Extended Forecast Discussion NWS Weather Prediction Center College Park MD 249 PM EDT Sat May 07 2022 Valid 12Z Tue May 10 2022 - 12Z Sat May 14 2022 ...Blocky pattern next week to support record heat over the Southern Plains/Lower Mississippi Valley into the Midwest along with episodes of central U.S. convection and an emerging onshore rains focus to the Carolinas... ...Overview... It remains the case that guidance shows a blocky pattern during the period. Mean ridging aloft will support a prolonged period of well above normal temperatures (challenging daily records over some areas) over the Mississippi Valley/central-southern Plains extending into New England. At the same time mean troughing containing a couple separate features will produce chilly and unsettled weather over the West, with waves/fronts and a dry line promoting episodes of rain and thunderstorms between these two regimes. Meanwhile a retrograding upper ridge over the Atlantic will likely push an initial western Atlantic upper low back toward the southern part of the East Coast, bringing an increase of moisture to central and southern portions of the East late week into the weekend. ...Guidance Evaluation/Preferences... Guidance differences in the forecast are mainly with features within the western mean trough, with varying ideas for the combination of a sharper trough/embedded low along or just inland from the West Coast through midweek and trailing northeastern Pacific energy that should reach the Northwest after midweek. Even with some differences in handling of the trailing energy among recent runs, the GFS and especially the CMC have been on the faster side of the envelope with ejection of the leading system. ECMWF runs have been slower, but has been gradually trending faster in recent runs due to trending faster with the trailing flow. Recent UKMET runs have been the slowest to bring in the upstream troughing, leading to slow progression of the leading feature and a rather far south track of the upper low by late Thursday. By day 7 Saturday there has also been quite a bit of variability in the models for how amplified the trailing energy becomes as it crosses the West. Overall, the WPC medium range product suite was mainly derived from a blend of the near middle of the forecast envelope cluster of the 06 UTC GFS/GEFS mean, 00 UTC ECMWF/ECMWF ensemble mean, WPC continuity and the 13 UTC National Blend of Models. Latest 12 UTC guidance trends generally remain in line. In the East, the upper low retrograding back to the East Coast, along with associated surface low and warm front, exhibit typical detail differences in the guidance, with the aforementioned consensus approach also offering a reasonable starting point and decent continuity. However, a manual adjustment was applied to limit some northward expansion of organized QPF heading into next weekend considering a southward closed low trend. ...Weather/Threat Highlights... Strong upper ridging will lead to persistent much above normal temperatures over the central-southern Plains and most of the Mississippi Valley through the week, with some of this warmth also expanding through the Great Lakes and Northeast. Expect a broad area of highs/morning lows 10-20F above normal with some localized areas reaching higher than that. Numerous daily records are likely through the week over the southern Plains and Lower Mississippi Valley, and may also extend into the central Plains/Midwest especially around Tuesday/Wednesday. Temperatures could also challenge record highs over a few places in the Northeast late in the week. Highest heat index values will be over southern Texas on Tuesday but the combination of heat and humidity may still produce maximum heat index values over 100F at some locations just inland from the western Gulf Coast through at least Friday. Excessive heat, gusty winds, and dry conditions should lead to an elevated fire weather threat across parts of the southern High Plains through at least mid-week. The West will see unseasonably cool and unsettled conditions next week, albeit with a gradual moderating trend as upper troughing becomes less amplified. Coldest anomalies will be on Tuesday/Wednesday over California and the western Great Basin with highs 10-20F or so below normal. Temperatures may rebound to moderately above normal levels over California and the Southwest by next Saturday while locations farther north may be see only single-digit negative anomalies for highs. Periods of rain and higher elevation snow will tend to be light to moderate, but a system arriving late in the week may enhance totals somewhat over favored terrain in the Northwest. Details of this system are still somewhat uncertain though. Between these two regimes, the central U.S. will see periods of rain and thunderstorms. A leading front should stall between the Upper Midwest and central Plains, lifting back was a warm front ahead of a ejecting from the West after midweek. This system will then push a north-south front into the Plains, with timing and wave details of the front dependent on specifics of trailing dynamics flowing through the West. These fronts and the ejecting western system may produce at least localized areas of significant rainfall that would be a runoff threat over northern tier areas that already have very wet soil. The late-week system requires monitoring for strong convection, with details becoming more evident in coming days. Most rainfall with the system off the East Coast should stay just offshore early next week, but is likely to expand back to the west and lead to some moderate amounts late next week as the vertically stacked low and warm front to the north of the surface low retrograde back toward the coast and inland into next Saturday. Schichtel/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