Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 1206 PM EDT Fri May 11 2018 Valid May 11/1200 UTC thru May 15/0000 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 12Z model evaluation including preferences and forecast confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Western Closed low that stalls over Great Basin ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-CMC blend Confidence: Above average A broad closed Western low is starting to settle into an expected locked position across the Great Basin over the next day. Internal shortwave features will rotate at a moderate distance from the parent center point of the low with broad diffluent pattern downstream over the Central Rockies. Models continue to show remarkable agreement even including the strength and timing of the internal waves as they rotate around the center. If there was a significant departure within the guidance suite...the 00z CMC is initially a bit west and south of the tight cluster by late Saturday. As the low fills slowly into late Sun/Mon, the CMC is also a bit stronger with the internal waves for a bit longer allowing for a slow northward drift back toward northern stream influences. The 12z NAM as is typical by the end of the forecast period is resolving a bit deeper of heights but this may be more related to the grid-scale that an apparent true mass/evolution difference. A general model blend could easily be supported but given the CMC slight divergent solution (with no readily apparent synoptic reason) will support a non-CMC blend as preference at above average confidence. Daily ejected shortwaves into the Plains crossing to confluent flow over Great Lakes/Mid-Atlantic...associated surface waves along flat W-E stationary front from KS to PA/MD... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Slightly above average Northern stream flow becomes confluent with return southwesterly flow around the closed lover over the Midwest into the Great Lakes. This pattern is quite stable. Within the southwesterly flow, the mean shortwave ridging and diffluence aloft with upslope return moisture/mass flow supports a daily surface low response in the lee of the Rockies. Convection will also support upscale development of a shortwave that will progress into the confluent flow and help track a weak surface ripple along the stationary front from KS to the Mid-Atlantic through the forecast period. Convection will be the main driver to mass differences, and driven in the meso-scale; while the large scale pattern is very agreeable and therefore supports maintaining a general model blend. Confidence is very high in the large scale, but low in precise details due to the contingency on the convective responses. Broad upper trough growing upscale over Easter Gulf of Mexico/FL by early Sun ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-UKMET blend Confidence: Slightly above average GOES-16 WV suite shows a weak shortwave/baroclinic leaf near/NW of the Yucatan peninsula, while surface analysis depicts a easterly wave/eastward tracking tail of the Sargasso Sea frontal zone across the FL peninsula into the eastern Gulf. The large scale pattern will support increasing ridging across the Southeast with peripheral evacuation/outflow channels. The combination will allow for a favorable convective environment to develop across the Eastern Gulf of Mexico to the Bahamian Bank/Gulf Stream. This will support the shortwave to amply into a closed upper low late Sunday into Monday, as well as a weak surface inflection. Deterministic guidance has been handling this similarly...though the 12z GFS and 00z ECMWF may be a bit deeper and concentric (GFS faster west, ECWMF a bit slower east), the UKMET depicts a more subtropical shape/evolution with a convective response along the east coast of FL that may be a bit too much latent heat release to pull the surface response further north and east from the stronger ensemble clustering. The CMC and NAM are a bit more broader with the convective response with more isolated clusters and are a slower/weaker response initially compared to the GFS/ECMWF; yet still good representations of potential outcomes. As such will favor a non-UKMET blend, only to tighten/strengthen the signal vs smearing it out (though its evolution provides overall confidence to the eventual evolution of the closed low itself). Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Gallina