Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 108 PM EDT Fri May 22 2020 Valid May 22/1200 UTC thru May 26/0000 UTC ...See NOUS42 KWNO (ADMSDM) for the status of the upper air ingest... 12Z NAM/GFS Evaluation Including Preliminary Preferences and Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Cut-Off Low impacting the OH Valley and northern Mid-Atlantic into Sunday morning... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Above average Only minor timing differences exist with this system with the 00Z UKMET slowest and 12Z GFS fastest. The differences are greatest after the system moves offshore of the East Coast and so a general model blend should suffice for this system. ...Deep upper trough/closed low reloading over the Western U.S... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 12Z NAM/00Z ECMWF blend Confidence: Slightly below average The 12Z NAM and 12Z GFS stand out early in the period (Saturday evening) with a faster progression with a mid-level closed low tracking across the Great Basin. By Sunday morning The 12Z GFS pulls away from all of the guidance with a faster progression toward the Northern Plains. Meanwhile, additional energy will be diving into the western U.S., helping to amplify a longwave trough axis through Monday. While trends have been slower over the past 24-48 hours with the mean trough axis translating eastward, the 12Z GFS remains faster to take the system east, while the 00Z UKMET is slowest. The 12Z NAM has some similarities to the 00Z UKMET by late Monday but when blended with the faster 00Z ECMWF, a favorable blend is reached. The latest ensemble mean's position for the trough axis is a little slower than the 00Z ECMWF but ahead of the 12Z NAM. Given trends in the ensemble guidance, prefer to be slightly slower than the means, especially when considering the potential cutoff nature of this system moving forward. ...Shortwave lifting northeast toward the Great Lakes through Saturday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 12Z NAM, 00Z ECMWF, 00Z CMC blend Confidence: Above average Generally the models are in good agreement with this system, but the 12Z GFS is slightly quicker and the 00Z UKMET is notably slower. Prefer the middle ground at this time which is near the 12Z NAM, 00Z ECMWF and 00Z CMC. ...Zonal flow into Pacific Northwest with embedded weak shortwaves... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: non-12Z GFS blend Confidence: Average There are some minor differences within a region of zonal flow sandwiched between a large closed low centered over the Aluetians and broad ridging in the eastern Pacific. Low amplitude shortwaves are forecast within the zonal flow with impacts mainly to British Columbia. However, a slightly more defined shortwave is expected to near Vancouver Island early next week, associated with cold front a the surface which will edge nearer to the coast. The NAM/GFS have been trending stronger with this wave since yesterday but the 12Z GFS stands out with lower heights into the Pacific Northwest which is against the remaining model consensus. A non-12Z GFS blend is preferred at this time. ...Upper level shortwave lifting northeast across the southern Plains Sunday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: non-12Z GFS blend Confidence: Slightly below average As the deep trough and associated closed low evolves across the West this weekend and slowly encroaches on the High Plains, there will be a shortwave trough rounding its base, lifting northeast across the southern Plains on Sunday. There is potential for thunderstorms to convective enhance the existing shortwave or develop a separate convectively generated vorticity max. Some of the guidance is a bit more bullish with this idea such as the 00Z UKMET and 00Z CMC, but only the 12Z GFS stands out enough with a faster timing (similar to the above systems) to be excluded from the preference. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Otto