Prognostic Meteorological Discussion
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000 FXUS02 KWBC 171856 PMDEPD EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 255 PM 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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