Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 106 PM EDT Fri May 08 2020 Valid May 08/1200 UTC thru May 12/0000 UTC ...See NOUS42 KWNO (ADMSDM) for the status of the upper air ingest... 12Z Model Preliminary Evaluation Including Latest Preferences and Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Deepening surface low moving along the Northeast Coast... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 12Z GFS/NAM, 06Z GEFS, 00Z ECENS/ECMWF/UKMET although less weight on the ECMWF/UKMET Confidence: Average Surface low pressure consolidating near New Jersey will lift northeast into the Gulf of Maine late tonight and then continue northeast into the Canadian Maritimes late Saturday into Sunday. This low will deepen in response to an anomalously strong closed low over SE Canada (-4 standard deviations from climo) interacting with the shortwave energy driving the surface low, as well as increasing diffluence of an atypical jet streak (+3 standard deviations from climo.) The 00Z/CMC is the only clear outlier with the track of this low, as it is E/SE of the consensus, and has ensemble spread even further east of its operational run. This is likely due to a much weaker 250mb jet streak with less robust pole-ward extent of the jet core rotating around the upper low, allowing the surface low to displace to the east. The 00Z/ECMWF has an 850mb low which is a little slow compared to consensus despite its similar placement, and since the trend has been for a faster ejection of this system, the slower ECMWF seems less likely, especially with the ECENS speed more in line with the remaining guidance. The GFS/NAM/GEFS/ECENS are very well clustered for D1 and create the preferred blend for this system, although some weight of the ECMWF and UKMET which are just a bit slow or a tad SE can be included, but will temper the overall great agreement of the strongest preferences to account for model shifts the past 24 hours. ...Dual shortwaves dropping through the Northern Plains and racing east to the Mid-Atlantic... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 00Z ECMWF/ECENS/UKMET/CMC, 12Z NAMNest, 06Z GEFS through D2. Non-00Z ECMWF D3 Confidence: Average Two shortwaves dropping quickly into the Northern Plains will eventually come together D3 over the Mid-Atlantic. The lead shortwave is most important as it digs through North Dakota southeast towards southern Lake Michigan Saturday into Sunday as this produces a wave of low pressure and associated cold front with precipitation across the region. The 12Z NCEP guidance (GFS and NAM) are both strong with the lead shortwave showing sharper troughing into Minnesota. This drives the surface low a bit SE of consensus, but also leads to stronger mid-level stretching/deformation to the NW. This leads to stronger forced precipitation and likely a heavier band of snow where thermals support frozen precipitation. The 00Z/CMC becomes a bit stronger with its 500mb amplification into Sunday, but is usable Saturday /D1./ The remaining guidance including the ECMWF/ECENS/UKMET and high res NAMNest all are well clustered with the surface low, and the GEFS is also west of the GFSop, well in line with the preferred consensus. Despite somewhat weaker deformation than shown in the NAM/GFS, a stripe of moderate snow is likely from ND into MN. This shortwave should weaken on D2 as it becomes sheared into the westerlies around the base of the trough in the east, but is likely to re-intensify Monday as the second shortwave begins interaction with the lead which is weakening. Sharpening of this mid-level wave towards the east coast may lead to secondary surface low development Monday south of New England. The ECMWF by this time is a clear slow outlier compared to the consensus which is otherwise in good agreement. For this reason the ECMWF is removed from the blend for D3. ...Shortwave energy crossing southern CA by Sun morning... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Above average A weak shortwave lifting onshore southern CA Saturday will weaken as it shifts eastward into the Desert Southwest on Sunday. Although the 12Z/GFS is a bit amplified compared to consensus, and the 00Z/CMC is subtly flat, the overall spread envelope is minimal. A general model blend remains the preferred option. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Orrison/Weiss