Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 241 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: General model blend Confidence: 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. 07z update: The 00z UKMET shifted toward the common ground solution near the initial preference. The 00z ECMWF/CMC only further tightened the overall agreement with the 00z GFS/NAM to support a general model blend at this point at above average confidence. ...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 NAM/ECMWF/CMC 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. 07z update: A slight jog north with a slightly deeper solution brings the 00z ECMWF into a better agreement with the 00z NAM...and while the 00z CMC is a bit faster and further southeast, there is enough similarity (especially in the lowest levels) to have better confidence in this growing consensus. This also makes the 00z GFS appear much too deep, especially with the negative tilt orientation and fairly deep surface result to suggest either elimination or significant reduction in weighting in the preference. At this point will favor a 00z NAM/CMC and ECMWF blend at average confidence. ...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 NAM/UKMET/ECMWF blend Confidence: Average 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. 07z update: The 00z UKMET trended a bit slower to wrapping up the base of the trof through the Red River valley; and while it is a bit deeper than the NAM and more so the ECWMF, there is good timing agreement and lower level mass field response to consider it within the blend. The 00z CMC trended stronger and less flat through the Southern Plains into the lower MS Valley, but like the GFS becomes a shade too fast rounding the base in comparison. The 00z ECMWF is a bit weaker and remains south compared to the NAM/UKMET, but still timed well with initial thinking...enough to supports a 00z NAM/UKMET/ECMWF blend 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. 07z update: A slightly increase spread manifests at the end of day 3 as the closed low reaches N CA/SW OR, a slight slowing in the ECMWF and more significant slowing by the UKMET at the same time frame the NAM shifted a bit faster to increase this spread. At this point, the spread is modest enough to keep initial, general model blend support 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: General model blend weighted toward ECMWF Confidence: 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. 07z update: A significant shift south by the 00z CMC brings it into alignment with the ECMWF especially after early Friday. This while the UKMET also shifted south. The 00z GEFS also trended south and faster with the wave and transition across to the eastern side of the large scale trof on Sat, even more than the 00z GFS. This increased agreement puts the northern focused NAM/GFS on the outside of the spread but not significantly so. As such will favor a general model blend though weighted toward 00z ECMWF. The reduction in spread increases confidence to average. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Gallina