Extended Forecast Discussion NWS Weather Prediction Center College Park MD 315 PM EDT Sat Jun 18 2022 Valid 12Z Tue Jun 21 2022 - 12Z Sat Jun 25 2022 ...Expansive area of heat to shift from the Midwest into the South next week... ...Overview... The predominant feature in the medium range period will be a broad upper level ridge/high centered across the Lower Mississippi Valley early next week slowly drifting westward into the southern High Plains while it expands across much of the South. Meanwhile, weak and progressive systems should ride overtop the ridge across the northern tier, while parts of the West and East Coasts could see periods of troughing on either side of the high as well. This pattern will allow for hot temperatures across much of the lower 48 east of the Rockies, along with some monsoon type rainfall in the Four Corners states and rounds of rain in the central U.S. with the weak shortwaves/frontal systems and along the East Coast. ...Guidance/Predictability Evaluation... Model guidance remains in good agreement with the broad ridge feature expanding across much of the southern U.S. as the week progresses. A shortwave rounding the ridge while moving through the north-central U.S./south-central Canada to start the period Tue-Wed appears well handled, and compact shortwave energy hovering near California shows fair agreement, though the GFS runs weaken that feature earlier than the ECMWF/UKMET. Additional energy coming in upstream toward the Northwest later in the week shows some differences in the details and some run to run inconsistencies but nothing too egregious for the late medium range period. However, the main challenge with this forecast cycle was the possibility for energy to split off from the narrow trough east of the ridge in the Atlantic and form an upper low, and where that low may track. Details of energy within the trough are quite uncertain as features even on the spatial scale of MCSs/MCVs could provide sufficient energy differences leading to differing upper patterns. Recent models mostly show an upper low breaking off from the trough in the Atlantic, then tracking southward somewhat and retrograding toward the East Coast. This has been a new trend since the 12Z UKMET was basically the only model to show this a day or so ago. But now, non-NCEP guidance shows this trend, though GFS runs are not so much. The 00Z ECMWF, the 00Z UKMET and many of the EC ensemble members indicate a closed upper low retrograding toward the Mid-Atlantic for the latter part of next week, and the 12Z EC shows a similar pattern. The CMC runs are also similar initially but end up south by Fri-Sat with the upper low off the Southeast coast, farther south than even the EC ensemble members. However, most GEFS members as well as the operational GFS runs (00Z and the new 12Z in particular) show a cutoff low forming initially, moving toward the Northeast, but then getting picked up by the main flow and tracking eastward. While the stagnant pattern of ridging with a narrow trough to the east would support some potential for a cutoff low to form, the details are quite uncertain at this time. Thus the WPC forecast was based on a multi-model blend early on but blending in considerable amounts of the ECMWF and GEFS means, particularly the EC mean that was a good compromise, as the period progressed to lessen the dependence on any particular deterministic model, considering their differences particularly in the East. This was done to indicate some potential for troughing and possibly an upper low coming into the East but not buying in on the more aggressive solutions yet, pending further model runs. ...Sensible Weather/Threat Highlights... Anomalously high monsoonal moisture for this time of year, likely tied at least in part to eastern Pacific tropical systems like Blas, will continue streaming into the Four Corners states at least through midweek and possibly later into the week as well. Model guidance and forecasts show that rainfall may focus in north-central New Mexico on Tuesday in particular, which could lead to flash flooding concerns considering large burn scars in that area as well as terrain-driven enhancements, so a Slight Risk was added to the experimental Excessive Rainfall Outlook in coordination with WFO Albuquerque. A showery/stormy pattern could also set up around Tuesday to Friday along central parts of the Plains and Mississippi Valley as a front becomes aligned west-east, with some potential for rain and storms to train in some areas. Farther east, a round of rain is forecast near a warm front in the Northeast Tuesday-Wednesday, and rain is forecast to spread into the Mid-Atlantic by Wednesday and beyond. How much rain lingers across the East Coast will be dependent on the aforementioned upper low/trough feature's existence and track. The broad ridge will act as a heat dome and spread anomalously warm temperatures across the Plains eastward next week. The greatest anomalies (generally 10-15+ above normal) will shift from the Great Lakes on Tuesday into the South Wednesday-Friday, with high temperatures in the mid-upper 90s and low 100s and lows well into the 70s meaning little relief overnight in many areas. Daily record high max and min temperatures are likely to be widespread. Relatively cooler temperatures with well below normal highs are forecast particularly for New Mexico given the monsoonal clouds and rain coverage, while near normal temperatures stretch through the Rockies. Tate 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