Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 1225 AM EDT Fri May 25 2018 Valid May 25/0000 UTC thru May 28/1200 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 00Z Model evaluation including final preferences and forecast confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Surface low development over the Gulf of Mexico into Sun... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 18z GEFS/12z ECENS mean blend Confidence: Slightly below average Complex evolution for potential sub-tropical or tropical cyclone across the East-central Gulf of Mexico late Saturday into Sunday. A broad weak upper trof will exist across the central Gulf reinforced by very weak subtle shortwave energy at the base/trailing edge of trof crossing the Great Lakes. Current WV/IR Goes-16 loops already denote modest dry air environment likely to form into Dry conveyor belt as the Upper level low gels around convective complex/development along in the Costa Maya Region in the next day or so. Sea surface temperature analysis also suggests a good gradient generally north the the tip of Yucatan that will play a pivotal role in overall convective development that feeds upscale to the upper low. It is the importance of this mesoscale to storm-scale upscale growth and its magnitude that shows the greatest run to run variation in the the evolution pattern. 12z ensemble suite showed very strong clustering with GEFS and ECENS members which are generally focused along that SST gradient (as convection develops on the eastern side with the pool of instability) feeding surface cyclogenesis. The 12z ECMWF shows greatest convective feedback and vertical stacking suggestive of greatest "tropical" look... which given the Dry air conveyor and strength/shape of Warm conveyor belt from the NW Caribbean through the FL peninsula, seems a bit over-aggressive. The 00z NAM, like the ECMWF is fast in drawing the sfc low north out of the tropical wave, though with similar tracks to the ensemble suite near/along the SST gradient which seem reasonable. Though do not fully believe the high tropical/near surface low convective development. The 12 UKMET continues to be very sub-tropical in nature and very slow in drawing the surface wave north and generally favoring a strong warm conveyor/convective processes along the FL peninsula/Western Cuba. This is likely too slow and least likely. The overall 12z CMC solution looks quite reasonable with and exception of cyclogenesis over the Yucatan Peninsula leading to a westward track relative to the ensemble suite/ECMWF/NAM. Would favor this solution greatest IF it was about 30-50 miles east overall. The 00z GFS has trended faster emerging with the incipient surface low north of the Yucatan in the same vicinity of the SST gradient/ECMWF/NAM; however, by 00z Sunday, strong convective development along the nose of the dry conveyor/western edge of the warm conveyor in the NE Gulf spurs a secondary surface response at the effective triple point, which becomes dominant and keeps the low level mass field divergent from other/ensemble suite solutions. The 18z GEFS is slower than the 12z ECENS/00z NAM but shows a similar track near/along the SST gradient and is more reasonable...like the old 24/*12z* GFS. At this point, the ensemble means are more in line with current thinking than use of any deterministic guidance member, though the CMC would be if further west. Confidence is slightly below average in this blend. ...Shortwave and surface wave clipping New England late Fri/Sat... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-UKMET blend Confidence: Above average Tightly packed heights and fairly flat northwesterly flow will buckle into a weak cyclonic trof over Southeast Canada clipping New England late Friday into Saturday. Here only the UKMET is a bit too amplified with the trailing back edge of the shortwave trof delaying the surface low/frontal zone a bit too much from the remaining guidance. As such a non-UKMET blend is supported at above average confidence. ...Short waves tracking along the Canadian border today thru Sun with associated surface low across Great Lakes Sat/Sun...Surface wave development in near Cape Cod by 00z Mon ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend through 28/00z 00z GFS and 12z ECMWF/UKMET blend thereafter Confidence: Slightly above average GOES-WV suite depict the elongated string of shortwave features across south-central Canada with the strongest/most amplified feature in southern Manitoba. Mean larger scale flow will drive the elongated trof eastward into general confluence with some weak blocking over the northeastern Great Lakes. As such the surface wave will slowly drift east through the Boundary Waters slowly weakening with time. As the features break down the block, into Sunday/Monday, the height-falls will support coastal cyclogenesis from Long Island to Cape Cod before shifting the new wave eastward. Here the NAM/GFS are a bit north with the surface boundary and faster toward closing a surface circulation than the CMC/ECMWF and UKMET. The NAM and CMC (north and south respectfully) eventually are a bit more amplified and slower through the 7-85H layer making them a bit less favorable east of the mountains, though fine across the Great Lakes. As such a general model blend will suffice through about 28/00z but will trend away from the NAM/CMC thereafter. This is a small variation difference from the preferred UKMET/GFS and ECMWF to increase signal fidelity in the blend and therefore a bit higher confidence (slightly above average in the blend). ...Closed low off central West Coast reaches CA by early Fri before crossing the Great Basin Sun into Mon... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Above average The 00z NAM and GFS both remain within the strengthening model consensus as the upper low reaches CA and translates into the Great Basin. The 00z NAM and 12z UKMET are a bit more aggressive with the shortwave rounding the eastern side of the upper low through the Rockies on late Sun into Monday morning. Both tend to shift the interaction in the deformation zone (between the upper low and the northern stream shortwave across southern Canada) a bit further east into the High Plains with likely some upscale enhancement from convection. The GFS/ECMWF and CMC also suggest this but not to the magnitude of the NAM/UKMET. All in all a general model blend should suffice through this late period minor difference. Given small spread and good timing of internal shortwave features, confidence is above average in this preference. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Gallina