Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
1258 AM EDT Sun Apr 29 2018

Valid Apr 29/0000 UTC thru May 02/1200 UTC

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

00Z NAM/GFS evaluation...with preliminary preferences
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...Closed low moving through the Northeast today...
...Surface low over New England through Monday...
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Preference: General model blend
Confidence: Above average

The models show similarly with this system.


...Closed low over the Pacific Northwest today, opening into a
trough over the Intermountain West by Monday...
...Lee cyclogenesis over the Plains by Monday with attendant cold
front...
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Preference: Blend of 12Z ECMWF; 00Z GFS
Confidence: Average

As the upper trough moves into the central/northern Plains on
Monday and Tuesday, the 00Z GFS is noted to be a bit quick
compared to the remaining model guidance and contrary to the past
3 cycles of the ensembles which show slowing as well. However,
this difference seen in the 00Z GFS diminishes into
Tuesday/Wednesday while the other models begin to show some timing
differences.

At the surface, a series of lows are forecast with the first in
the vicinity of North Dakota Monday morning. There is generally
good model agreement with this low as it moves into southern
Ontario. A second low is forecast to originate over South Dakota
late Monday and move toward Minnesota by Tuesday morning. The 00Z
NAM is a tad stronger and north with this low compared to the
remaining model consensus. Ensemble scatter low plots show a wide
range of possibilities, with the 12Z ECMWF and 00Z GFS nearest to
the middle of the spread. Meanwhile, the 12Z UKMET/CMC stand out
with stronger vorticity maxima within the southwest flow over the
north-central U.S., affecting surface low strength and placement
by Tuesday evening, lying toward the edges of the ensemble spread.

Overall, a 12Z ECMWF/00Z GFS blend is preferred with average to
above average confidence with the large scale evolution, but with
below average confidence with the details of the individual
vorticity maxima and surface low placement/timing.


...Shortwave digging from the Gulf of Alaska to California by
Monday with low possibly closing off aloft by Wednesday morning...
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Preference: non 12Z CMC blend
Confidence: Average

The latest ensemble means show good agreement through 12Z/02 over
the Southwest, with the 12Z CMC deviating faster than the means
and remaining deterministic consensus. Outside of the 12Z CMC,
there are minor differences seen with a developing closed low over
the Southwest, but a blend is recommended at this time to smooth
out these smaller scale differences, while excluding the faster
12Z CMC.


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

Otto