Preliminary Forecasts
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FXUS02 KWNH 170632
PREEPD

PRELIMINARY EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
231 AM EDT THU AUG 17 2017

VALID 12Z SUN AUG 20 2017 - 12Z THU AUG 24 2017

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

MODELS AND ENSEMBLE MEANS AGREEABLY ADVERTISE A TRANSITION FROM A
FAIRLY FLAT MEAN PATTERN AT THE START OF THE PERIOD TOWARD A
HIGHER AMPLITUDE REGIME THAT TYPICALLY LENDS ITSELF TO GREATER
PREDICTABILITY.  NORTHERN PACIFIC FLOW THAT INCLUDES THE
EXTRATROPICAL REFLECTION OF FORMER TYPHOON BANYAN WILL COMBINE
WITH MAINLAND ALASKA ENERGY TO YIELD A NORTHEASTERN PACIFIC TROUGH
(LIKELY WITH AN EMBEDDED CLOSED LOW AT A YET TO BE DETERMINED
LATITUDE) HEADING TOWARD THE WEST COAST.  THE LEADING RIDGE
BUILDING INTO WESTERN/CENTRAL CANADA AND ROCKIES/HIGH PLAINS WILL
HELP TO AMPLIFY TROUGHING OVER THE EAST.  ELSEWHERE, ENERGY WITHIN
A TROUGH NEAR CALIFORNIA MAY BEGIN TO EJECT INLAND LATE IN THE
PERIOD WITH THE APPROACH OF THE NORTHEASTERN PACIFIC TROUGH WHILE
GUIDANCE CONTINUES TO SHOW A WESTERN ATLANTIC RIDGE ALOFT
RETROGRADING ACROSS THE SOUTHEAST/NORTHERN GULF AND INTO THE
SOUTHERN PLAINS.

COMPARED TO 24 HOURS AGO GUIDANCE NOW SHOWS LESS SPREAD FOR THE
TROUGH APPROACHING THE WEST COAST, BY WAY OF A SLOWER TREND IN THE
ECMWF MEAN AND FASTER TREND IN THE 18Z GFS.  BY DAY 7 THU THE 18Z
GFS/GEFS MEAN AND 12Z ECMWF/ECMWF AS WELL AS 12Z CMC ARE
REMARKABLY SIMILAR FOR A FORECAST THAT FAR IN ADVANCE.  THE EARLY
PERIOD STREAM INTERACTION THAT LEADS TO THE TROUGH AS WELL AS
CONTINUED SPREAD AMONG INDIVIDUAL ENSEMBLE MEMBERS-- ALBEIT LESS
EXTREME THAN YESTERDAY-- SUGGEST THERE IS STILL SOME RISK THAT
TROUGH EVOLUTION MAY ULTIMATELY DIFFER FROM LATEST CONSENSUS.
SIMILAR HANDLING OF THIS TROUGH IN RECENT GFS RUNS AND 12Z ECMWF
ALSO PRODUCES GOOD AGREEMENT IN TIMING THE EJECTION OF THE LEADING
TROUGH NEAR THE CA COAST.  FARTHER TO THE EAST THERE IS A GROWING
SIGNAL THAT SHORTWAVE ENERGY WILL CROSS THE SOUTHERN ROCKIES FROM
NORTHERN MEXICO BUT ITS RELATIVELY SMALL SCALE WILL LIKELY KEEP
CONFIDENCE IN SPECIFICS LOW UNTIL THE SHORT RANGE TIME FRAME.

IN PRINCIPLE THERE IS VERY GOOD AGREEMENT FOR THE SOUTHERN CANADA
TROUGH THAT INITIALLY SUPPORTS THE COLD FRONT PUSHING
SOUTHEASTWARD FROM THE NORTHERN TIER.  ENERGY ROUNDING THE BASE OF
THE TROUGH MAY CREATE SOME WAVINESS ALONG THE FRONT AT TIMES,
INCLUDING HINTS OF CANADIAN LOW PRESSURE REDEVELOPMENT AROUND
TUE-WED.  EVEN LATE IN THE PERIOD AS THE FRONT REACHES THE EAST
COAST AND SOUTHERN TIER STATES, TIMING DIFFERENCES ARE FAIRLY
SMALL RELATIVE TO TYPICAL MARGIN OF ERROR FOR HOW FAR OUT IN TIME
IS BEING FORECAST.

BASED ON THE CLUSTERING OF GUIDANCE THROUGH THE 12Z/18Z CYCLES,
THE LATEST FORECAST INCORPORATED AN OPERATIONAL MODEL BLEND (MORE
18Z GFS/12Z ECMWF THAN 12Z UKMET/CMC) INTO DAY 5 TUE.  THEN THE
STARTING POINT TRANSITIONED TO AN EVEN MODEL/ENSEMBLE MEAN WEIGHT
OF THE 18Z GFS/12Z ECMWF AND THEIR MEANS.


...WEATHER HIGHLIGHTS...

EXPECT CONVECTION OVER THE WEST TO BE MOST ACTIVE OVER THE
SOUTHERN ROCKIES WHERE ENERGY ALOFT FROM NORTHERN MEXICO MAY
PROVIDE ADDITIONAL SUPPORT FOR A PERIOD OF TIME.  LIGHTER/MORE
SCATTERED ACTIVITY IS POSSIBLE OVER THE REST OF THE INTERIOR WEST
BUT WITH POTENTIAL FOR INCREASED COVERAGE TOWARD WED-THU AS ENERGY
ALOFT NEAR CA EJECTS INLAND.  THE COLD FRONT PUSHING SOUTHEAST
FROM THE NORTHERN PLAINS WILL GENERATE SHOWERS/STORMS THROUGH THE
PERIOD.  THE CENTRAL PLAINS TO UPPER MIDWEST REGION HAS THE BEST
POTENTIAL FOR SEEING ONE OR MORE EPISODES OF HEAVY RAINFALL.
BESIDES THIS FRONT, OTHER CONTRIBUTING INGREDIENTS INCLUDE THE
TAIL END OF AN EAST COAST FRONT EXTENDING BACK TO THE CENTRAL
PLAINS EARLY IN THE PERIOD PLUS MOISTURE/IMPULSES ALOFT EMERGING
FROM THE ROCKIES.  OVER THE SOUTHEAST AND VICINITY, AN UPPER LEVEL
FEATURE MAY ENHANCE GULF COAST CONVECTION EARLY IN THE WEEK AND A
TROPICAL WAVE CURRENTLY FORECAST TO BRUSH THE SOUTHERN FL
PENINSULA MAY BRING A PERIOD OF MORE ORGANIZED/HEAVIER RAINFALL
AROUND TUE-WED.

THE MOST PROMINENT ANOMALIES FOR TEMPS SHOULD BE OF THE POSITIVE
VARIETY AHEAD OF THE NORTHERN PLAINS TO EAST COAST FRONT.  FOR ONE
OR MORE DAYS SOME LOCATIONS IN THE NORTHEASTERN QUADRANT OF THE
LOWER 48 MAY REACH UP TO 10-15F ABOVE NORMAL FOR MIN AND/OR MAX
READINGS.  COOLER AIR BEHIND THE FRONT WILL LIKELY BE A FEW
DEGREES F BELOW NORMAL.  THE SOUTHERN TIER SHOULD SEE PERSISTENTLY
WARM MINS BUT MORE MIXED ANOMALIES FOR HIGHS DEPENDING ON
CLOUDS/CONVECTION.  THE INTERIOR NORTHWEST/NORTHERN ROCKIES WILL
SEE A BRIEF WARM SPELL EARLY-MID WEEK WITH HIGHS 5-10F OR SO ABOVE
NORMAL.

RAUSCH

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