400
FXUS10 KWNH 101913
PMDHMD
MODEL DIAGNOSTIC DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
313 PM EDT MON AUG 10 2020
VALID AUG 10/1200 UTC THRU AUG 14/0000 UTC
...SEE NOUS42 KWNO (ADMSDM) FOR THE STATUS OF THE UPPER AIR
INGEST...
12Z NAM/GFS EVALUATION INCLUDING PRELIMINARY PREFERENCES AND
FORECAST CONFIDENCE
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
...LONGWAVE TROUGHING OVER THE NORTHWEST FROM TUESDAY THROUGH
THURSDAY...
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
PREFERENCE: NON 12Z NAM BLEND
CONFIDENCE: SLIGHTLY ABOVE AVERAGE
..19Z UPDATE
ONLY MINOR TIMING DIFFERENCES WERE NOTED WITH THE 12Z
ECMWF/UKMET/CMC COMPARED TO THEIR PREVIOUS 00Z CYCLES, BUT THEY
WERE ALL NOT IN FAVOR OF THE NAM. A NON-12Z NAM BLEND REMAINS THE
PREFERENCE.
..PREVIOUS DISCUSSION FOLLOWS
THROUGH THE MIDDLE OF THE WEEK, THE MODEL CONSENSUS IS FOR A
LONGWAVE TROUGH TO AMPLIFY AS IT REACHES WESTERN CANADA INTO THE
NORTHWESTERN U.S. THE 12Z NAM IS THE FASTEST OF THE DETERMINISTIC
GUIDANCE TO PROGRESS THE TROUGH EASTWARD, NOTED WEDNESDAY INTO
THURSDAY AND THE NAM ALSO LIES ON THE FASTER EDGE OF THE LATEST
ENSEMBLE GUIDANCE. AMPLIFIED FLOW UPSTREAM SHOULD TEND TO LIMIT A
FASTER PROGRESSION DEPICTED BY THE NAM AND EVEN TRENDS IN THE
ENSEMBLES HAVE BEEN SLOWER OVER THE PAST FEW CYCLES.
..REMAINDER OF THE CONUS
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
PREFERENCE: GENERAL MODEL BLEND, LEAST WEIGHT ON THE 12Z NAM
CONFIDENCE: AVERAGE
..19Z UPDATE
THE DIFFERENCES NOTED WITH THE 12Z ECMWF/UKMET/CMC COMPARED TO
THEIR PREVIOUS 00Z CYCLES WERE WITH RESPECT TO CONVECTIVELY
GENERATED VORTICITY MAXIMA. THE 12Z NAM CONTINUES TO APPEAR AS THE
STRONGEST OF THE DETERMINISTIC GUIDANCE, BUT TIMING FAVORS A
MIDDLE GROUND OF THE LATEST GUIDANCE, WHICH INCLUDES THE NAM AT
TIMES.
..PREVIOUS DISCUSSION FOLLOWS
THERE IS WEAK LOW TO MID-LEVEL TROUGHING IN PLACE OVER THE
SOUTHEASTERN U.S. TODAY, DOWNSTREAM OF A LOW TO MID-LEVEL RIDGE
OVER THE LOWER MISSISSIPPI VALLEY. THERE IS A SECONDARY AREA OF
RIDGING LOCATED OVER THE SOUTHERN AZ/NM BORDER, FORECAST TO EXPAND
AND SLOWLY LIFT NORTHWEST THROUGH THURSDAY. UP NORTH,
QUASI-AMPLIFIED FLOW ACROSS THE NORTHERN U.S. INTO SOUTHERN CANADA
WILL IMPACT THE MOVEMENT OF A COUPLE OF COLD FRONTS INTO THE U.S.
THERE IS FAIRLY GOOD AGREEMENT ON A DEPARTING MID-UPPER LEVEL
TROUGH AXIS FROM SOUTHEASTERN CANADA AND THE ATTENDANT COLD FRONT
INTO THE GREAT LAKES AND NORTHEAST, WHICH EXTENDS WESTWARD THROUGH
THE OH VALLEY INTO THE GREAT PLAINS.
THE GREATEST DISAGREEMENTS ARE WITH SMALL SCALE, CONVECTIVELY
DRIVEN VORTICITY MAXIMA WHICH MOVE AND AMPLIFY/DEAMPLIFY WITHIN
THE RELATIVELY WEAK STEERING CURRENTS OVER THE CENTRAL AND EASTERN
SECTIONS OF THE COUNTRY. WHILE NO MODELS APPEAR AS AN OUTLIER
COMPARED TO ENSEMBLE SPAGHETTI PLOTS, THE 12Z NAM APPEARS TOO
STRONG, ESPECIALLY WITH A MID-LEVEL SHORTWAVE/VORTICITY MAX
FORECAST TO BE OVER MO/IA THURSDAY MORNING. OVER-AMPLIFICATION OF
MESOSCALE FEATURES CAN BE A COMMON BIAS IN THE NAM, SO FOR NOW,
LESS NAM WEIGHTING IS PREFERRED WITHIN A GENERAL MODEL BLEND
ACROSS THE REMAINDER OF THE CONUS (OUTSIDE OF THE NORTHWEST TROUGH
DISCUSSED IN THE TOP SECTION).
MODEL TRENDS AT WWW.WPC.NCEP.NOAA.GOV/HTML/MODEL2.SHTML
500 MB FORECASTS AT WWW.WPC.NCEP.NOAA.GOV/H5PREF/H5PREF.SHTML
THIS PRODUCT WILL TERMINATE AUGUST 15, 2020:
WWW.WEATHER.GOV/MEDIA/NOTIFICATION/PDF2/SCN20-58PMDHMD_TERMINATION.
OTTO
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