Extended Forecast Discussion NWS Weather Prediction Center College Park MD 240 PM EDT Sat May 23 2020 Valid 12Z Tue May 26 2020 - 12Z Sat May 30 2020 ...Heavy rainfall possible over the Southern Plains/Texas into the Lower Mississippi Valley... ...Multiple days of much above normal temperatures over the West... ...Overview... The heaviest precipitation focus will likely continue to be near/ahead of an upper low that should close off over the Southern Plains Tue-Wed and remain over the region (most likely Texas) through the end of the period next Sat. On either side of the upper low, expect separate upper ridges to promote much above normal temperatures over the West and northern parts of the East. Numerous record highs will be possible over the West (more isolated in the East) while both the West and Great Lakes/Northeast could see record warm morning lows. A low-latitude upper low forecast to approach northern California by next Sat should bring a cooler trend to the West Coast states late in the period. Meanwhile one or more fronts across the northern tier states may provide some rainfall focus at times. Especially after midweek frontal specifics have low confidence due to guidance becoming erratic/diverse with the pattern over the Northeast Pacific. ...Weather/Hazard Highlights... The best potential for highest five-day rainfall totals will be over the Southern Plains into parts of the Lower Mississippi Valley, supported by the persistent Plains upper low along with weakening wavy surface front and contribution of low level moisture from the Gulf. Confidence is low in specifying precise locations of heaviest rain given the likely dependence on mesoscale processes that can be a challenge to forecast a day in advance let alone three or more days out in time. This activity will be part of a much broader swath of moisture around the eastern U.S. upper ridge. Sufficient moisture and energy aloft may produce periods of locally moderate-heavy activity over Florida, while some of the Plains/Mississippi Valley moisture may extend into the Midwest/Great Lakes and possibly into the Ohio Valley/Northeast with rainfall of varying intensity. Surface front/wave details continue to be uncertain thus tempering confidence in locating highest totals within this area of rainfall. Over the West expect the coverage and intensity of very warm to hot temperatures to peak in the Wed-Fri time frame. Already on Tue parts of central/northern California and possibly a few nearby areas should see highs 15-20F above normal. Then by mid-late week a considerable area will likely see highs 15-25F above normal with the greatest anomalies extending from northern California into portions of Oregon/Nevada/Idaho. Readings in the low 100s are probable over the San Joaquin Valley with 110s over the inland deserts of southeastern California into the lower Colorado River Valley. The upper low nearing the West Coast late in the period should greatly erode the heat over California and western Oregon by Fri or Sat. At the same time the progression of the northern part of the upper ridge should raise highs up to 15-20F above normal over the northern Rockies/High Plains by Fri-Sat. Best potential for record highs will start over California and then gradually progress/expand eastward and northeastward. The forecast continues to show the Great Lakes/Ohio Valley/Northeast regions seeing the warmest temperature anomalies in association with eastern ridging aloft and low level flow around Atlantic/eastern U.S. surface high pressure. Highs and morning lows should both be 10-20F above normal but the lows will tend to be a little more anomalous than the highs, leading to a better opportunity for some daily record warm lows. Arrival of a cold front should bring a moderating trend around Fri-Sat. Easterly low level flow containing varying amounts of moisture will promote highs near or slightly below normal over southern portions of the East. The upper low and associated clouds/rain lingering over the Southern Plains will keep highs moderately below normal, by up to 5-10F or so. ...Guidance/Predictability Assessment... By far the greatest uncertainty in forecast specifics currently extends from the Northeast Pacific across the northern U.S. and southern Canada. The 00Z/06Z cycles of model and ensemble guidance reverted to a more defined system south of Alaska midweek with downstream timing implications/changes. This solution at least had more ongoing support despite the recent wobble in the guidance but confidence is low in the flow pattern along/north of 50N next week. In spite of the uncertainty leading up to next Sat there is a decent consensus toward a broad trough aloft reaching the Great Lakes/Northeast by the end of the period, though that has been trending quicker in recent ensemble runs. Elsewhere the forecast has been more stable. Guidance is maintaining better definition of the upper low that settles into the Southern Plains with an average position fairly close to continuity. Spread appears typical for such features and an intermediate solution seems reasonable. The upper ridges over the West and East have not changed significantly in most respects, though latest clustering hints that a little more of the western ridge could eventually extend into the north-central Rockies/Plains by late in the period--potentially having some influence on the path of the Southern Plains upper low. GFS/ECMWF runs and their means agreed reasonably well for the low-latitude upper low expected to reach just offshore northern California by early next Sat with the upper ridge to the east deflecting the feature toward northern California and eventually southwestern Oregon next Sat. Through the 00Z/06Z cycle a deterministic blend sufficed to start the forecast period with the exception of the 00Z UKMET due to a very different E Pacific forecast. Thereafter, transitioned to a slight majority ensemble weighting due to the large uncertainty over higher latitudes offset by the better agreement over southern latitudes. 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