Prognostic Meteorological Discussion
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FXUS02 KWBC 201538
PMDEPD

EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1137 AM EDT SUN AUG 20 2017

VALID 12Z WED AUG 23 2017 - 12Z SUN AUG 27 2017


...GUIDANCE EVALUATION/PREFERENCES...

MODELS/ENSEMBLES SHOW GENERALLY GOOD AGREEMENT WITH RESPECT TO THE
LARGE-SCALE PATTERN, WITH AN AMPLIFIED NORTHERN STREAM FLOW
PATTERN PERSISTING THROUGH THE MEDIUM RANGE, AND SUBTROPICAL
RIDGING GENERALLY PERSISTING ACROSS THE SOUTHERN TIER. TROUGHING
ACROSS THE GREAT LAKES/NORTHEAST BENEATH AN UPPER LOW CROSSING
QUEBEC WILL LIFT OUT SOMEWHAT SLOWLY THROUGH THE PERIOD AS A
NUMBER OF SHORTWAVES CONTINUE TO TRAVERSE THE FLOW, KEEPING
HEIGHTS LOW. MODELS SHOW ABOVE AVERAGE AGREEMENT WITH RESPECT TO
THE GENERAL EVOLUTION OF THIS TROUGH/UPPER LOW, BUT SOME
DIFFERENCES REMAIN WITH RESPECT TO SMALLER SCALE DETAILS SUCH AS
THE INDIVIDUAL SHORTWAVES. A PACIFIC UPPER LOW OFF THE COAST OF
BRITISH COLUMBIA ON DAY 3 (WED) IS FORECAST BY ALL GUIDANCE TO
MOVE INTO WESTERN CANADA AND THE CANADIAN HIGH PLAINS BY LATE IN
THE WEEK. HEIGHT FALLS AHEAD OF THIS SYSTEM WILL HELP TO PUSH A
COLD FRONT ACROSS THE NORTHERN ROCKIES AND INTO THE PLAINS BY
SAT/SUN. MODEL AGREEMENT WITH RESPECT TO THIS SYSTEM IS VERY HIGH
COMPARED TO JUST A FEW DAYS AGO, WITH ONLY THE UKMET BEING AN
OUTLIER, SHOWING A PROGRESSION INLAND THAT IS LIKELY TOO FAST.
FINALLY, MODELS 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 BY DAYS 5-7
(FRI-SUN).


...WEATHER HIGHLIGHTS...

SOUTH OF THE COLD FRONT, SHOWERS AND STORMS FOCUS WED AND THU
ACROSS THE SOUTHERN MID ATLANTIC/SOUTHEAST. 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. AS THE REMNANTS OF TROPICAL STORM HARVEY MOVE INTO EAST
CENTRAL MEXICO BY FRI/SAT, MOISTURE ON THE NORTHERN FRINGE OF THE
SYSTEM COULD INCREASE PRECIP CHANCES FOR EXTREME SOUTHERN TEXAS.

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

RYAN/PETERSEN


$$





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