Preliminary Forecasts
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FXUS02 KWNH 210638
PREEPD

PRELIMINARY EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
237 AM EDT FRI JUL 21 2017

VALID 12Z MON JUL 24 2017 - 12Z FRI JUL 28 2017

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

THE SYNOPTIC PATTERN ACROSS THE U.S. WILL BE AMPLIFIED AND RATHER
PROGRESSIVE IN NATURE. INITIALLY ON DAY 3/MONDAY...A SHORTWAVE
PUSHING THROUGH THE GREAT LAKES REGION WILL BE RESPONSIBLE FOR AN
AREA OF LOW PRESSURE WHICH IS FORECAST TO MOVE INTO THE
NORTHEASTERN U.S. MANY SOLUTIONS FAVOR A SECONDARY CYCLONE TO FORM
OFF THE ATLANTIC COAST AS NOTED IN THE ENSEMBLE SCATTER LOW PLOT.
IT APPEARS THIS ELONGATED REGION OF UPPER TROUGHING SHOULD HOLD ON
UNTIL TUESDAY AS MOST ENSEMBLE MEMBERS DEPICT. IN ITS
WAKE...MODELS GENERALLY DE-AMPLIFY THE UPSTREAM RIDGE AS A MUCH
SHARPER UPPER TROUGH WORKS ITS WAY THROUGH ONTARIO. THIS IS BY FAR
THE MOST ANOMALOUS FEATURE ON THE MAP WITH 500-MB HEIGHT ANOMALIES
IN THE 1 TO 1.5 SIGMA BELOW AVERAGE RANGE. THE CORRIDOR OF LOWER
HEIGHTS SHOULD REMAIN NORTH OF THE CANADIAN BORDER ALTHOUGH WEAK
HEIGHT FALLS MAY GRAZE THE GREAT LAKES/NEW ENGLAND BY MID-WEEK.
TURNING FARTHER UPSTREAM...THERE ARE SOME MODEL UNCERTAINTIES IN
HANDLING AN UPPER LOW PUSHING INTO THE PACIFIC NORTHWEST BY
WEDNESDAY/THURSDAY. THE GFS RUNS AS WELL AS THE PAST COUPLE OF
CYCLES OF THE CMC WERE QUICKER IN BRINGING THIS SYSTEM INLAND.
MEANWHILE...THE 00Z/12Z UKMET REMAIN SLOWER WHILE THE ECMWF SEEMS
TO OSCILLATE BETWEEN RUNS. THE IMPACT OF THIS SYSTEM ON THE
DOWNSTREAM PATTERN IS EVIDENT IN THE EXTENT OF UPPER RIDGING
ACROSS THE INTERMOUNTAIN WEST NEXT WEEK. ENSEMBLE SPAGHETTI PLOTS
SHOW A MYRIAD OF RIDGE STRENGTHS WHICH SUGGESTS THIS ASPECT OF THE
FORECAST IS STILL UP FOR DEBATE.

THE FORECAST WAS DRIVEN BY A COMBINATION OF THE 18Z GFS/GEFS
MEAN...12Z ECMWF/ECMWF ENSEMBLE MEAN...AND 12Z NAEFS MEAN.
UTILIZED A SIGNIFICANT CHUNK OF THE DETERMINISTIC 18Z GFS/12Z
ECMWF EARLY IN THE FORECAST TO MAINTAIN SOME DETAIL IN THE
PRESSURE PATTERN. THIS ALLOWED FOR MODEST SURFACE WAVES TO BE IN
PLACE ON DAYS 3/4...MONDAY/TUESDAY OVER THE NORTHEASTERN U.S.
DEEPER INTO THE FORECAST PERIOD...STARTED TO FAVOR A MORE ENSEMBLE
MEAN BASED APPROACH BUT STILL RETAINED 30 PERCENT OF THE 18Z
GFS/12Z ECMWF INTO DAY 7/JULY 28.


...WEATHER HIGHLIGHTS...

OVERALL...TEMPERATURES ACROSS THE U.S. SHOULD NOT BE TOO ANOMALOUS
IN NATURE WITH FORECAST HIGHS/LOWS BEING WITHIN 5 TO 10 DEGREES OF
CLIMATOLOGY. THE WARM SPOTS ON THE MAP WILL BE ACROSS THE DESERT
SOUTHWEST WITH READINGS EXCEEDING 110 DEGREES ON MANY DAYS WHILE
HIGHS IN THE UPPER 90S ARE LIKELY OVER THE SOUTHERN/CENTRAL
PLAINS. ACROSS THE GREAT LAKES AS WELL AS THE INTERIOR
NORTHEAST...GENERAL TROUGHING COUPLED WITH A FRONTAL PASSAGE
SHOULD AFFORD READINGS TO STICK CLOSER TO THE 70S. LOOKING AT
PRECIPITATION...IT SHOULD BE AN ACTIVE WARM SEASON PATTERN AS
MONSOONAL MOISTURE PUMPS INTO THE DESERT SOUTHWEST/FOUR CORNERS
REGION AND EVENTUALLY INTO THE HIGH PLAINS. ACTIVITY WILL MOVE
EAST AND EVENTUALLY SOUTHEAST AROUND THE ANTICYLONIC CIRCULATION
FORMED BY THE CENTRAL U.S. RIDGE. THIS SHOULD RESULT IN A NUMBER
OF MESOSCALE CONVECTIVE SYSTEMS WHICH SHOULD MOVE FROM THE
NORTH-CENTRAL U.S. TOWARD THE OH VALLEY. DETAILS OF COURSE ARE
VAGUE AT BEST BUT THESE SHOULD BECOME MORE APPARENT ONCE THE
FORECAST MOVES INTO THE SHORT RANGE. MEANWHILE...PLENTY OF PULSE
THUNDERSTORMS ARE LIKELY EACH DAY ACROSS THE SOUTHERN U.S. WHILE
GENERALLY FORCED BY DIURNAL HEATING...SOME INCREASED LOW-LEVEL
CONVERGENCE ALONG THE APPROACHING COLD FRONT SHOULD ENHANCE
AMOUNTS SOMEWHAT.


RUBIN-OSTER


$$





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