Extended Forecast Discussion NWS Weather Prediction Center College Park MD 103 PM EDT Sat Jun 06 2020 Valid 12Z Tue Jun 09 2020 - 12Z Sat Jun 13 2020 ...Heavy Rainfall Threat into the Upper Midwest/western Great Lakes Tue from Cristobal Merging with a frontal wave... ...Overview/Guidance/Predictability Assessment... The models and ensembles show the remnants of Cristobal merging with a frontal wave and corresponding high amplitude mid-upper trough moving east from the Plains on Tue. The merging post-tropical low and frontal wave lead to a strong area of low pressure that crosses Iowa and Wisconsin to Lake Superior and up into Canada. This system poses a locally heavy rainfall threat along with windy conditions along and adjacent to the low track. The departure of the combined low pressure area/upper trough drives a secondary front across the Upper MS Valley and northern Plains/upper Great Lakes Thu 11 Jun. The deamplifying upper trough leads to a slower frontal progression across the Great Lakes and Appalachians Thu and Fri. By Fri morning, the pattern reamplifies as as an upper trough over the east Pacific moves towards the west coast. More precipitation and cooler temperatures will infiltrate the Pac NW via a slow-moving front. The amplifying trough results in a downstream amplifying upper ridge in the northern Rockies and then central Rockies to northern Plains. In turn there a reinvigorating shortwave that reamplifies the Great lakes to Ohio Valley trough. The 00-06Z models and ensembles generally agreed on the pattern evolution with a few exceptions. The 00z Canadian formed a closed low on the central Plains Tue night that led to a lower forward progression of the system into the upper MS Valley and upper Great Lakes. The GFS remained quicker to lift Cristobal northward compared to the other models, but once inland an increase in forward speed is quite certain. Given good clustering among the 06z GFS/06z GEFS/00z ECMWF/12z ECMWF/00z ECMWF Ensemble Mean, a blend of these forecasts was used for mass fields and gridded temperatures, dewpoints, etc. By Day 7, Sat 13 June, the western trough moves inland across the Rockies and Great Basin, possibly reaching as far east as the northern Plains. Typical timing and amplitude differences exist, with enough agreement to use a consensus based approach among the ECMWF/GFS and their respective ensemble means. Likewise, in the Great Lakes and Ohio Valley across the northern Appalachians, mid Atlantic and northeast, slight timing/phasing differences in the upper trough drifting east from the Great Lakes impact the timing and whether low pressure develops along the front in the northeast. To mitigate differences, a blend of the 06z GFS/06z GEFS Mean /00z ECMWF/12z ECMWF/00z ECMWF Ensemble Mean was used for the gridded forecasts and QPF. ...Weather/Hazard Highlights... Rainfall may be heavy over the Corn Belt Tue to the western Great Lakes Wed as Cristobal's remnants merge with the frontal circulation. Several inches of rain are possible in Iowa and Wisconsin due to both the unusually strong low pressure and resultant low level jet, combined with the tropical system advecting above normal moisture into the region. Over the Northwest, lower elevation rain showers high elevation snow showers will accompany the front as it pushes inland next Friday to Saturday. In the east, showers focus near the slow moving frontal boundary from the northeast to the mid Atlantic, Appalachians, and southeast, where embedded thunderstorms are possible. Ahead of the front/Cristobal will be an area of warmer than normal temperatures to the Great Lakes into the Northeast. Several record high temperature occurrences are possible in southern Texas and also the Great Lakes on Tue and Wed 10 June, as well as coastal southern California. Below normal temperatures over the Great Basin/Rockies on Tuesday will be replaced by warmer than normal temperatures Thursday and Friday as ridging builds over the area. While warmer than normal temperatures continue inland on Friday, the arrival of the Pacific front leads to more clouds and cooling in western WA/OR/northwest CA. On Saturday 13 June, the cooler air continues inland into the Great Basin and northern Rockies, with above normal temperatures on the other side of the front further east in the northern Plains . Persistence of the upper trough supports below normal temperatures developing on Friday and continuing Saturday 13 June in the Great Lakes. Petersen Additional 3-7 Day Hazards information can be found on the WPC medium range hazards chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation, winter weather outlook probabilities and heat indices are at: https://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif https://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif https://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html https://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml