Extended Forecast Discussion NWS Weather Prediction Center College Park MD 256 PM EDT Tue May 10 2022 Valid 12Z Fri May 13 2022 - 12Z Tue May 17 2022 ...Lingering blocky pattern to extend record heat from the Southern Plains/Lower Mississippi Valley to the Midwest/Northeast late week along with episodes of central U.S. convection and an emerging onshore rainfall focus to the Carolinas... ...Developing heat threat for California and the Southwest from the weekend into next week while heat persists eastward over far southern tier areas... ...Overview... The blocky large scale pattern over the short range period will persist through the early medium range period with an upper low retrograding over the southeastern U.S. with ridging atop the low, while upper troughing across the western U.S. gradually gives way to higher heights. The pattern should transition toward an eastern Pacific trough, western U.S. into High Plains ridge, and eastern U.S. trough early next week. A broad area of much above normal temperatures into late this week from the southern Plains into Great Lakes/Northeast, with numerous daily records possible, will eventually moderate over the northern two-thirds of the country as a wavy front over the Plains (a possible focus for at least some locally heavy rainfall) pushes farther eastward ahead of the approaching trough. Meanwhile an upper low reaching the Southeast by Friday will quickly open up as it lifts northward, spreading rainfall into and north from the Carolinas. The upper ridge building into/through the West will moderate initially chilly temperatures over the Northwest while bringing a period of very warm to hot weather across the southern half of the West Saturday onward, connecting with the heat expected to persist over and near the southern Plains. Highs may challenge daily records over some locations across these areas. ...Guidance Evaluation/Preferences... Recent guidance continues to show reasonably good agreement with the overall pattern described above, but with lingering differences in the smaller scale details. One such smaller-scale feature is a shortwave located over the Pacific Northwest early Friday that tracks eastward to potentially join or at least interact with a small upper low forming over south-central Canada near the U.S. border by Saturday. Even for the end of this week, the shortwave variations lead to differences in the QPF over the Pacific Northwest (which this updated forecast trended a bit heavier with) and a surface low pressure system. The 00Z CMC especially and to some extent the UKMET continued to be a bit deeper with the shortwave feature than the GFS and ECMWF runs and the bulk of ensemble members. Going forward into early next week, the depth of the 00Z CMC leads to a farther south and slower upper low. The newer 12Z CMC seems to be trending somewhat closer to consensus. Other aspects of the forecast agree fairly well in principle as an upper high grows its ridging influence across parts of the southwestern and south-central U.S. early next week, while troughing is likely across the Northwest though again with some detail differences. Thus the WPC forecast began with a blend of the (latest available) 00Z ECMWF and 00Z/06Z deterministic GFS runs with smaller portions of the 00Z UKMET and CMC, phasing out the latter models as the period progressed in favor of the GEFS and EC ensemble means. ...Weather/Threat Highlights... Within the broad area of above normal temperatures from the southern Plains through Great Lakes and Northeast on Friday, the best potential for some daily records will be across the southern tier and Great Lakes/Northeast. Highest anomalies of plus 10-20F or slightly higher should be across northern areas from Friday into the weekend. Meanwhile potential record heat could persist over/near eastern Texas into next week, and may expand over more of the southern Plains/South early next week, with temperatures well into the 90s and 100s in some areas. The ridge building over the West will lead to a pronounced warming trend during the weekend, with broad coverage of highs 10-15F above normal over the southern half of the region on Saturday into Sunday and persisting into Tuesday. This heat could challenge daily record highs over some southern areas and ultimately increase temperatures over the southern Plains once again next week. The Pacific Northwest and northern Rockies will be chilly on Friday with highs 5-15F below normal, followed by some moderation. Shortwaves ejecting from the eastern Pacific upper trough and associated surface fronts may produce multiple episodes of precipitation over the Northwest. Some of this activity could be locally moderate/heavy depending on exact details that have low predictability at this time. Moisture streaming in ahead of a couple of frontal systems could bring moderate to heavy showers and thunderstorms over or just west of the Mississippi Valley late this week. The fronts should consolidate and become more progressive as upper troughing heads toward and into the East. Some rainfall along the front could still be fairly intense but the greater progression may temper maximum totals over the East. The trailing part of the front should stall and may lift back as a warm front early next week, potentially serving as a focus for some convection. The Southeast upper low that quickly opens up Friday into Saturday will spread an area of rain into the Carolinas and areas to the north. Some localized enhancement will be possible over the Carolinas/Virginia but rapid opening of the upper low should tend to decrease the focus of moisture farther northward. Tate/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, experimental excessive rainfall outlook, 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/#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