Extended Forecast Discussion NWS Weather Prediction Center College Park MD 418 PM EDT Tue Apr 21 2020 Valid 12Z Fri Apr 24 2020 - 12Z Tue Apr 28 2020 ...Extended Period of Much Above Normal Temperatures Across the Southwest and Southern Rockies... ...Overview... The synoptic pattern is expected to evolve over the medium range period into a western ridge/eastern trough configuration and with this pattern, temperature anomalies will become the main weather highlight featuring much above normal readings across the southern half to two-thirds of the West and below normal values across the Great Lakes, Mid-Atlantic, and Northeast. Shortwave energy will move quickly through the flow, bringing precipitation to the Pacific Northwest and extreme northern Rockies while the eastern U.S. will see a pair of weather systems bring unsettled weather through early next week. Despite the better than average agreement in the model guidance for the large scale pattern, there remains detail differences early in the medium range period. ...Guidance/Predictability Assessment... At the start of the period, day 3/4 Fri/Sat, there remains some spread with the latest deterministic guidance with respect to the speed of the low pressure system near the East Coast. The mid/upper level shortwave energy is faster in the 00Z ECMWF/06Z GFS compared to the much slower 00Z CMC/UKMET and the faster solution was favored for the blend preference on day 3. Otherwise, the model spread was reasonably tight/clustered through day 5 with the evolving western ridge/eastern trough such that heavy components of the latest ECMWF/GFS were incorporated. Beyond day 5 (Mon/Tue next week), model spread and uncertainties were high enough to incorporate a majority of the blend toward the ECENS and GEFS means with some minor inclusion of the deterministic ECMWF and GFS. ...Weather/Hazard Highlights... Northeastward-tracking low pressure currently expected to be near the Mid-Atlantic coast as of early Fri should bring rain to the Mid-Atlantic and parts of New England during the day Fri. Guidance is still having difficulty in resolving the precise strength/track/speed of this system, leading to ongoing uncertainty over rainfall amounts and northward extent. Florida should see some rainfall along the trailing cold front. The northern/central Rockies will see some lingering rain/higher elevation snow late this week with the shortwave energy aloft that ultimately amplifies into the central-eastern U.S. This amplifying energy and associated low pressure will spread an area of precipitation from parts of the Plains and Mississippi Valley on Fri onward through the eastern U.S. during the weekend with some activity possibly lasting into next week over the Northeast depending on system progression. The most likely axis for highest totals currently extends from the Midwest and south-central Mississippi Valley through the Ohio Valley, Mid-Atlantic, and Northeast. The potential remains for some higher elevation snow over northwestern New England and the Adirondacks. Two primary shortwaves/weakening surface systems coming into the Northwest should bring periods of rain and possibly high elevation snow to parts of the region during the weekend and early next week. There is a good signal for the Olympics and Cascades of Washington to see the highest totals during the period but with the magnitude very much in doubt. The forecast is consistent in showing above normal temperatures with some plus 10-15F anomalies over and near California late this week with expansion of very warm to hot readings across the southern half or so of the West during the weekend/early next week. Some of this heat should extend into the southern High Plains by next Mon or Tue as well. Within the broad area of plus 10-15F and locally higher anomalies, some daily records for highs/warm lows may be possible. System progression and some areas of precipitation in the northern parts of the West will keep temperatures closer to normal. Over the East expect the upper trough settling over the region to promote multiple days with below normal highs. Coolest anomalies in the minus 10-15F range should progress from the Midwest into the east-central U.S. and Northeast during Sat-Mon. Rausch/Taylor 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 Hazards: - Heavy precipitation across portions of the Pacific Northwest and the Northern Great Basin, Sun-Mon, Apr 26-Apr 27. - Heavy rain across portions of the Middle Mississippi Valley, the Lower Mississippi Valley, the Great Lakes, and the Ohio Valley, Fri-Sat, Apr 24-Apr 25. - Heavy rain across portions of the Mid-Atlantic, the Northeast, and the Central Appalachians, Sun-Mon, Apr 26-Apr 27. - Flooding possible across portions of the Northern Plains. - Flooding occurring or imminent across portions of the Middle Mississippi Valley, the Lower Mississippi Valley, the Upper Mississippi Valley, and the Northern Plains. - Flooding likely across portions of the Southeast and the Tennessee Valley. - Much above normal temperatures across portions of the Central Plains, the Central Rockies, the Central Great Basin, the Southern Plains, the Northern Great Basin, the Southern Rockies, California, the Pacific Northwest, and the Southwest, Fri-Tue, Apr 24-Apr 28. - Much below normal temperatures across portions of the Northeast, the Central Appalachians, the Tennessee Valley, the Mid-Atlantic, the Southern Appalachians, the Great Lakes, and the Ohio Valley, Sun-Mon, Apr 26-Apr 27. - Much below normal temperatures across portions of mainland Alaska, Fri-Sun, Apr 24-Apr 26. 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