Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 235 PM EDT Fri Apr 24 2020 Valid Apr 24/1200 UTC thru Apr 28/0000 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 12Z Evaluation with Final Model Preferences and Forecast Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Low pressure crossing Mid-Atlantic on Friday ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Slightly above average GOES-East WV suite depicts a shearing wave moving across the Mid-Atlantic currently, with a very strong warm conveyor belt along the Gulf Stream. The surface wave associated with the stacked but filling low will slide through VA today and give way to the developing coastal low. The 12z NAM is a bit stronger throughout the cycle with respect to the wave, but is nicely timed with the rest of the guidance suite. This gives some pause to its inclusion but given the low is a bit stronger than some of the other guidance, will include the NAM; and support a general model blend at slightly above average confidence. Low pressure tracking from Texas to the Ohio Valley ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-GFS blend Confidence: Slightly above average 19z update: Overall, the late arriving models including the 12z GEFS all generally slowed a bit, particularly by day 3 and the overall depth of mass differences remain very small overall. The shift brings the GFS back into question, but the strength of convection along the western periphery/deformation zone across the Lower Ohio/Mid-MS valley suggests this latent heat injection still is troublesome with inclusion in a blend, but only lesser so. So if one is reticent of point, GFS could be incorporated in a blend elsewhere. Sill, WPC preference will continue to be a non-GFS based blend at slightly above average confidence. ---Prior Discussion--- In the wake of the departing wave along the east, broad scale troughing remains across the Mid-MS valley into the Ohio Valley. This allows a shortwave emerging from the Colorado Rockies today to sharpen and develop a enhance a surface wave in the Southern Plains that moves through the Mid-MS Valley Sat and into the Ohio/TN Valley by early Sunday. Upstream shortwave features/small jet impulses out of the broad northwest flow across the Rockies into the Plains, will enhance the trof, but lead to increased binary interaction, which can make precise smaller scale features more difficult to predict in this range. This is particularly the case late Sunday into Monday, when the shortwave over the Northern Plains swings under the base of the former. Eventually, the combined binary trof expands over the Mid-Atlantic on Monday and transfers toward a coastal low development that slides through the Southeast of New England. The 12z NAM/GFS are initially stronger emerging from the Southern Plains, with the GFS a tad north, even compared to the GEFS solutions. The 00z UKMET/ECMWF/CMC are a bit weaker, faster but also less prone to the tricordial wobbles that manifest, particularly in the 12z GFS. When the upstream shortwave merges on Sunday and Monday, the GFS wobbles become even larger and move it out of the otherwise solid clustering. 00z UKMET trends toward a stronger initial wave through the Ohio Valley, holding onto the wave/surface low a bit longer than the NAM/CMC/ECMWF and the ECENS/GEFS solutions, but it doesn't move too much out of tolerance for a Day 3 forecast to have in removed fully. As such will support a Non-GFS blend eventually reducing influence of the UKMET by the end of the forecast period. Given the amount of influence of binary interaction/convective upscale influence to the timing of the wave...the other agreement is fairly strong to have slightly above average confidence in this non-GFS blend. Pair of troughs affecting the northwestern U.S. ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General blend, Exception(s): Non-UKMET after 60hrs with 2nd wave Limit NAM on day 3 in ID/WY (QPF mainly) Confidence: Above average 19z update: The evolution, timing of the lead wave continues to be solid agreement to continue a general model blend for that wave. With the second wave, the 12z UKMET is so quick in rotating the top of the wave into the larger parent cyclone, older vorticity hub, drops southeast and captures or delays the trailing edge of the trof through the Northeast Pacific on Monday. This leads to a slowing of the frontal zone and reloads a stronger trof aloft. This is a minor mass difference that has limited sensible effects; however, will remove the UKMET after 60hrs in the Pacific NW for a cleaner/tighter blend preference. ---Prior Discussion--- An elongated global scale trof will start to slide southeast to anchor in the Gulf of AK by the weekend as a strong Pacific shortwave moves through toward Vancouver Island early Saturday. This initial wave will stretch initially losing some of the top of the wave to the global trof, but the remaining base quickly translates across the Canadian Rockies. This wave will clip the Northern Tier with limited sensible effects until late Sunday across the Upper Midwest and Upper Great Lakes. The guidance is fairly agreeable with the evolution and can support a general model blend through the forecast period with this system. Upstream, a second Pacific shortwave will translate along the base of the consolidating closed low over the Gulf of AK. This wave is drawn more northward with only the base rapidly swinging more negative tilt through Vancouver Island late Sunday into early Monday. The trailing cold front and moisture plume press across the Coastal Range and into the Northern US Rockies with solid model agreement in mass fields. The only strange outlier in the 12z NAM being quite dry relative to the other solutions especially over eastern ID into the Ranges surrounding Yellowstone and the Jackson Range. As such a general model blend could be supported through would remove the NAM for any QPF/snow reasons on Day 3. Confidence is above average. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Gallina