Preliminary Forecasts
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FXUS02 KWNH 270558
PREEPD

PRELIMINARY EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
157 AM EDT THU JUL 27 2017

VALID 12Z SUN JUL 30 2017 - 12Z THU AUG 03 2017

...PATTERN OVERVIEW...

FLOW WILL BECOME QUITE AMPLIFIED OVER THE LOWER 48 LATE THIS
WEEKEND AND THROUGH THE MIDDLE OF NEXT WEEK AS UPPER RIDGING
BUILDS IN OVER THE WEST AND A DOWNSTREAM TROUGH SETS UP ACROSS THE
EASTERN HALF OF THE NATION.  AT THE SURFACE, THE TAIL END OF A
FRONTAL BOUNDARY PUSHING OFF THE EASTERN SEABOARD THIS WEEKEND
WILL BE SLOW TO DISSIPATE WHILE IT STALLS AND LINGERS ACROSS
NORTHERN FLORIDA AND THE CENTRAL GULF COAST FOR MUCH OF NEXT WEEK.
 FARTHER NORTH, WEAK SHORTWAVES STREAKING THROUGH SOUTHERN CANADA
COULD HELP PRESS A SERIES OF COLD FRONTS FROM THE UPPER MIDWEST TO
THE NORTHEAST THROUGHOUT THE MEDIUM RANGE PERIOD.


...GUIDANCE EVALUATION/PREFERENCES...

DETERMINISTIC AND ENSEMBLE GUIDANCE REMAINED IN VERY GOOD
AGREEMENT WITH THE LARGE SCALE SYNOPTIC PATTERN SETTING UP OVER
THE CONUS NEXT WEEK, ESPECIALLY WITH THE EXPANDING HIGH PRESSURE
IN THE WEST, WHERE MODELS APPEAR LOCKED INTO A CLOSED 500MB HIGH
CENTERING OVER THE CENTRAL GREAT BASIN BY 12Z THURSDAY AUG 03.
THERE REMAINS SOME UNCERTAINTY AND SPREAD WITH THE DETAILS OF WEAK
SHORTWAVE IMPULSES WHILE THEY ROUND THE NORTHERN PERIPHERY OF THE
WESTERN U.S. RIDGE AND TRACK EASTWARD ACROSS SOUTHERN CANADA OR
SOUTHEASTWARD THROUGH THE HIGH PLAINS.  SINCE THE MAJORITY OF
12Z/18Z DETERMINISTIC GUIDANCE FELL WITHIN THE 12Z ENSEMBLE
SPREAD, THE WPC DAY 3-7 FORECAST USED A GENERAL BLEND OF
DETERMINISTIC GUIDANCE, ENSEMBLE MEANS, AND CONTINUITY IN ORDER TO
SMOOTH OUT SOME OF THESE SMALLER SCALE DETAIL DIFFERENCES.


...WEATHER HIGHLIGHTS...

LOCATIONS ACROSS THE SOUTHERN/CENTRAL ROCKIES AND ADJACENT HIGH
PLAINS SHOULD SEE A HEIGHTENED RISK FOR HEAVY RAINFALL NEXT WEEK
AS IMPULSES OF ENERGY DROPPING SOUTH/SOUTHEASTWARD AROUND A
BUILDING WESTERN U.S. RIDGE INTERACT WITH AN AXIS OF HIGHLY
ANOMALOUS MOISTURE ACROSS THE REGION.  HOWEVER, THERE REMAINS A
FAIR AMOUNT OF DISCREPANCY AMONG DETERMINISTIC GUIDANCE WITH THE
EXACT LOCATION AND MAXIMUM AMOUNT OF RAINFALL, EVEN WHEN COMPARING
48-HOUR FORECAST PRECIPITATION TOTALS.  ELSEWHERE, PERSISTENT
MONSOONAL MOISTURE SHOULD KEEP LOCALLY HEAVY RAINFALL A THREAT
ACROSS THE FOUR CORNERS REGION, WHILE A FRONTAL BOUNDARY LINGERING
FROM FLORIDA TO THE CENTRAL GULF COAST SHOULD PROVIDE A FOCUS FOR
MULTIPLE DAYS OF CONVECTION AND HEAVY RAINFALL.

A MEAN EASTERN U.S. TROUGH SHOULD KEEP TEMPERATURES BELOW NORMAL
FROM THE PLAINS TO THE EAST COAST THIS WEEKEND AND INTO MUCH OF
NEXT WEEK, WHILE AN EXPANDING RIDGE IN THE WEST SHOULD BRING
CONSECUTIVE DAYS OF ABOVE NORMAL TEMPERATURES TO LOCATIONS FROM
THE WEST COAST TO THE ROCKIES.  AN EXCEPTION TO THE HEAT IN THE
WEST WILL BE ACROSS THE FOUR CORNERS REGION, WHERE MONSOONAL
CONVECTION SHOULD KEEP DAYTIME TEMPERATURES CLOSER TO, OR EVEN
BELOW, SUMMERTIME AVERAGES.


GERHARDT

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