Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 AM EDT Fri May 06 2022 Valid 12Z Mon May 09 2022 - 12Z Fri May 13 2022 ...Blocky pattern next week to support record heat over the Southern Plains/Lower Mississippi Valley into the Midwest along with episodes of north-central U.S. convection and northern Great Basin/Rockies snow... ...Overview... The first part of the week will feature a pattern close to an Omega-style block, with an amplified western mean trough, a strong ridge extending from Mexico into southeastern Canada, and an upper low just off the East Coast. With time the central/southern part of the ridge should weaken while the northern part may ultimately connect with a retrograding Atlantic ridge that should push the upper low toward the Southeast coast. Within the western trough, a leading shortwave will eject from the northern High Plains and then upstream energy should sharpen into midweek before ejecting northeastward (possibly leading to strong low pressure). Another system may reach the Northwest late in the week. This pattern will lead to very warm to hot weather over the central/east-central U.S., chilly and unsettled weather over the West, and periods of rain/thunderstorms between these two regimes. The retrograding upper low may also bring some moisture into the East late in the week. ...Guidance Evaluation/Preferences... The updated forecast started with a composite of 12Z/18Z operational models for about the first half of the period and then transitioned to increasing weight of 18Z GEFS/12Z ECMWF means, reaching 60 percent total by day 7 with remaining input from the 12Z ECMWF and somewhat less of the 18Z GFS. The most significant forecast uncertainty during the period will involve details of the western U.S. mean trough aloft, specifically how energy that sharpens into midweek ejects out of the West in response to an upstream northeastern Pacific system that may contain a fairly deep closed low. The 12Z CMC strayed most from consensus in that it held its northeastern Pacific upper low farther southwest, leading to flatter flow coming into the West and in turn a flatter/faster shortwave ejecting from the West mid-late week. The new 00Z CMC has adjusted closer to remaining guidance but still appears to have additional trending to do. Remaining guidance from recent days along with latest ensemble members suggest a fair degree of uncertainty with specifics of each feature. 18Z/00Z GFS runs lean to the amplified/southern side of the spread for upper low reaching the West by next Friday and could be a tad fast with the ejecting shortwave ahead of it, while either the 12Z ECMWF or 18Z/00Z GFS handling of the incoming feature could support slightly faster ejection than the 12Z ECMWF. The late-period model/mean preference provides a reasonable intermediate solution for the ejecting system and trended to the conservative side in depicting the incoming energy by day 7 Friday. However the new 00Z ECMWF has trended somewhat slower with each feature, widening the spread somewhat and highlighting the uncertainty with this overall evolution. The late-period model/mean preference provides a reasonable intermediate solution for the ejecting system and trended to the conservative side in depicting the incoming energy by day 7 Friday. Meanwhile an average of latest GFS/ECMWF runs and their means offers the most stable solution for the upper low near the East Coast. Other models differ at times due to differences in handling embedded impulses. ...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 gradually expanding through the Great Lakes and Northeast as well. 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 around Tuesday/Wednesday. Highest heat index values will be over southern Texas early in the week but the combination of heat and humidity may still produce maximum heat index values over 100F at some southern locations into late week. In contrast, the West will see unseasonably cool and unsettled conditions next week. Coldest anomalies should be early in the week when highs will likely be 10-20F or so below normal. Best potential for some daily records for cold highs will be on Monday. Temperatures may approach normal over southern areas by next Friday but locations farther north should remain at least moderately below normal. Periods of rain and higher elevation snow will tend to be light to moderate, but there could be a period of enhanced activity along and inland from the West Coast late in the week depending on details of an arriving northeastern Pacific system. Between these two regimes, the central U.S. will see periods of rain and thunderstorms as ejecting western shortwaves produce multiple lows, along with a Rockies to Upper Midwest front that ultimately stalls and then lifts northward ahead of another system that brings a cold front into the Plains late in the week. Deceleration of the leading front and then the late week system may produce areas of significant rainfall that would be a runoff threat over northern tier areas that already have very wet soil. The Storm Prediction Center highlights a concern for severe weather over parts of the northeastern Plains/Upper Midwest early in the week, while the late-week system could merit watching for strong convection as well. Most rainfall with the system off the East Coast should just offshore early in the week, but is likely to expand back to the west late in the week as the vertically stacked low retrogrades back toward the Southeast coast. 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