MODEL DIAGNOSTIC DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 215 PM EST SAT JAN 25 2014 VALID JAN 25/1200 UTC THRU JAN 29/0000 UTC ...SEE NOUS42 KWNO (ADMNFD) FOR THE STATUS OF THE UPPER AIR INGEST... ============================================== 12Z MODEL EVALUATION AND PREFERENCES ============================================== NAM/GFS INITIALIZATION ERRORS DO NOT APPEAR TO SIGNIFICANTLY AFFECT THEIR LARGE SCALE OUTPUT. ============================================== ...NEXT ARCTIC OUTBREAK LED BY LOW PRESSURE OVER THE GREAT LAKES... PREFERENCE: 12Z GEFS MEAN OR 12Z ECMWF CONFIDENCE: AVERAGE PROGRESSIVE CYCLOGENESIS WITHIN A RELATIVELY DRY POLAR/ARCTIC AIRMASS WILL OCCUR OVER THE GREAT LAKES ON DAY 1. THE LOW WILL THEN PASS THROUGH NORTHERN NEW ENGLAND AND SOUTHEAST CANADA. TO THE WEST...THE NEXT EXPANSIVE 1040 MB HIGH WILL CARRY ANOTHER ANOMALOUSLY COLD AIRMASS...2.5 STANDARD DEVIATIONS FROM CLIMATOLOGY...INTO THE CENTRAL AND EASTERN U.S. THERE ARE VERY FEW MODEL DIFFERENCES THROUGH DAY 2. THE NAM/GFS ARE A LITTLE FLAT WITH THE MID LEVEL SHORTWAVE OVER THE GREAT LAKES. THE NAM IS CORRESPONDINGLY SEVERAL MILLIBARS WEAKER WITH THE SURFACE LOW THROUGHOUT ITS LIFETIME...AS IS THE GFS BEYOND 27/00Z. THE NAM/GFS DO...HOWEVER...SHOW THE SAME TIMING/PROGRESSION AS THE OTHER MODELS THROUGH DAY 2...28/00Z. OUR PREFERENCE IS BASED MAINLY ON DAY 3...WHEN THE GFS BECOMES FAST WITH THE SURFACE FRONT AND MOVEMENT OF THE HIGH...THE NAM BECOMES AN OUTLIER IN INDUCING A PRONOUNCED LOW LEVEL WAVE OVER THE EASTERN GULF OF MEXICO...AND THE CANADIAN MODEL FALLS TO THE SLOW SIDE BACK IN THE CORE OF THE COLD DOME OVER THE PLAINS. CONSENSUS IS BEST REPRESENTED BY THE 12Z GEFS MEAN AND 00Z ECMWF. ...DEPTH OF TROUGHING OVER THE SOUTHERN PLAINS BY TUESDAY... ...LEADING TOWARD A PRECIPITATION EVENT IN THE SOUTHEAST STATES... MASS FIELD PREFERENCE: 12Z GEFS MEAN OR 12Z ECMWF PRECIPITATION PREFERENCE: BLEND OF 12Z GEFS MEAN AND 00Z ECMWF ENSEMBLE MEAN...APPROXIMATED BY WPC QPF CONFIDENCE: AVERAGE THE LONG WAVE TROUGH IN THE EAST IS FORECAST TO RETROGRADE AS A BROADLY DEFINED SHORTWAVE DIGS TOWARD THE SOUTHERN PLAINS ON DAY 3. AT A GLANCE...MASS FIELD DIFFERENCES WOULD NOT SEEM UNUSUALLY LARGE...BUT THE PRECIPITATION FORECAST IS APPARENTLY VERY SENSITIVE TO THESE DIFFERENCES. FROM DAY 3 INTO DAY 4...THE 12Z GFS AND 00Z ECMWF WERE VERY DRY...WITH NEAR ZERO PRECIPITATION INLAND OF THE CENTRAL GULF COAST...WHEREAS THEIR RESPECTIVE ENSEMBLE MEANS ARE MUCH WETTER. THE UKMET HAS TENDED TO OFFER DRY SOLUTIONS AND THE CANADIAN WET SOLUTIONS...ALTHOUGH TIMING IN THE CANADIAN IS VARIABLE FROM RUN TO RUN. THE NAM/SREF GUIDANCE IS WETTEST OF ALL...BUT THESE SOLUTIONS ARE ALSO MORE SHARPLY AMPLIFIED WITH THE SHORTWAVE DIGGING IN THE SOUTHERN PLAINS AND ASSOCIATED RESPONSE DOWNSTREAM. IT MAY BE THAT A RESPONSE WILL OCCUR DOWNSTREAM...BUT NOT LIKELY AS FAST AS THE NAM IS PREDICTING. PATTERN RECOGNITION SUGGESTS PRECIPITATION WILL LIKELY BREAK OUT IN THE ENTRANCE REGION OF AN EASTERN U.S. JET ON DAY 3...AND THEN BECOME ENHANCED TOWARD DAY 4 WHEN THE SHORTWAVE IN THE PLAINS ROUNDS THE BASE OF THE TROUGH TOWARD...MOVING TOWARD THE GULF COAST AND CAROLINAS. MODEL BIASES WOULD SUGGEST THAT THE NAM IS TOO AMPLIFIED AND PLACING TOO MUCH PRECIPITATION IN THE COLD SECTOR. THE NAM APPEARS EVEN MORE AN OUTLIER WITH THE REST OF THE 12Z GUIDANCE HAVING CONVERGED CLOSER TO THE MIDDLE OF THE PRE-EXISTING SPREAD. THAT THE GFS/ECMWF/UKMET ARE ALSO PRONE TO BEING TOO DRY IN THE JET ENTRANCE REGION AT THESE LONGER LEAD TIMES...BUT THE ECMWF AT LEAST DID TREND TOWARD GREATER COVERAGE AND INTENSITY OF QPF AT THE LATITUDE OF THE GULF COAST. WE STILL PREFER THE NOTION THAT PRECIPITATION WILL SPREAD EVEN FARTHER NORTH...AS DEPICTED BY THE ENSEMBLE MEANS. QPF FROM THE 12Z GEFS MEAN WAS SPATIALLY SIMILAR TO THE 06Z GEFS MEAN...BUT WITH SOMEWHAT LESSER MAGNITUDE...WHICH BROUGHT THE GEFS MEAN CLOSER TO THE ECMWF ENSEMBLE MEAN. AFTER DISCUSSION WITH THE WPC QPF...WINTER WEATHER...AND MEDIUM RANGE DESKS...WE FAVOR A BLEND OF QPF FROM THE 12Z GEFS MEAN AND 00Z ECMWF ENSEMBLE MEAN. THIS CONSENSUS IS NOT WELL REPRESENTED BY ANY PARTICULAR OPERATIONAL MODEL...BUT WILL BE REPRESENTED IN WPC QPF AND WINTER WEATHER PRODUCTS. ...SOUTHERN STREAM TROUGH PASSING SOUTHERN CALIFORNIA SUN/MON... PREFERENCE: BLEND OF 12Z ECMWF/UKMET CONFIDENCE: AVERAGE THE MULTI-CYCLE TREND IS TOWARD A GREATER LIKELIHOOD OF CLOSED LOW FORMATION AND MAINTENANCE...WITH THE LOW DROPPING TOWARD BAJA AND WESTERN MEXICO. IN PAST CASES THIS WINTER THE ECMWF...WHICH TENDS TO FAVOR GREATER DEPTH AT LONGER LEAD TIMES...WAS THE LEADER IN DEPICTING THIS SCENARIO. THE OPERATIONAL ECMWF...HOWEVER...OFTEN ERRS TO THE DEEP SIDE OF THE ENSEMBLE MEAN. THE 12Z RUN OF THE ECMWF TRENDED BACK TOWARD OUR PREFERENCE...A LITTLE FASTER THAN ITS 00Z RUN...AND CLOSER TO RECENT RUNS OF THE UKMET. THE LATEST TREND IN THE NAM/GFS IS TOWARD FASTER PROGRESSION...BUT WE HAVE LESS CONFIDENCE IN THIS TREND GIVEN THAT SEVERAL GEFS MEMBERS FAVOR THE ECMWF/UKMET. ...PACIFIC NORTHWEST... PREFERENCE: BLEND OF 12Z GFS/ECMWF CONFIDENCE: AVERAGE THE MODELS ARE MOVING TOWARD GENERAL AGREEMENT THAT A SHORTWAVE TROUGH SHOULD SURVIVE ITS VOYAGE INTO THE MEAN RIDGE POSITION ON DAY 3. IT IS A SMALL SCALE FEATURE...SO THERE IS A LOT OF VARIABILITY IN HOW THE MODELS ARE DEPICTING IT. GENERALLY...THE WAVE WILL BE WEAKENING AS IT ARRIVES...BUT DOES HAVE POTENTIAL TO BRING ONSHORE FLOW AND A LIGHT PRECIPITATION EVENT TO THE PACIFIC NORTHWEST. ASIDE FROM THE SLOWER CANADIAN...THE MODELS AT LEAST FORECAST SIMILAR TIMING OF THE WAVE INTO BRITISH COLUMBIA AND WASHINGTON. CONSENSUS AS TO THE STRENGTH OF THE WAVE COULD BE REPRESENTED BY A BLEND OF THE STRONGER GFS WITH WEAKER ECMWF. www.wpc.ncep.noaa.gov/html/model2.shtml... BURKE