Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 232 PM EDT Tue Oct 15 2019 Valid Oct 15/1200 UTC thru Oct 19/0000 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 12Z Model Evaluation with latest Preferences and Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Great Lakes closed low and developing Northeast coastal surface Low ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend thru 17.12z Non-NAM weighting toward 12z ECMWF/GFS thereafter Confidence: Slightly above average thru 17.12z...average after 19z update: The 12z UKMET/CMC held fairly solid consistency with their prior runs and the initial preference; however, the 12z ECMWF shifted a bit more toward the 12z GFS supporting a weaker Mid-Atlantic surface low in favor of the coastal Carolina low, as it lifts north as the new triple point (pressing the warm conveyor belt further offshore) and the deformation zone a bit further east through northern New England. The 12z GEFS continues to show a moderate spread after 12z in trying to handle which surface wave will dominate in the deepening along the Northeast coast. All in all, the spread is minor with this slight evolution difference and the sensible weather impacts further support reduced probability of the NAM coming to fruition. As such a non-NAM blend is supported after 17.12z at average confidence, weighting more to the ECMWF/GFS given better alignment in the smaller scales. Note: For clarification, record for low pressure are for non-tropical cyclone pressures in October...Post-Tropical Sandy is included in this database, so records across the lower Hudson and Mid-Atlantic are at much lower risk of being broken...please see www.wpc.ncep.noaa.gov/research/roth/SLPrecords.html . ---Prior Discussion--- Currently an amplifying closed low is tracking into the Great Lakes with a northerly jet streak descending the western side along with strong but relatively flat subtropical jet crossing the TN Valley. As the northern stream jet rounds the base, the subtropical jet stream supports an increasing diffluent pattern over the Mid-Atlantic to support rapid surface cyclogenesis late Wed into Thursday morning. Strong warm conveyor belt enhanced by the Gulf Stream and convection will accelerate and further deepen the low across Long Island into New England, potentially reaching record low pressure values for October particularly across SE New England. Guidance is in fairly solid agreement through this time period (17.12z) to support a general model blend at slightly above average confidence. After 17.12z, the latent heat release, rapid deepening is handled differently in the transfer of the closed low energy out of the Great Lakes into the coastal system. The 12z NAM continues to show the most vertically stacked and deepest solution, which may be related to better higher resolution of warm Gulf Stream air, but this very strong signal departs rapidly from the other guidance members and ensemble suite/trend, leaving it out of the preference quickly (yet may not be fully unrealistic). The GFS which has been typically weak, has trended a bit deeper and shows a stalling over SE New England like the ECMWF/UKMET/CMC solutions, but while the mass fields aloft translate northeast into SE Canada in line with other guidance, the GFS continues to support a surface wave through the Gulf of Maine and Nova Scotia well on the eastern side of the ensemble suite. The GEFS mean, helps to adjust for the operational run, but the moderate spread in the suite denotes the GFS overall uncertainty after this point. The ECMWF/ECENS show moderate spread of solutions a bit later in the forecast period further into Canada and have support of the CMC/CMCE and UKMET. As such a Non-NAM blend is supported after 17.12z, with more GEFS than GFS in the weighting. Confidence is average after 17.12z. Short wave energy affecting Texas through midweek shearing thru eastern Gulf States by Fri ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 12z NAM/CMC/UKMET blend Confidence: Average 19z update: The 12z CMC/UKMET trended a bit slower and therefore west, but still not as slow/weak in the TX shortwave as the 12z ECMWF which continues to lag. As such will continue to favor a 12z NAM/CMC/UKMET blend. ---Prior Discussion--- GOES-WV suite denotes a positive-tilt trof across the desert Southwest of E AZ, W NM, with a weak connection to the northern stream across the TX panhandle, that continues to sever with time. Eventually (early Wed), the remaining shortwave energy stalls in in Western TX/Big Bend region, with the GFS showing typical fast(eastward) bias within the overall suite. By late Thursday, the compact wave will lift shortwave energy out of the tropics along the western periphery of the subtropical ridge across the SE Gulf/Cuba. This will support increased convection and weak lower level wave development with weak surface wave in the Central Gulf by 84hrs. The GFS being east, lifts the tropical shortwave energy north and into the Coastal Louisiana, while the NAM/CMC/UKMET are more flat/sheared into the more confluent flow in the Eastern Gulf. The ECMWF shows typical slow progression with eventually weakening the shortwave too much allowing for the surface wave/convection too far west in the suite by the end of the forecast period. As such will favor a 12z NAM and 00z UKMET/CMC blend at average confidence. Western U.S. longwave trough (Wed-Thurs) shifts into Rockies with Northern Plains cyclogenesis Thurs into Fri ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Slightly above average 19z update: The 12z CMC/ECMWF show similar evolution and progression of the trof with the GFS/NAM, prior preference. However, the 12z UKMET shifted more energy further south in the long-wave trof, weakening the energy crossing the northern High Plains, leading it to outpace the other guidance after crossing the Continental Divide 18.00z, while also supporting a stronger base to the trof through the Lower Colorado before accelerating into the Southern Plains by 00z Sat. As such a non-UKMET blend is support at slightly above average confidence. ---Prior Discussion--- Gulf of Alaska wave will progress to the persistent ridge across the West, slowly breaking it down. While the main inner core of the shortwave breaks down and lifts north into the panhandle of AK, the remaining long-wave trof continues to press eastward under the influence of strengthening broad but flat Pacific flow upstream. This allows for a constant eastward progression through the Rockies by late Thursday into Friday; supporting northern Plains cyclogenesis. The guidance has been very persistent and mildly agreed upon, with only typical timing bias noted...ie GFS/NAM and UKMET a bit fast the CMC/ECMWF slow. This timing also supports a bit more curvature to the base of the trof across the Four Corners into the Central Rockies by 84hrs, compared to the ECMWF/CMC being a bit more sharp and south. Overall, this is a favorable setup/evolution for a general model blend, mainly to slow the fast guidance, and speed up the slow ones. Confidence is slightly above average in this blend. Pacific Northwest reinforcing shortwave late Thurs into Fri with broad/strong westerly Pacific jet thereafter ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-UKMET blend Confidence: Slightly above average 19z update: Both the UKMET and ECMWF trended a bit weaker with their reinforcing shortwave entering the Pacific Northwest. This put the ECMWF in better timing/strength with the GFS/NAM/CMC and therefore tightened the overall blend and the UKMET being still too strong (and too fast with the preceding wave), suggests a continuation of a non-UKMET blend at slightly above average confidence. ---Prior Discussion--- In the wake of the initial longwave height-falls, a fast/zonal and stormy flow will setup with nearly all guidance developing a broad, flat 140-150 25H jet across the Northern Pacific by the end of the forecast period. At the nose of the jet, a wave will track into the Pacific Northwest late Thursday into Friday, the ECMWF, being slow with the initial height-falls sees a slightly faster solution than the GFS/NAM with a greater negative tilt. This is not too out of phase/amplification with the GFS/NAM/CMC to suggest keeping it in the preference. However, the 00z UKMET is extremely over-amplified, though it the correct position, this looks like a typical negative over-amplification bias noted with the UKMET and should be removed from the preference at this time. Model trends at www.wpc.noaa.gov/html/model2.html 500 mb forecasts at www.wpc.noaa.gov/h5pref/h5pref.shtml Gallina