Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 AM EDT Wed Apr 12 2023 Valid 12Z Sat Apr 15 2023 - 12Z Wed Apr 19 2023 ...Overview... Upper-level troughing over the central U.S. this weekend looks to spin up into an upper low near the Great Lakes early next week, and at the surface, a low pressure system pushes a cold front across the eastern half of the CONUS. Wintry precipitation is possible across northern parts of the Upper Midwest behind the surface low while rain and thunderstorms abound along and ahead of the front, with areas of enhanced rainfall near the central Gulf Coast and in the Midwest causing some flooding concerns. Precipitation chances should spread into the Northwest this weekend and early next week in response to a northeastern Pacific upper low. ...Guidance/Predictability Assessment... Weak energy emerging from the Gulf Coast upper low in the short range looks to be lifting through the central Appalachians toward the Northeast on Saturday with good model agreement. Model guidance then shows reasonable agreement with the trough axis in the central U.S. Saturday and the fact that it should develop into a closed low early next week, but differences remain with the details as the energy distribution within the trough varies. The GFS/GEFS mean/ECMWF/EC mean generally cluster pretty well in developing one main upper low on Sunday, so this was favored over the 12Z CMC and UKMET runs that held some energy to the north and spilled some to the south for dual lows. The newer 00Z UKMET seemed to come into a bit better agreement in producing one upper low but the newer 00Z CMC still indicates a northern and a southern low. CMC runs have thus been showing the track of the surface low persistently farther south than the GFS/EC suite consensus as well. Farther west, there is good agreement for ridging to come in behind this trough and as an upper low spins over the northeastern Pacific. There are some differences in the ridge's amplitude by around Tuesday as it moves eastward due to some differences in evolution of the Pacific energy. Through around day 5/Monday models are compatible with the upper low's position, but beyond that, there are considerable variations among models with how energy within the low will evolve, possibly even basically breaking up the upper low per the 18Z GFS. The variability creates differences in timing of energy and therefore frontal systems coming into the western U.S. through the first half of next week. Leaning toward the GEFS and EC ensemble means seemed like the best bet to minimize the conflicting details. The WPC forecast thus favored a blend of the 12/18Z GFS and 12Z ECMWF early in the forecast period with a bit of CMC that was gradually phased out. Transitioned to a little over half GEFS/EC ensemble means by the end of the period amid increasing uncertainty. The newer 00Z model cycle seems to be pretty consistent with these choices, except that they seem to show a faster track northeast of the low that is over the Great Lakes Sunday into southern Canada. The timing has been waffling in model guidance in recent days. ...Weather/Hazards Highlights... Showers are possible across the Mid-Atlantic Saturday with the weakening upper trough/surface low feature in the area. But the larger concerns will be with the low pressure/frontal system crossing the central U.S. Saturday and continuing eastward early next week. Cold air filtering in behind the surface low could support some light to moderate springtime snow across northern parts of the Upper Midwest and Upper Great Lakes over the weekend and lingering into Monday. Just ahead of the front, convection may be enhanced across the Mississippi Valley given good upper-level support, instability in place, and anomalously high moisture. A Marginal Risk of excessive rainfall was added in this forecast cycle for Day 4/Saturday across portions of the Upper Midwest where convection looks to focus. Northern portions of this area may be particularly sensitive to any rain, if rain falls on snowpack or areas that are saturated from snow recently melted, while higher rain amounts may be seen in the southern portion of the Marginal. A Marginal Risk also remains in place across southeastern Louisiana for Saturday, where there is once again potential for slow-moving convection with high rain rates. As the system moves eastward on Sunday, convection looks to spread across the East. Considered a Marginal Risk for portions of the northern Mid-Atlantic/New York as storms could contain high rain rates in the same setup as farther west, but held off since the region has been dry lately and because storms could be fast-moving ahead of the front. 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 chances will increase for the Pacific Northwest on Saturday as a weak atmospheric river channels into the region on the southern side of the northeastern Pacific upper low. Precipitation in the form of lower elevation rain and higher elevation snow is likely to expand east into the northern Rockies and south into northern California early next week. Then some moisture return could allow for some rain chances across the south-central U.S. by Monday or Tuesday. The most anomalous temperatures as the period begins Saturday should extend through the Great Lakes region with readings exceeding normal by at least 15-25F. High temperatures as high as the low 80s over snow covered grounds across the northern tier states would promote snowmelt. These temperatures should gradually moderate closer to normal through early next week after an initial backdoor front and then the main cold front comes through. Meanwhile the trough pushing through the western and central U.S. and its attendant cold front will help produce cooler than normal temperatures especially in terms of highs across the central U.S. by the weekend and shifting into the Great Lakes/Ohio Valley with time. The West and then the central U.S. are expected to flip back above average early next week as upper ridging comes in behind the first trough. 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