Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 AM EDT Thu Apr 13 2023 Valid 12Z Sun Apr 16 2023 - 12Z Thu Apr 20 2023 ...Overview... A closed upper low will spin up near the Great Lakes early next week, while at the surface, a low pressure system pushes a cold front across the East. Wintry precipitation is possible across northern parts of the Upper Midwest/Great Lakes behind the surface low while rain and thunderstorms abound along and ahead of the front. Upstream, there remains some questions regarding the flow across the West, but in general precipitation chances should spread from the Pacific Northwest southward into northern California and eastward into the northern Rockies and Plains as the week progresses. ...Guidance/Predictability Assessment... Model guidance shows reasonable agreement for the large-scale pattern at the start of the medium range period Sunday as an upper low develops atop the Midwest and another spins in the northeastern Pacific, with ridging in between. The first upper low will help a surface low develop that looks to track slowly northeastward from the Great Lakes region into southern Canada. The 12/18Z model cycle shows some relatively minor differences in the track and timing of the low and on the whole looks a little faster around Monday than the newer 00Z cycle is. CMC runs have seemed to struggle over the past few days with the development of the upper low, spilling less energy south compared to consensus. A GFS/ECMWF blend with some UKMET clustered better. While generally the upper low looks to slowly lift away from the U.S. by Wednesday-Thursday, the newer 00Z GFS in particular tracks the upper low more south than the previous consensus. Model guidance has been persistent with a broad upper low in the eastern Pacific next week, with what begins as relatively typical differences in placement of embedded shortwave energies. Some significant differences arise though by Tuesday-Thursday as at least some energy is likely to track into the Northwest, and run-to-run consistency has been poor with little confidence in how exactly this pattern will evolve at this time. The 12Z and newer 00Z ECMWF start to divide the Pacific energy by Wednesday-Thursday, maintaining some closer to the Gulf of Alaska while spilling notable energy forming troughing with an axis around the Rockies-High Plains, with ridging in between over the West Coast by Thursday. On the other hand, GFS runs tend to keep most energy offshore of British Columbia to maintain one upper low near its early week location. There is considerable spread among the ensemble members but with a trend toward the members and thus the means looking like their deterministic counterparts for the ECMWF, GFS, and CMC. This lessens confidence in which way to lean, with a middle ground/blend approach favored, not as aggressive as the ECMWF with troughing in the central U.S. but with some indication that energy could make it that far east. This uncertain forecast will continue to be refined with time. Given the above considerations, the WPC forecast was based on a blend of deterministic 12/18Z guidance favoring the GFS and ECMWF early on with some inclusion of the UKMET. With increasing spread as the period progressed, added in and increased the proportion of the GEFS and EC ensemble means to over half by day 7 to lessen the reliance on individual models. ...Weather/Hazards Highlights... Low pressure at the surface and aloft will help support precipitation across the Midwest/Great Lakes Sunday, and cold air filtering in behind the surface low could support some springtime snow in northern portions of these areas, lingering into Monday. Meanwhile convection should spread across the East early next week. The central Appalachians/northern Mid-Atlantic into the Northeast may see the best chance for heavy rain in convection given the deepening closed low, instability in place, and anomalously high moisture, but the progressive nature of the storms and the dry antecedent conditions cast doubt on the chances for appreciable excessive rainfall. Precipitation could linger across the Great Lakes to northeastern U.S. into Monday-Tuesday with the slow-moving upper low, with some lake enhancement. Light snow is possible across the Great Lakes to interior New York/New England in this precipitation activity. Precipitation in the form of lower elevation rain and higher elevation snow is likely in the Pacific Northwest on Sunday, with some expansion into the northern Rockies beginning Sunday as well and potentially lasting there through around midweek. Northern California may see increasing precipitation chances too around Monday-Tuesday. By Tuesday into midweek, precipitation could extend into the Plains as well. Some return flow moisture could allow for some rain chances across the south-central U.S. by Monday or Tuesday It is worth noting that uncertainty significantly increases given the timing of surges of upper-level energy later into the period, leading to less confidence in the location and timing of daily precipitation chances in these areas. However, the pattern should keep precipitation chances in place across the region overall. Warmer than normal temperatures by 10-20F are likely across parts of the Great Lakes/Appalachians on Sunday before a cold front comes through Monday and cools temperatures to near or below normal through midweek. The interior West can expect temperatures 5-10F above average Sunday ahead of a possible cooling trend as upper-level energy could come in. After a relatively cool pattern in the central U.S. behind the cold front to begin the week, the ridge aloft will support temperatures warming to around 10-20F above average in the Plains for Monday and beyond, with above normal temperatures stretching toward the East Coast by Wednesday-Thursday. Tate 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