Extended Forecast Discussion NWS Weather Prediction Center College Park MD 254 PM EDT Sat May 29 2021 Valid 12Z Tue Jun 01 2021 - 12Z Sat Jun 05 2021 ...Record warmth likely Tuesday into Thursday across parts of the West... ...Locally heavy rain possible mid-week from the southern Plains into the Mississippi Valley... ...Overview and Guidance Evaluation/Preferences... The upper level pattern should become less amplified over the course of the period. A shortwave reaching the Pacific Northwest around next Thursday will begin to erode the strong ridge initially over parts of the West and upstream energy will likely bring additional height falls to the Northwest toward the end of the week. Farther east, expect troughing over the Plains-Mississippi Valley to deepen/sharpen a bit through midweek due to the early-period strength of the western ridge. The trough should eventually accelerate somewhat and weaken, respectively due to flatter mean flow to the west and a ridge over the western Atlantic. Leading shortwave energy over the Great Lakes/Northeast will depart early in the period. By next Saturday the latest guidance indicates the possibility that some combination of energy from northwestern Mexico and what may filter through/around the western U.S. ridge could yield a weak trough/upper low over the southern High Plains. Guidance comparisons for the 00Z/06Z cycles were again sufficiently good for an operational model blend to serve as the basis of the forecast during the first half of the period. Increasing shortwave detail uncertainties thereafter--from the Pacific Northwest across southern Canada, as well as over the East and southern High Plains--favored incorporating some ensemble mean guidance (06Z GEFS/00Z ECens) along with the 06Z GFS/00Z ECMWF and small weight of the 00Z CMC by days 6-7 Friday-Saturday. This consensus yielded two noticeable continuity changes, one toward a somewhat weaker and more progressive trough emerging from the Plains/Mississippi Valley after Thursday (with a corresponding weaker surface reflection) and the other being the introduction of a weak trough/upper low over the southern High Plains next Saturday, a low-predictability evolution. In both cases a majority of the trend has occurred in the latest cycle, tempering confidence some, but with enough agreement to support at least an intermediate blend. New 12Z guidance is mixed for the Plains/Mississippi Valley trough, with the 12Z ECMWF adjusting back at least halfway to pre-00Z runs but the GFS/UKMET are still somewhat weaker/faster. Trends are mixed as well over the High Plains by next Saturday. Also of note most 12Z models aside from the ECMWF are suggesting greater separation of energy reaching the West Coast Thursday. ...Weather Highlights/Hazards... The main focus for rainfall will be along and ahead of the Central U.S. upper trough as moisture pools along a wavy frontal system. To start the period on Tuesday, the best chance for areas of moderate to locally heavy rainfall will be from near southwestern Texas into parts of Arkansas/Missouri. Much of this region has already seen much above normal rainfall in the past couple of weeks. From Wednesday onward, the activity should shift eastward into the Tennessee/Ohio Valleys and the Northeast along a leading warm front and trailing stationary front that eventually dissipates. Even with a lack of frontal forcing by the end of the week, lingering shortwave energy and moisture may be sufficient to produce showers/thunderstorms of varying intensity into the southern Appalachians and Southeast. After Tuesday confidence decreases in resolving any particularly favorable areas of heaviest rainfall but some pockets of moderate to heavy rain may be possible on any day. Meanwhile the West should initially stay dry beneath upper level ridging. Later next week incoming shortwave energy may produce areas of scattered precipitation, mainly from the Northwest into portions of the Rockies. The most extreme temperature anomalies during the medium range period will be during Tuesday-Wednesday and to a lesser extent Thursday from California and the Northwest into the Great Basin where daytime highs of 15-25F above normal and possibly locally higher may meet or exceed daily records. Morning lows may be warm enough to challenge daily records across this same region into Thursday as well. Weakening/progression of the western upper ridge will yield a moderating trend over the Northwest in particular late in the week along with a warming trend farther east across the northern tier. By next Saturday there may be an axis of highs at least 10-15F above normal from central California and the Great Basin through the northern portions of the Plains/Great Lakes/New England. The southern half of the Plains will be below normal (up to minus 10-15F anomalies for highs) under cloudy and stormy skies Tuesday and Wednesday followed by some lingering moderately below normal readings over Texas. Much of the eastern U.S. should see near-normal temperatures through the period aside from the warming trend in the north by the end of the week. Rausch/Santorelli 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, 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