Extended Forecast Discussion NWS Weather Prediction Center College Park MD 306 PM EDT Wed May 31 2023 Valid 12Z Sat Jun 03 2023 - 12Z Wed Jun 07 2023 ...Broad multi-day thunderstorm and local heavy rain threat continues from the Northern Rockies to the Southern Great Plains... ...Overview... A strong upper ridge/high developing over the Upper Midwest and south-central Canada this weekend will persist into next week, supporting prolonged above normal temperatures from the northern Plains into middle/upper Mississippi Valley through midweek. Broad upper low energy over the Intermountain-West through the southern High Plains will weaken this weekend, but then a well-advertised southern stream Pacific low should track into southern California on Monday. These features will maintain periods of convection over the Rockies and parts of the central/southern Plains, gradually expanding to the Sierra Nevada, through the weekend. Expect above normal temperatures over the northwestern states through at least midweek, though a front ahead an ejecting Pacific upper low should provide a brief reprieve near the Pacific Northwest coast during the weekend. Downstream from the central upper ridge, guidance continues to vary though a consensus is moving toward anomalous low pressure over the Northeast developing this weekend and persisting into or through midweek which would bring cool and wetter conditions to New England and perhaps the northern Mid-Atlantic. ...Guidance Evaluation/Predictability Assessment... The 12Z GFS has backed off a bit from its recent outlier (and ignored in the initial model blend today) solution of a stronger central ridge that allowed a cutoff low to track across the Northeast this weekend and over the Midwest by Sunday. The 12Z consensus of global deterministics are to contain this upper low to the Northeast this weekend through Monday with divergence then about how strong the reinforcing shortwave trough is with the 12Z CMC much stronger, developing a second low, and the 12Z UKMET now digging much farther south to the southern Appalachians while the GFS and ECMWF limit the progression to the Mid-Atlantic (with the GFS being farther south of the two). By next Tuesday-Wednesday, the latest ensemble means suggests the low/trough will be anchored over southern New England or the northern Mid-Atlantic. Meanwhile, the National Hurricane Center continues to monitor a disturbance over the Gulf of Mexico associated with a southern tier upper trough that looks to shift east to the Atlantic this weekend with the motion dependent on upper feature details to the north. The forecast farther westward is less chaotic in most respects. Guidance has been consistent in weakening initial shortwave energy/one or more upper lows over the West over the course of the weekend. There have been some recent trends toward a little farther eastward extent of height falls ahead of the upper low ejecting from the northeastern Pacific, with corresponding effects on the leading cold front and Pacific Northwest temperatures for a time. The majority cluster of guidance still holds up well for the upper low to track into southern California next week, with most recent cycles having one or two stray solutions that ultimately trend closer to the mean. Based on the array of guidance from this morning, the forecast progs were based on a non-06Z GFS consensus that increasingly favors the 00Z ECENS mean through time. ...Weather/Hazards Highlights... The pattern keeps a persistent threat for showers and thunderstorms over parts of the Rockies and Plains during the period, with diurnally favored showers/storms also likely to become more common with time from the Sierra Nevada-southern Cascades into the Great Basin as the southern stream Pacific upper low approaches/reaches southern California Monday. Gradual weakening of upper impulses/lows initially over the West/High Plains should lead to somewhat lower maximum rainfall totals Sunday/Monday than in the shorter term, but localized flash flood potential will likely continue given wet ground conditions from recent or soon to occur rainfall. The Days 4-5 Excessive Rainfall Outlooks (covering Saturday through Sunday night) depict Marginal Risk areas across locations from the northern Rockies into parts of the southern Plains to reflect the persistent moisture/overall pattern that could still produce some areas of locally heavy rainfall. It will likely take into the shorter range time frame to resolve areas of more pronounced focus/confidence for any embedded Slight Risk areas. The Day 5 outlook incorporates the Sierra Nevada into the overall Marginal Risk area with moisture expected to increase across the region. Meanwhile the Day 4 outlook continues to have a Marginal Risk area for some lingering heavy rainfall focused over the lower Florida Peninsula. Expect the potential for some showers/storms with a couple cold fronts dropping south over the Great Lakes/Northeast during the period, with a potential trend toward diurnally favored pockets of rainfall in the Northeast as upper troughing deepens next week. Most of any moisture still crossing Florida during the weekend should progress offshore thereafter, though still with some lingering uncertainty given the guidance spread for the pattern farther north. The area of well above normal temperatures from the northern Plains through Upper/Middle Mississippi Valley into the central Appalachians as of Saturday should get steadily trimmed away on the eastern side as upper troughing develops over the East with increasing coverage of near to somewhat below normal readings. The weekend should start with decent coverage of highs 10-20F above normal, with some daily records possible, followed by most plus 10F or greater anomalies getting confined to the northern Plains by next Wednesday. The Northwest will be the other focus for very warm temperatures, with highs 10-20F or so above normal especially Monday-Wednesday. An ejecting Pacific upper low/leading front should temper the warmth during the weekend over western Oregon/Washington though. Clouds and periods of rainfall should keep highs 5-15F below normal over the southern half of the Rockies/High Plains for most of the period and the upper low reaching southern California Monday will also bring a cooling trend there for early next week. Jackson 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