Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
256 PM EDT Tue Jul 21 2020
Valid Jul 21/1200 UTC thru Jul 25/0000 UTC
...See NOUS42 KWNO (ADMSDM) for the status of the upper air
ingest...
12Z Final Model Evaluation Including Preferences and Forecast
Confidence
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...Northern tier of the CONUS...
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Preference: Model blend early then decreasing GFS influence
Confidence: Average
...18Z Update...No change to previous preference given decent
run-to-run agreement from non-NCEP models
Fast flow atop an expansive ridge will persist across the northern
tier of the CONUS going through the middle of the week. A series
of weak to moderately-strong shortwaves embedded within the larger
scale flow will traverse west to east, causing at least subtle
amplification of troughs into the ridge located to the south. The
NCEP models remained in good agreement for most of the period,
with the 21/12Z GFS still being a tad stronger and faster with its
shortwave evolution across the northern tier of states. A general
model blend should work for most of Day 1...then an blend with
less GFS influence is preferred on Day 2...and a non-GFS blend by
Day 3.
...California/Southwest U.S...
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Preference: 12Z NAM/ECMWF/UKMET
Confidence: Average
Through 36 to 48 hours, a general model blend is acceptable as the
mass fields spread is minimal. However, a complex evolution begins
early D2 as a northern stream trough digs into WA state, while a
closed low pivots near central CA, and a third vort lobe ejects
northeast from the Four Corners towards the Northern Rockies. The
21/12Z GFS is likely too strong...similar to previous GFS
runs...with the vort dropping into WA which then phases too much
energy of the closed low near CA rendering it mostly dissipated,
or as just a weakness. The NAM/ECMWF/UKMET are all in reasonable
agreement with all three impulses to comprise the blend.
It is worth noting that this energy over the Southwest coupled
with an influx of deepening monsoonal moisture from northern
Mexico and Rio Grande Valley is expected to help drive an uptick
in the coverage and intensity of monsoonal convection across the
Four Corners region through the middle of the week.
...Mid-level easterly waves impacting the southern CONUS...
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Preference: NAM/GFS/ECMWF
Confidence: Slightly above average
A series of mid-level troughs/easterly waves with at least a
modest surface reflection will move across the Gulf Coast region
through midweek. The UKMET and CMC are both much flatter/weaker
with this wave activity, but the remaining guidance is in
reasonably good agreement in tracking this energy across the Gulf
of Mexico and into Texas...with a growing consensus for weak/broad
low pressure forming in the western Gulf on Thursday.
Also noted was a closed low at 250 mb making its way along 30N
over the waters of the western North Atlantic Ocean...approaching
the southeast U.S. coast by 24/12Z in the GFS and ECMWF. Given
the fact the low has little reflection by 500 mb...no preference
needed here although it is a feature worth probably worth watching
in subsequent runs.
Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml
500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml
This product will terminate August 15, 2020:
www.weather.gov/media/notification/pdf2/scn20-58pmdhmd_termination.
pdf
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