Extended Forecast Discussion NWS Weather Prediction Center College Park MD 335 PM EDT Fri May 17 2024 Valid 12Z Mon May 20 2024 - 12Z Fri May 24 2024 ...Overview... An upper trough and surface low will be exiting the East Coast by the start of the period, but may linger in the western Atlantic for a day or two before getting pushed out to sea by the next system behind it. Meanwhile, a shortwave will push into the northern Plains/Upper Midwest early in the week along with a modest surface reflection and potential for at least locally heavy convection across the north- central U.S.. An upper low is forecast to drop into the Northwest behind this system around midweek bringing another cold front and unsettled weather to the West. ...Guidance/Predictability Assessment... Models are in reasonable agreement on the overall synoptic pattern across the CONUS during the medium range period. A general model blend consisting of the 00z EC/UK/CMC and 06z GFS were used on days 3 and 4, with higher weighting toward the EC and GFS. The 00z ECE and 06z GEFS were introduced to the blend on day 5 in place of the favored EC/GFS weighting as well as the UKMET. The 00z CMCE replaced the 00z CMC on day 6 and was continued through the end of the period with the ensembles making up the bulk of the blend on days 6 and 7. ...Weather/Hazards Highlights... An upper shortwave and leading cold front pushing slowly through the Plains/Upper Midwest early in the week will help fuel showers and thunderstorms from the north-central Plains into the Great Lakes, with potential for at least locally heavy rainfall given ample anomalous moisture and instability present. Both the Days 4 and 5 (Monday-Tuesday) Excessive Rainfall Outlooks show just broad marginal risks across this area, and in some places may be dependent on activity towards the end of the short range period as well. Farther West into the Rockies, some moderate snowfall may fall in the higher elevations. After Tuesday, expect northeastward progression of the surface low to push the trailing cold front and accompany rain/storms farther east and south with some more uncertain potential for heavy rainfall in the eastern and southern U.S. for Wednesday-Thursday. Approach/arrival of another upper low as currently advertised by midweek or so would produce somewhat more organized precipitation across the Northwest, and potential for heavy snow in the mountain, and snow levels dependent on the depth of the upper low. Expect South Texas to see multiple days of hazardous heat during the period with highs persistently running 10-15F above normal with max heat index values possibly reaching at least 110F. Highs near or over 100 degrees could stretch farther north across the southern High Plains at times as well. Some daily records for highs/warm lows will be possible. Above normal highs will also track East across the Midwest and Northeast Monday-Wednesday but should moderate by Thursday as the cold front pushes through the region. The forecast pattern will favor below average highs over the Northwest to northern Plains for most of next week. Kebede/Santorelli 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 forecast (QPF), excessive rainfall outlook (ERO), winter weather outlook (WWO) probabilities, heat indices, and Key Messages can be accessed from: 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 https://www.wpc.ncep.noaa.gov/#page=ovw