EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
158 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 RIDGING OVER THE
CENTRAL CONUS WITH SLIGHT TROUGHING IN THE PAC NW AND NORTHEAST
BUT STILL GENERALLY ABOVE AVERAGE HEIGHTS CONUS-WIDE. THE INTENSE
HEAT IN THE EAST WILL EASE AS THE UPPER HIGH REASSERTS ITSELF BACK
TOWARD NEW MEXICO, 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 OSCILLATING FRONTAL BOUNDARY
THROUGH THE CENTRAL PLAINS AND MID-MS VALLEY AS WELL AS THROUGH
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.


...GUIDANCE EVALUATION/PREFERENCES...

THE ENSEMBLES HAVE COME INTO BETTER AGREEMENT WITH THE FLOW 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 (70% TO 30%
OF THE GFS/GEFS) AS THE GFS MAY BE TOO STRONG/SLOW IN THE
NORTHEAST WHILE THE TREND IN THE PAST FEW WEEKS HAS GENERALLY BEEN
TOWARD MORE PROGRESSIVE UPPER FLOW (E.G. THE FLOW OUT OF THE GULF
OF ALASKA THAT THE ECMWF WAS FIRST TO PICK UP ON MOVING IT INTO
CANADA).


FRACASSO