Extended Forecast Discussion NWS Weather Prediction Center College Park MD 256 AM EDT Tue Apr 21 2020 Valid 12Z Fri Apr 24 2020 - 12Z Tue Apr 28 2020 ...Overview... Latest models and ensembles continue to show the large scale flow aloft settling into a western ridge/eastern trough configuration. As this pattern becomes established expect a broadening area of above to much above normal temperatures across the southern half to two-thirds of the West, and likely reaching into the southern High Plains by next Mon or Tue. Meanwhile shortwave energy rounding the ridge should bring some precipitation to the Pacific Northwest and extreme northern Rockies. This energy should temporarily erode the upper ridge to some degree but most guidance shows the upper ridge starting to rebuild next Tue. The eastern U.S. will see a cool and unsettled period as the developing upper trough supports surface low pressure that should depart from the East during the first half of next week. Even with the good large scale agreement, there are still significant detail uncertainties embedded within and ahead of the developing upper ridge/trough pattern. ...Guidance/Predictability Assessment... Even at the start of the period early day 3 Fri there is still significant timing spread for the system near the East Coast at that time. A composite of the 18Z GFS/GEFS mean and 12Z ECMWF/ECMWF mean continue to provide the most stable solution for this system while the 12Z UKMET and to some degree CMC were slow as has been the case for multiple runs. This relationship holds into the new 00Z cycle as well. Given the guidance spread and variability up to this point, the 18Z GFS-GEFS mean and 12Z ECMWF/ECMWF mean/CMC clustered remarkably well for the evolution of the upper trough amplifying into the East and associated low pressure. This scenario would have low pressure tracking from the Midwest into the Ohio Valley/Lower Great Lakes with a triple point center in the Mid-Atlantic becoming the dominant low that ultimately tracks near the New England coast. Over recent days ECMWF runs have been particularly inconsistent, varying between a disjointed upper pattern with one or more weak/progressive surface lows versus strong inland development at the surface and aloft. GFS runs have varied on location of strongest development but have been somewhat more agreeable in general. The main trend of note in the means over the past couple days is in the GEFS mean which has been adjusting farther north with the track of eastern U.S. low pressure in response to a deeper and more closed upper system. Ensembles as a whole still exhibit some spread with the surface low(s) but clustering is much improved over 24 hours ago. In the new 00Z runs the GFS has strayed to the fast side of consensus but the CMC/ECMWF compare well to ongoing consensus. Plus the UKMET is fairly close through the end of its run. That model has compared poorly to most other solutions over the eastern half of the country in recent days. This latest stability in the consensus scenario is helping to improve confidence now. Around the northern side of the western ridge there is increasing agreement that two primary shortwaves should flow into the northwestern U.S. and western Canada, one around late Sat and the another around Sun night-Mon. Thus far the GFS/GEFS mean have been persistent with being more amplified for both shortwaves than the ECMWF/ECMWF mean/CMC. Typical biases would suggest an intermediate solution tilted a bit more in the ECMWF/CMC direction. The 00Z ECMWF has lowered its heights a bit, adding support to this approach. By day 7 Tue the models and means in varying ways suggest this shortwave energy should begin to amplify into the central U.S. as the upstream ridge begins to rebuild, with a corresponding surface frontal system over the Plains. Based on overall forecast preferences the updated forecast started days 3-5 Fri-Sun with highest weight of the 18Z GFS/12Z ECMWF and lesser input from their means and the 12Z CMC. Ensemble weight increased to 30-40 percent by days 6-7 Mon-Tue while the three operational runs continued to provide the remaining input. ...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. While the guidance is coming closer to resolving the individual shortwaves, there is still uncertainty for their amplitude and thus the southward extent of precipitation. 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 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