Extended Forecast Discussion NWS Weather Prediction Center College Park MD 234 AM EDT Sun May 27 2018 Valid 12Z Wed May 30 2018 - 12Z Sun Jun 03 2018 ...The decaying remains of what is currently Subtropical Storm Alberto should bring unsettled conditions to the Great Lakes and Ohio Valley by mid-week... ...Synoptic Overview... At the onset of the forecast period, May 30th, Alberto should be a tropical depression based on the latest National Hurricane Center forecast with a location over the lower Ohio Valley. It should be in the process of getting absorbed by a mid-latitude shortwave with a gradual eastward adjustment the following couple of days. Without significant steering flows in its vicinity, the upper reflection of this system could linger over the Eastern Seaboard into next weekend. Elsewhere, the pattern is a bit less clear given a series of uncertainties which prevail offshore over the Pacific. On Thursday/Friday, an upper trough is forecast to dig into the western U.S. with the southern extent of height falls crossing northern Arizona by early Friday. This system should carry its influences into the Great Plains over the weekend while another upper low looms offshore of Vancouver Island. The state of each of these mentioned systems is somewhat in flux into next weekend given large-scale model differences. It does appear safe to say some form of negative height anomaly will linger off the British Columbia coast into next Sunday, June 3rd. ...Model Guidance/Preferences... The latest 00Z model suite has shown a slight northward adjustment in the position of Alberto as indicated by the GFS/UKMET solutions. Will see if the 09Z advisory makes any major adjustments to the medium range forecast track. Regardless, it appears the remnant circulation will eventually become absorbed by an upstream mid-latitude trough in the vicinity of the Great Lakes by Thursday. At this point, there is general model agreement in dragging a broad mid-level weakness toward the Eastern Seaboard by next weekend. Given significant distance from the stronger belt of westerlies across eastern Canada, the system could continue to linger while gradually shearing. Elsewhere, a modest shortwave will be in the process of moving through the Dakotas and Upper Midwest on Wednesday. Solutions continue to vary with surface strength as the 12Z ECMWF remained the weakest, possibly on the order of 6 to 10 mb. Recent GFS forecasts favor keeping this cyclone center very near the remains of Alberto on Thursday while other global models lift the upstream circulation well up into western Ontario. Gradually the synoptic-scale focus will shift westward to amplified flow setting up over the southwestern U.S. on Thursday/Friday. Ensemble spaghetti plots had been quite noisy but the 12Z cycle offers a bit more certainty in the pattern through Thursday or so. Like preceding shortwaves and closed lows crossing this region in recent times, most have lifted sharply north-northeastward with this system likely following suit. Uncertainty grows from Friday onward with what state this feature will be in. By Saturday morning, scenarios are variable as indicated by the notable ensemble spread. Considering recent trends, the 00Z GFS/CMC have moved in opposite directions with the latter significantly slowing while the 12Z ECMWF sits well south and east of other global models. As this system wobbles about the middle of the country, uncertainty across the Pacific looms over the forecast. Run-to-run model continuity is rather poor at best as they cannot seem to agree on whether a trough or a ridge will sit over Vancouver Island early next weekend. Ensemble means seem to favor a flattening trough which would be a nice compromise from the myriad of solutions at hand. Took a multi-operational model consensus through Day 4/Thursday, utilizing a combination of the 12Z GFS/ECMWF/CMC/UKMET. From Day 5/Friday onward, favored a solution along the lines of the 12Z GFS/ECMWF and 12Z GEFS/ECMWF/NAEFS ensemble means with more emphasis on ensemble solutions deeper into the forecast. Confidence becomes slightly below average next weekend given the messy western North America pattern. ...Weather/Threats Highlights... It should be a rather wet period for much of the Ohio Valley and Great Lakes region as Alberto pushes 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 diffulent flow in advance of the upper low across the central Great Basin will foster the development of heavy rainfall over the Northern Rockies into the Dakotas. The 00Z GFS/12Z ECMWF show 24-hour totals possibly in the 2 to 4 inch range given slow progression of precipitation elements. This signature slowly drags eastward into the Upper Midwest by the weekend. Excessive heat will dominate the forecast throughout much of next week. While locations over 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. 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