Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 1239 PM EDT Mon Mar 12 2018 Valid Mar 12/1200 UTC thru Mar 16/0000 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 12Z model evaluation including model preferences and forecast confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Closed shortwave energy crossing the southern Appalachians with developing coastal low off Carolinas that will track up the East Coast reaching Nova Scotia Wed enhanced by phasing height falls out of Great Lakes... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-UKMET blend to (14/12z)...Non-GFS thereafter Confidence: Slightly above average to 15/00z...Average thereafter GOES-16 WV suite depict vigorous shortwave exiting the southern Appalachians currently as well as digging jet streak across the Upper Great Lakes sharpening the base of the lead height falls. This sharpening has become better agreed upon including the spacing/phasing with the southern stream wave to build much higher confidence overall in the near (1-2 day term). The 12z NAM has backed up slightly in timing to further tighten agreement; while it is a bit quicker to the lowest heights across New England with the northern stream energy in the 5-7H layer it is not out of the realm of possibility to continue to remove it from WPC preference. The 12z GFS while much better than the 06z run remains on the eastern/northern side of the surface to 7H low cluster after 36hrs. The 00z UKMET is showing a stronger more wound up solution/deeper at the surface to make it suspect for inclusion,especially for a tighter/more confident blend. As the system transfers from peak maturity after early Wed...there is some modest spread in guidance as the coastal low loops back cyclonically to the northwest in New Brunswick/St. Lawrence Seaway. Here the 00Z ECMWF/UKMET both remain best clustered and faster with the GFS having been shifted east of the main clustering at landfall in Nova Scotia further compounding with slower NW then W rotation within the larger parent vortex. The 12z NAM is more central to the two factions with a very tight wobble respectively. All considered would reduce/remove more of the GFS influence further in time within the blend. Overall, this supports a 00z ECWMF dominant weighting blend with the UKMET removed early and the GFS removed late in the period; while the 12z NAM/00z CMC are solid contributors. Non-UKMET blend up to 48hrs (14/12z) and Non-GFS blend after 48hrs. Given the reduced spread/increased consensus of the low over the water; confidence is slightly above average but breaks down to average after landfall (15/00z). ...Remaining upstream shortwave/closed low energy through western Great Lakes Tues swinging through Mid-Atlantic/N Carolinas late Wed with surface trof/weak low along Gulf Stream... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 12z GFS and 00z ECMWF/UKMET blend Confidence: Above average Lingering sheared vorticity energy at the apex of the jet streak over N Ontario combined with Arctic stream shortwave energy begins to amplify over the western Great Lakes by early Tuesday rounding out the western periphery of the developing closed low that will dominate SE Canada/New England through the forecast period. This feature is coming into stronger agreement especially between the 00z ECMWF and 12z GFS which show remarkable timing/shape evolution through the Mid-Atlantic as well. While there are some difference in the center interface owning to differences with the lead wave (see section above)...confidence with this feature is good. This agreement is further supported with good agreement in timing with the UKMET; though it is a shade stronger. The 00z CMC on the other hand breaks from consensus midday Wed and shows greater amplitude of the trof as it swings through; and in doing so drags the centroid of the main inner core of the upper low further south. This negatively affects the speed of the dangling frontal zone and the weak developing wave that forms offshore making it not favorable. The 12z NAM while nicely timed with the trof is actually a bit broader through the width as it moves offshore and likewise negatively affects the frontal zone/surface wave. Still even though the CMC/NAM are not tremendously different; their removal in a 12z GFS and 00z ECMWF/UKMET blend increase confidence it above average ...Shortwave digging over the western Great Lakes by Thursday and preceding weakening surface wave... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 12z NAM and 00z ECMWF/CMC blend Confidence: Slightly above average As the preceding shortwave amplifies, upstream shortwave ridging slows further upstream Arctic stream shortwave which amplifies over S Nunavut Tues. This process and the consolidating/exiting upper low downstream, allows for this next shortwave to drop south but stretch in the narrow weakness across Ontario. The 00z ECMWF having a bulk of the s/w energy on the south side with a stronger jet streak, accelerates through this stretching process into the Great Lakes quicker than other guidance. The 12z NAM is a bit slower to reaching this point is shows equal strength of the broad wave by early Thurs, followed closely by the 00z CMC. The 12z GFS remains generally weak and slides through the overall trof in the Great Lakes opposite to the consensus of the ECMWF/NAM and CMC. The 00z UKMET is most amplified prior to entering the shear zone shifting it eastward respective to the other guidance; which also seems to kick the closed low. As such a 00z ECMWF/CMC and 12z NAM blend is preferred at slightly above average confidence. ...Shortwave over the Yellowstone vicinity shearing southeast across the High Plains by Monday...Arkansas by Tues... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Above average The models all take a shortwave trough out of the Great Basin and then over the northern Rockies through early Monday. Thereafter, the guidance takes the energy around the crest of the large scale ridge axis developing over the West and drops the energy down over the High Plains where it will gradually shear out over Arkansas Tuesday. The models show decent agreement with timing and depth and a general model blend will be preferred at this point. ...Large eastern Pacific closed low (part 1)... ...Shortwave and associated surface reflection emerging from central Pacific and lifting parallel to the NW coast Tuesday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 12z GFS and 00z ECMWF/CMC blend Confidence: Slightly above average The large deep layered closed low extending from the Southeast Gulf of Alaska off the west coast. The base is reinforced with a strong shortwave emerging from the central Pacific today; supporting a deepening surface wave. This s/w and surface wave reach 130W and lift north Tuesday. Overall the guidance is fairly well timed with this wave but the amplification differences are large enough to remove the very deep solution of the 00z UKMET. The 12z NAM trended slower but still remains a bit faster than the overall consensus and while much weaker than the ECMWF/UKMET/CMC...there is support from a more elongated/sheared north-south oriented feature presented by the GFS. Last four cycle ensemble trends generally favor something between the GFS/ECMWF...and as such will also favor a 12z GFS and 00z ECMWF/CMC blend at slightly above average confidence. As the wave lifts north Wed and interacts with the main inner core energy, model spread radically spreads and as such desired to split the features into separate sections please see sections below. ...Large eastern Pacific closed low (part 2) ...Trailing shortwave energy with height falls through N California early Wed...translating into N Rockies Thurs with string of lee surface lows from MT to CO by Thurs... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~ Preference: 12z GFS/00z ECMWF blend Confidence: Average to slightly below average As the central Pacific shortwave energy lifts north (see section above) on Tuesday, the overall size of the central Pacific wave is vast enough to support a sharpening of the trailing edge. Global scale eastward shift of the downstream ridge allows for this portion of the trof to also shift eastward. The base of the trof expands the height falls into SW OR/N and central CA by midday Wed with ideal angle of onshore moisture/flow to the Sierras (please see QPFPFD for additional details). The 12z NAM shows some typical bias of being a bit faster and more consolidated from N CA into ID and eventually western MT. While plausible, given the large spread in guidance this leading/stronger edge is less preferred. The 00z ECMWF shows some of its typical slow bias and in doing so has some negative feedback from the upstream wave leading the overall height pattern to be more meridional in nature compared to the NAM/GFS. The 12z GFS is a nice compromise of the NAM and the ECMWF. The 00z CMC while bringing in weak leading height falls timed with the ECMWF/GFS; it also holds back a bulk of the energy leading to significant departure from the ensemble suite (including quite a few of its own ensemble members). The 00z UKMET, like other portions of the upper low seems much too strong with vorticity features and lower level low reflections to be considered at this time. All considered an average to slightly below average confidence blend of the 12z GFS and 00z ECMWF best represents a good compromise for the upper level pattern/moisture flux as well as surface development pattern in the lee of the Rockies by Thurs. ...Large eastern Pacific closed low (part 3)... ...Main inner core vorticity center descends south Wed angling toward Pacific NW coast by Thur with associated surface low... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~ Preference: 12z GFS, 06z GEFS and 00z ECENS blend Confidence: Below average As the central Pacific shortwave lifts north on Tuesday, it begins to interact with the strong inner core of the mean larger scale upper low off the BC coast in the southeastern Gulf of Alaska. This strong inner core is fairly well agreed upon through Tuesday but rapidly shears north to south under the influence of this southern wave...however the more sheared southern stream waves (GFS/NAM) interact more toward the middle section of the main core. Given a more centralized binary interaction, the two waves rotate closer and more consolidated in a larger scale sense descending in latitude by Thursday and spurring the a deeper surface reflection. The 12z NAM goes into its typical day 3 over amplification/rapid deepening that is clearly an outlier while the GFS is more sensible and north reaching SW OR by 00z Fri. This is opposed by the UKMET/ECMWF and lesser so CMC where being a bit more consolidated, tend to rotate cyclonically to the base of the shearing inner core. This does one of two things that lead to a drastically different evolution: the binary interaction is significantly reduced and as such as the wave wants to descend in latitude Thurs does so very elongated an much weaker...supporting a much weaker and well north surface reflection off the WA/OR coast. In reduced interaction...a shortwave emerging from the Aleutians/Bristol Bay is drawn southward late Wed keeping the core of the upper low much further north. While the operational ECMWF/CMC and UKMET support each other the ensemble suite including the 00z ECENS mean suggest the GFS solution is more viable. Additionally, WPC medium range analysts suggest the ECMWF/CMC/UKMET have been over performing with this Aleutian wave...and is much less favorable into the medium range forecast as well. As such will favor a 12z GFS and use the 06z GEFS and 00z ECENS means in the blend but confidence is well below average in this evolution coming to fruition. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Gallina