT IN THE NORTHERN PENINSULA AND A TROPICAL
WAVE BRINGS FURTHER INCREASE IN MOISTURE.

THE COMBINATION OF THE WEAKENING UPPER LOW/TROUGH PUSHING INTO
SOUTHERN CALIFORNIA AND THE UPPER RIDGE FOCUSING OVER TEXAS SHOULD
SUPPORT INCREASED SHOWERS/CONVECTION OVER THE SOUTHERN ROCKIES
LATE IN THE WEEK AND THEN AS THE RETURN SOUTHERLY FLOW DEVELOPS
SHOWERS AND STORMS DEVELOP IN TX AND INTO THE SOUTHERN AND CENTRAL
PLAINS.
THE SLOWING FRONTAL BOUNDARY IN THE NORTHERN PLAINS AND UPPER MS
VALLEY ALLOWS MOISTURE TO SURGE AND SHOWERS TO DEVELOP NEXT
WEEKEND INTO MON 28 AUG.

AS THE REMNANTS OF TROPICAL STORM HARVEY MOVE INTO EAST CENTRAL
MEXICO BY FRI/SAT, MOISTURE ON THE NORTHERN FRINGE OF THE SYSTEM
SHOULD INCREASE PRECIP CHANCES FOR SOUTHERN TEXAS AND UP THE RIO
GRANDE VALLEY.

WITH THE UPPER TROUGH ACROSS THE GREAT LAKES LAKES/OH
VALLEY/NORTHEAST, TEMPERATURES WILL BE SEVERAL DEGREES COOLER THAN
NORMAL LATER THIS WEEK THROUGH NEXT WEEKEND. THE RETURN OF ABOVE
NORMAL HEIGHTS WILL LEAD TO WARMER TEMPERATURES NEXT WEEKEND INTO
EARLY NEXT WEEK IN THE NORTHERN HIGH PLAINS/NORTHERN ROCKIES TO
THE GREAT BASIN AND DESERT SOUTHWEST.

PETERSEN/RYAN



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