Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 237 PM EDT Sun May 13 2018 Valid May 13/1200 UTC thru May 17/0000 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 12Z model evaluation including preferences and forecast confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Base of the eastern Canada upper low/trough...stationary front in the northern Mid-Atlantic/New England states through Wed. ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Average Aloft there is strong model consensus with the broad global scale trof across Eastern Canada and the confluence zone over the Great Lakes/Mid-Atlantic/Northeast. The spread will increase as moisture/energy is injected from the closed low over the Eastern Gulf surging through the Carolinas toward the Mid-Atlantic Tues into Wed but only in a minor sense aloft. The mass fields at the surface/lower elevations are another story. While initially, surface front and weak surface waves are easier to depict. the larger scale shortwaves become weaker/less defined exiting out of the Central Plains with the daily convective maxima and further lose cohesion along the ill-defined frontal boundary. Overall the placement of the front is generally agreeable, with exception of the CMC toward late Mon into Tues, when it is the only model to resolve a convective complex out of the Southern Plains, and draws the closed low over the eastern gulf north. This disrupts the otherwise good placement of the frontal zone on Tues/Wed. As such a non-CMC blend is preferred. Confidence is average as the signal becomes too weak and convectively driven to have high confidence. 19z update: While the 12z CMC continues to have a weak eastern Gulf system lifting toward the boundary a bit earlier...the strong convective response shortwave in the Southern Plains has resolved more in line with other guidance members (such as 12z GFS/ECMWF) to bring back into the blend. Confidence is increased slightly but remains average given the weak flow/convective influence that will remain a bit elusive at longer time frames. Developing closed upper low and weak surface reflection in Eastern Gulf of Mexico...lifts into Southeast toward Wed ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-CMC blend Confidence: Average Goes-16 satellite suite shows the amplifying sub-tropical wave over the Southeast Gulf with a weak surface low developed just off the NW Cuban coast with the easterly wave/trof extending through the Keys toward the Northern Bahamas at this time. Convective development will be key in upscale evolution of the closed low and surface wave(s) with time. There remain three solution "clusters" within the deterministic suite. The 00z CMC remains the most out of phase, with a weaker overall development that lifts quite quickly north. Additionally, the model insists on a strong convective response from the Mexican Plateau into the Southern Plains, which draws the closed low faster north too, which continues not to be represented by other guidance/ensemble members in the GEFS/ECENS clusters. The 12z GFS and 00z ECMWF continue to be clustered in evolution favoring a western/less elongated solution that is more vertically stacked and more traditional/supported by analogs. The 12z NAM and 00z UKMET while not significantly out of phase with the GFS/ECWMF favor greater response to the convection with a surface low tucked toward west coast of FL...with a greater SW-NE elongation of the wave as a whole providing more moisture/forcing across the Carolinas. Toward the end of the forecast period, both are a bit stronger than the GFS/ECWMF overall...though the NAM shifts toward the GFS in the central northern Gulf while the UKMET lifts toward the FL panhandle. All in all, it is difficult to precisely choose one solution over another given the meso/storm scale influence of low level moisture/latent heat release into the synoptic pattern. As such will favor a slight weighting toward the more traditional GFS/ECMWF solutions though will include the NAM/UKMET. As such a Non-CMC blend is preferred at average confidence...though 2/3rds to GFS/ECMWF and 1/3rd NAM/UKMET. 19z update: Little change from the 12z CMC, perhaps even a bit weaker and faster lifting north. The 12z ECMWF trended toward the UKMET, which tamped down some of the negative concerns (ie convective/latent heat response on the east coast of FL) with it. While the surface shifts toward this NAM/UKMET/ECMWF solution, the upper level mass fields remain solid enough to support a non-CMC blend, perhaps at a more equal blend between the remaining deterministic guidance. Great Basin Upper low slowly spins down by early Tues before remnants drift northeast to northern High Plains ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-NAM blend Confidence: Above average Closed low remains over northeastern Great Basin with peripheral shortwave features continuing to rotate around the stationary parent center. This will continue with small pieces of the wave slowly shedding off into the northern flow late Sun into Mon before finally weakening enough to lift north and east into MT by the end of Day 3. The ECMWF/CMC both retain a bit stronger inner core by the end of Wed, but this is fairly typical and combined with the faster solutions to weaken..ie GFS/UKMET will work out well. The 12z NAM continues to be the only piece of guidance that as the low lifts north late Tues into Wed, deflects westward around the ridge and becomes entangled in the approaching coastal low/trof. While possible, this is growing more unlikely given the weakness of the ridging through the Northern Rockies. As such a non-NAM blend is preferred at above average confidence. 19z update: No change in thinking here, as the 12z UKMET/GEFS/CMC and ECMWF all remain fairly similar to initial preference/respective earlier runs. Surface low streaking north of US/Canadian border Tue/Wed ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-UKMET blend Confidence: Slightly above average A quick moving clipper wave will track out of the central Canadian Rockies early Tuesday and skirt the US/Canadian boarder across ND/MN back toward a secondary lee sfc wave that moves into MT by the end of the forecast period. The 12z NAM and GFS are best clustered to the ensemble suite mean and are generally a bit slower and flatter than the 00z UKMET/CMC, which eventually are much deeper and north over SW Quebec at 00z Thurs. The ECMWF is the weakest and therefore is bit faster dropping south with the front. Overall the differences in timing a quite good and only small depth issues lead to a split in preference/confidence. All considered a 12z GFS/NAM and 00z ECMWF blend will allow for the strongest retention of the wave(s) signal. Confidence is slightly above average in this blend. 19z update: The 12z UKMET shifted south slightly but remains much more amplified and faster than the remaining guidance. The 12z CMC, remains the slowest but has shifted a bit south to be nestled into better clustering with the NAM and ECMWF. As such a non-UKMET blend is preferred. Upper low approaching CA Tuesday night ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Slightly above average Goes-W WV shows the beginning phases of a compact closed low developing near 34N141W. The ECMWF has been consistently generating this tight feature and rolling it toward the CA coast in Tues/Wed overnight period but has also been generally south and faster than any other guidance member as each slowly latch toward its solution over the last few days. This faster position is an atypical location for the ECMWF in the deterministic suite. However, other issues upstream with northern portion of the longer wave trof seem to manifest with the 00z ECMWF run as it reaches the blocking ridge over 120W. This leads to an equally compact and strong vort center that does not seem too logical meteorologically speaking with negative consequences to mass fields and sensible weather for NW CA by the end of the forecast period. So while anchoring the idea of the guidance, the 00z ECMWF cannot be included mainly after 00z Wed. The 12z NAM and GFS have slowly migrated to the compact faster wave but remain a bit north and slightly outpace the UKMET...this is likely related to the UKMET being faster drawing the lingering northern stream energy around the western periphery faster than the GFS/NAM. Still this seems minor. The NAM has some downstream issues in drawing the prior Western closed low over into this wave, so will weight the NAM a bit lower in the blend, favoring the GFS as a better representation with the 00z ECENS mean and UKMET. The CMC while also like the ECMWF is south and slower than the UKMET/ECENS mean/GFS/NAM, but also becomes too south, too slow and too broad with the larger scale trof by the end of the forecast period compared to the ensemble suite to aid the blend. 19z update: The 12z ECMWF has trended toward a more sensible strength of the northern trof energy around the western periphery of the larger low. Additionally, the ECMWF/CMC/UKMET all cluster very close to the 12z GFS/NAM position enough to support a general model blend at slightly above average confidence. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Gallina