Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 PM EDT Fri Apr 14 2023 Valid 12Z Mon Apr 17 2023 - 12Z Fri Apr 21 2023 ...Overview with Guidance/Predictability Assessment... Latest model consensus appears to have hauled the gradual slowdown of the negatively-tilted upper trough/closed low and the associated surface cyclone across the Great Lakes early next week. This is followed by a slightly faster exit of the system into the Canadian Maritimes through midweek. Farther west, the models and ensembles agree fairly well early in the week as a fairly vigorous upper trough moves from the eastern Pacific into the western U.S. However, solutions diverge in varying ways by mid-late week, both for how leading energy evolves as it reaches the Rockies/Plains and with eastern Pacific flow details that will affect how upstream energy could feed into the larger scale western trough. Ensemble means generally indicate a ridge-trough-ridge pattern across the mainland U.S. by the end of next week, with a Rex Block pattern setting up across Canada. The EC mean and CMC mean are more pronounced regarding this pattern whereas the GEFS is less amplified. There appears to be a general agreement for a northwest-southeast oriented upper jet to push onshore into the western U.S. while embedded shortwaves will periodically be ejected into the large-scale trough over the mid-section of the country. The overall effect of this pattern appears to advance a positively-tilted trough relatively slowly eastward across the central U.S., with a relatively long fetch of Gulf moisture streaming up the Mississippi Valley under the return flow of a warm high pressure system as it slides off the East Coast late next week. In the meantime, the blocky pattern seen from the northeastern Pacific to North America may lead to further adjustments in the forecast pattern for late next week. Given the above considerations, the WPC medium-range forecasts begin with 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. ...Weather/Hazards Highlights... Initial Great Lakes low pressure and a leading frontal system should produce areas of rainfall especially over the Northeast early in the week. Some of this activity could be locally heavy, but it will likely be fairly progressive and antecedent ground conditions are on the dry side so the risk of excessive rainfall should be fairly limited/isolated. Cold air behind the surface low could support snow across some areas from the Great Lakes into interior New England. Meanwhile, rain and higher elevation snow will spread from the Pacific Northwest/northern California into the northern half of the Rockies early in the week. Precipitation may persist over some of these areas beyond midweek while moisture increases over the central U.S. from Tuesday onward. One area of rainfall may progress across parts of Texas on Tuesday while other locations in the central/east-central U.S. could see one or more episodes of moderate to heavy rainfall mid-late week depending on uncertain surface low/front details. With fairly low confidence, the best signal for highest totals currently exists over the Midwest/Middle Mississippi Valley. There will also be the potential for some snow to extend into the northern High Plains but again with low confidence in any specifics at this time. Areas along and just south of the initial Great Lakes upper low will see a day or two of chilly daytime temperatures early in the week, with highs 10-20F below normal from the Great Lakes/Ohio Valley into central Appalachians. On the other hand, parts of the Plains will be very warm early-mid week with highs up to 10-20F above normal. Expect the warmth to shift more into the East late in the week while the central U.S. trends cooler, including below normal readings over the northern Plains. Meanwhile expect below normal temperatures to spread across most of the West with some highs 10-15F below normal. A few locations could challenge record cold highs, especially on Tuesday. 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