Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
226 PM EDT Mon Oct 21 2019

Valid Oct 21/1200 UTC thru Oct 25/0000 UTC

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

12Z Model Evaluation with Preferences and Confidence
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Central U.S. cyclone; secondary low from Carolinas to New England
Tue-Wed.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Preference: General model blend
Confidence: Above average

1830z Update: No changes in preference with this system. The 12z
ECMWF did indeed speed up a bit over New England and is now more
in line with the other guidance.

...Original Discussion...

The low over the upper MS valley will lift northward into Canada
over the next couple days. Slight strengthening of the low will
continue today, however by tonight the system will become stacked
and fill in as it lifts northward. A secondary low will form over
the Mid Atlantic Tuesday and lift northeastward into New England
through Wednesday. Overall guidance is in good agreement with the
evolution of this system. The 0z ECMWF is probably a tad slow with
the front moving across New England Tuesday night into Wednesday,
otherwise a general model blend should suffice for this system.


Stronger shortwave dropping through Rockies into the southern
Plains early to mid week
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Preference: Blend of the 12z ECMWF/ECMWF mean/CMC/UKMET
Confidence: Slightly below average

1830z Update: The 12z non NCEP models have backed off a bit from
their very deep/slow trough over the southern Rockies/Plains by 0z
Friday...however they still remain much slower/deeper than the 12z
NAM/GFS. The new 12z ECMWF/CMC are actually now more in line with
the 0z ECMWF ensemble mean...although the spread in the ECMWF
ensemble members is high. Overall think blending the 12z
ECWMF/ECMWF mean/CMC/UKMET probably results in a good, most
probable, forecast at this time. Of course given we are 3 days out
and the spread remains high, some changes are likely over the
coming days. But for now the aforementioned blend seems like the
best option. The 12z GFS/NAM still seem too progressive, and while
not impossible, remain a lower probability outcome. Given the
lingering spread will leave the confidence at slightly below
average.

...Original discussion...

Models are in pretty good agreement though 12z Wed (23rd) with the
shortwave energy dropping southward through the Canadian Rockies
and into the northern Plains. Thereafter spread increases
significantly. The non NCEP guidance (ECMWF/UKMET/CMC/EC MEAN) all
show a stronger wave and a trough digging deeper to the southwest
across the Rockies and Plains...eventually resulting in the
development of a cutoff low. The 12z GFS and NAM show a weaker and
more progressive system.

The last several runs of the ECMWF have been very consistent with
the deeper and slower trough...and with the other non NCEP models
trending in this direction as well...it would appear this
slower/deeper solution is becoming increasingly likely. Thus WPC
will continue to prefer a non NCEP model blend for this system
after 12z Wed. Confidence has increased some, but is still
slightly below average given the spread...however if the new 12z
non NCEP models remain consistent with their deeper/slower
solutions, then confidence would increase towards average for this
system.


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

Chenard