Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 1249 AM EDT Thu Apr 02 2020 Valid Apr 02/0000 UTC thru Apr 05/1200 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 00Z NAM/GFS Evaluation with Preliminary Preferences and Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Western Atlantic closed low and offshore surface low... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General Model Blend Confidence: Above average An elongated trough will move off the Mid-Atlantic/Northeast coast through the period, eventually consolidating into a strong deeply stacked ocean cyclone NW of Bermuda. There is a highly amplified synoptic blocking pattern that will set up over the higher latitudes resulting in a deep and slow moving 500mb trough over the Western Atlantic Ocean. A surface low will develop offshore and retrograde as strong mid level vorticity rounds the stacked low. The surface low will make its closest approach to land on Friday. Model differences are fairly minor overall with the 12Z GFS a bit farther west with its surface low feature as compared to the rest of the deterministic models. However, the latest 00Z GFS has come more in line and could be used within the blend. There is some variability with respect to QPF, but this is largely a result of a warm frontal placement, low level jet influences and mid-level impulse propagation. ...Upper Midwest/Great Plains/Mississippi Valley... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 12Z EC/ECENSmean/GEFSmean Confidence: Average A large scale trough across the western US associated with a gyre anchored in Alberta will help to eject multiple impulses across the northern Rockies/Plains and into the Upper Midwest/Great Lakes region. The first impulse will shift from the northern Rockies/Plains into the Upper Midwest through early Sunday and the second impacting the northern Rockies Saturday into Sunday. Within this section we will discuss the first impulse that will help to develop a weak surface low in the Upper Midwest region with a trailing cold front through the Plains edging east into the MS Valley region Friday through Sunday. The 00Z NAM/GFS is a bit too fast with both the impulse and thus surface features. Additionally, the axis of vorticity from the 00Z GFS is more longitudinal in nature which results in the energy propagating too quickly by the end of the period. Meanwhile the 12Z UKMET/CMC are too slow. Therefore, the EC/ECENSmean/GEFSmean seems like a reasonable middle ground and is the suggested model blend of choice. There are differences among the models in terms of mesoscale features that will result in variability with respect to QPF. Another aspect of this broad scale trough is the increasing Gulf moisture interacting with a front and seasonably strong westerly flow/southern stream jet which will yield thunderstorms in the southern Plains. The 00Z NAM QPF intensity continues to drop off with southward extent. The other models do display some spatial spread which is a product of the surface boundary placement and mid-level energy interaction, but all are more indicative of robust precipitation, so the NAM appears to be a dry outlier, especially across south Texas. So while the 00Z NAM should be removed from any blend within this region, the remaining QPF differences will be improved by higher resolution guidance that can handle these mesoscale features. For now, the EC seems a bit too far south the the higher QPF amounts, thus, in terms of a model blend for QPF, the 00Z GFS/12Z UKMET/12Z CMC seems like a logical choice. ...Northern Rockies... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-00Z GFS Blend Confidence: Average With respect to the large scale synoptic trough pattern situated across the western US as discussed above, the second impulse rounding the trough will swing through the northern Rockies/Plains Saturday into Sunday. Models are in fairly good agreement with this feature. However, the 00Z GFS is slightly farther south and is thus producing more precipitation south into the ranges of NW WY. Therefore, with regard to this features moving through, would opt for a non-00Z GFS blend. ...West Coast... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Blend of the 12Z ECMWF/ECENSmean and 00Z GFE/GEFSmean Confidence: Average Another strong impulse will drop south and east toward the west coast around the aforementioned trough anchored over the western half of the country. All models depict this feature well, but there are differences with respect to intensity and overall placement. The 12Z UKMET is too slow and weak with this energy. Meanwhile, the 12Z CMC is too far south, especially with respect to energy ejecting west into CA. While the 00Z NAM seems to be within the model cluster, the look of the large scale synoptic pattern by this point is a bit questionable with the energy on the southern periphery of the consensus. Thus, this leaves the 12Z EC/00Z GFS and their respective means for the model blend of choice. There are subtle hints the GFS is starting to become too fast with this feature, which is a known bias among the model. But it is within tolerance. Therefore, these models are a good middle ground for now. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Pagano