Extended Forecast Discussion NWS Weather Prediction Center College Park MD 343 PM EDT Fri Jun 09 2023 Valid 12Z Mon Jun 12 2023 - 12Z Fri Jun 16 2023 ...Heavy rain and strong storms for portions of the High Plains and into the Mid-South as deep summer heat builds from Texas to Alabama... ...Synoptic Overview... A quite troublesome closed upper low has been anchored in place across New England downstream of a highly amplified central continent upper ridge. The low has been a main part of a long-standing blocky flow pattern over the nation, but will slowly lift away across the Canadian Maritimes by Monday, with another well-defined low developing across the Great Lakes as a shortwave drops south from the Hudson Bay region. This will spur surface cyclogenesis across the Ohio Valley early in the week with a trailing cold front reaching the East Coast around Tuesday, and this will likely bring widespread showers and storms from the Mid-Atlantic to New England. A second main closed upper low is expected to linger across the southwestern U.S. into early next week and associated energies that should reach the central/southern Plains by midweek will likely support a surface frontal low, and this will also tend to increase the prospects for widespread showers and storms from northern Texas/Oklahoma eastward with a focus across the Mid-South/vicinity and with an uncertain axis downstream toward the southern Appalachians Wednesday through Friday. ...Guidance Evaluation/Predictability Assessment... The WPC medium range guidance suite was primarily derived from a composite blend of reasonably well clustered guidance from the 06 UTC GFS and 00 UTC ECMWF/UKMET/Canadian models along with the 13 UTC National Blend of Models valid for early to mid next week. This solution has favorable ensemble support and maintains good WPC product continuity in an overall pattern with seemingly above normal predictability. Forecast spread/uncertainty remain lower than normal into later next week, but prompted inclusion of some of the latest GEFS/ECMWF ensemble guidance in place of the 06 UTC GFS that seemed too progressive at these longer time frames within less certain southern stream flow system and convective focus details. The 12 UTC GFS has trended slower. ...Weather/Hazards Highlights... A daily risk for heavy showers and strong to severe thunderstorms from the Southern Plains to the northern Rockies and westward into the Great Basin is expected to be slow to dislodge through about mid-week, as tropical Pacific moisture advects northward between a strong central U.S. upper ridge and an upper low/trough parked over California/Southwest early in the week. A weakening boundary through the region should also help to enhance rainfall totals, with particular concerns over areas that have seen well above average rainfall over the past two weeks (northern and central Rockies/western Great Plains). A Slight Risk area remains in place on the WPC Day 4 Excessive Rainfall Outlook across northeastern Colorado and small portions of neighboring states, with a broad marginal risk extending from the Intermountain West to the south-central Plains. The closed low over the Great Lakes and trailing surface cold front will advect copious moisture northward across the Mid-Atlantic and Northeast, and in combination with ample instability will likely produce storms with locally heavy rainfall, with a Marginal Risk of excessive rainfall in effect for Monday-Monday night, with lingering wrapping into northern New England into Tuesday. Antecedent dry conditions and the progressive nature of the front preclude anything more than a broad Marginal Risk at this time. Elsewhere, ejecting shortwaves from the Southwest also appear to favor additional rounds of heavy rainfall and convection again around mid-week for parts of the south-central Plains to the Mid-South and vicinity with copious Gulf return moisture and instability intersecting a quasi-stationary/warm front. There will be two main areas of the U.S. that will likely have above normal temperatures next week, with the first being across the Dakotas and into Minnesota, Wisconsin, and Iowa where readings are expected to be 5-15 degrees above mid-June averages. The second and more impactful area should develop and spread across Texas into the central Gulf Coast region as upper ridging becomes better established over the region and temperatures slowly increase to the south of a frontal boundary and associated convection. Widespread mid-upper 90s are expected, with values well into the 100s across the normally warmer locations across southern Texas, along with increasing humidity levels. Some daily record highs are possible, mainly mid-late next week. This heat wave will likely have some staying power beyond next Friday and also expand into the following week toward the Southeast U.S. as per latest guidance from the Climate Prediction Center. Elsewhere, underneath closed upper low development over the Great Lakes/Midwest and also over the Southwest/California, temperatures should feel more like the month of May next week. Schichtel/Hamrick 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 Hazards: - Heavy rain across portions of the west-central Plains, the Mid-Atlantic, the Northeast, and the Great Lakes, Mon, Jun 12. - Heavy rain across portions of the Lower Mississippi Valley, the Central Appalachians, the Tennessee Valley, the Middle Mississippi Valley, the Southern Appalachians, and the Ohio Valley, Wed-Thu, Jun 14-Jun 15. - Flooding possible across portions of the Northern Plains. - Flooding occurring or imminent across portions of the Northern Rockies and the Northern Great Basin. - Flooding likely across portions of the Central Great Basin, the Northern Plains, and the Northern Great Basin. - Excessive heat across portions of the Lower Mississippi Valley and the Southern Plains, Tue-Fri, Jun 13-Jun 16. - Enhanced wildfire risk across portions of the Southern Rockies, the Southern Plains, and the Southwest, Mon, Jun 12. 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