Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 1212 PM EDT Fri May 01 2020 Valid May 01/1200 UTC thru May 05/0000 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 12Z Model Evaluation Including Final Preferences and Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Deep trough/closed low impacting the East through Friday and early Saturday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~ Preference: Non-UKMET blend or General model blend Confidence: Slightly above to above average High amplitude trof along the East Coast is a north-south stack of two smaller scale features, the broader/southern of which will continue to narrow and weaken, while the northern will elongate but strengthen across SW Quebec into the Northeast early Saturday. The model spread is fairly small and agreeable with the overall evolution including the newer surface cyclone that develops along 40N east of 70W early Sat. The UKMET is a tad slow with the surface wave along East Coast today and shifts the development of the 2nd surface wave further north. So for a tighter blend a non-UKMET blend is supported but a general model blend may be viable, to handle some of the uncertainty if desired. Confidence is slightly to above average ...Shortwave/surface low tracking near the U.S./Canadian border through Sunday, frontal zone through Ohio Valley; Northeast reinforcement/surface redevelopment Sunday to Monday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-CMC blend Favoring: 00z UKMET Canada/Great Lakes/Northeast 12z NAM/GFS and ECMWF across Ohio Valley to Mid-Atlantic Confidence: Average to slightly below average (by end of d3) In the wake of larger scale trough along the East Coast, generally flatter flow exists across the West into the Northern Plains. Smaller scale shortwave features initially in the Northern Rockies and Canadian Rockies will be start to expand and interact across the Southern Prairies of Canada into the Northern Plains and Upper Great Lakes by Sat morning. This elongation and magnitude of interaction between the features along with influence of convective upscale/latent heat into the system leads to lower predictability scenario unfolding across the Great Lake into the Northeast the the end of the weekend, especially as a subtle shortwave/speed max emerges out of the Central Rockies and races across the Ohio Valley Sat into Sunday. Initially, however, the merger of the waves across SW Ontario by late Sat/Sun is in moderate agreement though with typical timing issues. The GFS/GEFS are fast and the 12z NAM shifted faster to join its NCEP brethren, while the ECMWF is slower. The CMC is already well north and offset to remain out of phase and therefore out of preference throughout the remainder of the forecast period. The UKMET is between the two camps, but favors the NAM/GFS just generally weaker through early Sunday. As the NAM/GFS rapidly develop the surface low and starts the occlusion process earlier as it progresses through the Great Lakes on Sunday, the upstream (northern Canadian Rockies) wave swings through the base and presses the frontal zone across the Ohio Valley. Given the ECMWF was weaker/slower with the initial wave the initial surface wave is weaker and south, but the upstream kicker is stronger and faster supporting a triple-point low to form over the Northeast by late Sunday, pressing the frontal zone further east with time, even more aggressively than the bulk of ECENS members. So there is large spread by 60hrs in the Northeast with the focus of stronger surface cyclone therefor wind fields. The UKMET is middle ground, and frankly ideal in placement of the Canadian wave, but being weaker/elongated initially...did not press the frontal zone south through the Ohio Valley. By day 3, there is no ideal solution/preference but a compromise something closer to the UKMET north in Canada/northeast but something closer to the timing/orientation of the frontal zone across the Ohio Valley/Mid-Atlantic Monday by a NAM/GFS/ECMWF blend. Confidence is average initially becoming slightly below average by day 3. ...Deep trough offshore the West Coast today and Saturday with remnant shortwave energy moving through the Northern Rockies Sunday morning, northern High Plains surface low... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend thru 03.12z Non-CMC compromise afterward Confidence: Above average thru 03.12z Average to slightly below average afterward Developing Gulf of Alaska cyclonic gyre will be shifting southeast with an initial shortwave that is lifting north along 135W today. Quick on its heals, another potent shortwave further shifts the gyre eastward, with the leading shortwave energy deepening into a stacked deep surface wave with a frontal zone that will press through the Pacific NW by midday Saturday with little model difference to support a general model blend through the Pacific NW. The base of the trof will accelerate eastward into the Northern US Rockies within the left exit of a weakly cyclonically curved upper level jet. With very strong 130-150kt upstream energy, there could be potential for vigorous development as it detaches from the gyre and moves into the Northern High Plains late Sunday. By this time, typical timing differences start to manifest, with the GFS/NAM and UKMET shifting faster than the ECMWF/CMC. In combination of timing of the exiting shortwave and return of shortwave ridging downstream, as well as some vorticity stretch in the lee of the MT Rockies, the GFS/NAM both rapidly deepen the shortwave into a compact closed low that lifts north into Alberta/Saskatchewan by Monday; with the UKMET initially a bit slower in development and therefore having the opportunity to press further east by 00z Tuesday. The slower CMC is very weak and moves outside of the ensemble suite and is quickly dismissed. The ECMWF is slower and therefore weaker remaining south as it closes off Monday, this will help with stronger upslope flow and closer proximity to better instability and moisture across the Central Plains. Additionally and potentially more important for the downstream evolution (as well as upstream see section below) in the Northern and Central Plains, the NAM/GFS and lesser extent so UKMET show a strong trailing shortwave entering N CA at 04.00z. The ECMWF/ECENS solutions depict this feature much weaker if at all, which only increases the spread and uncertainty 72-84hrs, as the NAM/GFS both show a strong surface response with a new surface wave along the front crossing into SD by 84hrs. This falls into line with both the NAM/GFS fast biases as well as the ECMWF slower solution bias; ensemble suite are very similar to their parent operational model, providing little help one side or the other. So a compromise of non-CMC solutions seems prudent after a general model blend through 60hrs. Confidence is above average through 60hrs, but rapidly shifts to average or slightly below average in the compromise thereafter with likely sizable run to run variation expected with upcoming model cycles. ...Next embedded shortwave in Gulf of AK gyre, clipping NW Monday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-CMC blend Confidence: Average The next Pacific shortwave within the parade along the southern edge of the Alaskan gyre, will press eastward by Sunday. The uncertainty here, is how much of the initial shortwave will be shed into the zone flow before the wave deepens Sunday out west of 140W. The GFS/NAM being faster, shed more before the wave buckles the flow (see last paragraph of section above), relative to the slower ECMWF. In retaining more energy, this allows for the ECMWF to be stronger and therefore dominate the binary interaction with the larger scale gyre...and shift it eastward to the Pacific Coast late Monday into Tuesday. While the GFS/NAM being a bit weaker, relative to the EC lifts northward toward the center of the gyre and therefore slows relative to prior runs, with barely and QPF reaching the West coast by the end of the forecast period. The 00z UKMET, typical of its bias, while fast, is also strong in its evolution and splits the difference, between the ECWMF and NAM/GFS. The CMC remains out of phase, well east of the ensemble solutions and with downstream issues elsewhere that are important to this system's evolution...it is discarded. So a Non-CMC blend is supported and with milder spread, confidence is 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