Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 PM 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 a closed 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... The two main features of the upper-level pattern over the CONUS at the start of the medium-range period (12Z Saturday) are a weakening, open wave moving up the East Coast and a second upper-level trough moving over the eastern Rockies and western Plains. The 00Z deterministic and ensemble mean guidance remain in good agreement with both the initial placement and amplification of these features. The East Coast wave is forecast to continue weakening and depart over the eastern Atlantic while the trough to the west rapidly deepens with a closed low developing. This feature appears first in both the most recent 00Z/06Z runs of the GFS/CMC while developing in the 00Z ECMWF/UKMET about a day later. The 00Z UKMET/CMC differ from the 00Z/06Z GFS and 00Z ECMWF by mid-period in depicting a slightly more southern placement of the closed low that is generally centered over the Ohio Valley as opposed to a more northern position over the Great Lakes. This is likely tied to a second northern stream wave in both the 00Z UKMET/CMC passing over southern Canada, displacing the closed low a bit further southward. The latest few runs of the GFS and ECMWF have also shown at least generally good run-to-run consistency in the placement of the feature with more significant differences in how deep the closed low gets, which has varied back and forth. Specifically, the 06Z GFS is much deeper compared to the 00Z run after both the 00Z GFS and ECMWF runs had weakened compared to their respective prior 12Z runs from the previous day. The closed low continues to remain further north in the 06Z/00Z GFS and 00Z ECMWF compared to the 00Z CMC towards the end of the period, with the low slowly migrating from the Great Lakes into southern Ontario Province Canada. The low in the CMC remains further south over the Upper Ohio Valley/Lower Great Lakes. It is notable that the CMC ensemble mean is also further north compared to the deterministic CMC run, more similar in placement with the 00/06Z GFS and 00Z ECMWF runs and the corresponding 00Z GEFS and ECens means. Further west, the deterministic and ensemble guidance show the least consistency with the timing and phasing of embedded shortwave energy dropping southward over the Northwest associated with a closed low over the northeastern Pacific mid- to late period. The run-to-run consistency of the individual guidance is also very poor with little confidence in how exactly this pattern will evolve at this time. The updated WPC forecast began with a general blend of the deterministic model guidance given the small but notable differences between the guidance on the initial evolution of the developing closed low moving towards the Midwest, with no specific preference on the timing of the deepening of the low. More emphasis is placed on the 00Z/06Z GFS and 00Z ECMWF by mid-period given the notable difference in the evolution of the northern shortwave over southern Canada depicted in the CMC and UKMET guidance, and the CMC ensemble mean suggesting a preference towards the GFS/ECMWF deterministic and GEFS/ECens solutions over the CMC deterministic run. A contribution from the 00Z GEFS/ECens means is introduced as a replacement. By late in the period, the forecast steadily relies more on the ensemble means specifically because of the differences in the pattern evolution over the Northwest late in the period, along with some increasing divergence amongst all guidance with respect to the placement of the closed low to the east at the end of the period. ...Weather/Hazards Highlights... Showers are possible across the Mid-Atlantic Saturday with the weakening upper wave and an associated surface low in the area. But the larger concerns will be with a 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 the deepening upper-level trough/closed low, instability in place, and anomalously high moisture. A Marginal Risk of excessive rainfall is in place for Day 4/Saturday across portions of the Upper Midwest where convection looks to focus ahead of a couple surface waves. 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 a frontal boundary lingers in the vicinity. As the system moves eastward on Sunday, convection looks to spread across the East. A Marginal Risk has been considered for portions of the northern Mid-Atlantic/New York as storms could contain high rain rates in the same setup as farther west, but dry antecedent conditions and the progressive nature of the front 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. Some return flow moisture could allow for some rain chances across the south-central U.S. by Monday or Tuesday as well. 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 with precipitation amounts trending upwards across the region, particularly in the Monday-Tuesday time frame. Some locally moderate to heavy snow is possible in the Cascades. Precipitation chances may also spread southward into northern California. It's worth noting that uncertainty significantly increases given the timing of surges of upper-level energy later into the period, with less confidence in the location and timing of daily precipitation chances at any given location from the Pacific Northwest east into the northern Rockies/Plains. However, the pattern should keep precipitation chances in place across the region overall. 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 upper low over the Great Lakes. Putnam/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