Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 1234 AM EDT Mon Apr 23 2018 Valid Apr 23/0000 UTC thru Apr 26/1200 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 00Z model evaluation including preferences and forecast confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Strong closed low over the Lower MS Valley today, gradually opening into a trough and lifting into the Mid Atlantic and Northeast, phasing with northern-stream shortwave digging through the Great Lakes Tuesday Night into the Northeast... ...Weak surface low over the interior Southeast, redeveloping along the Atlantic coast by mid-week... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 00z GFS/12z CMC blend Confidence: Slightly below average Models continue to show solid agreement through the Southeast at least through early Wed when the trof opens up and there is increasing influence of the digging trof crossing the Great Lakes. By late Wed into Thursday, the timing/amplification and spacing of the two features lead to large model differences across the Mid-Atlantic into the Eastern Great Lakes/Northeast. The 12z UKMET is a clear outlier depicting a much sharp but much further south trof advancing through the northern Ohio Valley. This wave undercuts the weakening/opening shortwave through the Mid-Atlantic resulting in a broader developing closed low feature with maximized DPVA through the Mid-Atlantic coast Thursday resulting in a deep surface reflection and strong moisture conveyor into ME. The 00z NAM shows a similar evolution but rather than the upstream wave being strong, its the coastal wave that dominates the interacting pair. This is well timed and matches a solid clustering of ECENS members but stronger wind fields/moisture and deep surface low show signs of typical day 3 NAM over-amplification issues. The 00z GFS has shown a solid shift in timing out of the Cumberland plateau toward the Carolinas with the opening trof. This slowing here leads to a southeastward shift with the track as it lifts northeast. Additionally, this matches timing as the Great Lakes trof starts to swing negative tilt and close off into a weak concentric low, which is generally favorable in the mean timing of the ensemble suite. The 00z CMC, while a bit stronger with the upstream wave than the GFS shows a similar evolution in timing and seems a good compromise over the faster/stronger NAM/UKMET. The 12z ECWMF is very slow and strong with the Great Lakes wave, and closes off, helping to sling the opening wave through the Mid-Atlantic and presenting a very deep surface reflection. This is not even supported well in the ECENS...as about half to 2/3rds are not this robust with closing off the mid-level low. As such the ECMWF cannot be supported in the blend (especially after Wed), though the ECENS mean could help account for some of the possibilities. Still, a 00z GFS/12z CMC blend is preferred at slightly below average confidence. ...Shortwave entering northern Great Basin, amplifying into the northern Rockies through Monday Night, and then digging through the Central Plains Wed/Lower MS Valley by Thurs... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 00z GFS and 12z ECMWF/CMC blend Confidence: Slightly above average Global guidance shows a solid agreement with the amplification of the norther Great Basin/Oregon Desert shortwave across the Northern Rockies tonight and sliding south and east along the high terrain as the northern stream wave lifts away reducing influence. The GFS finally backed away from the faster/stronger more concentric wave through the MO river valley and now tracks similarly to the ECMWF/CMC Wed into Thurs. The wave will amplify nearing the base of the larger scale trof across the eastern third of the US as it enters the Lower MS Valley. The ECMWF/CMC both are bit broader and south, intersecting moisture return off the Gulf a bit more readily than the GFS but still very close. The 00z NAM while much better than the slower/closed low with the 18z run by 12z Thursday, still is a bit too slow to the better consensus and remains a bit more wound up. The 12z UKMET in the most connected with the northern stream trof, so the top of the trof digs deeply across the Great Lakes/Ohio River Valley while leaving the base of the trof languishing across the Colorado Rockies. This has little to no backing support from other guidance and ensemble members...so is not considered for a blend. WPC preference will favor a 00z GFS and 12z ECMWF/CMC blend at slightly above average confidence. ...Northern stream portion of Gulf of Alaska trough breaking off and digging through the Canadian Rockies Wed, continuing into the northern Plains Thursday, with associated surface cold front pushing southeast... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 00z GFS and 12z UKMET/ECMWF blend. Confidence: Slightly below average A weak trof connecting a deep low over the Bering Sea and the developing closed low near 35N 135W (section below)...will progress out of the eastern Gulf of AK under the influence of an approaching strong upper-level jet and shorten the wavelength. An amplified ridge pattern across the Western US will help deflect the energy through BC though as it over tops the ridge breaking it down slightly on Wed...trof will descend into the Northern US Rockies and progress into the High and Northern Plains toward Thursday. The 00z NAM is a bit stronger and broader with the wave hinting at some over-amplification/compact cyclogenesis issues typical of day 3 bias. The 12z CMC is also a bit broader and therefore does not extend south (southwest) into the North Central Rockies. The 12z ECMWF which shows a bit stronger overall ridging is therefore a bit faster emerging out of the Pacific though also with further west ridging does extend into the Rockies more than other guidance. The GFS had been very weak with the ridge showing significant suppression as the trof passed, thought the 00z iteration was less so and therefore extended into the terrain more. The GFS while not as broad as the ECWMF shows good agreement with a narrower/similarly timed 12z UKMET. As such will favor a 00z GFS and 12z ECMWF/UKMET blend though modest spread and recent moderate shifts in timing/strength lead to average confidence in this blend. ...Southern stream portion of Gulf of Alaska trough breaking off and developing into a closed low around 35N/135W by Tuesday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 12z UKMET/CMC blend Confidence: Slightly below average Strong downstream amplification of western US ridge will support the base of the larger scale trof to dig south and close off a broad low out near 35N and 135W by Tuesday. Ensemble trends suggest a strong ridge with good consistency. The 12z ECMWF/ECENS mean are a bit more bullish and shifted a bit west of the ensemble consensus but are supported by the UKMET/CMC and CMCE members though GFS/GEFS members are out of phase suggesting the northern stream shortwave will suppress the ridge allowing for eastward migration of the closed low as the internal shortwave features are a bit more robust and set wider to the center of the center of the parent closed low. Conceptually, with orientation of jet axis and location to higher terrain...this appears a bit too aggressive. The 00z NAM is similar to the 18z GFS but supports a slightly enhanced ridge than the GFS. The 00z GFS did strengthen the ridge slightly and shift a bit west with the closed low but still remains on the eastern side of ensemble suite. The UKMET and CMC are more central to the ensemble suite and run to run continuity though also closer to the ECMWF. WPC will prefer a 12z UKMET/CMC blend to best represent the ensemble trends/mean. The breadth of the spread leads to slightly below average confidence at this time. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Gallina