Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 1214 PM EDT Sun Oct 20 2019 Valid Oct 20/1200 UTC thru Oct 24/0000 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 12Z Model Evaluation with Preferences and Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Post-tropical cyclone Nestor ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-CMC blend Confidence: Slightly above average The remnants of TS Nestor continue to progress through the Carolinas into SE VA and is expected to shift offshore later today. High vertical shearing will separate the low level circulation and mid-level forcing at the start of the week along 70W, though there remains some small spread especially in the lower levels/surface. The CMC remains north of the cluster while the NAM has finally latched on to similar solutions presented by the ECMWF/GFS and UKMET. The remaining low level circulation will lift north in advance of the approaching frontal zone Wed, again with the CMC the only stronger and further west solution, so a Non-CMC blend is preferred at slightly above average confidence. Rapidly deepening Central Plains cyclone, lifting into Great Lakes by Tues. Second surface wave lifting along front in Carolinas into New England 12z Wed. ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Slightly above average The strong jet and diffluent pattern across the Western Northern Plains is supporting rapid cyclogenesis currently and the overall guidance suite has come into very strong agreement with it. The trailing shortwave trof that had been leading to some uncertainty with the strong surface reflection and low level surge of moisture/instability over the past few days, has come deeper into the more dense data network and so as it lifts through the MS Valley on Monday, there is much stronger agreement. The overall spread is tight with the only minor exception being the 12z GFS which, at the surface, shows a greater cyclonic wobble to the occluded low center, still this is not an issue overall. Even the next surge of upper level jet energy/weak shortwave inflection drops along out of the Rockies into the base of the growing large scale trof, by late Monday into Tuesday, eventually spurring a surface wave to develop along/southeast of the Appalachians. This surface wave will lift along the progressive front into New England, also under fairly solid model agreement. As such a general model blend is supported at slightly above average confidence throughout the forecast period. Fast moving 'clipper' shortwave entering Northern US Rockies/MT High Plains late Tuesday and Midwest by late Wed ushering in cold air plunge across the Northern Plains. ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 00z ECMWF/CMC blend with some 06z GEFS Confidence: Average While there was some cold air pulled south very near the upper low/surface cyclone with the prior system, a strong but fairly potent shortwave is expected to exiting the Gulf of AK, over-top the ridge and roll along the spine of the Canadian Rockies. By late Tuesday, this will spur a Clipper surface low, that will skirt through the northern Plains, but in its wake usher a strong cold surge from an open Arctic stream of low level flow. The guidance, has some moderate spread, mainly in the strength of the lead shortwave energy...the 12z NAM is particularly strong supported by the UKMET, but is much wetter than the UKMET, making it a bit suspect toward its negative day 3 bias. The 12z GFS is uncharacteristically slow, relative to the other guidance, but that seems to manifest from initialization of the wave in the Pacific, favoring the second upstream shortwave over this lead one. Given the ECWMF/CMC both are a equal split of the stronger NAM/UKMET and the 06z GEFS is more ECMWF/ECENS mean-like in strength/timing will favor an ECWMF/CMC blend at average confidence. Model trends at www.wpc.noaa.gov/html/model2.html 500 mb forecasts at www.wpc.noaa.gov/h5pref/h5pref.shtml Gallina