Prognostic Meteorological Discussion
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FXUS02 KWBC 010741
PMDEPD

EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
118 AM EDT SAT AUG 01 2015

VALID 12Z TUE AUG 04 2015 - 12Z SAT AUG 08 2015

...OVERVIEW...
SLOW-MOVING MIGRATORY UPPER-LEVEL TROUGHS WILL PREVAIL IN BRITISH
COLUMBIA AND CENTRAL QUEBEC PROVINCES THIS MEDIUM RANGE PERIOD.
BENEATH THESE TROUGHS...TEMPERATURES WILL BE BELOW AVERAGE FOR
EARLY AUGUST. NESTLED IN BETWEEN THESE MIGRATORY TROUGHS---A LONG
WAVE RIDGE---THAT WILL DOMINATE THE WEATHER PATTERN ACROSS THE
SOUTHWEST...INTERMOUNTAIN WEST AND PLAINS STATES. THE STRUCTURE OF
THE RIDGE WILL ULTIMATELY---DICTATE THE DOWNSTREAM TRAJECTORIES OF
EXITING PACIFIC MOISTURE AND PRECIPITATION DISTRIBUTIONS FROM THE
CONTINENTAL DIVIDE TO THE ATLANTIC COAST.

...MODEL PREFERENCES/UNCERTAINTY ASSESSMENT...
CHOSE TO BLEND THE 31/12Z ECENS MEAN AND 31/18Z GEFS MEAN --- WITH
SOME OF DETERMINISTIC ECMWF TO CAPTURE THE SMALLER-SCALE FEATURES
OF THE PATTERN UNTIL THE END OF DAY 5 (7/00Z). THOUGHT THIS
DETERMINISTIC GUIDANCE APPEARS TO BEST FIT THE MEANS---WITH
RESPECT TO THE UPPER-LEVEL TROUGH AXES OVER NORTH AMERICA INVOF
85W AND 125W BETWEEN DAYS 3-6.

THIS BLEND WAS NOT PERFECT---BUT DID KEEP THE CONTINUITY WITH
SHORTWAVE FEATURES IN THE WEST AND MAINTAINS THE `MEANDERING`
OVER-WATER TRAJECTORY OF ITS MID-LEVEL CIRCULATION.

THE MODEL DIFFERENCES PROVIDED A VARIED QPF AXIS EAST OF THE
ROCKIES WITH SOUTHERN SASKATCHEWAN/NORTH DAKOTA BEING THE REGION
OF CENTRAL NORTH AMERICA  WHERE THE DOWNSTREAM FLOW (DOWNWIND SIDE
OF THE LONG WAVE RIDGE) BECOMES NORTHWESTERLY AND ENTERS THE BASE
OF THE EASTERN CANADA TROUGH AXIS.

BETWEEN DAY 4 AND DAY 6--- THE REGIONAL FOCUS FOR HEAVY RAINFALL
POTENTIAL APPEARS TO `ELONGATE` INVOF 40 NORTH LATITUDE THEN
SOUTHWARD---ALONG THE BASE OF THE EASTERN NORTH AMERICAN
UPPER-LEVEL TROUGH AXIS AND THE LOW-LEVEL CANADIAN FRONTAL
BOUNDARY RESPECTIVELY. AND ALTHOUGH THE 31/12Z-31/18Z
DETERMINISTIC GUIDANCE DID NOT APPEAR TO BE `PICTURE PERFECT` BY
7/00Z---THE DETERMINISTIC ECMWF SEEMED TO HAVE MORE SUPPORT FROM
THE ENTIRE ENSEMBLE GUIDANCE SUITE (IN GENERAL) OVER NORTH
AMERICA.

AND SO LIKE YESTERDAY...THE DAY 4-7 QPF CHALLENGES EXIST OVER THE
MISSOURI VALLEY...OHIO VALLEY AND APPALACHIANS. OPTED TO SIMPLY
USE THE PREFERRED BLEND TODAY WITH DAY4-5 MCS DEVELOPMENT AND
TRAJECTORY DICTATED BY THE 31/12Z DETERMINISTIC ECMWF SOLUTION.
DAY6-7 QPF WAS A STRICT 70/30 BLEND OF THE ECENS MEAN AND GEFS
MEAN.

VOJTESAK

$$




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