Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 310 PM EDT Fri Apr 06 2018 Valid Apr 06/1200 UTC thru Apr 10/0000 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 12Z model evaluation... with preferences and confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Shortwave crossing the Great Lakes and Northeast through early Saturday... ...Accompanying surface low crossing the Saint Lawrence River Valley... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Above average The models are in good agreement with the details of the northern stream shortwave and accompanying surface low that crosses the Great Lakes region and lifts across the Northeast tonight through early Saturday. This will drive a strong cold front across the Ohio Valley, Mid-Atlantic and Northeast. A general model blend will be preferred. ...Southern stream shortwave crossing through the southern Plains and lower Mississippi Valley today through tonight... ...Surface low tracking across the Gulf Coast/Southeast tonight through Saturday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Blend of the 12Z GFS, 12Z UKMET and 12Z ECMWF Confidence: Slightly above average As a longwave northern stream trough becomes better established over the Eastern U.S. over the next couple of days, there will be a southern stream shortwave that crosses the southern Plains today and the lower Mississippi Valley through tonight. This will develop a wave of low pressure along a cold front which will cross the Gulf Coast states tonight and the Southeast U.S. on Saturday. Multi-day deterministic and ensemble model trends have been to substantially slow down and suppress the storm track of this low center. There are some modest timing and depth differences with periodically the NAM and CMC slower than the GFS, UKMET and ECMWF as the energy crosses the South, and then as the low center exits offshore the East Coast late Saturday, the CMC is bit stronger than the model consensus. Will prefer blending the GFS, UKMET and ECMWF at this point. ...Strong closed low and trough impacting the West Coast through Saturday... ...Atmospheric river impacting the West Coast... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-CMC blend Confidence: Above average The latest guidance agrees in bringing a strong closed low and associated surface low center in toward the Pacific Northwest tonight through Saturday. The 12Z CMC is a bit weaker and more progressive with the height falls as the energy arrives late Saturday and this translates to its surface reflection as well. The remaining multi-model suite and ensemble guidance favors a stronger and slower evolution. In any event, a well-defined atmospheric river event along with highly anomalous moisture transport is noted out ahead of this troughing and the guidance brings this moisture well inland ahead of a cold frontal passage through early Saturday. Will prefer a non-CMC blend for the model mass fields. Please consult the latest QPFPFD and QPFERD for details on heavy to excessive rainfall potential associated with the atmospheric river. ...Shortwave energy traversing the Plains on Sunday... ...Advancing across the Mississippi and Ohio Valleys Monday... ...Surface low activity from the Plains to the Gulf Coast and Southeast U.S... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Blend of the 12Z GFS and 12Z ECMWF...through 60 hours 12Z UKMET/12Z CMC/12Z ECMWF blend...after 60 hours Confidence: Average...becoming below average The models take the remnants of the Pacific Northwest closed low quickly off to the east in two parts through Sunday and Monday. A lead shortwave is expected to kick east out of the across the Plains on Sunday which will drive a wave of low pressure down across the immediate northern High Plains and across South Dakota before then crossing the upper Midwest by Monday. The 12Z NAM is the weakest with this energy, with the 12Z UKMET, 12Z CMC and the 12Z ECMWF clustered toward the stronger side of the spread. The 12Z GFS is in between camps with depth. This energy though will be tending to dampen out as it moves east of the Mississippi Valley and toward the Ohio Valley by late Monday. Upstream behind this energy will be another shortwave that will arrive across the Plains and toward the Midwest by Monday evening. Spread with this secondary energy is fairly modest now, but the CMC is generally more progressive with with ECMWF a little slower as compared to the model consensus. By the end of the period, the 12Z NAM and to a lesser extent the 12Z UKMET, 12Z CMC and 12Z ECMWF all suggest sufficient energy advancing downstream for there to be a weak area of low pressure developing off the Southeast coast. There is support from the 00Z ECENS mean for this too, but the 12Z GFS is notably flatter at this point. Based on the latest model clustering and ensemble guidance, will prefer a blend of the GFS and ECMWF through 60 hours, and then lean closer to a blend of the UKMET, CMC and ECMWF thereafter. Confidence is limited by Monday as the energy reaches toward and east of the Mississippi Valley. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Orrison