Preliminary Forecasts
Issued by NWS

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FXUS02 KWNH 220652
PREEPD

PRELIMINARY EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
251 AM EDT SAT JUL 22 2017

VALID 12Z TUE JUL 25 2017 - 12Z SAT JUL 29 2017

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

THE LARGE-SCALE PATTERN ACROSS THE U.S. WILL FEATURE A SERIES OF
SHORTWAVES WHICH GENERALLY TRACK VERY NEAR THE U.S./CANADIAN
BORDER THROUGHOUT THE PERIOD. COMPARISON OF THE LAST SEVERAL
ENSEMBLE SPAGHETTI PLOTS SHOW A QUICKER TREND...PARTICULARLY
DURING THE 12Z MODEL CYCLE. AN ELONGATED AREA OF TROUGHING
EXTENDING ALONG THE NEW ENGLAND COAST ON TUESDAY MORNING SHOULD
QUICKLY GIVE WAY TO MODEST SHORTWAVE RIDGING IN THE WAKE. THE
QUICKER TREND IN THE GUIDANCE IS NOTED HERE AS THE PARENT SURFACE
WAVE IS FARTHER OFF THE COAST AT 25/1200Z THAN CYCLES THE PREVIOUS
DAY. LOOKING TOWARD CENTRAL CANADA...A SHARPENING BAND OF HEIGHT
FALLS WILL CONTINUE TO MARCH EASTWARD TUESDAY THROUGH THURSDAY. IT
IS THE MOST ANOMALOUS FEATURE ON THE MAP AS THE GFS PORTRAYS
500-MB HEIGHT DEPARTURES IN THE 1 TO 1.5 SIGMA BELOW AVERAGE
RANGE. LIKE THE PRECEDING SYSTEM...THIS SHORTWAVE TOO HAS TRENDED
IN THE QUICKER DIRECTION WHICH EVENTUALLY RE-ESTABLISHES A
NEGATIVE HEIGHT ANOMALY ACROSS EASTERN CANADA INTO THE
NORTHEASTERN U.S. FOR LATER IN THE WEEK. MODELS ARE IN DECENT
AGREEMENT WITH THE EVOLUTION OF THIS TROUGH WHILE MANY OF THE 12Z
ECMWF ENSEMBLE MEMBERS FAVOR MORE AMPLIFICATION. IT SEEMS LIKE THE
STRONGEST HEIGHT FALLS SHOULD REMAIN NORTH OF THE CANADIAN BORDER
THOUGH. AND FINALLY THE LAST MAJOR FEATURES WILL GENERALLY IMPACT
THE PACIFIC NORTHWEST WITH DOWNSTREAM RIDGING ACROSS THE CENTER OF
THE COUNTRY. LIKE THE DAY BEFORE...GFS RUNS HAVE BEEN QUICKER TO
ADVANCE WEAK HEIGHT FALLS INTO THE REGION WITH SUPPORT FROM THE
00Z/12Z CMC. HOWEVER...OF NOTE...THE 00Z GFS CAME IN SLOWER WHICH
APPEARS TO MIMIC THE 12Z CYCLE OF THE ECMWF. THE ECMWF SOLUTIONS
CONTINUE TO OSCILLATE WITH BEING QUICKER/SLOWER WHEN COMPARING THE
12Z/00Z RUNS. THE SLOWER RUNS ARE MORE CONSISTENT WITH THAT OF THE
12Z UKMET. IT CONTINUES TO BE THE MOST COMPLEX SECTOR OF THE MAP
GIVEN THE NUMBER OF SOLUTIONS POSED BY RECENT GUIDANCE.

AS MODELS WERE IN DECENT AGREEMENT FOR THE FIRST COUPLE OF DAYS OF
THE PERIOD...STUCK WITH A MORE DETERMINISTIC SOLUTION
APPROACH...12Z GFS/ECMWF/UKMET. DID MAINTAIN A CHUNK OF THE
ENSEMBLE MEANS TO ACCOUNT FOR MODEL VARIABILITY. MOVING TOWARD THE
LATTER PART OF NEXT WEEK...ENOUGH SPREAD IN THE MODELS IS NOTED TO
LEAN MORE HEAVILY ON AN ENSEMBLE MEAN BASED APPROACH. DID FAVOR
THE 12Z GFS OVER THE 18Z CYCLE AS THE LATTER DID NOT BUILD HEIGHTS
ON DAYS 6/7 ACROSS THE CENTRAL U.S. THE WAY ALL ENSEMBLE SOLUTIONS
SUGGESTED.


...WEATHER HIGHLIGHTS...

FORECAST TEMPERATURES ACROSS THE NATION SHOULD REMAIN WITHIN
ROUGHLY 10 DEGREES OF CLIMATOLOGY THROUGHOUT THE PERIOD. THE COOL
SPOTS ARE FORECAST TO BE OVER THE GREAT LAKES/INTERIOR NORTHEAST
WITH HIGHS EXPECTED TO REMAIN IN THE 70S GIVEN THE SERIES OF
FRONTAL PASSAGES. AND ALONG THE WEST COAST...ANY PERIOD OF ONSHORE
FLOW OFF THE COOLER PACIFIC WATERS SHOULD HELP KEEP TEMPERATURES
DOWN. THE USUAL HOT SPOT ON THE MAP WILL BE ACROSS THE DESERT
SOUTHWEST AS HIGHS SOAR TO NEAR 110 DEGREES WHILE VALLEY LOCALES
APPROACH THE CENTURY DIGIT MARK. OTHERWISE...THE SOUTHERN/CENTRAL
PLAINS SHOULD SEE NEAR 100 DEGREE READINGS WITH THE RIDGE OVERHEAD.

ANOMALOUS MONSOONAL MOISTURE PUMPING INTO THE DESERT
SOUTHWEST/FOUR CORNERS REGION SHOULD KEEP THE REGION WET
THROUGHOUT THE PERIOD. ANTICYCLONIC FLOW AROUND THE RIDGE WILL
EVENTUALLY CARRY THESE FEATURES THROUGH THE NORTH-CENTRAL U.S. AND
INTO THE GREAT LAKES/OH VALLEY...POSSIBLY AS MULTIPLE MESOSCALE
CONVECTIVE SYSTEMS. FAIRLY ROBUST QPF AMOUNTS ARE NOTED IN MODELS
BUT IT WILL REMAIN DIFFICULT TO IRON OUT ANY SPECIFICS THIS DEEP
INTO THE FORECAST. ELSEWHERE...THE SOUTHERN TIER OF THE COUNTRY
SHOULD BE WET...ESPECIALLY DURING THE PEAK IN THE DIURNAL HEATING
CYCLE. SOME ENHANCEMENT IS LIKELY AS A FRONTAL ZONE LINGERS OVER
THE REGION MUCH OF THE PERIOD.



RUBIN-OSTER


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