Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 1213 AM EDT Sun Apr 22 2018 Valid Apr 22/0000 UTC thru Apr 25/1200 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 00Z Model Evaluation including preferences and forecast confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Lingering trough over the Northeast US, with progressive shortwave moving it through the Northeast by Monday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Above average General consensus appears to be made with the shortwave crossing the Northeast on Sunday/Monday...sharpening a bit into the Northwest Atlantic. As such general model blend is supported at above average confidence. ...Strong closed low over the southern Plains, pushing into the Southeast by Monday and then opening into a trough through Central Ohio Valley Wed... ...Inverted surface trough in the western Gulf Coast region, with weak surface low developing in the central Gulf Coast states Sunday...GA Tuesday and Carolinas Wed... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Average There is fairly good agreement with the mass fields through 23/12Z Monday, as the 12z ECMWF/CMC shift a bit south and east likely under influence of upscale growth/enhancement from stronger latent heat release. This also shifts the surface low/triple point south compared to the NAM/GFS which favor the deeper occluded sfc low across TN. Eventually, there is a transition back toward better consensus by late Tuesday as the surface low lifts north through the Carolinas. Spread aloft is increases a bit as the ECMWF slows under greater influence of a slightly faster/sharper upstream trof through the Great Lakes. This likely directs stronger moisture flux further south and slows northward progression by the end of the forecast period; this while the 12z UKMET/CMC and 00z GFS/NAM both lift faster through the central Appalachians into the southern Mid-Atlantic. While each have a small issue, a general model blend appears a good compromise to handle the smaller spread/timing issue...at least mass-wise. Confidence is average in this blend as the blending likely flattens/smooths out the features a bit, so shifting the features while retaining the magnitude of the sensible weather to that central position would be best to keep integrity of said magnitude. ...Secondary shortwave reaching the Pacific Northwest early Sunday; amplifying and pushing into the Northern Rockies Monday and northern Plains Tuesday before sliding south in Central Plains Wed... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 12z UKMET/ECMWF blend Confidence: Slightly below average A fast/subtle shortwave approaches the OR/N CA coast by 12z Sunday and starts to amplify and broaden across the Northern US Rockies. A growing consensus is emerging toward this broader more cyclonically symmetric solution toward midday Monday providing confidence through this time period. However, as the wave exits the terrain, into the high plains, a shortwave in the northern stream inserts influence in either amplifying or sliding past leading to a split in evolutionary paths. The 00z GFS/NAM both are stronger further north initially (likely due to bias of being initially fast), this allows for increased amplification of the wave crossing into the Northern Plains before dropping south through the MO River Valley. The 00z GFS in fact even trended more amplified and faster translating through the Midwest into the lower MO Valley by Wed. The ECMWF/CMC both are less influenced and the shortwave is "back loaded" toward the SW portion of the wave through the Rockies, and as such the shortwave is slow to exit the terrain and slides southwest with increased positive tilt and stretching through the Central Plains by Wed. The UKMET while more wrapped up exiting Absaroka Range Monday or "front loaded" with the shortwave...quickly shears/stretches under influence of the northern stream wave...allowing for a more ECMWF/CMC evolution to manifest with the wave by Day 3. These features appear well agreed upon to exist and are fairly timed well in the guidance suite, but it is clear small differences in strength/amplification lead to large end output, so confidence is reduced. Unfortunately, the ensemble suite generally favors their parent deterministic solutions and an overall trend is not apparently clear in the last 4 cycles in spaghetti analysis. Placement of jet streak/overall flow regime and conceptual meteorological model would suggest a faster northern stream and less influence, favoring something close to the 12z ECMWF/UKMET. As such a 12z UKMET/ECMWF blend will be the initial WPC preference, but reserve the right to shift toward any growing consensus with the 00z GEFS/UKMET/ECMWF/CMC solutions. ...Sharpening/digging shortwave in the middle section of the larger scale trof in the Great Lakes by Wed... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 12z UKMET Confidence: Slightly above average In the wake of strong central Canadian cyclone lifting north and east toward the southern Hudson Bay/James bay by Tuesday...an upstream shortwave digs southeast splitting and sharpens as it reaches the axis of the larger scale global trof over Ontario to TN Valley. The bulk of shortwave energy will be focused around the L.P. of Michigan by the end of the forecast period. Uncharacteristically, the 12z ECMWF/CMC are both faster with the shortwave feature likely under reduced influence of the Pacific shortwave across the central US/Midwest (see section above). The 12z UKMET is also less influenced by this wave but also a bit slower than the ECMWF/CMC, more in line with the 00z NAM/GFS. The 12z ECENS members suggest the ECMWF may be a bit too fast as well. The 18z GEFS mean is also a bit sharper and faster than the 00z GFS looking very similar to the UKMET providing additional confidence toward it over the deterministic 00z GFS. As such preference will be the 12z UKMET anchored by the 18z GEFS/12z ECENS means. Inclusion of the other deterministic models may be ok, but would likely lead to a broader/weaker wave, that is not likely to verify given agreement in the sharp nature of the trof. Confidence is slightly above average, as the differences are small and the UKMET is supported by the ensemble averages. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Gallina