Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 220 AM EDT Wed Jun 17 2020 Valid Jun 17/0000 UTC thru Jun 20/1200 UTC ...See NOUS42 KWNO (ADMSDM) for the status of the upper air ingest... 00Z Model Evaluation Including Preferences and Forecast Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Eastern U.S. closed low and coastal front... Returning frontal zone/moisture confluence axis across FL... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend until 20.00, 00z NAM/ECMWF/UKMET thereafter. Confidence: Above average except in FL (average) 07z update: Increasing spread as the UKMET has trended a bit faster toward weakening and melding into the approaching westerly flow on late Friday into Sat. While the ECMWF starts lifting faster, the CMC languishes a bit more centering near VA by the end of Day 3. So will favor a 00z NAM/ECMWF/UKMET compromise to remove the outliers in the field and account for the known timing biases of the GFS/CMC after 20.00z. ---Prior Discussion--- The models continue to show very good agreement with the closed low through the bulk of the forecast period, with mesoscale convective alignment resulting in the greatest uncertainty. However, as the low starts to weaken and feel the affects of the approaching northern Plains low, the 00z GFS shows a typical negative bias of weakening too early and lifting out by the end of Day 3, contradictory to even the bulk of 18z GEFS members. So will support a general model blend through about 72 hrs (20.00z), hedging toward a non-GFS blend remaining at above average confidence. The greatest uncertainty may not be really affected by the closed low itself, but by its weakening influence and the strength of returning flow around the Bermuda high. This results in a northward advancement of the old "frontal" zone which is more of a moisture discontinuity boundary/axis. This will increase the threat of thunderstorms across/near the coastal zones of FL by the end of the forecast period. As the GFS is lifting out a bit faster with the close low, the return may be a bit too fast here, as well. Yet, the uncertainty is greater than further north but the same preference of general model blend shading away from the GFS after 72 hours is preferred. Closed low moving into the Northwest Tuesday into the Northern High Plains by Thursday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-CMC blend (hedging more GFS/NAM in Central Plains D3) Confidence: Slightly above average for most mass fields Below average in Central Plains Fri night-Sat (QPF/outflow boundary) 07z update: While the 00z CMC trended initially a bit faster, and therefore trends north-east relative to the 12z run, after 19.00z it once again starts wobbling west of the growing consensus to the east with respect to the closed low and frontal zone. The ECMWF/UKMET continue to be similar to their prior runs with the mass fields. Yet the UKMET seems to split the difference evolving a strong/heavy rainfall complex dropping south into OK, but then redeveloping further north along the cold front like the ECMWF. As such will continue to keep initial preference/thinking of a Non-CMC blend, but hedge more toward the GFS/UKMET/NAM in the QPF sense on Day 3. ---Prior Discussion--- The closed low across OR/ID is starting to open/stretch into a more positive tilt orientation as it crosses the Rockies tonight into tomorrow. The GFS is a bit fast, more elongated than the other guidance, but not resulting too much offset to the overall evolution. This includes up to and including the interaction with the Arctic/northern stream in Canada drawing it southward by Thursday resulting in a new closed low across the Southern Canadian Prairies. The 12z CMC is slow with this and does not draw any northern stream energy south, leading to a different evolutionary path, much further south and west with the close low and not favored. Upstream jet/shortwave energy will round the base and fill out a weakly formed trof across the Central Rockies by Friday, resulting in solid upslope flow regime. From a synoptic point of view, the mass fields remain in good agreement, the uncertainty lies in the placement of the outflow boundary from the convection the prior night across the Central Plains. The GFS and NAM are stronger/faster with pressing the resulting outflow boundary southward, which leads to a southward shift in the Friday evening/Sat morning convective complex compared to the UKMET/ECMWF, given better differential heating, reinforcing the boundary. Synoptically, the mass fields would yield a similar result in a blend to support a non-CMC blend overall. However, the QPF differences are significant but also highly contingent on the placement of the boundary; traditionally, the faster, southward outflow propagation from the prior night would favor the GFS/NAM, but if the convection is strongly forced because of slower exiting of the wave the prior night as the ECMWF would be apt to do, the resultant convective complex would delay with stronger mean flow toward the NE and not result in faster southward propagation, not dropping as far south. So overall a non-CMC blend is supported at slightly above average confidence, yet the QPF uncertainty late Day 2 into Day 3 yields a below average confidence scenario. Surface front/associated moisture approaching the Pacific Northwest by late Friday night into Sat... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 00z NAM/GFS/ECMWF blend Confidence: Slightly above average 07z update: The ECMWF and UKMET both trended a bit faster, but this means the ECMWF is closer to the initial preference of the 00z NAM/GFS while the UKMET is in step with the slower/flatter, eastward shifting CMC. So, will support a NAM/GFS/ECMWF blend at slightly above average confidence. ---Prior Discussion--- Positive tilt ridging just off-shore the Pacific NW will continue to erode/flatten with the approach of a larger scale closed low that will slowly meander in the NW Gulf of AK. A short-wave lobe will press southeast and move near Haida Gwaii late Friday. This will help to press the frontal zone toward Vancouver Island and increase southwesterly moist flow ahead of it. The UKMET is the only clear outlier being very slow with the shortwave spurring the weak surface wave along the front. The 12z CMC is a bit weaker but also broader with the gyre, suppressing the ridging a bit more than the GFS/NAM/ECMWF to at least press the frontal zone more southeast and nearer the coast. So it helps build confidence in this evolution but may be a bit too far east/broader/flatter for inclusion to a 00z GFS/NAM and 12z ECMWF blend. Confidence is average to slightly above average. Gallina Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml