Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
223 PM EST Thu Dec 06 2018

Valid Dec 06/1200 UTC thru Dec 10/0000 UTC

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

12Z Model Evaluation...with Final Preferences and Confidence
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...Overall Pattern Across the CONUS...
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Preference: General model blend
Confidence: Above average

The primary weather maker through the period is low pressure
currently off southern California. This low will drift southeast
to northern Mexico as it opens into a neutrally tilted trough
through Friday before reaching the lower MS Valley late Saturday
and the southeast coast Sunday. The combination of Pacific and
Gulf moisture allows precipitation to blossom over the southern
Plains and lower MS Valley Friday through Saturday as surface low
pressure develops. Threats of heavy rain near the Gulf Coast and
wintry weather inland on the north side of the precipitation
shield are expected.

There is relatively good agreement with the upper level open wave
into Saturday as it reaches TX. The trough deepens again Saturday
night over the lower MS Valley with some indication of it closing
again at 500mb; the 12Z NAM and ECMWF have a brief close over TX
while the 12Z GFS and NAM closes over KY/TN. Influence of the next
low descending from the northern Great Plains on the initial low
over the southeast begins Sunday. That secondary low is fully in
continental air with little to no associated precipitation until
it phases in with the primary low Monday, extending precip over
the southeast. The focus is on the southeastern CONUS trough that
will be a notable winter storm. The 12Z GFS is still the most
progressive model with the surface low off the SC coast Sunday,
though (like yesterday) the 12Z run is less progressive than the
06Z. The 12Z CMC shifted the surface low back farther south and
into the consensus envelope and is no longer an outlier.

An equal general model blend will suffice for precipitation
purposes.

The other area of notable precipitation in the CONUS in the 3-day
forecast range is with a trough reaching the coastal Pac NW
Sunday. A few differences become apparent Sunday with the 12Z
NAM/GFS more amplified with more precip for WA/OR. The 12Z
ECMWF/UKMET/CMC are in good agreement with Vancouver Island having
the Day 3 QPF max. A non-NCEP blend is preferred for the Pac NW on
Day 3.

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

Jackson