Extended Forecast Discussion NWS Weather Prediction Center College Park MD 238 PM EDT Mon Aug 16 2021 Valid 12Z Thu Aug 19 2021 - 12Z Mon Aug 23 2021 ...Emerging North-Central U.S. Heavy Rain Threat ahead of cold front and upper trough... ...Weather Pattern Overview... A well defined upper level trough with potential for a cut-off low should be in place across the Intermountain West and Northern Rockies to begin the forecast period Thursday, and an upper level ridge situated over the Southeast U.S. in the wake of Tropical Storm Fred. The upper trough then reaches the northern Plains and drives a cold front and surface low eastward, with a building ridge across the Northeast states. A couple of systems shifting through should maintain troughing across the Northwest through much of the medium range. ...Model Analysis and preferences... Latest guidance continues to show some timing uncertainties with the northern Plains trough/cold front. The 06z GFS is much faster than most of the other models, and while the 00z run was still a little fast, it's slower than the 06z run so more in line with the ECMWF, UKMET, and CMC. A general model blend between the ECMWF/CMC/UKMET and the 00z GFS gives a good middle ground solution. Another deeper trough should enter the Pacific Northwest around next Sunday-Monday. Guidance has trended towards a more amplified system (possible closed low) but some uncertainty remaining around timing and details. A blend towards the ensemble means for later in the period should help mitigate the differences until details can be further resolved. Finally, the guidance shows good agreement that a more robust ridge axis developing over the Gulf Coast region should tend to steer what should be Tropical Storm Grace by Thursday farther south and westward over the far southern Gulf. This should pull the most significant impacts farther south into central Mexico. ...Sensible weather and potential hazards... Residual tropical moisture from what will become the remnants of Tropical Storm Fred will fuel scattered to numerous showers and thunderstorms across the Northeast U.S. on Thursday prior to exiting the region, along with some lingering showers on Friday. Some instances of flooding may be possible, particularly across areas that have received above average rainfall already. Moderate to heavy rainfall is also becoming more likely across portions of the central/northern Plains Thursday and Friday. Exact amounts and location will be dependent on developing mesoscale convective complexes. Another corridor of enhanced showers and storms may materialize across portions of the mid-Mississippi Valley to Tennessee/lower Ohio Valleys owing to a slow moving front lingering into the weekend. For temperatures, daytime highs up to 20 degrees below normal are likely across portions of the northern Rockies and northern High Plains for the end of the week, with some snow possible for the highest elevations. Temperatures trend a little warmer this weekend into early next week out west, but still should be below normal as mean upper troughing is maintained across the region. Elsewhere, above normal temperatures from the Upper Midwest/Great Lakes into portions of the northeast are likely ahead of the cold front Thursday-Friday, but much of the rest of the country should be near normal through the entire medium range period. Santorelli/Hamrick 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