MODEL DIAGNOSTIC DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 251 PM EDT TUE APR 21 2015 VALID APR 21/1200 UTC THRU APR 25/0000 UTC ...SEE NOUS42 KWNO (ADMNFD) FOR THE STATUS OF THE UPPER AIR INGEST... ...AN EVALUATION OF THE 12Z MODEL GUIDANCE ALONG WITH FINAL PREFERENCES/FORECAST CONFIDENCE... THERE DID NOT APPEAR TO BE ANY INITIALIZATION ERRORS WITH THE 12Z NAM/GFS DEEMED SIGNIFICANT ENOUGH TO IMPACT THEIR FORECASTS. ...DEEP CYCLONIC FLOW AFFECTING THE GREAT LAKES/NORTHEASTERN U.S... ...ASSOCIATED SURFACE CYCLONES/SECONDARY COLD FRONT... FINAL PREFERENCE: BLEND OF THE 12Z NAM/ECMWF FORECAST CONFIDENCE: SLIGHTLY ABOVE AVERAGE WATER VAPOR IMAGERY THIS MORNING SHOWED AN EXPANSIVE UPPER LOW SPINNING OVER THE UPPER GREAT LAKES WITH THE CYCLONIC CIRCULATION EXTENDING FROM THE GREAT PLAINS TO THE EAST COAST. OVER THE COURSE OF THE NEXT COUPLE OF DAYS...THIS VORTEX SHOULD SLOWLY LIFT TO THE NORTH AND EAST BECOMING CENTERED OVER THE ONTARIO/QUEBEC BORDER BY 23/1200Z. A SERIES OF SHORTWAVES WILL CONTINUE TO SPIRAL AROUND IN A COUNTERCLOCKWISE FASHION CARRYING ADDITIONAL THREATS FOR PRECIPITATION ACROSS THE REGION. CONSIDERING SOME OF THE AVAILABLE GUIDANCE...THE 12Z/06Z GFS CONTINUE TO BE DISPLACED TO THE NORTHWEST LEADING TO A DIFFERENCE IN THE POSITION OF ITS SURFACE LOW AND MID-LEVEL CYCLONE CENTER. THE 12Z NAM/ECMWF FORM A SOLUTION CLOSER TO WHAT THE ENSEMBLE MEANS ARE SHOWING SO THE RECOMMENDATION IS A BLEND OF THE 12Z NAM/ECMWF. ...INITIAL UPPER LOW SPINNING ACROSS SOUTHERN CA... ...BROAD UPPER TROF SLIDING FROM THE SOUTHWESTERN U.S. TOWARD THE SOUTHERN HIGH PLAINS... FINAL PREFERENCE: MODEL COMPROMISE BETWEEN THE 12Z NAM/GFS/ECMWF FORECAST CONFIDENCE: AVERAGE THE TYPICAL SPRING BLOCKY PATTERN CONTINUES AS A PAIR OF CLOSED LOWS AFFECT THE SOUTHWESTERN U.S. THE INITIAL COMPACT CIRCULATION AFFECTING SOUTHERN CA SHOULD SLOWLY DRIFT NORTHWARD TOWARD SOUTHERN NV BEFORE GIVING WAY TO A BROADER TROF MOVING IN FROM THE EASTERN PACIFIC. AT FIRST THERE IS QUITE A BIT OF VARIABILITY IN THE HEIGHT FIELDS AMONG THE MODELS BUT THIS DIMINISHES ONCE THE TROF BEGINS TO ACCELERATE EASTWARD. RELATIVE TO YESTERDAY THE GUIDANCE AS A WHOLE SEEMS TO HAVE A BETTER HANDLE ON THIS EVOLUTION. MUCH OF THE UNCERTAINTY SEEMS TO DO WITH THE WEST-EAST PLACEMENT OF A FRONTAL ZONE STRETCHING FROM SOUTHERN PLAINS ALONG THE GULF COAST TO FL. THE 12Z GFS RECENTLY JOINED THE 12Z/00Z CMC IN DEVELOPING A SURFACE LOW ACROSS THE FL PANHANDLE BY 24/0000Z WITH MINIMAL SUPPORT NOTED IN THE ENSEMBLE LOW PLOTS. HOWEVER...THE 12Z GFS DOES APPEAR REASONABLE WITH THE PATTERN UPSTREAM IN TERMS OF THE TROF PLACEMENT AND LOW DEVELOPMENT OFF THE HIGH PLAINS BY FRIDAY EVENING. AT THIS TIME PREFER TO UTILIZE SOME FORM OF COMPROMISE BETWEEN THE 12Z NAM/GFS/ECMWF. ...SECONDARY SHORTWAVE TRACKING THROUGH SOUTHERN CA BY FRIDAY... FINAL PREFERENCE: BLEND OF THE 12Z GEFS/00Z ECMWF ENSEMBLE MEANS FORECAST CONFIDENCE: AVERAGE IN THE WAKE OF THE INITIAL HEIGHT FALLS MOVING FROM THE DESERT SOUTHWEST TO THE GREAT PLAINS WILL BE SECONDARY SHORTWAVE MOVING TOWARD SOUTHERN CA BY 25/0000Z. THE MODEL SPREAD HAS GROWN PRECIPITOUSLY SINCE THE 00Z CYCLE. THE ENSEMBLE MEANS APPEAR LIKE A REASONABLE COMPROMISE GIVEN THIS INCREASE IN UNCERTAINTY. ...MID-LEVEL HEIGHT FALLS MOVING ACROSS THE PACIFIC NORTHWEST TONIGHT... FINAL PREFERENCE: BLEND OF THE 12Z GFS/00Z ECMWF FORECAST CONFIDENCE: AVERAGE A STRETCHED OUT BAND OF VORTICITY CURRENTLY EXTENDING ACROSS THE NORTHEAST PACIFIC WILL REACH THE WA COAST BY LATER TONIGHT. SOME OF THE MODELS CARRY THIS SYSTEM AS A COMPACT UPPER LOW WHICH MAY BE SLOW MOVING AT TIMES. THE 06Z GFS HAD BEEN ONE OF THE SLOWER SOLUTIONS AS IT CLOSED OFF THE UPPER CENTER. HOWEVER...THE 12Z CYCLE HAS TRENDED QUICKER LIKE OTHER MODELS FEATURING MORE OF AN OPEN WAVE SOLUTION. BY 23/1200Z...THE 12Z NAM/GFS/CMC CLUSTER BUT ARE A BIT SLOWER THAN THE QUICKER 12Z/00Z ECMWF. MOST RECENTLY...THE 12Z ECMWF HAS SINCE TRENDED QUICKER WITH THE OVERALL PROGRESSION OF THE PATTERN ACROSS THE NORTHWEST SECTOR OF THE CONTINENT. THIS TAKES IT OUTSIDE THE ENSEMBLE SPREAD SO WILL RETAIN THE PREVIOUS PREFERENCE. ...STRONG CLOSED LOW DESCENDING FROM THE NORTHEAST PACIFIC ON THURSDAY/FRIDAY... FINAL PREFERENCE: BLEND OF THE 12Z GFS/00Z ECMWF FORECAST CONFIDENCE: SLIGHTLY BELOW AVERAGE A BROAD UPPER LOW WILL SLOWLY WORK ITS WAY DOWN FROM THE NORTHEAST PACIFIC. THERE ARE A NUMBER OF SOLUTIONS PRESENTED HERE WITH THE ENSEMBLE SPAGHETTI PLOTS DEPICTING QUITE A BIT OF SCATTER. THE 00Z ECMWF ENSEMBLES ON THE 540-DM PLOT CONTINUE TO SUGGEST A MORE SOUTHWARD DISPLACED SOLUTION RELATIVE TO THE 00Z GEFS/CMC ENSEMBLE MEMBERS. MOST RECENTLY THE 12Z GFS HAS TAKEN A SOUTHWARD TREND TOWARD THIS ECMWF CAMP. AND AS MENTIONED IN THE PREFERENCE IN THE PARAGRAPH ABOVE...NOT SURE OF THIS MOVE SOUTH AND EAST IN THE 12Z ECMWF AS IT PULLS THE SOLUTION OUTSIDE OF THE ENSEMBLE SPREAD. PLAN ON RETAINING THE 12Z GFS/00Z ECMWF BLEND. www.wpc.ncep.noaa.gov/html/model2.shtml... www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml... RUBIN-OSTER