Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 1232 AM EDT Thu May 24 2018 Valid May 24/0000 UTC thru May 27/1200 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 00Z Model evaluation including preferences and forecast confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Surface low development over the Gulf of Mexico by Saturday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 12z ECMWF/00z NAM blend Confidence: Slightly below average A tropical easterly wave is currently located in the Gulf of Honduras up the Yucatan Peninsula with a weak upper low just north of it with a strengthening ridge over the Western Gulf/Eastern Mexico. Cycle to cycle ensemble trends show an increasing suggestion of the surface wave to lift northward and develop toward Saturday as the upper low broadens over the Gulf. The 12z ECMWF continues to develop off the NE tip of the Yucatan and lift north through the central Gulf supported by a bulk of ECENS members that are also clustering tighter with time. The 00z NAM continues to track close to the ECMWF with some typical Day 3 bias toward a convectively influenced deeper solution. The 12z UKMET is most out of phase suggesting the wave/surface low remain south in the NW Caribbean with convective response over Cuba; which is becoming further from growing consensus. The GFS which has been also well distant (east along the FL coast, due to greater convective response in that direction) continues to slowly shift toward the ECMWF/NAM but remains on the eastern fringe of the solution suite (even compared to the bulk of GEFS members...those that is lessening too). The 12z CMC shows typical tropical cyclone bias of strong cyclogenesis and shows greatest upper level deepening/support as well. As such will favor a 12z ECWMF/00z NAM blend at this time. While the ECMWF is consolidating on a general track solution there remains large model/ensemble spread, quite contingent on convective response to have slightly below average confidence in this blend. ...Shortwave and associated surface wave clipping northern New England late Fri/Sat... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Above average confidence After the exit of the shortwave crossing northern New England today, tightly packed height-lines usher in strong northwest flow with a broader longwave cyclonic curvature noted. This curvature starts to buckle and sharpen a bit toward late Friday, and as such a broad but sharpening surface pressure falls will occur across northern New England and SE Canada. Model guidance shows mesoscale variability but overall the mass fields are strongly aligned with good timing/shape to consider a general model blend at above average confidence as the wave passes Sat connecting back to the wave over the Great Lakes (see section below). ...Multiple shortwave features across Northern US Rockies that will slide eastward along US/Canadian border with associated low into the western Great Lakes by Sat/Sun... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend (along US/Canadian Border) Non-NAM blend across Lower MO valley/Central MS Valley Sat/Sun Confidence: Slightly above average Goes-WV suite depict three compact shortwave features over the Northwest US into Southern Canadian Rockies with strongest/tightest circulation over MT lifting north. A surface low has developed in response across the MT/Dakota's border region. The vorticity centers will continue to rotate around each other as they over-top the larger scale ridge and start to slide eastward into confluent flow along the Central US/Canadian boarder. Generally, there is west to east stretching of the centers through this time period...but there is strong model agreement through this time period. The only significant departure is more related southward across E KS/MO into the Central MS river valley with the 00z NAM. Here significant convective development (not supported well by other global guidance), leads to mass field feedback with an eventual 1006mb surface low early Sat in MO. Further north a general model blend is likely sufficient, but the NAM needs to be removed across the central US. ...Development of closed low off Central West Coast that reaches CA by late Thurs/early Fri before moving into Great Basin by Sun... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Slightly above average Goes-W WV shows the incipient stages of the developing closed low at the base of the Northeastern Pacific/Gulf of AK trof slowly advancing eastward. Strong model agreement exists within the ensemble and deterministic suite with small differences in timing remaining. The 00z NAM and GFS both are a bit faster and stronger with an internal shortwave feature rotating through the base of the parent circulation on late Fri through Sat as the low crosses CA. This leads to a slightly more elongated closed low entering the Great Basin while showing typical faster bias and both are are paired with the faster 12z UKMET. The 12z ECMWF is slower but more concentric as it does not enhance the internal shortwave as dramatically as the GFS/NAM. All in all, a general model blend is likely a good compromise and more likely to verify, Confidence is Slightly 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