Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 242 PM EDT Mon Mar 23 2020 Valid Mar 23/1200 UTC thru Mar 27/0000 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 12Z Model Evaluation with Preliminary Preferences and Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Nor'easter developing off New England tonight... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Above average ...19Z update... Minor changes were observed with the 12Z ECMWF/UKMET/CMC compared to their previous cycles. No changes are recommended with a general model blend continuing to represent the best option for this system. ...previous discussion follows... Minor placement differences exist with this system, perhaps the 12Z GFS stands out the greatest early tonight with its surface low track being displaced a little farther offshore. However, the differences in the GFS disappear beyond 12Z Tuesday, with a general model blend from start to finish looking fine for this system as the differences noted with the GFS are well within the tight ensemble spread. ...Mid-level shortwave tracking from the Southwest today to the East Coast Wednesday morning... ...Surface low tracking offshore of the northern Mid-Atlantic coast Wednesday into Thursday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 12Z NAM, 12Z GFS blend Confidence: Slightly below average ...19Z update... The 12Z ECMWF trended slower with the mid-level shortwave and is in better agreement with the timing of the other models. However, the 12Z CMC continues to be slower than the remainder of the guidance. No major trends were seen in the 12Z UKMET. The 12Z NAM and 12Z CMC are closest to the coast with the surface low and the 12Z GFS/ECMWF/UKMET are more offshore. This seems to suggest the earlier trend in the ensembles is the new direction to lean. but some incorporation of the 12Z NAM is still preferred to account for the continued model spread. The 12Z CMC is considered least likely to verify with a continued stronger and more westward track. ...previous discussion follows... The 00Z The timing of the 00Z ECMWF mid-level shortwave is noted to be fastest as it nears the Ohio Valley Tuesday evening and offshore of the East Coast Wednesday evening. The 00Z ECMWF mean is noted to be a little slower than the deterministic ECMWF. The remaining model show minor differences with the shortwave timing but more notable differences shift to the related surface low. Ensemble scatter low plots had been consistent over their past few cycles, valid 12Z/26, with a centroid just east of the 40N/70W benchmark. However, the latest (00Z/23) ensemble cycle has shifted the ECMWF/GEFS/CMC surface lows southeast of the benchmark or more offshore. It is unclear if this trend is a temporary shift or a new direction that the guidance will all shift toward. For now, prefer to play the middle of the two camps, or by blending the 12Z NAM and 12Z GFS. The 12Z GFS is supported by the 00Z UKMET, while the 00Z CMC which is stronger and closest to the coast is not considered less likely to verify given lack of support, especially Wednesday evening. ...Longwave trough over the western U.S... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 12Z ECMWF/CMC blend Confidence: Average ...19Z update... The 12Z ECMWF/UKMET/CMC slowed the upper trough timing into the West compared to their previous cycles. This makes the faster 12Z NAM/GFS stand out more with their quicker progression. The preference will be updated to include the 12Z ECMWF/CMC which represents a nudge to the west with the upper trough. ...previous discussion follows... The amplified pattern over the eastern Pacific will end up producing a positively tilted longwave trough axis which will work down the West Coast and eventually move inland into the Southwest Friday morning. There haven't been many trends in the ensembles and there is a moderate amount of spread with the timing by Thursday evening. The 12Z GFS and 12Z NAM are just a little faster than the majority of ensemble spaghetti height plots while the 00Z UKMET is toward the slower side. Prefer to stay toward the middle, or near the 00Z ECMWF/CMC at this time, with thinking that the 12Z NAM/GFS are less likely to verify with a faster progression within the amplified flow regime. ...Negatively titled mid-level shortwave crossing southern Ontario/Quebec on Thursday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 12Z NAM/ECMWF/UKMET blend Confidence: Average ...19Z update... The 12Z CMC adjusted faster with the shortwave Thursday evening while the 12Z ECMWF/UKMET only made minor changes compared to their previous cycles. Thinking remains the same as the previous discussion, with the updated preference excluding the faster 12Z CMC. ...previous discussion follows... The 12Z GFS appeared to be a faster outlier here which is an often seen bias in the GFS concerning shortwave trough progression. Given only the GEFS supports the faster timing of the 12Z GFS by late Thursday, a non 12Z GFS blend will be preferred. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Otto