Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 1250 AM EDT Wed Apr 25 2018 Valid Apr 25/0000 UTC thru Apr 28/1200 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 00Z model evaluation including preferences and forecast confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Closed low over the Cumberland Plateau, gradually opening into a trough and lifting into the Mid Atlantic and Northeast while phasing with northern-stream shortwave closing off through the Great Lakes late Wednesday before lifting into the Northeast Thursday... ...Surface low over Carolinas translating into Mid-Atlantic/Northeast later today into Thurs... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-UKMET blend Confidence: Slightly above average After days of consternation over model preference with respect to timing/phasing of northern stream closed/nearly closed shortwave and the decaying TN/OH valley upper low...there appears to be a growing common solution. The 12z UKMET remains a clear outlier in the tightening ensemble spaghetti analysis plots, deeply closed/west of the remaining solution. The 12z ECMWF/CMC continue to be a bit more wound up with the wave into the Eastern Great Lakes and suggest a faster acceleration of the decaying Southeast wave into the northeast...while the 00z NAM/GFS are more aligned/phased with a greater negative tilt orientation to the combined trof through the Northeast. The 00z GFS shifted slightly deeper and slowed compared to the NAM/GEFS/18z GFS to further tighten the cluster. At this point a Non-UKMET blend is supported, leaning ECMWF/CMC if subtly so. Confidence is slightly above average. ...Elongated trough from Central Rockies to Mid-MO Valley drops southeast to the lower MS Valley Thursday before weakening through the Southeast by Friday with surface low crossing South early Thurs to Mid-Atlantic by late Friday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 00z GFS/NAM and 12z ECMWF blend Confidence: Average Energy crossing the northern Rockies and High Plains will drop southeast over the next couple of days and dig in toward the southeast U.S. by Friday. The 12Z UKMET becomes generally a weaker and faster outlier in digging the energy across the South, with the best model clustering generally residing with the 00Z NAM, 00Z GFS and 12Z ECMWF. All of this energy is expected to drive an area of low pressure across the interior Southeast by Thursday and then northeast up toward the Mid-Atlantic region by Friday. However, there is some notable spread with respect to the placement of low pressure at the end of the period as the guidance differs with degree of possible northern stream interaction at that time. The 00z NAM and GFS are a bit deeper rounding the base of the larger scale trof through the interior Southeast and so are a shade north with a tighter radius of rotation; however both (more so GFS) did trend wider and further southeast. The 12z ECMWF is also quite quick to break down the upper low and even though it is moving into larger scale confluence/ridging in the Carolinas/Mid-Atlantic...it may be uncharacteristically too fast in doing so. The 12z CMC is not too bad showing a similar track to the ECMWF, but is as usual a bit too slow to be considered at this time. ...Northern stream portion of Gulf of Alaska trough breaking off and sliding southeast through Northern Rockies early Thursday then starting to anchor the Global scale trough from the Great Lakes to the lower MS Valley late Friday with base swinging off southeast Coast Sat. ...Frontal zone through northern Plains Wed/Thurs stalling in Great Lakes Fri... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 00z GFS/NAM and 12z ECMWF thru 28/00z 00z NAM/12z ECMWF thereafter Confidence: Slightly above average thru 28/00z Average afterward GOES-WV mosaic depicts the incipient development of the next trof over-topping the western ridge in the Canadian Rockies. By tomorrow, the wave will expand in a full-latitude but narrowing trof that slides southeast from the northern Plains to Central Plains. With time, the northern and southern portion of the trof have stretched to the point of breaking with the base of the trof beginning to collect into a cyclonic/consolidated wave, especially by early Friday entering the Lower MS valley. The 12z UKMET, however was earliest to do so, and is nearly mature by reaching NE TX by 12z Fri. The 00z CMC on the other side of the spectrum is timed ok, but remains more stretched/elongated with positive tilt orientation...which further compounds late Fri into Sat in the Southeast. The 00z NAM/GFS and 12z ECMWF remained timed well until this time late Fri at the base of the parent trof. The 00z GFS/NAM are both north but the GFS shows typical fast bias by the end of day 3, exiting the SE coast, while the NAM/ECWMF are slowed. The ECMWF was always south of the other guidance cluster, and so may be too far south into the far NE Gulf, but would prefer this timing with the NAM over the GFS. As such a 00z GFS/NAM and 12 ECMWF blend is preferred at slightly above average confidence up to 28/00z shifting to NAM/ECMWF after at average confidence. ...Closed low evolution toward the West Coast by Friday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Slightly above average Now that the closed low has matured with the main shortwave placement relative to the center of the main vortex...timing, strength and overall shape of the upper low have come into starkly better agreement even by day 3. While there remains smaller scale internal details yet to be resolved, many of those details are typical of known guidance biases and are best handled by blending them to a common solution. As such a general model blend is preferred at slightly above average confidence. ...Fast moving clipper wave across south central Canada toward N MN by early Friday...amplifies to anchor the northern portion of the Long Wave trof over Ontario Sat...transfer of surface wave to deep low over Ontario/Quebec late Fri/Sat... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-CMC blend Confidence: Average to slightly below average A strong fast moving jet will over-top the larger scale ridge in north central Canada Thursday and rapidly accelerate SE supporting a subtle surface wave that reaches N MN early Friday. The wave eventually amplifies to anchor the larger scale broad trof over Ontario into the weekend. As such the stalled surface front from the prior trof (2 sections above) will buckle and support a deepening low across SE Ontario and SW Quebec as well as congeal with the surface low lifting out the Mid-Atlantic (see 3 sections above). The 12z is uncharacteristically fast with the shortwave/nose of the jet into the western Great Lakes but this is likely due to a delayed/slower downstream trof that is also much stronger/deeper by Sat. This leads the CMC to have significant differences in the surface pattern and makes it less favorable overall. The 12z ECMWF is initially a bit south of the remaining guidance and is a bit faster reaching the CONUS with a bulk of the ensemble suite cluster (including some CMCE/GEFS). The 12z UKMET and 00z GFS/NAM are all a bit deeper at the surface but also further north. Still, the ECMWF slows at the base of the large scale trof while the GFS/UKMET and lesser so NAM transfer to the developing SE Ontario low a bit quicker and northeast. While the spread is modest...the overall shape, evolution look similar enough to support a Non-CMC blend at this point from validation. Confidence is average to slightly below average due to the spread within the blend. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Gallina