Prognostic Meteorological Discussion
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000
FXUS02 KWBC 201544
PMDEPD

EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1143 AM EDT THU JUL 20 2017

VALID 12Z SUN JUL 23 2017 - 12Z THU JUL 27 2017

...PATTERN OVERVIEW AND WEATHER HIGHLIGHTS...

UPPER FLOW WILL TRANSITION AWAY FROM A DOMINANT HIGHER LATITUDE
UPPER LOW OVER NORTHERN HUDSON BAY (THAT SUPPORTS THE STRONG UPPER
HIGH IN THE CENTRAL/EASTERN STATES THIS WEEKEND) TOWARD LOW
AMPLITUDE FLOW ACROSS MOST OF NORTH AMERICA. THE MOST ANOMALOUS
FEATURE WILL BE A STRONG UPPER HIGH (500MB HEIGHTS ABOUT 3
STANDARD DEVIATIONS ABOVE AVERAGE) OVER THE WESTERN ALEUTIANS.
TELECONNECTIONS FROM THIS ANOMALY SUPPORT MODEST BUT BUILDING
UPPER RIDGE OVER THE CENTRAL CONUS AND EXTENDING UP INTO CANADA
WITH A POSSIBLE CLOSED 594DM HIGH OVER CO INTO THE CENTRAL PLAINS
BY NEXT THURS. MEANWHILE, SLIGHT TROUGHING IN THE PAC NW AND
NORTHEAST CAN BE EXPECTED, BUT FOR THE MOST PART ABOVE AVERAGE
HEIGHTS WILL EXTEND CONUS-WIDE. THE INTENSE HEAT IN THE EAST WILL
EASE AS THE UPPER HIGH REASSERTS ITSELF BACK TOWARD THE MIDWEST,
ALLOWING COOLER AIR TO SWOOP INTO THE UPPER MIDWEST/GREAT
LAKES/NORTHEAST LATER NEXT WEEK (BUT ONLY NEAR TO JUST BELOW
AVERAGE FOR LATE JULY). PRECIPITATION SHOULD MOSTLY FOCUS ALONG
AND TO THE SOUTH OF AN FRONTAL BOUNDARY THAT PUSHES THROUGH THE
EAST AND BECOMES QUASI-STATIONARY BACK INTO THE CENTRAL PLAINS AND
MID-MS VALLEY, WHILE MOISTURE ALSO AFFECTS THE SOUTHWEST AND
INTERMOUNTAIN WEST ON THE BACKSIDE OF THE UPPER HIGH. THE RAIN
THREAT SHOULD PUSH A BIT FARTHER WEST NEXT WEEK INTO THE GREAT
BASIN AS THE UPPER RIDGE PUSHES WESTWARD AND TROUGH REACHES THE
NW.

...GUIDANCE EVALUATION/PREFERENCES...

THE ENSEMBLES HAVE COME INTO BETTER AGREEMENT WITH THE WESTERLIES
OUT OF THE GULF OF ALASKA AND THROUGH CANADA, WHICH AFFECTED THE
FRONTAL/PRECIPITATION TIMING ACROSS THE NORTHERN TIER. THE ECMWF
AND ITS ENSEMBLES HAVE LED THE WAY WITH THIS EVOLUTION, BUT THE
GFS/UKMET WERE CLOSE ENOUGH IN THEIR RECENT RUNS SO THAT A BLEND
OF THAT CONSENSUS OFFERED A REASONABLE STARTING POINT. GAVE THE
ECMWF/ECENS MEAN A MAJORITY WEIGHTING BY NEXT WED/THU (55% TO 45%
OF THE GFS/GEFS) AS THE GFS MAY BE TOO STRONG/SLOW WITH THE TROUGH
IN THE NORTHEAST LIFTING OUT, WHILE THE ECMWF PERHAPS IS TOO
PROGRESSIVE WITH LOWERING HEIGHTS NEXT WED/THURS INTO THE NWRN
QUAD OF THE COUNTRY AND INTO WESTERN CANADA. OVERALL THOUGH THE
THEME WILL BE THE RE-DEVELOPMENT OF CENTRAL RIDGE AND EXPECTED
RING OF FIRE.


FRACASSO/MUSHER


$$




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