Extended Forecast Discussion NWS Weather Prediction Center College Park MD 330 PM EDT Wed Jun 19 2024 Valid 12Z Sat Jun 22 2024 - 12Z Wed Jun 26 2024 ...Heat Wave persists for Mid-Atlantic Sunday with elevated heat over the South/Plains through much of next week... ...Heavy rainfall threat from the Upper Great Lakes to northern New England Saturday-Sunday... ...Overview... Flow pattern becomes more zonal over the CONUS this weekend with the jet focus over southern Canada and subtropical ridging over the southern states. A shortwave trough crossing the far northern Plains to Quebec Saturday through Sunday night brings a heavy rainfall threat from the Upper Great Lakes to northern New England during the weekend which will also moderate the area of hazardous heat extending from the Midwest into the Mid- Atlantic through Sunday. However heights over the southern tier will remain somewhat above normal, leading to fairly broad coverage of above normal temperatures over the central/southern states next week. There remains potential for second western Gulf feature (after Alberto) that could bring additional rainfall into far southern Texas this weekend. ...Guidance/Predictability Assessment... Decent agreement among 00Z/06Z global guidance on the northern stream shortwave trough moving from the northern Plains to Quebec Saturday through Sunday night and the next wave off BC Monday warranted a general deterministic model blend for preferences through Day 5. The breakdown came with the BC wave being faster in the 00Z CMC, though the 12Z is much more in line with the 12Z GFS/ECMWF. A blend of the 00Z ECMWF/06Z GFS with their ensembles was used for Day 6/7 given their similarities. That said, the zonal flow pattern does make for inherent uncertainty in what is sure to be impulses tracking with the flow near the U.S./Canadian border. Days 4-7 QPF was generally derived from a blend of the 13Z NationalBlend with the 00Z ECMWF/06Z GFS with local enhancements from the 00Z GDPS and UKMET. 13Z NBM temperatures in the Mid- Atlantic on Sunday were generally tempered a bit using a global bias corrected blend. ...Weather/Hazards Highlights... Focus for heavy rain ahead of the northern Plains shortwave trough Saturday has focused a bit south in the global guidance consensus which warrants focusing the Day 4 Excessive Rainfall Outlook south of Lake Superior with central Wisconsin the most likely spot for a second round of heavy rain after a first round on Friday. The Marginal Risks into the central Plains where merely trimmed a bit for timing. Progressive, but possibly repeating activity looks to be more focused over northern New England Sunday, so the Day 5 Slight Risk was trimmed a bit from eastern Maine. The Marginal Risks were maintained over the Upper Ohio Valley and Northeast. The enhanced moisture into far southern Texas allows the Day 4/5 Marginals to be focused over the lower Rio Grande Valley south of the Sand Sheet. Meanwhile some scattered diurnal convection looks to persist over parts of the Four Corners states. Anomalously high precipitable water values suggest some activity could be locally heavy but so far guidance has not been quite organized enough to merit a risk area. Parts of the Southeast/Florida may also see locally heavy rainfall but again without enough organization to favor a risk area. During the early- middle part of next week the next frontal system reaching the Plains/Mississippi Valley may produce showers and thunderstorms of varying intensity. The Mid-Atlantic should continue to have temperatures 10-15F above normal Sunday with a dozen or two daily records for max/warm min temperatures forecast. The front progressing into the East will suppress the heat southward Monday with max temperatures around 10 degrees above normal persisting through midweek over the southern Plains, Midwest and into the Southeast. Temperatures 5-10F above normal over the Desert Southwest is also significant. Jackson 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