Extended Forecast Discussion NWS Weather Prediction Center College Park MD 249 AM EDT Wed Apr 13 2022 Valid 12Z Sat Apr 16 2022 - 12Z Wed Apr 20 2022 ...Overview... Lower amplitude mean flow aloft should prevail over the lower 48 at least through the weekend, behind a departing deep upper low which may take its time drifting across eastern Canada into next week. Guidance generally shows a shortwave and associated rain/snow progressing into and through the West during the weekend then into the Plains/Mississippi Valley by early next week. Meanwhile, an upper trough reaching the eastern Pacific by Monday may serve to amplify the overall flow with potential for a closed low over the Great Lakes/Midwest around next Monday-Tuesday and into the Northeast by Wednesday. Within what has been a decently agreeable forecast of the large scale pattern, persistent significant detail differences at the surface and aloft keep confidence lower than desired for coverage and intensity of precipitation--especially over the eastern half of the country from Sunday onward but also to some degree over the West. ...Guidance Evaluation/Predictability Assessment... The first half of the period started with a composite of latest operational runs. This part of the forecast involved primarily timing issues, with the GFS beginning to stray a bit faster than most other guidance for the surface front and supporting dynamics crossing New England during the weekend. There's also some initial spread/variability persisting for the system reaching the West by Saturday. The western system has some degree of complexity, involving potential interaction of a trough/upper low off the Pacific Northwest/Vancouver Island and separate shortwave just to the south. The 18z GFS and 12z UKMET both were faster than the ECMWF and CMC. The UKMET is also the quickest/farthest south to close off a low over the Ohio Valley and was not included in the blend for tonight. By days 6-7, the ECMWF and the ECENS mean both become noticeably slower with the upper system through the Northeast, with the CMC continuing to be a nice middle ground (along with the NAEFS mean). The new 00z runs actually look a little more agreeable, with the GFS slowing a bit and the ECMWF a little faster than the previous run. The 00z UKMET remains a fast/south outlier. The next system enters the West Coast early next week with again, the GFS a fast/stronger outlier and wanting to close off an upper low over the Northern Plains/Upper Mississippi Valley next Wednesday. Prefer a blend towards the slower ECMWF/CMC along with modest inclusion of the NAEFS/ECENS ensemble means for the later periods. ...Weather Highlights/Threats... The departing low pressure system north of the Great Lakes should continue to produce some generally light to moderate precipitation across the Great Lakes/Northeast as a secondary surface low lifts off the coast. Moisture and instability interacting with a warm front lifting through the Southeast and Lower Mississippi Valley this weekend and approach of a cold front from the north should bring areas of moderate to locally heavy rainfall to parts of those areas, but for now with low confidence in exact placement of heaviest totals. Farther west, a system forecast to reach the West by Saturday should continue to focus rain/mountain snow over the central and northern West Coast into the northern Rockies late this week into the weekend. Moisture should spread east beyond the Rockies Sunday onward as low pressure/fronts reach the central/eastern U.S.. At least some precipitation across the northern tier may be in the form of snow while rainfall coverage/intensity over the East will depend on currently uncertain system details. The central Rockies/High Plains could see a brief period of strong winds around Sunday with system passage. Expect another episode of precipitation from the Pacific Northwest/northern California into the northern Rockies early next week, with some enhanced totals possible over favored coastal/terrain areas. The core of below normal temperatures across the northern High Plains will likely persist into early next week, with daytime highs generally 20-30F below normal, and moderating slowly with time. Over the West, temperatures should moderate in between systems as a period of upper ridging moves through. Saturday will feature chilly readings over the northern half of the region (minus 10-15F anomalies for highs) and closer to normal temperatures farther south. As the upper ridge builds over the region by the start of next week, northern locations should trend toward normal and southern areas above normal. The Great Basin and Southwest could see highs as warm as 10-15F above normal. The East should stay near normal on Saturday, but the system potentially reaching the Midwest/East early next week could draw some of the chilly northern Plains air southward and eastward behind it, with expanding coverage of minus 10-20F anomalies. The southern Plains will see highs 10-15F above normal next weekend followed by a cooling trend. Santorelli/Rausch Additional 3-7 Day Hazard information can be found on the WPC medium range hazards outlook chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation, experimental excessive rainfall outlook, 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/#page=ero https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml