Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 254 PM EDT Wed Apr 29 2020 Valid Apr 29/1200 UTC thru May 03/0000 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 12Z Model Evaluation Including Latest Preferences and Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Deep trough/closed low impacting the East... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-NAM blend Confidence: Slightly above average The models show good agreement in taking the deep upper trough and associated closed low over the Midwest gradually off to the east through tonight, with low pressure focusing predominantly over the Great Lakes region over the next 24 hours. However, as additional shortwave energy rounds the base of the trough over the Gulf Coast states, TN Valley and southern Appalachians early on Thursday, the closed low itself should begin to re-consolidate down across the OH Valley later Thursday and then over the central Mid-Atlantic region by Friday. This will allow a new surface low to develop and concentrate over the VA/NC Piedmont area before then lifting northeast toward southern New England on Friday. On Saturday, the system should begin to exit offshore of the Northeast. Multi-model trends in the guidance favors the closed low evolution over the Mid-Atlantic region to be a tad farther south, and the latest 12Z global model suite has clustered in agreement on that as the 12Z GFS trended farther south from its 00Z run. The UKMET which had been the one non-NCEP model that was north of the model consensus has also trended south with its 12Z run, and is now well clustered with global model suite. The 12Z CMC came just a tad north, and the 12Z ECMWF is very close to its previous run. These trends allow for a well-clustered global model consensus. The 12Z NAM did trend notably south compared to its 00Z run, but it may be a tad too far south, and especially after 60 hours as the deep trough/low center edges offshore of the Northeast. By then the NAM also appears to be too slow. So, at this point, will simply suggest a non-NAM blend in favor of the global model consensus. ...Weak shortwave clipping the Pacific Northwest later today... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Above average A modest and rather progressive shortwave trough will come in off the Pacific Ocean and clip areas of the Pacific Northwest and southern British Columbia later today and tonight. A weak area of low pressure and attendant front is expected to accompany this energy. Model spread is very modest with this feature, and so a general model blend will be preferred. ...Shortwave crossing the Northwest on Thursday... ...Clipping the Northern Plains on Friday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Blend of the 12Z GFS and 12Z ECMWF Confidence: Slightly above average The models bring another shortwave trough in across the Northwest on Thursday and clip the northern Plains on Friday as surface low pressure crosses southern Canada and a trailing front extends south across the Plains. The 12Z UKMET gradually ends up a tad farther south and slower with its energy and the focus of its surface low relative to the model consensus. The 12Z NAM came in a bit on the north side of the model spread. The 12Z GFS and 12Z ECMWF show good model clustering and are close to the model consensus through the period, so a blend of these two solutions will be preferred. ...Surface low over the Southern Plains by Saturday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend...through 72 hours Non-NAM blend...after 72 hours Confidence: Above average A surface low center is expected to be focused across the southern High Plains by Saturday along the tail end of a front. The guidance is in good agreement generally through about 72 hours with the placement of the low and the front, but thereafter, the 12Z NAM tends to displace its surface low and front a tad farther to the southeast (closer to northwest and north-central TX). There is a little bit of support from the 12Z CMC and 12Z UKMET at the end of the period for a low center a tad farther east of the global model consensus (more over the OK/TX Panhandles), but the NAM is still likely too far to the southeast with its low and frontal placement. So, a general model blend will be preferred through 72 hours, and then a non-NAM blend thereafter. ...Deep trough offshore the West Coast by Saturday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 12Z GFS/ECMWF and 12Z GEFS/00Z ECENS mean blend Confidence: Average A deep trough is forecast to be evolving offshore of the West Coast by Saturday along with a strong surface low reflection. Much of the energy is expected to be confined to the Gulf of AK, but a portion of the trough/height falls will pivot across the West Coast and impact especially the Pacific Northwest by late Saturday. A cold front is expected to be approaching, and the guidance is now showing good agreement with aside from the 12Z UKMET which is a slower outlier. The bigger differences tend to be more centered on a possible wave of low pressure developing and deepening around the east-side of the larger synoptic scale low offshore as shortwave energy rounds the base of the trough. The NAM, GFS and UKMET all favor a reasonably well-developed low center developing along the aforementioned front that will be approaching the West Coast. The CMC and ECMWF show less development. The ensemble means are a tad split with the GEFS favoring the NAM/GFS/UKMET camp, and the ECENS mean favoring the CMC/ECMWF camp. Will recommend a compromise between the camps with this and continue to favor a blend of the deterministic GFS/ECMWF and GEFS/ECENS means for the time being. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Orrison