Prognostic Meteorological Discussion
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FXUS02 KWBC 180538
PMDEPD

EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
137 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...

THE MODELS AND ENSEMBLES SHOWED GOOD ENOUGH CLUSTERING NEXT
MON/TUE THAT A CONSENSUS BLEND OF THE 18Z/12Z GFS, 18Z GEFS AND
12Z ECMWF/ECENS MEAN OFFERED A GOOD STARTING POINT. QUESTION
THEREAFTER IS HOW DEEP/QUICKLY A SFC LOW INTENSIFIES/MOVES THROUGH
THE EASTERN GREAT LAKES AND INTO SE ONTARIO/W QUEBEC AS THE UPPER
PATTERN STARTS TO AMPLIFY. 12Z GFS WAS QUICKER/WEAKER THAN THE
RATHER DEEP/SLOW 18Z GFS, WHILE THE 12Z ECMWF LIED IN BETWEEN BOTH
SPATIALLY AND IN STRENGTH. USED THE ECMWF AS A GOOD GUIDE FOR NOW.
IN THE PAC NW, HOWEVER, THE ECMWF LIED ON THE SLOWEST/DEEPEST SIDE
OF THE ENSEMBLE SPREAD WITH THE INCOMING UPPER LOW. GFS RUNS
SEEMED TOO QUICK TO LIFT IT NORTHEASTWARD GIVEN THE OVERALL
AMPLIFICATION BUT NOT WILLING TO BITE ON THE SLOW ECMWF. USED THE
ECMWF ENSEMBLE MEAN AS MAJORITY WEIGHTING BY NEXT FRIDAY WHICH WAS
IN BETWEEN THE ECMWF AND THE GEFS MEAN.


...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. LOWERED
TEMPS A FEW DEGREES ALONG THE PATH OF TOTALITY FROM THE CENTRAL
PLAINS SOUTHEASTWARD GIVEN THE LACK OF SUNSHINE NEAR THE MAX
HEATING TIME OF DAY, THOUGH TEMPERATURES SHOULD RECOVER MOST OF
THE LOSS BEFORE 4-5PM LOCAL TIME.

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.


FRACASSO


$$





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