Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 AM 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... Guidance continues to show the large scale upper pattern transitioning from a blocky regime late this week toward an eastern Pacific trough, western U.S. into High Plains ridge, and eastern U.S. trough. 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... The 12Z/18Z operational model composite used as the basis of the updated forecast early in the period transitioned to a blend of the 18Z GFS/12Z ECMWF and their ensemble means by mid-late period. The UKMET/CMC have continued their general trend toward recent GFS/ECMWF ideas for the storm expected to reach just north of the northern Plains as of early Friday. However the UKMET/CMC have been straying from the other guidance for the ultimate evolution of trailing shortwave energy initially over the Northwest, pulling it farther south than consensus. This trait of those models contrasts with recent GFS/ECMWF and GEFS/ECMWF mean trends toward somewhat less amplitude of the eventual eastern trough aloft and a stronger upper ridging building back over the southern Plains--leading to greater persistence of the heat over that region. Meanwhile individual models and ensemble members diverge for details of eastern Pacific shortwave energy pushing into western North America. In general the latest GFS runs have leaned toward bringing lower heights into the West versus most other guidance but not to an extreme degree versus typical predictability/error ranges at the time frame of interest. Also the new 00Z UKMET bears some similarity to the GFS. These shortwave details will be important for determining the coverage and intensity of precipitation over the Northwest. ...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. 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 by 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 which have low predictability at this time. A wavy front extending south from an initial storm system over southern Canada may hang up over the Upper Midwest through southern Plains for a time late this week as it awaits the arrival of dynamics from the Northwest, leading to the possibility of some moderate to heavy showers and thunderstorms over or just west of the Mississippi Valley. Then this front will 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. 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