Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 245 AM EDT Sat Apr 18 2020 Valid Apr 18/0000 UTC thru Apr 21/1200 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 00z Model Evaluation with Final Preferences and Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Short wave energy tracking from the Mid MS Valley into Southern New England... ...Surface low pressure crossing the Ohio Valley into the northern Mid Atlantic... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Slightly above average The 00z NAM/GFS are close to the consensus taking short wave energy from IA/MO across the OH Valley into Southern New England by this evening. The short wave aids in spinning up surface low pressure on a frontal boundary extending across the southern OH Valley, which tracks to the southern New England coast. The surface low deepens off the New England coast after 20/12z, and the 00z ECMWF/CMC become a bit slower than the consensus once this occurs. The differences are greatest once the surface low is well off the coast, so the sensible weather with the system is offshore. Based on this, a general model blend is preferred, with slightly above average confidence, influenced by differences late in the period. ...Short wave energy in the southern stream... ...Surface low tracking from the Southern Plains to the Mid Atlantic coast... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 00z GFS/00z ECMWF blend through 20/12z, 00z GFS/00z GEFS/12z ECMWF mean thereafter Confidence: Average The 00z NAM/GFS are close to the consensus taking the short wave energy in the southern stream from AZ/NM into the Southern Plains by 20/00z. After this time, the 00z NAM becomes flatter and faster with the mid level wave, which results in a slightly northward displacement of the surface low track that forms over the Southern Plains. The 00z GFS has been the most consistent with the track of the short wave and developing surface low over the past three model cycles. It is closely mirrored by the 12z ECMWF through about 20/00z. The 00z ECMWF remains flatter and slower with the mid level short wave, which results in the surface low lagging the consensus, as well as being displaced too far north. Because of this, the 00z ECMWF remains out of the preference after 20/12z, and the model preference becomes the more consistent 00z GFS, which has the support of the 00z GEFS and 12z ECMWF ensemble. Given the amount of spread in the guidance, especially with the track of the surface low, forecast confidence is average. ...Short wave energy in the northern stream from Manitoba into the Upper Great Lakes... ...Attendant cold front crossing the Northern Plains into the Upper Great Lakes... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Slightly above average While there is generally good model agreement with the short wave in the very fast mid level flow tracking from central Manitoba into the Upper Great Lakes between 20/12z and 21/12z, the 00z NAM becomes slower than the consensus with the closed low that forms over eastern Ontario by the end of the period. There is also fairly good model agreement with timing of the surface cold front that crosses the Northern Plains into the Great Lakes. Despite some timing issues in the six hour forecast periods with the front, there is enough model clustering to favor a general model blend with slightly above average forecast confidence. ...Short wave over the Pacific Northwest into Sun... ...Short wave approaching the central CA coast late... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 00z NAM/00z ECMWF/00z UKMET blend Confidence: Slightly below average The 00z GFS/NAM are close to the consensus with the short wave that drops south from western British Columbia across the Pacific Northwest through 19/12z. After that time, the 00z NAM becomes faster and flatter with the initial short wave as it kicks out over the Northern Rockies. The 00z GFS remains closer to the consensus by ejecting the initial short wave more slowly across the Great Basin. The next short wave in the flow tracks toward the central CA coast after 20/12z. There is quite a bit of model spread concerning the timing of this short wave, as it undercuts mid level ridging across the Pacific Northwest. Though there is a loose consensus taking the mid level system over NV/AZ by the end of the period, each solution takes a slightly different route getting there. After reviewing the 00z non-NCEP guidance, as well as the 00z GEFS, the 00z GFS now appears to be a fast outlier with the mid level system. Considering that much of the rest of the guidance is showing a closed mid level by 21/12z, the 00z GFS has been removed in the preference, with the 00z NAM/UKMET inserted, with support from the 00z GEFS. It should be noted that there still spread in how the models get to the final destination, so the forecast confidence remains slightly below average. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Hayes