Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 AM EDT Sat May 18 2024 Valid 12Z Tue May 21 2024 - 12Z Sat May 25 2024 ...Overview... A fairly progressive pattern looks to evolve across the CONUS during the medium range period, with much of the activity focused across the West to north-central Plains and Upper Midwest. Early next week, a potent shortwave will send a modestly deep surface low into the Upper Midwest and potential for at least locally heavy convection across the region. It's attendant cold front will push across the Eastern U.S. midweek as the next system (possible closed upper low) reloads into the Northwest and tracks eastward (with much more uncertainty) into the northern Plains/Upper Midwest. Another shortwave/possible closed low may drop into the Northwest again late period. ...Guidance/Predictability Assessment... Models show reasonable agreement with the first shortwave that kicks out into the Upper Midwest Tuesday-Wednesday, though with some uncertainty in the timing and depth of the surface low. The first upper low into the Northwest shows a lot more uncertainty even as early as Day 5/Thursday. The GFS is the deepest/slowest with it as it progresses eastward through the north-central U.S., owing in part to weaker shortwave energy reloading the Western trough later in the week. The ECMWF and CMC suggest two separate upper lows into the Northwest (the first Wednesday/Thursday, the next Friday/Saturday) and has some support but there is a lot of spread in the details and timing of this per the latest ensembles and ECMWF- initialized Machine Learning models. The WPC forecast for tonight used a blend of the deterministic models for the first few days of the period, incorporating the ensemble means for the latter half of the period amidst plenty of spread. Used some ECMWF contributions late period for some added system definition. Overall, this maintained reasonable agreement with the previous WPC forecast. ...Weather/Hazards Highlights... An upper shortwave and leading cold front, with deepening surfact low, will push into the Upper Midwest Tuesday/Wednesday, helping to fuel showers and thunderstorms across this region, with potential for at least locally heavy rainfall given ample anomalous moisture and instability present. The Day 4 Excessive Rainfall Outlook continues to depict a broad marginal risk along the surface low track across the Upper Mississippi Valley/Great Lakes and attendant cold front southward into the middle Mississippi Valley. Models show fairly good agreement for an area of higher QPF near the surface low, but given somewhat drier antecedent conditions, uncertainties in moisture/instability on the backside of the low, and collaboration with the affected WFOs, opted to hold off on any sort of upgrade to a slight risk at this time. There is also potential for severe weather along the cold front. After Tuesday, expect northeastward progression of the surface low to push the trailing cold front and accompanying rain/storms farther east and south with some more uncertain potential for heavy rainfall in the eastern and southern U.S. for Wednesday-Thursday. There should be ample moisture and instability present along the cold front so did go ahead and introduce a marginal risk along the boundary for the Day 5 ERO period from northeast Texas into the Ohio Valley. Approach/arrival of another couple of systems as currently advertised into the Northwest by midweek and next weekend would produce somewhat more organized precipitation there, and potential for heavy snow in the mountains, with snow levels dependent on the depth of the upper low(s). 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 into 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. 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