Prognostic Meteorological Discussion
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151
FXUS02 KWBC 221600
PMDEPD

EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1159 AM EDT FRI MAY 22 2015

VALID 12Z MON MAY 25 2015 - 12Z FRI MAY 29 2015

OVERVIEW...

THE LARGE BLOCK INFLUENCING MUCH OF NORTH AMERICA IS EXPECTED TO
SHRINK AND WEAKEN EARLY THE MEDIUM RANGE PERIOD...WITH THE
WESTERLIES BECOMING MORE PROGRESSIVE AND LIGHTER ACROSS MUCH OF
THE U.S.  AS IS OFTEN THE CASE WITH PATTERN TRANSITIONS...MODELS
SHOW A LOT OF VARIABILITY IN THE TIMING/STRENGTH/
STRUCTURE OF THE TRANSITION THAT SHOWS ITSELF IN ABOVE NORMAL
SOLUTION SPREAD COMPARED TO THE PAST COUPLE DAYS.

MODEL PREFERENCES AND UNCERTAINTY...

THE TRANSITIONAL PATTERN AND ASSOCIATED HIGH SOLUTION SPREAD
SIGNIFICANTLY REDUCES CONFIDENCE IN DETERMINISTIC MODEL GUIDANCE.
THEREFORE...THE PREFERENCE IS MOSTLY TOWARD THE ENSEMBLE MEANS AND
SPECIFICALLY A BLEND OF THEM TO INCORPORATE THE LARGEST POSSIBLE
SOLUTION ENVELOPE.  THE IMPACTS OF AN ENSEMBLE MEAN APPROACH IS
SIGNIFICANT IN TWO WAYS.  THE FIRST WAY IS TO ADDRESS THE SPREAD
WITH PERTURBATIONS...WHICH IS VERY USEFUL AND INCREASES FORECAST
CONTINUITY.  UNFORTUNATELY...THE FLIP SIDE IS MORE AMBIGUITY WITH
THE DAY-TO-DAY DETAILS...AND THEREFORE THE FORECAST IS ALSO
ACCOMPANIED BY LOWER THAN AVERAGE CONFIDENCE.

SENSIBLE WEATHER HIGHLIGHTS...

WIDESPREAD RAINFALL...SOME LIKELY EXCESSIVE...WILL SPREAD ACROSS
PORTIONS OF THE SOUTHERN PLAINS AND LOWER/MIDDLE MS VALLEY EARLY
IN THE PERIOD AS THE LEADING EDGE OF THE UPPER TROUGH SHIFTS
EASTWARD FROM THE ROCKIES.  WITH THE TROUGH ALSO SLOWLY WEAKENING
AND LIFTING NORTHEASTWARD...RAIN IN THE WEST SHOULD GRADUALLY
LESSEN AND BECOME MORE SCATTERED WITH TIME.  MEANWHILE...THE EAST
COAST WILL EXPERIENCE A WARMING TREND IN ASSOCIATION WITH AN
EXPANDING BERMUDA HIGH...WITH THE SOUTHEAST ALSO REMAINING MOSTLY
DRY.

JAMES

$$





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