Extended Forecast Discussion NWS Weather Prediction Center College Park MD 249 PM EDT Fri May 22 2020 Valid 12Z Mon May 25 2020 - 12Z Fri May 29 2020 ...Heavy rainfall possible over the Southern Plains/Texas next week... ...Expanding area of much above normal temperatures over the West... ...Overview... Expect separate upper ridges building over the eastern and western U.S. to bookend a Southern Plains upper low whose energy will originate from a trough over the central-southern Rockies as of early Mon. This persistent upper low and associated gradually weakening wavy surface front will lead to a multi-day focus for areas of significant rainfall over the Southern Plains and possibly extending into nearby areas. The upper ridges will support well-above normal temperatures, with the West likely to see more anomalous daytime highs (with record highs probable) and the East (especially Great Lakes/Northeast) seeing somewhat more extreme morning lows. Meanwhile Pacific flow aloft will push a cold front into the Northwest early next week. This front will continue eastward across the northern U.S. during the rest of the week as an upper ridge building over western Canada gradually promotes upper troughing over east-central Canada and into the northern tier states. ...Weather/Hazard Highlights... The strongest signal for heavy rainfall during next week exists over the Southern Plains, with highest probabilities over parts of Texas and to some degree extending to the north/northeast. Persistent low level flow of Gulf moisture should interact with the upper low and wavy surface front settling over the region, leading to multiple days with heavy rainfall potential. Some locations could see several inches over the five-day period but this will likely be modulated by mesoscale processes that are unpredictable at this time range. A portion of this moisture may continue northeastward into the Midwest/Great Lakes/New England. Over these regions confidence is much lower in determining the location of any pockets of heavier rainfall. However an early-mid week front reaching the Midwest/Upper Great Lakes and another approaching from the Northern Plains late in the week will provide opportunities for some organized activity. During the latter half of the week, low level upslope flow behind the northern tier front could promote areas of rain over northern parts of the High Plains. The West will see an expanding area of well-above normal temperatures as upper ridging builds over the region. Parts of California into southern Oregon should already see highs 10F or more above normal on Memorial Day. By mid-late week expect broad coverage of plus 10F and greater anomalies with the core of greatest anomalies (highs 15-25F above normal) likely to be centered over portions of California/Nevada/Oregon. California may begin to moderate somewhat next Fri. Daily records for highs/warm lows will be possible especially from Tue onward which may be the warmest readings of the season for many. 100F temperatures may become widespread from the lower deserts (which may climb into the low 110s) of AZ into the central valley of California. Over the East, flow around Atlantic/eastern U.S. high pressure will promote highest temperature anomalies over the Great Lakes/Ohio Valley/Northeast, with highs up to 10-15F above normal and morning lows up to 15-20F above normal. Anomalies will decrease toward normal along the Southeast coast and Florida due to more influence from easterly Atlantic flow. Over the central U.S., some locations over the central High Plains may see highs 10-15F below normal on Memorial Day followed by generally single-digit negative anomalies for highs over the Southern Plains the rest of the week owing to abundant cloud cover and higher rain chances. ...Guidance/Predictability Assessment... Guidance through the 00Z/06Z cycles provided better than average agreement in most respects, allowing for a 00Z ECMWF/Canadian/UKMET and 00Z-06Z GFS blend to represent consensus for Mon before dropping the much slower UKMET for Tue-Wed. Given the inherent uncertainty of upper lows, most solutions seem quite plausible but even the more removed ones cannot truly be ignored--future adjustments are likely. Thereafter, transitioned to a mostly 00Z ECMWF blend with the 06Z GEFS/00Z ECMWF mean by days 6-7 Thu-Fri with uncertainty in the Pacific (especially toward the Gulf of Alaska) as well as over the East and into the Atlantic with the upper high orientation/strength and a weak upper low to its south. Questions regarding an upper low well offshore California increase by late next week but prefer to maintain strong ridging in place over the Southwest. This should act to keep the system away from at least southern California but perhaps not from northern areas just beyond the period. Meanwhile the current majority cluster for the broad upper trough that develops over the northern tier by late week represents a reasonable solution that was not as deep as some previous models advertised. Fracasso/Rausch Additional 3-7 Day Hazards information can be found on the WPC medium range hazards chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation, 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/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml