Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 232 AM EDT Sat May 26 2018 Valid May 26/0000 UTC thru May 29/1200 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 00Z model evaluation including final preferences and forecast confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Subtropical Storm Alberto... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: See NHC Forecast Nearest Guidance: 00z ECMWF/CMC blend Initial setup/placement of center development continues to remain key on timing/track path through the eastern Gulf of Mexico. The 12z UKMET remains slow and southeast for initial development and therefore is the greatest east track skirting the western FL peninsula before pivoting on Monday south of Panama City, FL. The 00Z NAM continues to be quite aggressive with fastest/northern deep cyclogenesis...so it is also fastest north and eventually most west as it reaches the Monday pivot solution into the Bird's Foot of LA. The 00z GFS, like prior runs and the NAM also suggests convective feedback to break down the initial surface low and jump northward later Sat into early Sun. The 12z ECMWF continues to show strong consistency and is slower to emerge from the Yucatan channel and lifts north (but west of the GFS track), before pivoting SE of the Bird's foot but also south of the GFS/NAM or even 18z GEFS mean solutions. The 12z CMC like the UKMET is very slow to emerge from the Caribbean but unlike the UKMET is more in line with the growing ensemble track; though there is much less of a forecast pivot toward Monday. The official NHC forecast is slower and east of the ECMWF and about 1/3 of the way to the 12z CMC, though initially (first 12-24hrs) are slower and east of nearly all guidance closer to CMC than the ECMWF. So to get closest to the NHC forecast a 2/3rds ECMWF & 1/3 CMC blend would likely match best. Please refer to official NHC forecast for more details. 07z update: The 00z UKMET shifted faster in consolidation of the low level center and emerges near the western tip of Cuba, but still slowest along with the 00z CMC. Unlike the CMC, the UKMET still takes the widest right track before angling back toward the pivot south of Panama City. The CMC while still weaker than the ECMWF, shows greater pivoting south of Mobile Bay, Monday morning but near the NHC forecast. The 00z ECMWF shows the low level uncertainty in the first 24hrs; this leads to a slower and eastward shift emerging from the Yucatan channel and is further east...nearly matching the NHC forecast. The 00z GEFS mean, like the NAM/GFS is fast and generally splits the difference between the two. The 03z NHC forecast matches closest to the 00z ECMWF/CMC blend. ...Shortwave and surface wave clipping New England late Fri into Sunday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Above average Shortwave energy and associated surface low appear very well aligned including the lingering frontal wave angling back to the next system (see section below) for late Sun into Mon. So a general model blend will be favored at above average confidence for this system. ...Lingering frontal zone with coastal surface low developing south of Long Island Late Sunday/early Monday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 00z ECWMF/NAM blend Confidence: Slightly above average Elongated shortwave energy along the US/Canadian border into the Great Lakes will shift through the low amplitude but solid ridging across the northern Great Lakes into Northern New England late Sat into Sun. The tail frontal zone of the preceding system will be draped along 40N. The weak height falls will support weak cyclogenesis along the boundary south of Long Island Monday. There is strong agreement in the latitude of the track/frontal orientation, but the timing of the wave is highly split driven mainly by the weakness of the energy getting through the block as well as rotating from the south. The 12z UKMET clearly lags the wave and is generally weak...quite uncharacteristic on both categories. Likewise, the 12z CMC is lags but just ahead of the UKMET, in a typical location relative to other guidance, but clearly slow compared to continuity/ensemble support. The 00z NAM has shifted faster and while still lagging the most central to ensemble means solution of the ECMWF, it is more comparable and could be included. The 00z GFS still is well faster than most guidance, typical of negative bias in this region of the world. Given the 18z GEFS is tightly packed to the ECENS mean and ECMWF, will favor that solution. 07z update: The 00z ECMWF trended a bit slower between the GEFS and NAM...but closer to the 12z ECENS mean. The CMC remains slow and is not preferred at this time. The UKMET did shift faster but is also much weaker with the surface wave to not favor it within the blend, but is closer. To account for this run to run variation, will include the 00z NAM with the ECMWF in the preferred blend for this wave. ...Northern stream shortwave amplifies enough to clip New England Tues...frontal passage... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-NAM blend Confidence: Slightly above average Compact shortwave from the Gulf of AK lifts over the longwave ridge in western Canada toward Hudson Bay by early Monday. Here, general troughing/weakness allows for sharpening/southward amplification with strong jet enhancement along the base in Quebec by early Tues. This supports a strong surface low with trailing cold front to press through northern New England at the end of the forecast period. For such a fast moving wave in the larger scale pattern, the system is fairly well agreed upon. The 00z NAM is a bit uncharacteristically weaker with the southward extension and weak frontal zone affects. The 00z GFS matches well to the remaining guidance and ensemble suite to support a non-NAM blend at this point. Confidence is slightly above average given fairly good agreement in typically more random environment/setup, suggesting modest predictability at this range in time. 07z update: No significant departures with the 00z GEFS, UKMET, CMC or ECMWF to keep initial preference and have slightly above average confidence in the blend. ...Closed low central CA coast...crossing the Great Basin Sun into Mon and lifting/breaking down into Northern Rockies on Tues... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Above average (slightly above average late Mon/early Tues) Models continue to be strongly agreed upon with the evolution of the closed low through the Great Basin Sun into Monday. Even as the upper low begins to break down and lift northeastward into the northern and north central Rockies Monday... the timing/shape and evolution of the wave are fairly well agreed upon including the interaction with northern stream connection across MT into late Monday/Tuesday. The 00z GFS/NAM and UKMET are a bit more aggressive with the convective complex and upscale enhancement to the upper low across the High Plains of MT/WY compared to the weaker response from the ECMWF, but this is fairly minor as the ECMWF/CMC both indicate some response...which is typical in comparison. Typically, a general blend between these two camps will verify well, and so WPC will continue to support a general model blend at above average confidence (though reducing slightly due to mesoscale/convective influences toward Tuesday). 07z update: While there remains some run to run, model to model mesoscale upscale/feedback influence on Day 3, a continued general model blend will account for this variation and lead to some better continuity for the longer time periods. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Gallina