MODEL DIAGNOSTIC DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 113 PM EDT SAT SEP 13 2014 VALID SEP 13/1200 UTC THRU SEP 17/0000 UTC ...SEE NOUS42 KWNO (ADMNFD) FOR THE STATUS OF THE UPPER AIR INGEST... =================================================== 12Z NAM/GFS EVALUATION AND PRELIMINARY PREFERENCES =================================================== THE NAM INITIALIZED 500 MB HEIGHTS TO BE 5 TO 17 METERS COOLER THAN RAOBS THROUGHOUT THE WESTERN US. THIS MAY CONTRIBUTE TO THE NAM HAVING A DEEPER SOLUTION IN THE EASTERN U.S. OVER TIME. =================================================== ...SHORTWAVE TROUGH EXITING THE EAST COAST BY SUNDAY MORNING... PREFERENCE: BLEND OF THE OPERATIONAL MODELS THIS SYSTEM WILL PRODUCE RAIN OVER THE NORTHEAST AND THUNDERSTORMS FROM THE CAROLINAS TO THE GULF COAST...BEFORE EXITING INTO THE ATLANTIC OVERNIGHT. THERE ARE NO LARGE SCALE MODEL DIFFERENCES. ...EASTERLY WAVES AFFECTING THE GULF COAST STATES... PREFERENCE: BLEND OF THE OPERATIONAL MODELS ONE WAVE WAS PRODUCING DEEP CONVECTION IN FAR SOUTH TEXAS EARLY SATURDAY. THE PARENT UPPER LEVEL FEATURE IS FORECAST TO WEAKEN...BUT MAINTAIN SOME IDENTITY AS A LOW AMPLITUDE SHORTWAVE ROUNDING THE WESTERN SIDE OF THE FLAT UPPER LEVEL RIDGE...AND COMING BACK EASTWARD ACROSS THE SOUTHERN U.S. ASSOCIATED MOISTURE MAY INFLUENCE CONVECTION OVER THE SOUTHEAST STATES BY DAYS 2/3. MEANWHILE...A BROAD EASTERLY WAVE IS FORECAST TO SLIDE FROM SOUTH FLORIDA TOWARD THE CENTRAL GULF OF MEXICO DURING THE NEXT COUPLE OF DAYS. INVERTED TROUGHING MAY EVENTUALLY EXTEND INLAND AND HELP TO FOCUS RAINFALL...ESPECIALLY IN SOUTHERN LOUISIANA AND ADJACENT PORTIONS OF MS/TX. THE GLOBAL MODELS ARE SIMILAR IN HANDLING THE PROGRESSION OF THIS SYSTEM...AND IN THEIR DISTRIBUTION OF RAINFALL. THE 12Z NAM FITS WITH THE GLOBAL MODELS. ALTHOUGH IN THE WESTERN GULF THE NAM DOES PRODUCE A MORE ROBUST DISTURBANCE WITH STRONGER WIND FIELDS BY DAY 3...ITS TIMING OF PRECIPITATION IS SIMILAR TO THE OTHER MODELS WHILE KEEPING THE HEAVIEST PRECIPITATION OFFSHORE THROUGH 17/00Z. ...WESTERN U.S... PREFERENCE: BLEND OF THE 00Z GEFS MEAN AND ECMWF ENSEMBLE MEAN BY TUESDAY A DEEP PACIFIC TROUGH WILL DIG ALONG 138 DEGREES WEST. THIS SETS UP AMPLIFIED SHORTWAVE RIDGING OVER THE INTERIOR OF THE WESTERN U.S. WEAK HEIGHT FALLS MAY BRING UNSETTLED WEATHER TO THE PACIFIC NORTHWEST...BUT MORE IMPORTANTLY...MOISTURE FROM HURRICANE ODILE IN THE EASTERN PACIFIC IS LIKELY TO OVERSPREAD PARTS OF THE SOUTHWEST U.S. BEGINNING AS EARLY AS TUESDAY AFTERNOON. THE ECMWF ENSEMBLE MEAN HAS TRENDED STEADILY EASTWARD WITH ODILE...KEEPING IT NEARER TO BAJA CALIFORNIA. THE GFS HAS NOT YET JOINED THE TREND...BUT THE 12Z NAM DID TREND EAST. REGARDLESS...THE LARGE SCALE PATTERN PRODUCES AN INCREASE IN MOISTURE THAT IS NOT DISSIMILAR BETWEEN THE GEFS AND ECMWF ENSEMBLE MEANS...WHICH ARE CURRENTLY A LITTLE MORE GENEROUS WITH PRECIPITATION THAN ARE THE OPERATIONAL MODELS...ESPECIALLY THE GFS. FOR THAT REASON WE WILL CONTINUE TO FAVOR THE ENSEMBLE MEANS. PLEASE SEE THE WPC MEDIUM RANGE DISCUSSION FOR MORE DETAILS. ...LARGE TROUGH AND RELATIVELY DRY/MILD CONDITIONS EAST OF THE ROCKIES... PREFERENCE: BLEND OF 12Z GFS WITH 00Z ECMWF/CANADIAN ON THE HEELS OF THE INITIAL HEIGHT FALLS TRACKING THROUGH THE GREAT LAKES...A LONG WAVE TROUGH WILL REMAIN ESTABLISHED ACROSS THE NORTHERN TIER OF STATES. ADDITIONAL SURGES OF SURFACE HIGH PRESSURE WILL REINFORCE GENERALLY COOL TO MILD CONDITIONS. THE MODELS SHOW GOOD AGREEMENT ON THE LARGE SCALE...BUT FOR THE DETAILS WE PREFER THE GFS/ECMWF. THE NAM APPEARED TOO DEEP/COLD ALOFT. THIS MAY OWE...IN PART...TO THE INITIALIZATION ERRORS NOTED ABOVE...ALTHOUGH DEEP/COLD NORTHERN STREAM TROUGHS ARE ALSO TRADITIONALLY A NAM BIAS. THE 00Z UKMET WAS MORE AGGRESSIVE WITH SURFACE HIGH PRESSURE AND LOW LEVEL COLD AIR INTO THE TENNESSEE VALLEY AND SURROUNDING STATES. www.wpc.ncep.noaa.gov/html/model2.shtml... www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml... BURKE