Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 257 PM EDT Tue Apr 24 2018 Valid Apr 24/1200 UTC thru Apr 28/0000 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 12Z model evaluation including preferences and forecast confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Closed low over the OH/TN Valley today, gradually opening into a trough and lifting into the Mid Atlantic and Northeast, phasing with northern-stream shortwave closing off through the Great Lakes late Wednesday before lifting into the Northeast Thursday... ...Weak surface low over the interior Southeast transition to Carolinas Wednesday/Thursday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Blend of the 12Z GFS and 12Z ECMWF Confidence: Average to slightly below average The models continue to advertise the phasing of southern stream troughing/closed low energy over the OH/TN Valley region with the arrival of a northern stream shortwave dropping southeast across the Great Lakes region. Differences with the southern stream energy are quite small as the energy lifts off to the northeast over the next 24 to 36 hours. However, the 12Z NAM and 12Z GFS are a bit faster to advance the northern stream energy off to the east through Wednesday, which allows for phasing to occur a bit farther east over New England Wednesday night and Thursday. The 12Z CMC, 12Z ECMWF and especially the 12Z UKMET are all slower, and consequently allow for phased low pressure to linger a bit farther back to the west over the interior of the Northeast. The GFS and ECMWF tend to show the strongest phasing and actually by Thursday night tend to come into somewhat better spatial/timing agreement along with the CMC solution, but the UKMET remains a slow outlier post-phasing. For now, an attempt to compromise between camps will be preferred given the ensemble spread, while also attempting to approximate the model consensus. Thus a GFS/ECMWF blend will be preferred for now. ...Elongated trough with embedded shortwaves crossing the Rockies/Black Hills... ...Energy dropping southeast to the lower MS Valley Thursday before weakening through the Southeast by Friday... ...Low pressure near the Mid-Atlantic by late Friday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Blend of the 12Z NAM, 12Z GFS and 12Z ECMWF Confidence: Average...becoming below average Energy crossing the northern Rockies and High Plains will drop southeast over the next couple of days and dig in toward the southeast U.S. by Friday. The 12Z UKMET becomes generally a weaker and faster outlier in digging the energy across the South, with the best model clustering generally residing with the 12Z NAM, 12Z GFS and 12Z ECMWF. All of this energy is expected to drive an area of low pressure across the interior Southeast by Thursday and then northeast up toward the Mid-Atlantic region by Friday. However, there is some notable spread with respect to the placement of low pressure at the end of the period as the guidance differs with degree of possible northern stream interaction at that time. The NAM and GFS show more northern stream interaction and focus low pressure more over the interior of the Mid-Atlantic on Friday. The UKMET, CMC and ECMWF are farther east although there has been some trend to focus the low pressure at least closer to the Mid-Atlantic coastal plain by the end of the period. The preference for now is to lean toward a compromise solution that is close to a blend of the NAM, GFS and ECMWF, which is also generally favored by the 12Z GEFS mean. ...Northern stream portion of Gulf of Alaska trough breaking off and sliding southeast through Northern Rockies early Thursday then starting to anchor the Global scale trough from the Great Lakes to the lower MS Valley late Friday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Average Energy emanating from the northern Gulf of Alaska will cross through western Canada and drop southeast through the northern Rockies and Plains states through Thursday and help lead the way for more of a longer wave trough evolution over the Eastern U.S. by late Friday. The guidance tends to agree on the larger scale with the details of the height falls evolution and agree with focusing the bulk of the energy farther north closer to the Great Lakes on Friday along with an area of low pressure. However, there remains some modest spread toward the base of the trough with the details of how much energy separation occurs and the timing of it. Most of the models are hinting at at least a weak closed low by Friday over the Gulf Coast states, but the UKMET tends to be the most emphatic with this at this time. Based on the latest spread of guidance and the ensemble means, a general model blend will be preferred with all the details of this height fall evolution. ...Closed low evolution toward the West Coast by Friday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Slightly above average The southern portion of an elongated trough axis offshore the West Coast and extending north through the Gulf of Alaska will separate out and advance east toward the West Coast by Friday as a well-defined closed low. On the larger scale the model differences are rather small with respect to timing and depth, and so a general model blend will be preferred for now. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Orrison