Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 PM EDT Sun Apr 16 2023 Valid 12Z Wed Apr 19 2023 - 12Z Sun Apr 23 2023 ...Overview with Guidance/Predictability Assessment... With the departure of a low pressure system into the Canadian Maritimes early in the medium-range period, much of the concern for the rest of the medium-range will be centered around how various shortwaves embedded within a broad trough moving across the western U.S. will eventually evolve and/or interact with one other as they reach farther downstream over the eastern and northeastern portions of the country. The blocky pattern extending from Siberia across Alaska and most of Canada will continue to inject a higher degree of uncertainty/variability to the pattern evolution across mainland U.S. Overall, models and ensembles appear to gradually converge toward an intermediate solution between the more amplified solutions as advertised by the ECMWF (and to a lesser degree the CMC), and the more progressive solution indicated by the GFS/GEFS for the latter half of next week over the central to eastern portion of the country. The EC mean appears to offer the most stable solution, depicting a low pressure system to develop over the central Plains midweek, then track it across the Great Lakes and the Northeast Friday and through the weekend. On the other hand, the less-amplified GFS and CMC solutions depict two surface cyclone to develop out of this pattern, with the lead system moving across the Great Lakes on Friday followed by a second low forming rapidly over the central Plains before moving once again into the Great Lakes on Saturday. The 12Z ECMWF has begun to hint at this scenario with a less-amplified upper pattern leading to a less-pronounced surge of cold air down the Plains late this week and a better opportunity for residual shortwaves in the western U.S. to be ejected into the central Plains and amplify. With the above considerations, the WPC medium-range forecasts were based on a blend of 40% from the 06Z GFS/GEFS, 40% from the 00Z ECMWF/EC mean, and 20% from the 00Z CMC/CMC mean Day 3 - 5, followed by a higher percentage from the ensemble means through Day 7. A higher percentage was given to the 00Z EC mean Days 5-6 to compose the moisture fields. ...Weather/Hazards Highlights... Locations from the Pacific Northwest into the Rockies will see periods of rain and higher elevation snow during the period, with some uncertainty over coverage and amounts on a day-to-day basis. Generally expect light to moderate amounts, though perhaps with some localized enhancement at times over the Pacific Northwest. Meanwhile the coverage and intensity of rainfall should be increasing over the central U.S. by midweek. The day 4 Excessive Rainfall Outlook (12Z Wednesday-12Z Thursday) maintains a Marginal Risk area over the portions of the Midwest as guidance continues to suggest that the frontal system nearing the region could slow down for a time as an upstream wave replaces the initial one. Farther south, guidance has backed off on the heavy rainfall potential across southeastern Texas. By day 5 (12z Thursday-12Z Friday) expect Gulf inflow ahead of the cold front to support a broader area of locally heavy rainfall, with a Marginal Risk area depicted over areas from the central Gulf Coast into parts of the Midwest. Rainfall of varying intensity should then continue across the eastern U.S., with the possibility of highest totals over the Northeast if there is a period of Atlantic inflow as some guidance is suggesting. Meanwhile, there will be the potential for some snow across the northern tier, with probabilities for 0.25 inch liquid in the form of snow extending across portions of the Dakotas into the U.P. of Michigan. Model spread for important details has kept these probabilities on the lower side in absolute terms thus far, but there are some scenarios for the upper low that could produce a period of heavy snow over some areas. System progression from midweek through next weekend will lead to an area of below normal temperatures initially over the West reaching into the Plains and then east-central U.S. The best potential for highs of at least 10-15F below normal should extend from the Great Basin and northern half of the Rockies/Plains and eventually into the Ohio Valley. The leading warm sector will see readings up to 10-20F above normal parts of the central/southern Plains on Wednesday, followed by this warmth pushing into the East later in the week through early Saturday. Locations across California, the Southwest, and southern Great Basin may see temperatures trend moderately above normal from late week through the weekend. Kong/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