Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
1211 PM EDT Fri Apr 26 2019

Valid Apr 26/1200 UTC thru Apr 30/0000 UTC

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

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

...Overall Pattern Across the CONUS...
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Preference: General model blend
Confidence: See below for each system
            but generally, slightly above to above average.


Strong Polar vortex is anchored over far north-central Canada
directing a fairly active northern stream along the northern tier
of the United States.  The first system currently making final
ongoing phasing with shearing southern stream wave across the
Upper Ohio valley into the Mid-Atlantic today, has come into very
strong agreement though the vast majority of guidance.

The second wave is taking shape with classic baroclinic leaf seen
in WV suite across WA into the Northern Rockies currently.  The
wave as a whole is a bit more elongated/broad with the mid-level
wave and so there is some model variance as it takes a more
concentric shape across the Midwest late Sat into Sun; mainly the
variance is in timing with the 12z NAM/GFS a bit faster relative
to the CMC/ECMWF and a slightly stronger UKMET.  Otherwise, the
evolution and magnitude/axis of QPF is solid enough to support a
general model blend here as well.

Quick on the second system's heels, a stretched/elongated
shortwave drops out of the Canadian Rockies/BC but finding a more
favorable environment in subtle shortwave ridging from the Polar
Vortex, intensifies rapidly into a strong compact wave with lee
surface wave developing in the MT High Plains late Sat into Sunday
that rolls into ND and N MN by Day 3.  The 12z NAM, typical of
late term forecast bias, is very strong/deep with the and while it
has support of the ECMWF/CMC it may a bit too strong.  The GFS had
been a bit weaker overall, likely too weak; however, the 12z GFS
run has trended a bit deeper and while still the weakest solution
of the global guidance is still in line.  Perhaps a general model
blend incorporating the stronger NAM and weaker GFS can offset
each other.  Models are also quite agreeable in the placement of
QPF/convection where the northern stream forcing intersects the
return Gulf moisture across the Central Plains into the Midwest,
enough for general model blend here as well.  Confidence is
average to slightly above average for this system.

The lingering tail end of the northern stream trough does settle
across the Snake River Plain by early Monday...this along with a
slow moving closed low out of the Southern stream will advance
into S California initially with little fanfare.  However, the
associated moisture and daytime heating and favorable jet ascent
pattern will support convection across the Upper Colorado River
Valley back into the terrain of S CA. Likewise to other CONUS
locations, the mass fields show similar depth, evolution to the
merging of these features late Monday, enough to support a general
model blend.  Small, but typical timing differences are noted,
with the CMC/ECMWF slower to advance while the 12z GFS/NAM are
faster and slightly earlier to weaken/shear into the growing
larger scale trof in the Four Corners region. 

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

Gallina