Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 AM 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... ...Overview... Guidance continues to show 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... Once again the most pronounced guidance differences in the forecast involve the features within the western mean trough, with significantly varying ideas for the combination of a fairly sharp 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/CMC have consistently been on the faster side of the envelope with ejection of the leading system. ECMWF runs have been slower but the 12Z run was a bit faster than the prior 00Z version due to trending faster with the trailing flow. The 12Z UKMET and to some degree the new 00Z run are slower to bring in the upstream troughing, leading to slow progression of the leading feature and a rather far south track of the upper low in the new run by late Thursday. Preference continues to be for an intermediate timing until solutions gravitate more to one side of the spread. The new 00Z ECMWF has adjusted noticeably faster, to a timing just ahead of the favored compromise. 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, with a blend/mean approach providing a reasonable starting point. Teleconnections relative to the core of positive height anomalies over southeastern Canada by late in the period support the residual western troughing. 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 a consensus approach among latest models/means offering a reasonable starting point and decent continuity. The 12Z GFS appeared to be a little quick to lift it northwestward and open it up late in the period though. If anything, latest trends seem to be nudging the upper low center a little farther south. The updated forecast started with a 12Z/18Z operational model blend on Tuesday but then phased out the UKMET due to question marks with its northeastern Pacific pattern and its eventual effect on the West, followed by the CMC by Thursday due to its overall faster progression. 18Z GEFS/12Z ECMWF mean weight steadily increased mid-late period while varying parts of the 12Z ECMWF and 18Z/12Z GFS (with some manual edits) helped center the western/central U.S. evolution. ...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 in the week, but is likely to expand back to the west late in the week as the vertically stacked low and warm front to the north of the surface low retrograde back toward the coast and possibly a little inland by next Saturday. Rausch/Santorelli 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