Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 247 AM EDT Sun Apr 29 2018 Valid Apr 29/0000 UTC thru May 02/1200 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 00Z model evaluation...with final preferences ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Closed low moving through the Northeast today... ...Surface low over New England through Monday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Above average The models show similarly with this system. ...Closed low over the Pacific Northwest today, opening into a trough over the Intermountain West by Monday... ...Lee cyclogenesis over the Plains by Monday with attendant cold front... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Blend of 12Z ECMWF; 00Z GFS Confidence: Slightly below average As the upper trough moves into the central/northern Plains on Monday and Tuesday, the 00Z GFS is noted to be a bit quick compared to the remaining model guidance and contrary to the past 3 cycles of the ensembles which show slowing as well. However, this difference seen in the 00Z GFS diminishes into Tuesday/Wednesday while the other models begin to show some timing differences. At the surface, a series of lows are forecast with the first in the vicinity of North Dakota Monday morning. There is generally good model agreement with this low as it moves into southern Ontario. A second low is forecast to originate over South Dakota late Monday and move toward Minnesota by Tuesday morning. The 00Z NAM is a tad stronger and north with this low compared to the remaining model consensus. Ensemble scatter low plots show a wide range of possibilities, with the 12Z ECMWF and 00Z GFS nearest to the middle of the spread. The 00Z ECMWF becomes a stronger deterministic outlier with a 500 mb shortwave over the U.S./Canadian border Tuesday evening, which has weak ensemble support but results in a stronger surface low. While the 00Z UKMET also has a stronger surface low, its shortwave timing is faster than the consensus. Meanwhile, the 00z CMC stands out with a different timing of the shortwaves across south-central Canada through Wednesday morning resulting in the CMC being an outlier with its surface evolution compared to the latest ensemble scatter low plots. Overall, a 12Z ECMWF/00Z GFS blend is preferred with average to above average confidence with the large scale evolution, but with below average confidence with the details of the individual vorticity maxima and surface low placement/timing. ...Shortwave digging from the Gulf of Alaska to California by Monday with low possibly closing off aloft by Wednesday morning... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Average The latest ensemble means show good agreement through 12Z/02 over the Southwest, with the latest 00Z deterministic guidance also showing reasonable agreement. While there are minor differences seen with the developing closed low over the Southwest, a blend is recommended at this time to smooth out these smaller scale differences. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Otto