Prognostic Meteorological Discussion
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000
FXUS02 KWBC 171455
PMDEPD

EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1054 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.

MODEL SPREAD HAS DECREASED COMPARED TO THIS TIME YESTERDAY WITH
RESPECT TO THE TROUGH REACHING THE WEST COAST LATE IN THE PERIOD,
WITH RUN-TO-RUN CONSISTENCY ALSO IMPROVING, ESPECIALLY IN THE
ECMWF. THE ECMWF HAS TRENDED TOWARD A SLOWER SOLUTION THAN SHOWN
YESTERDAY WHILE THE GFS HAS SPED UP A BIT. THE 00Z GFS WAS AN
OUTLIER SHOWING A MUCH FASTER SOLUTION BUT THAT 06Z GFS WAS MORE
IN LINE WITH THE ECMWF AS WELL AS OTHER PREVIOUS GFS RUNS.
ELSEWHERE, 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.

THE WPC FORECAST WAS INITIALLY BASED ON A MOSTLY DETERMINISTIC
BLEND INCLUDING THE 06Z GFS/00Z ECMWF/00Z CMC FOR DAYS 3-4
(SUN-MON). FROM DAY 5 ONWARD, THE FORECAST TRENDED TOWARD HEAVIER
ENSEMBLE WEIGHTING, ALBEIT TO A LESSER DEGREE THAN YESTERDAY GIVEN
THE REDUCED SPREAD ACROSS WESTERN NORTH AMERICA LATE IN THE PERIOD
AND THE ABOVE AVERAGE CONSENSUS WITH RESPECT TO THE EASTERN TROUGH.


...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
CONUS MAY REACH UP TO 10-15 DEG F ABOVE NORMAL FOR MIN AND/OR MAX
READINGS. COOLER AIR BEHIND THE FRONT WILL LIKELY BE A FEW DEGREES
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 5-10 DEG F OR SO ABOVE NORMAL.

RYAN/RAUSCH

$$




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