MODEL DIAGNOSTIC DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 148 PM EST SUN FEB 22 2015 VALID FEB 22/1200 UTC THRU FEB 26/0000 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... AS HAS BEEN THE CASE WITH MOST ARCTIC HIGHS...THE 12Z NAM/GFS WERE 3 TO 6 MB ABOVE THE OBSERVED SURFACE PRESSURES ACROSS THE NORTHERN PLAINS. ...BAROCLINIC ZONE SLOWLY EXITING THE SOUTHEASTERN U.S... FINAL PREFERENCE: A NON-12Z NAM/GFS MODEL CONSENSUS FORECAST CONFIDENCE: AVERAGE A SLOW-MOVING FRONTAL ZONE HAS BEEN THE FOCUS FOR SCATTERED RAIN SHOWERS ACROSS THE SOUTHEASTERN STATES EARLY THIS AFTERNOON. THE BOUNDARY SHOULD CONTINUE TO BE SLUGGISH TO EXIT THE REGION AS THE UPPER FLOW REMAINS GENERALLY OUT OF THE SOUTHWEST. IT IS WORTH NOTING THE 12Z/06Z GFS SHOW MUCH WEAKER LOW-LEVEL INFLOW OFF THE GULF OF MX WHICH LEADS TO MORE LIMITED SURFACE CONVERGENCE. THIS SEEMS TO EXPLAIN THE DIMINISHED QPF VALUES NOTED IN THESE GFS CYCLES. MEANWHILE...THE 12Z NAM SEEMS TO BE SOUTH OF THE CONSENSUS WITH THE COLD FRONTAL PLACEMENT. GIVEN THIS ASSESSMENT...WILL LEAN ON A NON-12Z NAM/GFS MODEL CONSENSUS. ...PAIR OF NORTHERN STREAM SYSTEMS AFFECTING THE GREAT LAKES/NORTHEASTERN U.S... ...STRONG SURFACE LOW CROSSING SOUTHEASTERN CANADA BY EARLY WEDNESDAY... FINAL PREFERENCE: BLEND OF THE 12Z NAM/ECMWF/UKMET FORECAST CONFIDENCE: SLIGHTLY BELOW AVERAGE AN ACTIVE NORTHERN BRANCH OF THE JET WILL CONTINUE AS A PAIR OF LONGWAVE TROFS MOVE THROUGH THE GREAT LAKES/NORTHEASTERN STATES THROUGH MID-WEEK. THE FIRST SET OF HEIGHT FALLS ARE ALREADY WORKING THEIR WAY THROUGH THE GREAT LAKES/MIDDLE MS VALLEY WITH A STRONG ARCTIC HIGH BUILDING IN ITS WAKE. AS NOTED IN THE POTENTIAL INITIALIZATION ERRORS...A MAJORITY OF THE GUIDANCE...EXCEPT THE 12Z/00Z UKMET WERE TOO HIGH WITH THE PRESSURES IN THE SURFACE RIDGE. OTHERWISE...THE DIFFERENCES WITH THE PASSAGE OF THIS INITIAL SYSTEM ARE MINIMAL. REGARDING THE SECONDARY TROF REACHING THE LAKE SUPERIOR BY 24/1200Z...THE 12Z GFS IS NOTABLY MORE AMPLIFIED/QUICKER BY TUESDAY AFTERNOON/EVENING. EVENTUALLY THE 12Z CMC SHIFTS TOWARD A DEEPER AND MORE AMPLIFIED SOLUTION AS WELL. THESE TWO SOLUTIONS APPEAR TO DIVERGE FROM THE FLATTER ENSEMBLE MEANS. SOME FORM OF BLEND BETWEEN THE 12Z NAM/ECMWF/UKMET IS PREFERRED AT THIS TIME AS THEY FIT MOST CLOSELY TO THE CENTER OF THE ENSEMBLE SPREAD. ...CLOSED LOW INITIALLY OVER THE SOUTHWESTERN STATES... ...OPEN WAVE EJECTING THROUGH THE SOUTHERN PLAINS ON WEDNESDAY... FINAL PREFERENCE: BLEND OF THE 12Z ECMWF/UKMET FORECAST CONFIDENCE: SLIGHTLY BELOW AVERAGE WATER VAPOR IMAGERY EARLY THIS AFTERNOON SHOWED A DEFINED SHORTWAVE MOVING TOWARD THE CENTRAL CA COAST WHICH HAS BEEN SPAWNING NUMEROUS SHOWERS UNDERNEATH THE MID-LEVEL CENTER. THE CLOSED SYSTEM SHOULD BRIEFLY STALL ACROSS INTERIOR CA/SOUTHERN NV BEFORE SLIDING SOUTH AND EAST EVOLVING INTO MORE OF AN OPEN WAVE. THE MULTI-DAY SPAGHETTI PLOTS HAVE SHOWN A LARGER AMOUNT OF STABILITY FOR THE FIRST COUPLE DAYS OF THE FORECAST. THE PAST THREE RUNS LOOK SIMILAR WHICH FOLLOWED THE QUICKER TREND INITIALLY NOTED. THROUGH 25/0000Z...THE 12Z CMC HAS CONTINUED ITS SLOWER EVOLUTION BEING WELL BEHIND THE OTHER GUIDANCE. AND WHILE THE 12Z GFS HAS TRENDED A BIT SLOWER RELATIVE TO ITS PRIOR RUN...IT STILL LOOKS TOO QUICK BASED ON THE ENSEMBLE SPAGHETTI PLOTS. FOR DAY 2 RUNNING THROUGH 25/0000Z...THE 12Z ECMWF/UKMET OFFER A REASONABLE COMPROMISE WITH THIS PATTERN WHILE THE 12Z NAM WAS REMOVED GIVEN IT SEEMS TO BE OUT OF PHASE WITH THESE SOLUTIONS. TOWARD DAY 3...THE 12Z UKMET/ECMWF BOTH TRENDED NORTHWARD WITH THE VORTICITY MAXIMA CROSSING THE DEEP SOUTH. GIVEN THEY BOTH ACCOUNT FOR THIS APPARENT TREND...WILL MAINTAIN A BLEND OF THEIR SOLUTIONS THROUGHOUT THE PERIOD. ...PAIR OF WEAK SHORTWAVES DROPPING THROUGH THE PACIFIC NORTHWEST BY TUESDAY/WEDNESDAY... FINAL PREFERENCE: THROUGH 25/0000Z: BLEND OF THE 12Z GFS/ECMWF...THEREAFTER: BLEND OF THE 12Z GEFS/00Z ECMWF ENSEMBLE MEANS FORECAST CONFIDENCE: AVERAGE...THEN BECOMING BELOW AVERAGE FOR DAY 3 AN UPPER RIDGE INITIALLY ANCHORING THE EAST PACIFIC AND EXTENDING UP FAR WESTERN CANADA WILL BREAK DOWN IN RESPONSE TO HEIGHT FALLS APPROACHING VANCOUVER ISLAND BY 24/1200Z. THE 12Z NAM IS TOO PROGRESSIVE WITH THE INITIAL SHORTWAVE. OVERALL...THE 12Z GFS/ECMWF OFFER A REASONABLE COMBINATION FOR THIS INITIAL IMPULSE ALTHOUGH THEY BEGIN TO DIVERGE BY DAY 3 ACROSS INTERIOR LOCATIONS OF THE WEST. THE 12Z GFS TAKES A MORE WESTERN ROUTE TOWARD SOUTHERN CA WHILE THE 12Z ECMWF CARRIES THE SHORTWAVES INTO THE FOUR CORNERS REGION. WILL LEAN ON A COMBO OF THE 12Z GEFS/00Z ECMWF ENSEMBLE MEAN TO MITIGATE THIS SPREAD FOR DAY 3. REGARDING THE NEXT SHORTWAVE ON THE HORIZON...A SLEW OF MODEL SPREAD IS NOTED WITH THE 12Z NAM LEADING THE WAY BEING QUICKER TO SPREAD PRECIPITATION INTO COASTAL WA. GIVEN THE LOW-AMPLITUDE NATURE OF THIS SYSTEM...UNCERTAINTY IS RATHER HIGH SO WILL ALSO FOCUS THE FORECAST ON ENSEMBLE MEANS HERE. www.wpc.ncep.noaa.gov/html/model2.shtml... www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml... RUBIN-OSTER