Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 131 PM EST Tue Feb 06 2018 Valid Feb 06/1200 UTC thru Feb 10/0000 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... ...12Z Model Evaluation with Preliminary Preferences/Confidence... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Weak Cyclogenesis from Mid-Atlantic into the Northeast with Trailing Cold Front Sweeping Through the Southeast Wed-Thu... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Blend of 12Z GFS, ECMWF, UKMET Confidence: Average ---18Z UPDATE--- No change to the preliminary preference. The 12Z CMC continues to have the weakest depiction of the trough, and the ECMWF and UKMET remain fairly close to the GFS in timing and intensity of the surface low (slightly closer to the coast than the GFS). With the best clustering continuing to be with the GFS, UKMET, and ECMWF, that continues to be the preferred blend. ---PREV. DISCUSSION--- As the upstream wave contributing to the gradual cyclogenesis pushes through the Great Lakes region early Wednesday, small differences begin to emerge. The 12Z NAM shows a slightly deeper trough which eventually leads to earlier surface low development, with a stronger surface low that is on the western periphery of model spread as it moves into the Northeast. The 00Z CMC, by contrast, shows slightly less amplification of the trough in the Northern Great Lakes, with more downstream ridging. This leads to one of the weaker surface lows depicted by operational models. The preference is to lean closer to the GFS, ECMWF, and UKMET which are more tightly clustered and their mass fields are reasonably close to the 00Z ECMWF Ensemble Mean. Overall differences with this system are relatively small, but even minor fluctuations in the strength or track of the surface low would have implications on the precipitation type forecast across portions of the Northeast. ...Gradual Deepening of a Broad Trough with Several Digging Waves Across the Northern Tier of States Thursday to Friday... ...Southward Progression of an Arctic High into Northern Plains... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Blend of 12Z GFS, ECMWF Confidence: Slightly Below Average ---18Z UPDATE--- No change to the preliminary preference. Despite all models agreeing on broad trough amplification, there remain substantial differences in the details. The preference is to maintain continuity with the original assessment. ---PREV. DISCUSSION--- Several digging waves will contribute to height falls and a gradual deepening of the broad trough across the northern tier of states late this week. One wave, currently north of the primary mid-upper level low over Nunavut, will round the low and become increasingly sheared out as it kicks into the Western Great Lakes on Friday. Model agreement is generally good with that particular wave, however greater differences are noted with a secondary wave that will be digging from the Gulf of Alaska into the Intermountain West. This digging wave will be the result of complex phasing of several elements in the vicinity of Alaska over the next 24 hours -- one shortwave rounding the top of an eastern Pacific ridge, and another shortwave undercutting the northward extension of the ridge over Alaska. As these two elements phase, model spread markedly increases, which has downstream implications. The 12Z NAM produces a much deeper trough over portions of the West, with the lead portion of the wave accelerating into the Plains by 10/00Z, well ahead of any other operational model. With a lack of support from other models and ensembles, the NAM is not included in this preference. The next fastest solution is represented by the 00Z UKMET/CMC which are notably ahead of the ensemble means. The 12Z GFS and 00Z ECMWF, by contrast, are closest to the ensemble means, and in particular the 06Z GEFS and 00Z ECMWF Ensemble. Therefore, the preference is for a blend of the GFS and ECMWF, with a slight lean in the direction of the ECMWF. The GFS has higher heights across portions of the west at the base of the trough than many other operational models, as well as its ensemble mean, while the ECMWF is better supported by a range of operational models and is more similar to both the GEFS and ECMWF Ensemble mean. ...Broad Southwesterly Return Flow and Associated Increasing Precipitation Chances Friday Over the Southeast... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Blend of 12Z GFS, ECMWF Confidence: Average ---18Z UPDATE--- No change to preliminary preference. The 12Z ECMWF trended closer to the GFS in terms of depicting a slower erosion of the low-level ridge, with associated changes in the precipitation across the Southeast. ---PREV. DISCUSSION--- Despite some similarity in the mass fields, there are differences in how the models handle precip across the Southeast on Friday. Differences appear to be partially related to the strength of the residual high and surface ridge along the East Coast and down the lee of the Appalachians, as well as how models handle the deepening of the upstream trough. Given the influence of the two primary short-term systems on conditions in the Southeast, the preference is leaning toward the models that handle those systems best: the GFS and ECMWF. The 12Z GFS shows a stronger ridge over the Southeast and restricts the return flow a little longer. Drier air and a slower timing of the ramp up in return flow result in less precip than the ECMWF, which erodes the influence of the ridge faster. Given uncertainties at this time range, the preference is for a blend of the two at this time. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Lamers