Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1139 AM EDT Sun Jun 24 2018 Valid 12Z Wed Jun 27 2018 - 12Z Sun Jul 01 2018 ...Overview... Latest guidance continues to show the mean pattern transitioning toward an amplified western ridge/eastern trough configuration. A notable trend over the past day has been toward greater northward amplitude of the eastern ridge by next weekend, with the center of the embedded upper high possibly drifting at least as far north as 40N latitude. This evolution would direct highest temperature anomalies originating from the central Plains/mid-upper MS Valley into the Northeast. Under the base of the ridge, weather should be unsettled across FL. The primary uncertainty with the western trough aloft is for its depth and the details of individual shortwaves. The latter will affect the forecast for what may be a convectively active period across the Northern Tier states. ...Model discussion/preferences... There are two issues in the forecast. One has to do with the longwave trough in the West and how quickly it ejects. After Friday, the 00z Canadian and 00z ECMWF eject it out quicker bringing a strong low into southern Canada at a faster clip than the other guidance. Normally, the guidance is too quick to eject the longwave trough and any shortwaves moving around its periphery tend to be weak. Since the ECMWF is quicker than its own mean, discounted it for next weekend. The other issue is the possibility for a baroclinically-initiated warm core/convective low near/offshore the Southeast coast. Most of the guidance brings convectively-induced shortwaves out of the southern Plains through the Mid-Mississippi Valley and Carolinas during the short range period, causing a low to form in the Carolinas along a weakening frontal boundary which moves offshore by Wednesday morning. The 00z ECMWF was quite aggressive with this system and strengthens it to hurricane strength by next Sunday -- likely too strong. Since there is a building ridge across the northern Mid-Atlantic states in all the guidance, whatever forms offshore the Southeast would move southwest towards/over FL sooner or later, and stuck with clustering within the southwest side of the 00z global ensemble guidance here which resulted in a slower westward movement than shown by yesterday/overnight WPC continuity across FL. At one point, 35 of the 90 ECMWF/Canadian/GFS 00z members have the low, so it can't be discounted. A couple similar developments (Clara 1977 & Claudette 1985) formed as a result of convective complexes moving off the Southeast coast; such a development is possible. For Wednesday into Friday, a compromise of the 06z GFS, 00z ECMWF, 00z Canadian, and 00z UKMET works out fine. Thereafter, phased out the use of the 00z ECMWF and 00z Canadian and replaced them with the 00z NAEFS and 00z ECMWF ensemble mean solutions. Used 00z global ensemble member clustering to place the low offshore the Southeast and track it into northern FL by next Sunday. Overall, this maintained reasonable continuity. ...Weather threats/highlights... The system initially over the Great Lakes will likely support some areas of moderate to heavy rainfall early in the period, most likely from eastern Great Lakes/upper Ohio Valley into western-northern New England. As the upper pattern becomes more amplified and a mean frontal boundary/low pressure persist over the Plains and vicinity, there will be the potential for multiple convective episodes and accompanying heavy rainfall from over or just east of the northern Rockies into the upper MS Valley/upper Great Lakes. Specifics will depend on less predictable details of shortwaves ejecting from the western trough aloft. Currently the best signal for highest 5-day rainfall totals exists over the upper MS Valley/upper Great Lakes. With lighter amounts, the western upper trough will bring scattered rainfall to other areas within the northern half of the region. About 20 degrees south of the amplifying mid-level positive height anomaly, the Southeast/Florida will see areas of locally heavy convection during the period, with enhancement expected from the retrograding surface low/trough forecast to move in from offshore the Southeast coast. An expanding area of hot/humid conditions will likely cover a large portion of the central-eastern U.S. by late this week and weekend. Currently expect the axis of highest min/max temperature anomalies of plus 10-20F to extend from the central Plains through the Great Lakes/Ohio Valley and into the Northeast. Daily maximum records appear to be more likely for warm lows than daytime highs, though a smattering of record highs are forecast across the northern Mid-Atlantic states and portions of New England. Increased low-level moisture will also lead to very high heat index values over many areas. Otherwise, the upper trough amplifying into the West will bring a pronounced cooling trend. Locations in the northern Rockies/High Plains should see highs as much as 10-15F below normal by Fri-Sun with less extreme anomalies elsewhere. Roth/Rausch WPC medium range forecasts of 500 mb heights, surface systems, weather grids, quantitative precipitation, and winter weather outlook probabilities are 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