Prognostic Meteorological Discussion
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FXUS10 KWNH 231833

Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
232 PM EDT Wed May 23 2018

Valid May 23/1200 UTC thru May 27/0000 UTC

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

12Z Model Evaluation...with Final Preferences and Confidence

---18Z UPDATE---

No change to the preliminary preference. The 12Z ECMWF has slowed
the progression of the low in the Gulf further, as compared to its
00Z run, which was somewhat expected. The overall track did not
change appreciably, though. The 12Z CMC and 12Z UKMET have moved
into better agreement with the ECMWF elsewhere across the CONUS,
with the GFS more distinctly different in the West -- still with a
faster progression of the trough.


...Surface low development over the Gulf of Mexico by Saturday...

Preference: Blend of 00Z ECMWF; 00Z ECMWF Ensemble Mean
Confidence: Slightly below average

The ECMWF appears to have trended in the right direction with
respect to the timing of the surface low over the Gulf. Two model
cycles ago (22.00Z run), it had a surface low approaching the
mouth of the Mississippi River by Friday evening (26.06Z), but now
delays that in its most recent 00Z run to about a day later
(27.06Z). Given the lack of significant development thus far,
would not be surprised by further trends in this direction.
However, the overall progression indicated by the model appears
more reasonable today. It is also fairly close in proximity to the
ECMWF Ensemble Mean and a blend of the two is the preference for
this system at this time.

Did not include the 12Z GFS as it still appears to tie the surface
low progression to a burst of convection, eventually tracking it
across the Florida Peninsula. The 00Z UKMET is furthest south and
southeast now and does not have a lot of other model support. The
12Z NAM and 00Z CMC are closer to the ECMWF, but the NAM is the
furthest west (which also doesn`t have a lot of support at the
moment) and the CMC is faster and deeper with the surface low
indicating a greater pace of development unsupported by
observational data.

...All other areas and systems of interest...

Preference: Blend of 00Z ECMWF; 00Z ECMWF Ensemble Mean
Confidence: Average

For the remainder of the CONUS over the next several days, models
generally agree on the large scale features -- dominated by a
trough and associated closed low progressing into the Great Basin
this weekend, downstream ridging in the Plains, and low amplitude
troughing evolving in the Great Lakes. The overall flow pattern
remains fairly weak, so there are also a variety of embedded
shortwaves (some perhaps convectively driven) that may also affect
sensible weather.

Model differences are not substantial, but are more noteworthy
with the progression of the trough across the West. The 12Z GFS
shows a typical bias for a faster trough (as does the GEFS Mean),
with the 00Z UKMET the slowest and the 00Z ECMWF, 12Z NAM and 00Z
CMC in the middle. The preference is to lean toward the ECMWF and
its ensemble mean overall, but the NAM and CMC also seem to be
close approximations that could be included in a blend as well.

Model trends at
500 mb forecasts at


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