Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
244 PM EDT Thu Apr 05 2018

Valid Apr 05/1200 UTC thru Apr 09/0000 UTC

...See NOUS42 KWNO (ADMNFD) for the status of the upper air
ingest...

12Z model evaluation with final preferences
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
NAM/GFS initialization errors do not seem to degrade their short
range forecasts.


...Amplifying shortwave trough into the West on Saturday with
accompanying surface cyclone...
...Surface lows emerging into the Plains Saturday night/Sunday
morning...
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Preference: 12Z GFS/ECMWF/UKMET blend through 06Z/08
            12Z NAM/ECMWF/UKMET blend 06Z/08 - 00Z/09
Confidence: Slightly above average

The 12Z CMC was significant different with the 500 mb reflection
(farther south) compared to the remaining deterministic guidance
and continued to appear the least likely model across the West.
Changes between the 00Z and 12Z CMC were small. Timing differences
among the non-CMC guidance had reduced since yesterday, but the
12Z NAM remained slightly quicker to bring the upper
trough/related cold front across the West Coast and with the
closed low into the Pacific Northwest Saturday night.

Beyond Sunday morning, timing differences appeared with the
shortwave which crosses the Intermountain West, to the south of
the weakening closed low across the Pacific Northwest. The 12Z GFS
and GEFS had been trending faster through the latest 00Z/05 cycle
across the Midwest, but the GFS/GEFS leaned toward the front edge
of the ensemble spread. The better agreement with the ensemble
means was toward the 12Z NAM/ECMWF/UKMET...hence the
preference...with the 12Z CMC considered too slow here.


...Southern stream shortwave crossing through the Plains Friday
night with East Coast troughing becoming better established on
Saturday...
...Surface lows tracking across the Southeast and offshore the
East Coast Saturday/Sunday...
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Preference: 12Z GFS/12Z ECMWF blend
Confidence: Average

While trends have continued to be to drop the surface low farther
south along the East Coast Saturday morning compared to
Wednesday's model runs, ensemble spread remained somewhat large
for a 48-72 hour forecast despite fairly good agreement today. The
12Z NAM, 12Z GFS and 00Z ECMWF were pretty much in agreement with
the surface low(s) evolution across the Mid-Atlantic into the
western Atlantic Ocean, but the 12Z GFS was slightly more
suppressed with the surface low located roughly 200 miles east of
the VA/NC border valid 00Z/08.

However, the 12Z ECMWF had also continued a more suppressed
(slower/south) low track trend consistent with what was observed
earlier and more similarly resembled the 12Z UKMET but the 12Z
UKMET was noted to be significantly slower/deeper with the upper
trough evolution across the Southeast and carried a surface low on
the western side of the ensemble spread valid 00Z/08. The 12Z
ECMWF fit better into the ensemble low clustering, while the 12Z
CMC was slightly faster aloft resulting in a surface low tracking
through the western Atlantic more quickly than the consensus.

The preference has been adjusted to nudge toward the trend seen in
the 12Z ECMWF to now include a 12Z GFS/12Z ECMWF blend.


...Disturbance moving through Michigan early Friday morning late
Thursday/Friday and across the Northeast by late Saturday
morning...
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Preference: non 12Z UKMET blend
Confidence: Above average

Outside of the faster/deeper progression at 500 mb noted in the
12Z UKMET, no significant differences were noted with this system.
The result in the 00Z UKMET was for the surface cold front to
progress more quickly through the Mid-Atlantic region Saturday
morning.


Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml
500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml

Otto