Preliminary Forecasts
Issued by NWS

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000
FXUS02 KWNH 230649
PREEPD

PRELIMINARY EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
248 AM EDT SUN JUL 23 2017

VALID 12Z WED JUL 26 2017 - 12Z SUN JUL 30 2017

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

THE SYNOPTIC-SCALE PATTERN ACROSS THE REGION WILL FEATURE A SERIES
OF SHORTWAVES TRACKING ACROSS SOUTHERN CANADA WITH ATTENDANT
HEIGHT FALLS GENERALLY GRAZING THE NORTHERN TIER OF THE U.S.
MEANWHILE...BROAD RIDGING SHOULD REMAIN IN PLACE FROM THE DESERT
SOUTHWEST TO THE SOUTHERN/CENTRAL PLAINS...A PLACE IT HAS BEEN
POSITIONED FOR LARGE CHUNKS OF THE SUMMER. CONSIDERING THE PATTERN
AS A WHOLE...THERE HAS BEEN A SLOWING TREND WITHIN THE ACTIVE BELT
OF WESTERLIES AFTER NOTING A PUSH IN THE FASTER DIRECTION JUST
YESTERDAY. THE INITIAL SHORTWAVE IN THE CHAIN SHOULD BE EXITING
NOVA SCOTIA ON WEDNESDAY MORNING WHILE MODEST RIDGING LIES IN THE
WAKE ALONG THE EASTERN SEABOARD. FARTHER UPSTREAM...A RATHER
POTENT UPPER TROUGH WILL BE ACCELERATING THROUGH ONTARIO WITH
500-MB HEIGHT DEPARTURES GENERALLY AROUND 1 STANDARD DEVIATION
BELOW CLIMATOLOGY. IT APPEARS THE MOST IMPRESSIVE GRADIENT WILL
REMAIN UP IN CANADA ALTHOUGH THESE BAND OF HEIGHT FALLS WILL HELP
SLIDE A FRONTAL ZONE SOUTH AND EAST. BY LATER IN THE WEEK...MODELS
CONTINUE TO SHOW SOME SORT OF NEGATIVE HEIGHT ANOMALY LINGERING
ACROSS THE EASTERN U.S. IT SHOWS UP IN MOST OF THE RECENT GUIDANCE
ALTHOUGH THE 12Z ECMWF IS BY FAR THE MOST AGGRESSIVE WITH THE
EXTENT OF TROUGHING AS WELL AS THE STRENGTH OF THE SURFACE CYCLONE
ALONG THE DELMARVA NEXT WEEKEND. ITS SOLUTION DOES HAVE DECENT
SUPPORT FROM THE 12Z CMC. WHILE WITHIN THE REALM OF
POSSIBILITIES...IT IS BEST TO PLAY THE FORECAST A BIT MORE
CONSERVATIVE. AND THE LAST SYSTEM OF INTEREST IS WITH GENERAL LOW
AMPLITUDE FLOW ACROSS THE NORTHWESTERN U.S. A MEAN UPPER LOW SEEMS
TO SITUATE ITSELF OVER THE NORTHEASTERN PACIFIC WHILE A SERIES OF
WEAK TO MODEST PERTURBATIONS SPIN OFF INTO THE PACIFIC NORTHWEST.
GIVEN THE SHEARED NATURE OF THESE IMPULSES...RIDGING SHOULD HOLD
SHAPE DOWNSTREAM ACROSS THE INTERMOUNTAIN WEST. THE 12Z GFS
ATTEMPTED TO BREAK THE NORTHERN EXTENT OF THIS RIDGE DOWN WHICH
DID NOT MAKE SENSE BASED ON THE PATTERN.

FOR THE FIRST COUPLE OF DAYS OF THE PERIOD...A COMBINATION OF THE
18Z GFS AND 12Z ECMWF/UKMET WERE PRIMARY MODELS UTILIZED IN THE
BLEND WITH MINIMAL CONTRIBUTIONS FROM THE AVAILABLE ENSEMBLE
MEANS. DID NOT UTILIZE TOO MUCH OF THE 12Z UKMET GIVEN IT WAS
STRONGER WITH RIDGING ACROSS THE SOUTHEASTERN U.S. TOWARD DAY
5/FRIDAY...GRADUALLY REDUCED USE OF THE 12Z ECMWF GIVEN ITS
HANDLING OF THE UPPER TROUGH ACROSS THE EASTERN U.S. INTO THE
WEEKEND. DID KEEP A SMALL FRACTION OF THE 18Z GFS THROUGHOUT THE
FORECAST IN CONJUNCTION WITH HEAVY USE OF THE GLOBAL ENSEMBLE
MEANS...18Z GEFS AND 12Z ECMWF/NAEFS.


...WEATHER HIGHLIGHTS...

FORECAST TEMPERATURE ANOMALIES FOR THE PERIOD SHOULD GENERALLY
REMAIN WITHIN 5 TO 10 DEGREES OF CLIMATOLOGY. THE HOT SPOTS IN THE
NATION ARE EXPECTED TO BE ACROSS THE DESERT SOUTHWEST WITH HIGHS
APPROACH THE LOW 110S WHILE OTHER CENTURY DIGIT READINGS ARE
LIKELY ACROSS THE INTERIOR VALLEYS OF CA. ELSEWHERE UNDERNEATH THE
UPPER RIDGE...EXPECT HIGHS APPROACHING 100 DEGREES INTO THE
SOUTHERN PLAINS AS WELL. MEANWHILE...SOME OF THE COOLER LOCATIONS
WILL BE ACROSS THE GREAT LAKES AND INTERIOR NORTHEAST WITH HIGHS
IN THE UPPER 70S AS WELL AS ALONG THE IMMEDIATE WEST COAST GIVEN
ANY PERSISTENT ONSHORE FLOW.

REGARDING PRECIPITATION...AN ACTIVE MONSOONAL PATTERN COUPLED WITH
ANY APPRECIABLE MOISTURE FROM DECAYING TROPICAL CYCLONES WILL KEEP
THE WEATHER ACTIVE ACROSS THE DESERT SOUTHWEST/FOUR CORNERS
REGION. BASED ON THE 00Z GFS...PRECIPITABLE WATER VALUES MAY REACH
THE 90TH TO 95TH PERCENTILE. ALL OF THIS ACTIVITY WILL MIGRATE
INTO THE SOUTHERN/CENTRAL ROCKIES AND EVENTUALLY INTO THE ADJACENT
GREAT PLAINS. MULTIPLE MESOSCALE CONVECTIVE SYSTEMS ARE LIKELY TO
PUSH EASTWARD INTO THE MIDDLE MS VALLEY/OH/TN VALLEYS WITH SUCH
DETAILS DIFFICULT TO PINPOINT AT THIS JUNCTURE. LOOKING TOWARD THE
SOUTHERN TO EASTERN U.S...THE FRONTAL ZONE SWEEPING THROUGH THE
REGIONS SHOULD AFFORD AN UPTICK IN RAINFALL...PARTICULARLY DURING
THE PEAK IN THE DIURNAL HEATING CYCLE.


RUBIN-OSTER



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