Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 241 PM EDT Sun Apr 01 2018 Valid Apr 01/1200 UTC thru Apr 05/0000 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 12Z Model Evaluation...Including Model Preferences and Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Shortwave crossing the central Plains today... ...Reaching the central Appalachians/Mid-Atlantic tonight... ...Surface low crossing the Mid-Atlantic by early Monday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Above average The 12Z NAM in a subtle fashion may still be just a tad too strong with its ejecting mid level shortwave that crosses the central Appalachians and Mid-Atlantic states tonight through early Monday. However, the overall model mass field spread is quite minimal, and the 12Z GFS made a favorable trend toward a slightly less progressive solution which puts it into better agreement with the NAM timing, but also the 12Z UKMET/CMC/ECMWF non-NCEP solutions which had been consistently slower than the GFS with this system over the last several model cycles. At this point, feel as though a general model blend will suffice for the mass fields with this system. ...Upper level trough digging into the Northwest today and reaching the Upper Midwest by Tuesday... ...Surface cyclogenesis over the Plains Monday with a deepening low moving into the Great Lakes Tuesday... ...Low lifting into Quebec with trailing cold front clearing the East Coast through Wednesday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Blend of the 12Z GFS, 12Z CMC and 12Z UKMET Confidence: Slightly below average Model spread with this system remains fairly high, beginning as early as Monday, with a variety of model surface low tracks stretching from the High Plains into the Great Lakes going through Tuesday. Early in the period (Monday), the 12Z NAM again insists on a stronger upper trough/closed low evolution through the Northwest which yields a much stronger inverted trough and separate area of low pressure impacting the northern Plains region compared to the global models which consolidate low pressure farther south down the High Plains before ejecting eastward across the central U.S. Going through Tuesday as low pressure ejects east across the middle Mississippi Valley and toward the lower Great Lakes region, the NAM is the most progressive with the low track and trailing cold front. Overall, the 12Z UKMET/CMC/ECMWF solutions are all clustered on the slower side of the model suite with the 12Z GFS a bit more progressive than the non-NCEP consensus but slower than the NAM. The ECMWF though is now overall the slowest solution and is even a little slower than the 00Z ECENS mean. On Wednesday, as low pressure lifts quickly across Ontario and Quebec, and a trailing cold front quickly reaches the East Coast, the NAM remains on the fast side of the guidance. The global models all maintain a relatively slower progression. Accounting for the stronger/faster NAM, and the now slower ECMWF solution with this system, the GFS, UKMET and CMC make for a more appealing model cluster to lean toward and especially since they are closest to the model consensus. A blend of the GFS, CMC and UKMET will be preferred as a result and also with consensus support from the latest GEFS mean and ECENS mean. ...Low-amplitude shortwaves approaching the Pacific Northwest by Tuesday Night and Wednesday Morning... ...Surface wave aiming for Vancouver Island by late Wednesday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Blend of the 12Z GEFS mean and 00Z ECENS mean Confidence: Below Average No change to the previous thinking for the Pacific Northwest as the latest guidance again supports a multitude of shortwave impulses over the eastern Pacific that will likely take aim on the region as they will be embedded within deep layer westerly Pacific flow. Again, as per the latest ensemble spaghetti diagrams of mid-upper level heights, there is little in the way of a coherent signal for timing and strength of individual shortwaves. However, there is broad agreement that low-amplitude shortwaves embedded within the relatively zonal flow pattern will impact the Pacific Northwest by Wednesday. The latest deterministic guidance does favor the idea of wave of low pressure advancing toward Vancouver Island by late Wednesday, but there is considerable spread with the timing and strength of this wave. Will therefore maintain continuity and favor a blend of the latest GEFS mean and ECENS mean. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Orrison