Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 PM EDT Sun Apr 04 2021 Valid 12Z Wed Apr 07 2021 - 12Z Sun Apr 11 2021 ...Overview and Guidance Evaluation/Predictability Assessment... Latest model guidance this morning made some notable adjustments to the overall pattern evolution across the U.S. during the medium-range period as the upper-level flow gets increasingly blocky across Canada by next weekend. The main areas of concern are (a) the eventual status of a low pressure system forecast to move across the Midwest/Ohio Valley to the Great Lakes, and (b) timing of various upper-level troughs/shortwaves forecast to move into the Pacific Northwest. Early in the period, there continues to be decent clustering for the upper low forecast to track over the Plains/Midwest. Meanwhile, the ECMWF has sped up the arrival of an upper-level trough from the northeastern Pacific into the west coast of Canada in excellent agreement with the GFS cluster. The faster forward motion of this upper trough eventually leads to its greater degree of interaction with the low pressure system moving into the Great Lakes by late next week. The ECMWF now shows this system lifting across the Great Lakes ahead of the upper trough approaching from the west by next Friday--more in line with what the GFS has been advertising. The CMC shows a similar trend as well. By next weekend, forecast uncertainty further increases as ensemble means indicate the formation of an omega block over eastern Canada. The WPC medium-range forecast package generally followed these latest guidance changes. This resulted in a noticeable departure from WPC continuity regarding the track of the low pressure system moving across the Great Lakes from Day 5 onward together with a faster eastward push of colder air across the Canadian prairies into the northern Plains. The low is forecast to slowly weaken or perhaps dissipate over the Great Lakes as the omega block sets up over eastern Canada. Meanwhile, a low pressure wave could linger near the North Carolina coast and could begin to develop next weekend. ...Sensible Weather and Potential Hazards... Some locally heavy rain may occur over parts of the Midwest around midweek, near the upper low and associated surface system affecting the region. Other areas of enhanced rainfall will be possible along the trailing front--most likely from near the south-central Mississippi Valley toward the southern Appalachians on Wednesday. The lingering front may provide a focus for additional showers and thunderstorms over parts of the South through at least Friday or Saturday. Many other portions of the eastern U.S. will also see one or more days of rain with varying intensity. The Northwest should see periods of rain and mountain snow, with highest totals likely over favored terrain in the Olympics and Washington Cascades. Locations from the Midwest into Ohio Valley and Great Lakes will see well above normal temperatures during mid-late week. Morning lows should be particularly anomalous with some areas seeing a day or two of 20-25F above normal readings. Warmest highs should be 10-20F above normal but they should tend to cool off some by the weekend. Clouds and precipitation should keep min temperatures above normal over the East into the weekend. Expect Texas to be very warm to hot during the period with highs 10-20F above normal most days. Above normal temperatures initially over a majority of the West will likely give way to below normal conditions penetrating further across the Northwest into northern Rockies. 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, 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/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml