Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 AM EDT Mon May 09 2022 Valid 12Z Thu May 12 2022 - 12Z Mon May 16 2022 ...Blocky pattern through late 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 rainfall focus to the Carolinas... ...Overview... The start of the period will feature a mean trough aloft over the West, ridging that extends from the southern Plains through Great Lakes/Northeast, and an upper low off the Southeast coast. Temperatures will likely challenge daily records under and near the upper ridge axis late this week while parts of the West will see chilly weather with rain and higher elevation snow. Between these regimes, a storm system emerging over the northern Plains may produce some areas of heavy rainfall where wet ground conditions already exist. Then during the weekend and early next week most guidance shows the pattern transitioning toward a western ridge/east-central U.S. trough configuration. This evolution should finally push the rainfall focus to the east of the Plains. The initial upper low tracking into the Southeast around Friday due to a retrograding Atlantic ridge should quickly lift northward and open up during the weekend, with associated rainfall spreading into and north from the Carolinas. The ridge building over the West will lead to increasing coverage of very warm to hot temperatures over the southern half of the West during Saturday-Monday. ...Guidance Evaluation/Preferences... Based on guidance available through the 12Z/18Z cycles, the 18Z GFS and 12Z ECMWF/CMC provided the most compatible and consistent cluster for the overall forecast through the period along with reasonable support from the ensemble means. The starting blend used those three operational runs for about the first half of the period and added in a moderate weight of 18Z GEFS/12Z ECMWF means thereafter. GFS/ECMWF continuity and trends of other guidance still seem to be pointing more toward some variation of a GFS/ECMWF scenario for the vigorous system expected to emerge from the West and track over the northern Plains into southern Canada late this week. Aside from some detail variability, GFS/ECMWF runs converged fairly well over the past day or so while the previously fast CMC has been gravitating in their direction. UKMET runs had been the most extreme with a southern track of the western upper low/trough but the new 00Z run is finally trending in a favorable direction. Behind this system, models and ensembles still show a moderate amount of spread for the character of trailing energy that consensus shows flowing through the Northwest and then sharpening over the Plains and east-central U.S. while ridging builds into the West. The favored model cluster mentioned above fit well within the ensemble mean template but the new 00Z cycle has introduced more spread in opposing directions, as the GFS has trended flatter with the trough (due to stronger depiction of shortwave energy reaching the West by Monday) while the CMC forms a closed low farther south than other guidance. Finally, latest trends for the upper low reaching the Southeast coast around early Friday have been toward faster northward progression after that time along with opening of the low by late Saturday. ...Weather/Threat Highlights... The strong upper ridge extending from the southern Plains through the Great Lakes/Northeast as of Thursday will promote much above normal temperatures from much of the Plains and Mississippi Valley through the Great Lakes and Northeast late this week. Highs/morning lows will be at least 10-20F above normal with some embedded plus 20-25F anomalies possible from the central Plains into Great Lakes, and locations across a fairly broad area may challenge daily records. After Friday expect the above normal temperatures to be more confined to the Great Lakes/Northeast and the far southern Plains/western Gulf Coast region. There will be a lingering potential for daily records over the Northeast into Saturday and over eastern Texas and vicinity through the end of the period. The frontal system and upper dynamics crossing the Northwest late this week will produce a brief period of enhanced rain and high elevation snow as well as highs 5-15F below normal. Meanwhile the southern part of the West should moderate to slightly above normal by Friday. Then arrival of the upper ridge will extend the warming trend over the West through the weekend. Highs of 10-20F above normal should expand from California eastward from Saturday onward and some locations could reach record highs by Sunday-Monday. Northern areas of the West should be closer to normal and one or more fronts may produce periods of light to moderate precipitation over the Pacific Northwest. Late this week the system emerging from the West and developing over the northern Plains before tracking into southern Canada, along with associated fronts, may produce areas of heavy rainfall over parts of the far northern tier where some areas are quite sensitive to added rain due to very wet ground conditions. The experimental Day 4 (Thursday-Thursday night) Excessive Rainfall Outlook has introduced a Slight Risk area over parts of northern North Dakota and Minnesota where there is the best overlap of guidance signals for significant rain and instability, along with high soil moisture. Depending on system evolution, areas farther west back to the northern High Plains could also see significant rainfall. Some locally moderate to heavy showers and thunderstorms will be possible farther south along the wavy trailing front (which may hang up for about a day as western energy catches up to it) but so far with less agreement for specifics. The Storm Prediction Center is also monitoring the potential for severe weather, so check the latest outlooks for updated information. Rainfall and the focusing front should become more progressive from Saturday onward, leading to a drier trend over the Plains. Retrogression of the western Atlantic upper low into the Southeast toward the end of the week should spread rain into the Carolinas and vicinity by around Friday with some of this moisture spreading northward thereafter. Highest totals should be over or near the eastern Carolinas. Amounts farther north should trend lighter due to decreasing focus of moisture as the upper low opens up while it lifts 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