Prognostic Meteorological Discussion
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000
FXUS02 KWBC 181506
PMDEPD

EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1105 AM EDT FRI AUG 18 2017

VALID 12Z MON AUG 21 2017 - 12Z FRI AUG 25 2017

...OVERVIEW...

UPPER PATTERN IS FORECAST TO TRANSITION TO A MORE AMPLIFIED
TROUGH/RIDGE/TROUGH ORIENTATION BY THE END OF NEXT WEEK AS THE
FLOW SHUFFLES AROUND NORTH AMERICA. THIS WILL CARRY A COLD FRONT
EASTWARD AND SOUTHWARD OFF THE NORTHEAST/MID-ATLANTIC COAST BY
NEXT THURSDAY AND INTO THE DEEP SOUTH ON FRIDAY. AS TROUGHING DIGS
INTO THE PACIFIC NORTHWEST, A COLD FRONT WILL WORK INTO WA/OR NEXT
THURSDAY.


...GUIDANCE EVALUATION/PREFERENCES...

CONSENSUS AMONG MODELS/ENSEMBLES WAS SUFFICIENT TO USE A 70%
DETERMINISTIC BLEND INITIALLY FOR DAYS 3-4 (MON-TUE), COMPRISED
LARGELY OF THE 00Z ECMWF AND A COMBINATION OF THE 00Z/06Z GFS.
SPREAD INCREASES A BIT BY DAY 5 (WED) WITH RESPECT TO HOW QUICKLY
A SURFACE LOW MOVING FROM THE EASTERN GREAT LAKES INTO
SOUTHEASTERN CANADA DEEPENS AND MOVES NORTHEASTWARD AS THE UPPER
PATTERN BEGINS TO AMPLIFY. THE 00Z GFS WAS QUICKER TO STRENGTHEN
THE LOW RELATIVE TO THE 06Z GFS, WHILE THE 00Z ECMWF WAS GENERALLY
IN BETWEEN THE TWO GFS RUNS. A SECOND AREA OF INCREASED
UNCERTAINTY EXISTS IN ASSOCIATION WITH THE TROUGH/UPPER LOW
AMPLIFYING ACROSS THE NE PACIFIC/WESTERN NORTH AMERICA BY DAYS 6-7
(THU-FRI). GIVEN THE COMPLEX UPSTREAM INTERACTION OF THIS SYSTEM
WITH THE REMNANTS OF TYPHOON BANYAN, MODELS HAVE SHOWN SIGNIFICANT
RUN-TO-RUN VARIABILITY OVER THE PAST SEVERAL DAYS, ESPECIALLY WITH
RESPECT TO HOW QUICKLY THE SYSTEM MOVES INTO WESTERN CANADA AND
THE NORTHWESTERN U.S. IN GENERAL SPREAD HAS DECREASED AS SOLUTIONS
HAVE SETTLED TOWARD THE CENTER OF THE SPREAD, HOWEVER, RUN-TO-RUN
VARIABILITY STILL EXISTS, WITH THE 06Z GFS LYING ON THE
SLOW/AMPLIFIED SIDE OF THE SPREAD ENVELOPE. THE 00Z GFS WAS MUCH
CLOSER TO CONSENSUS INCLUDING THE 00Z ECMWF/06Z GEFS MEAN/00Z
ECENS MEAN, AND THUS THE 06Z GFS WAS EXCLUDED FROM THE FORECAST BY
DAYS 6-7 IN FAVOR OF THE 00Z RUN.


...WEATHER HIGHLIGHTS...

MONDAY (ECLIPSE DAY) - FAIRLY QUIET WITH A FRONT DRAPED ACROSS THE
UPPER MIDWEST AND A WEAKENING ONE THROUGH THE SOUTHEAST BACK
THROUGH MISSOURI. SHOWERS (AND CLOUDS) WILL FOCUS AROUND THESE
BOUNDARIES BUT COULD BE LOCALLY MODULATED BY THE DECREASE IN SOLAR
HEATING/INSOLATION MIDDAY AND DROP IN SFC TEMPERATURES.

TUE-THU - FRONT WILL PUSH EASTWARD AND DROP SOUTHWARD AS THE SFC
LOW LIFTS INTO SOUTHERN CANADA. TEMPERATURES WILL WARM AHEAD OF
THE BOUNDARY WELL INTO THE 80S INTO NEW ENGLAND AND STAYING IN THE
90S AND LOW 100S OVER TEXAS. PRECIPITATION COULD BE HEAVIEST OVER
S MN AND N IOWA THROUGH S WISCONSIN/N ILLINOIS INTO LOWER MICHIGAN
AS THE SFC LOW DEEPENS. GEFS M-CLIMATE QPF LIES IN THE 99TH
PERCENTILE WHICH SIGNALS A HEAVY RAIN THREAT. FLORIDA SHOULD ALSO
SEE AN INCREASE IN RAINFALL AS A TROPICAL WAVE MOVES THROUGH THE
STATE WED/THU.

THU-FRI - PAC NW WILL SEE AN INCREASE IN SHOWERS AS THE COLD FRONT
PUSHES INLAND. COOLER TEMPERATURES ARE EXPECTED WITH THE INCREASE
IN CLOUD COVER. FARTHER SOUTH, COMBINATION OF THE WEAKENING UPPER
TROUGH PUSHING INTO SOUTHERN CALIFORNIA AND THE UPPER RIDGE
FOCUSING OVER TEXAS SHOULD SUPPORT INCREASED SHOWERS/CONVECTION
OVER THE FOUR CORNERS REGION, MOSTLY OVER NEW MEXICO PER THE
ENSEMBLES.


RYAN/FRACASSO


$$





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