Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1134 AM EDT Sun May 27 2018 Valid 12Z Wed May 30 2018 - 12Z Sun Jun 03 2018 ...Remnants of what is now Subtropical Storm Alberto expected to bring heavy rain potential to portions of the Tennessee/Ohio Valleys through the end of the week... ...Synoptic Overview and Model/Uncertainty Assessment... While there was some suggestion yesterday that the upper flow across North America might become a little less blocky during the medium range, it appears that trend was not meant to be. Look at a hemispheric perspective this morning, it now appears as if a number of strong and persistent height anomalies will continue to dominate the flow regime, keeping the slowing the progression of any smaller scale systems. Alberto is forecast by the NHC to be a tropical depression at 12Z on day 3 (Wed) across the Ohio Valley, before becoming post-tropical as the surface low moves north into the Great Lakes before eventually becoming absorbed into a northern stream frontal boundary. For the latest information and forecast for Alberto, please refer to products issued by the NHC. A block across the northern Atlantic Ocean, comprised of negative height anomalies east of the Canadian Maritimes and ridging south of Greenland will prevent the combination of Alberto and the northern stream energy from progressing eastward, and it appears that the only place the system will have to go is southeastward, with consensus among the guidance now that the shortwave will dig into the Mid-Atlantic region, possibly even cutting off for a period of time. Models also show a general consensus that a new surface low should develop by Fri or Sat near the Mid-Atlantic coast as the upper trough digs into the region. At this time, model variations surrounding the evolution of these features are not particularly large, with the primary differences by days 6-7 related to the amplitude of the eventual trough or cutoff low over the Mid-Atlantic. Farther west, a strong upper ridge appears set to expand across the southern plains over the by later this week, with a ridge axis extending north across the Upper Midwest and Great Lakes. Meanwhile, the evolution of an upper trough along the West Coast by Wed still remains somewhat uncertain, although there have been some notable trends. The persistence of the blockier pattern would seem to favor a slower progression of systems, with more potential for amplification, and this has been a trend among some of the models/ensembles over the past day. A sizable number of CMC ensemble members had previously shown a slower and more amplified upper trough across the western U.S. late this week, and this morning a notable number of solutions have trended in that direction, including the ECMWF, which was much more progressive with this feature yesterday. The slowest/most amplified solution is the 00Z CMC, which at this time cannot be ruled out given the trend and the broader pattern. At this time, a compromise is likely warranted, but with a bit more weight placed toward the slower and more amplified solutions. By later Sat into Sun, the system will likely begin to move east toward the plains, with the degree of amplification a bit uncertain at this time given the broad anticyclonic flow across the central U.S. The WPC forecast was initially based on a heavily deterministic multi-model blend including the 00Z ECMWF/CMC and 06Z GFS during days 3-4. By days 6-7, ensemble weighting (00Z ECENS/NAEFS) was increased to a majority of the blend, with a bit more weight placed to the NAEFS given its somewhat slower/deeper solution with the western U.S. system. ...Weather/Threats Highlights... It should be a rather wet period for much of the Ohio Valley and Great Lakes region as Alberto moves through the region on Wednesday/Thursday. Abundant tropical moisture spilling into the southern and eastern states will also raise rainfall chances through much of the week. In these latter locations, it will likely be more difficult to pinpoint where isolated heavier amounts will occur given unknown forcing mechanisms and placement of mesoscale features. Farther upstream, highly diffluent flow in advance of the western U.S. trough will foster the development of heavy rainfall over the northern Rockies into the Dakotas. This signature slowly moves eastward into the Upper Midwest by the weekend. Heat will dominate the forecast throughout much of the week. While locations along the West Coast can expect readings near or slightly below climatology due to the deep upper trough, this will truly be the anomaly in the pattern. Widespread temperature records are likely from the Great Plains eastward, particularly for the warm overnight minima. The hot spot should be the Southern Plains where numbers may exceed 105 degrees along the Rio Grande while century degree readings also encompass much the region surrounding the Red River. A true heat wave is in store as several days of such conditions are likely. Even surrounding locations will get in on the action as widespread 90s are expected across the central Plains as well as the lower Mississippi Valley. Ryan/Rubin-Oster WPC medium range forecasts of 500 mb heights, surface systems, weather grids, quantitative precipitation, and winter weather outlook probabilities can be found at: http://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif http://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif http://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html http://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml http://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4