Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 242 PM EDT Sat May 05 2018 Valid May 05/1200 UTC thru May 09/0000 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 12Z Model Evaluation...with Preferences and Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Upper level low near the Bahamas and Cuba this weekend... ...Inverted surface trough extending into Florida... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Above average Models are in excellent agreement with their mass fields for this system over the next couple days, and thus a general model blend is preferred. ...Shortwave trough ejecting from the Lower/Mid Mississippi Valley to the Mid Atlantic this weekend... ...Weak surface lows developing along a frontal zone across the region... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Blend of the 12Z GFS, 12Z UKMET and 12Z ECMWF Confidence: Slightly above average The models take a southern stream shortwave out of the lower to mid MS Valley region today and eject it rather quickly across the TN Valley and into the Mid-Atlantic region tonight through Sunday. The guidance supports multiple waves of low pressure developing along a frontal zone that will be draped from the TN Valley across the Mid-Atlantic and offshore New England. Model spread has been decreasing a bit over the last couple of model cycles and the 12Z GFS in particular has come into better agreement with the non-NCEP cluster of solutions which agree in having multiple lows develop and eject northeast up along the front over the Mid-Atlantic coastal plain and then offshore New England. The 12Z CMC and periodically the 12Z NAM appear a little slow with their low center evolution, so will prefer a solution toward the 12Z GFS, 12Z UKMET and 12Z ECMWF at this point. ...Upper level trough digging into the Great Lakes on Sunday and sweeping across the Northeast Monday... ...Associated cold front pushing through the Midwest and Northeast... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Blend of the 12Z GFS and 12Z ECMWF Confidence: Slightly above average The models have come into good agreement on a more progressive trough evolution across the Great Lakes, Northeast and through southeast Canada Sunday through Monday. This will drive a cold front down across the Midwest and Northeast and the models show rather good agreement with the timing of the front. Regarding the upper trough, the 12Z NAM is still a bit too slow it appears with its progression and also perhaps a little too amplified. The 12Z UKMET likewise tends to lean toward a somewhat slower and slightly more amplified solution. The 12Z GFS and 12Z ECMWF, along with their respective ensemble means show very good agreement now in being more progressive and less amplified. The 12Z CMC supports this as well, and so will favor the GFS/ECMWF consensus at this time. ...Upper trough advancing inland across the West Coast this weekend... ...Energy advancing downstream to the northern Plains by Tuesday... ...Surface low development in the lee of the Rockies... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Blend of the 12Z GFS, 12Z UKMET and 12Z ECMWF Confidence: Slightly above average Despite the complexity involved in the broad trough coming into the West Coast this weekend and interaction with a separate trough near southern British Columbia, the models are in generally good agreement on the larger scale height fall evolution. They all suggest the troughing will deamplify a bit as it traverses the Northwest and then begins dropping southeast across the northern Plains through Monday and Tuesday. The guidance also agrees in developing low pressure in the lee of the Rockies which will edge east across the Plains on Tuesday. The concerns with the models are mainly centered on the NAM and CMC at this point. The 12Z CMC tracks some of its mid level energy a bit north of the model consensus on Tuesday across the northern Plains which allows its surface low to also track a bit farther north. It is also a bit deeper with the surface wave. Also, the NAM is appearing to be a little too weak with its surface reflection and also may be a little too far north as well. We will lean toward the well clustered GFS, UKMET and ECMWF at this time which are a little farther south but also better supported by the latest ensemble guidance. ...Upper trough/closed low approaching the West Coast by Tuesday... ...Accompanying offshore surface low... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Blend of the 12Z GFS and 12Z ECMWF Confidence: Average The models bring another upper trough and associated closed low toward the West Coast by Tuesday. The 12Z GFS and 00Z ECMWF are very well clustered at the surface and aloft and are both a bit north of the overall model consensus. The 12Z NAM solution has a closed low feature that is the farthest south of any solution. The 12Z UKMET and 12Z CMC are north of the NAM solution, but generally a bit south of the GFS/ECMWF camp. The ensemble means are rather solidly behind the GFS and ECMWF, so will prefer a blend of the GFS and ECMWF at this time. ...Upper trough reloading over the Southeast U.S. by Tuesday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Average The models show very good agreement in dropping additional shortwave energy in across the Southeast U.S. through Monday and into Tuesday which reloads a rather broad upper trough across the region. Model agreement is good until the end of the period when the 12Z GFS becomes a bit quicker to lift the upper trough off to the northeast. The 12Z CMC becomes the slowest by comparison with the remaining guidance clustered in between. The overall model spread is rather modest though, and therefore will suggest a general model blend. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Orrison