MODEL DIAGNOSTIC DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 249 PM EDT SUN AUG 02 2015 VALID AUG 02/1200 UTC THRU AUG 06/0000 UTC ...SEE NOUS42 KWNO (ADMNFD) FOR THE STATUS OF THE UPPER AIR INGEST... 12Z MODEL EVALUATION INCLUDING ANALYSIS AND PREFERENCES ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ SHORTWAVE TROUGH TRACKING FROM NEVADA TO THE NORTHERN PLAINS ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ PREFERENCE: BLEND OF 12Z NAM/GFS/ECMWF CONFIDENCE: SLIGHTLY ABOVE AVERAGE THE 12Z NAM MOVES TOWARD THE STRONG EDGE OF THE GUIDANCE AFTER REACHING THE PLAINS TUESDAY NIGHT BUT OTHERWISE SHOWS REASONABLY GOOD AGREEMENT WITH THE 00Z ECMWF AND THE ENSEMBLE MEANS. THE 12Z GFS LIFTS THE SHORTWAVE ENERGY NORTH WITH A STRONGER INTERACTION WITH THE PACIFIC NORTHWEST CLOSED LOW...SHEDDING ENERGY INTO THE LOW MUCH MORE THAN OTHER GUIDANCE. THIS OVERALL REDUCES SOME OF THE DOWNSTREAM ENERGY ALLOWING IT TO WEAKEN AND SPEED UP EVENTUALLY OUTPACING THE 12Z NAM BY 00Z ON THE 6TH. THE 12Z ECMWF ALSO PICKED UP ON THE TREND WITH VERY GOOD AGREEMENT WITH THE SPEED LOCATION OF THE 12Z NAM BUT BETTER AGREEMENT WITH THE 12Z GFS IN STRENGTH/AMPLITUDE. AS SUCH A BLEND OF THE 12Z GFS/NAM/ECMWF REMAINS PREFERRED WITH SLIGHTLY INCREASED CONFIDENCE. SHORTWAVE TROUGH TRACKING FROM COLORADO TO THE MID MISSISSIPPI VALLEY ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~ PREFERENCE: BLEND OF 12Z GFS/00Z ECMWF CONFIDENCE: SLIGHTLY BELOW AVERAGE THE 12Z NAM INITIALIZED WITH 500 MB HEIGHTS 10-20 METERS TOO LOW OVER A BROAD EXPANSE OF THE FOUR CORNERS REGION. THIS IS MAGNIFIED DOWNSTREAM WHERE THE NAM BECOMES A BROAD AND DEEP UPPER LEVEL SOLUTION OVER THE PLAINS AND MID MISSISSIPPI VALLEY...DESPITE HAVING A REASONABLE SURFACE FORECAST. THE GFS/ECMWF SHOW GOOD AGREEMENT WITH EACH OTHER AND WITH THEIR RESPECTIVE ENSEMBLE MEANS UP TO WED MORNING WHEN THE 12Z GFS APPEARS TO HAVE CONVECTIVE FEEDBACK LIFTING NORTH ACROSS NORTHERN MO/IL BY THE END OF THE PERIOD. THE 12Z ECMWF HAS ALSO TRENDED A BIT MORE ROBUSTLY AS THE WAVE EJECTS ACROSS KANSAS ON WED. ADDITIONALLY TRENDING A SHADE FURTHER NORTH AS WELL. THE 12Z UKMET RAPIDLY DEVELOPS THE VORT MAX STRONGLY GENERATING A SURFACE LOW WELL TOO DEEP BY DAY 3. THE 12Z CMC FALLS IN BETWEEN THE 12Z UKMET AND 12Z ECMWF...TRENDING MORE CONSOLIDATED LIKE THE UKMET BUT WEAKER/MORE REALISTIC LIKE THE ECMWF. ALL CONSIDERED...A CONTINUATION OF CONTINUITY WITH THE TWO MIDDLE GROUND MODELS...THE 12Z GFS/12Z ECMWF...BLEND IS PREFERRED. INCREASED DIFFERENCES AND A TREND STRONGER CONFIDENCE IN THIS BLEND IS REDUCED TO SLIGHTLY BELOW AVERAGE. TROUGH AND RENEWED SURGE OF MILD AIR OVER THE GREAT LAKES / NORTHEAST ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~ PREFERENCE: BLEND 12Z NAM/GFS/ECMWF...TURNING TO LESS WEIGHT ON THE NAM AFTER 05/00Z CONFIDENCE: AVERAGE THE NAM/GFS/ECMWF ARE IN TIGHT AGREEMENT WITH THE ENSEMBLES ON THE LARGE SCALE THROUGH DAY 2. THE NAM IS THEN DEEPER AND SLOWER...MAINTAINING COOLER HEIGHTS OVER THE SOUTHERN AND EASTERN GREAT LAKES. THIS MAY NOT BE A BAD FORECAST...GIVEN THE RELATIVELY DEEP AND SLOWLY PROGRESSIVE LONGWAVE PATTERN. THE GFS/ECMWF MAY PROVE TO BE TOO ANXIOUS TO RAISE HEIGHTS. MUCH DEPENDS ON THE UPSTREAM SHORTWAVE TRACKING FROM NEVADA TO THE NORTHERN PLAINS...FOR WHICH THE GFS...ALTHOUGH IT IS ONE OF THE PREFERRED MODELS...IS A LITTLE NORTH AND EAST AND/OR FASTER THAN AN IDEAL CONSENSUS POSITION. THE NAM IS QUESTIONABLE GIVEN ITS LOWER-THAN-CONSENSUS HEIGHTS AROUND OTHER SHORTWAVES...SO CONFIDENCE IS LOWER ON DAY 3...AND WE RECOMMEND MORE WEIGHT ON THE GFS/ECMWF WHICH BETTER FIT THE AVAILABLE ENSEMBLE GUIDANCE. LOW PRESSURE OVER THE NORTHEAST GULF OF MEXICO ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ PREFERENCE: BLEND OF 12Z GFS/ECMWF CONFIDENCE: SLIGHTLY ABOVE AVERAGE THE 12Z NAM PATTERN ALOFT...LIKE ELSEWHERE...APPEARS TO BE A FEW DECAMETERS TOO LOW THROUGHOUT THE FORECAST PERIOD...REDUCING CONFIDENCE IN USING IT IN THE BLEND. THOUGH SURFACE FEATURES LOOK GOOD IN STRENGTH THOUGH MAYBE A SHADE FAST AS IT LIFTS UP THE COAST...THE 12Z NAM MAY BE USABLE AT THE LOWER LEVELS. THE 12Z GFS REMAINS SOLID IN CONTINUITY WITH ITSELF PRIOR ECMWF RUNS AND ENSEMBLE MEANS...INCLUDING THE MOST RECENT 12Z RUN. SIMILARLY THE 00Z CMC AND UKMET REMAIN STRONGEST/CONSOLIDATED IN COMPARISON. AS SUCH A BLEND OF THE 12Z GFS/ECMWF REMAINS PREFERRED BUT AT SLIGHTLY ABOVE AVERAGE GIVEN CONTINUED AGREEMENT WITH LITTLE CHANGE. TROUGH PROGRESSING INTO THE PACIFIC NORTHWEST ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ PREFERENCE: BLEND OF THE 12Z NAM/GFS/ECMWF CONFIDENCE: AVERAGE A CLOSED LOW WILL GRADUALLY MOVE ONSHORE INTO BRITISH COLUMBIA WHILE MEDIUM WAVELENGTH TROUGHING OVERSPREADS THE U.S. PACIFIC NORTHWEST AND NORTHERN ROCKIES. THE 12Z GFS/ECMWF AND RESPECTIVE ENSEMBLES SHOWED GOOD AGREEMENT AS TO THE EVOLUTION HERE...EVEN INTO THE BASE OF THE TROF WITH THE SHORTWAVE COMING ASHORE INTO NORTHERN CA/SW OR BY TUE NIGHT/WED MORNING. THE 12Z ECWMF ALSO PICKED UP A BIT OF EASTWARD SHIFT MORE IN LINE WITH THE 12Z NAM INCREASING STRENGTH IN THE RETAINED BLEND OF THE 12Z NAM/GFS/ECMWF WITH AVERAGE CONFIDENCE. THE 12Z CMC REMAINS THE NOTABLE OUTLIER IN POSITION AND STRENGTH...WHILE THE 12Z UKMET WAS A BIT SOUTH OF THAN CONSENSUS AS IT ROTATES THE MAIN VORT ENERGY FASTER TOWARD THE US/CANADA BORDER BY DAY 3. www.wpc.ncep.noaa.gov/html/model2.shtml... www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml... GALLINA/BURKE