Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 256 PM EDT Sat May 25 2019 Valid May 25/1200 UTC thru May 29/0000 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 12Z Preliminary Model Evaluation Including Preferences and Forecast Confidence... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Overall CONUS Overview... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-NAM days 1-2, ECMWF/GFS/UKMET on day 3 Confidence: Average 19Z Update: Most significant difference with the 12Z Non-NCEP suite was a trend upward in strength of the day 2 shortwave lifting across the middle of the country near 12Z Monday morning. ECMWF trended stronger, which matched the GFS and UKMET. The CMC also is a bit sharper with this feature than previous runs, leaving the NAM as the sole weak solution at this time. By day 3, the important difference in the CMC is how it handles the evolution of the closed low opening into the middle of the country. While uncertainty remains, the UKMET/GFS/ECMWF remain tightly clustered with a slower eastward translation of this feature. Previous Discussion: Flow across the CONUS remains anchored by an anomalously strong subtropical ridge across the Southeast downstream of a deep longwave trough over the West. While the guidance remains tightly clustered in the overall synoptic pattern, differences in mesoscale features as shortwave energy is repeatedly shedded to the northeast through the middle of the country, are beginning to arise. This is evident on day 1 /Sunday/ as the 12Z/25 NAM is stronger and further south with the shortwave lifting out of the Southwest. This is well south of the remaining guidance, and at the edge of the envelope. With the strength of the subtropical ridge across the Southeast, it seems unlikely this feature would traverse into the ridge as sharply as the NAM suggests. While this is a relatively minor feature overall, it plays significantly on the sensible weather and potential heavy rainfall in the middle part of the country. For this reason the NAM should be used with caution on day 1, but the remaining global suite is reasonable. By day 2, a secondary impulse sheds from the western trough and across the middle of the country once again as it gets caught up around the periphery of the dome of high pressure to the east. A more widely spaced envelope of solutions exists into Monday, but the NAM again is the model left out of the blend as it is much weaker than the rest of the guidance, and displaced from the consensus, likely in response to its difference on day 1 breaking down the ridge. The UKMET is on the strong edge of the solution envelope, but recent 12Z/25 GFS has trended stronger, and is in line with the CMC/ECMWF so can remain in the blend. By day 3, spread becomes more significant as the closed low across the west opens and begins to lift eastward towards the persistent ridge over the Southeast. The 00z/25 ECMWF remains a very consistent solution with respect to its prior two runs, and matches well with the 00z/25 UKMET and 00z/25 GEFS mean, and the new slower 12Z/25 GFS. Initially, the movement of this closed low will force downstream amplification of the ridge, which leads to jet-streak enhancement north of the Great Lakes in response to intensifying confluent mid-level flow. Uncertainty abounds into how this low will eventually break down the ridge, noting that continuous shedding of energy will be progressively sheared eastward in the strengthening flow to the northeast, potentially aiding in weakening the ridge. However, guidance tends to be too aggressive with breakdown of these strong ridges, so the consistent and slower ECMWF/UKMET/GEFS solutions become preferred by the end of this forecast period. Model trends at www.wpc.noaa.gov/html/model2.html 500 mb forecasts at www.wpc.noaa.gov/h5pref/h5pref.shtml Weiss