Prognostic Meteorological Discussion
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FXUS02 KWBC 250636
PMDEPD

EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
236 AM EDT TUE JUL 25 2017

VALID 12Z FRI JUL 28 2017 - 12Z TUE AUG 01 2017

...PATTERN OVERVIEW/GUIDANCE EVALUATION/PREFERENCES...

DURING THE UPCOMING FORECAST PERIOD...A PERSISTENT UPPER RIDGE
EXTENDING FROM THE DESERT SOUTHWEST TO THE SOUTHERN/CENTRAL PLAINS
SHOULD STRENGTHEN OVER THE WEEKEND AND INTO EARLY NEXT WEEK. TO
THE NORTH OF THIS ANTICYCLONE...GENERAL TROUGHING WILL REMAIN
ACROSS THE NORTHEASTERN PACIFIC WHILE A MUCH MORE PRONOUNCED
NEGATIVE HEIGHT ANOMALY ENCOMPASSES THE EASTERN U.S. THE LATTER
SEEMS TO LINGER THROUGHOUT THE FORECAST WITH THE POTENTIAL FOR AN
IMPRESSIVE SURFACE WAVE TO SWING THROUGH THE NORTHEASTERN U.S. AND
INTO THE ADJACENT COASTAL WATERS. ENSEMBLE SCATTER LOW PLOTS SHOW
QUITE A BIT OF SPATIAL SPREAD WITH A NUMBER OF INTENSITY
DIFFERENCES NOTED AMONG THE AVAILABLE GUIDANCE. THE 12Z
ECMWF...PREVIOUS RUNS OF THE CMC...AND NOW THE 00Z UKMET FAVOR THE
POTENTIAL FOR A SUB-1000 MB LOW TO MOVE OFF THE NORTHEASTERN U.S.
COAST EARLY IN THE PERIOD. HOWEVER...THE PAST FEW CYCLES OF THE
GFS ARE LESS ADAMANT WITH THE EXTENT OF DEEPENING. WHILE THIS
SYSTEM CARRIES OFF TOWARD NOVA SCOTIA AND POINTS
NORTHWARD...RESIDUAL LONGWAVE TROUGHING IS EXPECTED TO LINGER OVER
THE SOUTHERN U.S. RELEVANT ENSEMBLE SPAGHETTI PLOTS SHOW A LARGE
HOST OF MEMBERS FAVORING THIS SCENARIO AND THE LOWER HEIGHTS
LINGERING INTO EARLY NEXT WEEK. LOOKING TO THE WESTERN HALF OF THE
COUNTRY...WHILE DIFFERENCES ARE EVIDENT...THE LARGE-SCALE PATTERN
APPEARS TO BE HANDLED REASONABLY WELL. MID-LEVEL HEIGHTS BEGIN TO
INCREASE TOWARD THE END OF THE WEEKEND ACROSS THE
SOUTHWESTERN/SOUTH-CENTRAL STATES WITH THE 12Z CMC/UKMET BEING
STRONGER. AND TO THE NORTH...PERSISTENT TROUGHING OVER THE
NORTHEASTERN PACIFIC SHOULD KEEP GENERAL WEST-SOUTHWESTERLY FLOW
IN PLAY OVER THE PACIFIC NORTHWEST. THE STRONGEST HEIGHT FALLS
WILL LIFT INTO BRITISH COLUMBIA AND SHEAR AS MID-LEVEL RIDGING
HOLDS STRONG OVER THE INTERMOUNTAIN WEST/ROCKIES.

REGARDING THE FORECAST PREFERENCE...DECIDED TO STAY AWAY FROM THE
MORE AGGRESSIVE 12Z ECMWF AS IT SHOWED A 996-MB SURFACE LOW OVER
SOUTHERN NEW ENGLAND ON FRIDAY MORNING. WHILE POSSIBLE...IT WAS
ALSO 6-MB DEEPER THAN ANY OTHER SOLUTION AT THAT TIME. WENT WITH
THE PREVIOUS 00Z ECMWF RUN WHICH WAS NOT NEARLY AS FORMIDABLE
WHILE ALSO INCORPORATING A BIT OF THE 12Z CMC TO CAPTURE SUCH A
DEPTH. HOWEVER...TO PLAY IT MORE CONSERVATIVELY...ADDED THE 18Z
GFS ALONG WITH A FRACTION OF ENSEMBLE MEANS TO DAMPEN THE SOLUTION
A BIT. KEPT UP TO 50 PERCENT OF THE 18Z GFS/12Z CMC/00Z ECMWF IN
THE MIX THROUGH DAY 5/SUNDAY BEFORE GRADUALLY PUSHING TOWARD AN
ENSEMBLE MEANS BASED APPROACH. HOWEVER...THERE WAS ENOUGH
CERTAINTY IN THE SYNOPTIC PATTERN TO AFFORD A BIT OF THE 18Z
GFS/00Z ECMWF THROUGH THE END OF THE PERIOD.


...WEATHER HIGHLIGHTS...

AS USUAL...THE WARMEST TEMPERATURES ON THE MAP WILL BE ACROSS THE
DESERT SOUTHWEST WHERE READINGS SHOULD SIT CLOSE TO
CLIMATOLOGY...UPPER 100S/LOWER 110S. THIS HEAT WILL EXPAND INTO
THE INTERIOR VALLEY LOCALES OF CA WITH READINGS JUST ABOVE THE 100
DEGREE MARK EACH DAY. THE MOST ANOMALOUS HEAT IS EXPECTED UP
TOWARD THE PACIFIC NORTHWEST/UPPER INTERMOUNTAIN WEST/NORTHERN
ROCKIES GIVEN THE PERSISTENT RIDGE. FORECAST TEMPERATURE ANOMALIES
SHOULD BE AROUND 10 DEGREES ABOVE NORMAL WHICH EQUATES TO THE 90S
WHILE COOLER WEATHER IS LIKELY WEST OF INTERSTATE 5 IN WA/OR. THIS
WILL BE IN RESPONSE TO ANY ONSHORE COMPONENT OF THE WIND WHICH
WOULD TEMPER FORECAST HIGHS. OTHERWISE...EXPECT NEAR TO BELOW
AVERAGE READINGS ACROSS THE EASTERN HALF OF THE COUNTRY GIVEN THE
LINGERING UPPER TROUGH AND FRONTAL ZONE SWINGING TOWARD THE GULF
COAST.

ACROSS THE FOUR CORNERS REGION...AMPLE MONSOONAL MOISTURE LIFTING
NORTHWARD FROM THE SUBTROPICAL PACIFIC WILL AID IN THE DEVELOPMENT
OF WIDESPREAD CONVECTION EACH DAY. ADDITIONALLY...ANY ADDITIONAL
MOISTURE FROM WHAT IS CURRENTLY HURRICANE HILARY WOULD ALSO BE OF
INTEREST WHEN CONSIDERING TROPICAL CONNECTIONS. EVENTUALLY ALL OF
THE THUNDERSTORM ACTIVITY WILL TRANSLATE TOWARD THE
SOUTHERN/CENTRAL ROCKIES AND PERHAPS INTO THE GREAT PLAINS.
FARTHER EAST...THE SLOW MOVING FRONTAL ZONE WILL KEEP WET WEATHER
IN STORE FOR LARGE SECTIONS OF THE REGION. IN PARTICULAR...A
LOCALIZED HEAVIER FOCUS MAY BE POSSIBLE NEAR THE DEVELOPING
SURFACE WAVE. IF IT BECOMES AS STRONG AS SOME SOLUTIONS
SUGGEST...THERE COULD BE A FAIRLY PRONOUNCED COMMA-HEAD OF
RAINFALL IN THE NORTHEASTERN U.S.


RUBIN-OSTER


$$





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