Extended Forecast Discussion NWS Weather Prediction Center College Park MD 249 PM EDT Sat Jun 13 2020 Valid 12Z Tue Jun 16 2020 - 12Z Sat Jun 20 2020 ...Heavy rain possible over parts of the Carolinas and Mid-Atlantic next week... ...Overview and Weather/Hazard Highlights... At least before the 12Z model cycle, latest guidance continued to agree well for the large scale pattern evolution while medium to smaller scale differences and variability tempered confidence in specifics over some regions. Consensus maintains the idea that an upper low will settle over or near the Carolinas for most of the Tue-Thu time frame and possibly begin to weaken or lift out by late week or next weekend. This feature should have minimal incentive to move as it is embedded within a broader area of mean ridging between the Plains and the Atlantic Ocean. Models/ensembles are still struggling with the rainfall details--due in part to uncertainty over exact position of the upper low as well as small-scale influences that may not be resolved more than a day or so out in time. Confidence remains lower than desired but a slow moving upper low in the warm season always requires close attention for heavy rainfall potential, in this case over the Carolinas/Mid-Atlantic and perhaps into parts of the Appalachians. Meanwhile an upper ridge over northwestern Canada (likely closing off a high for a brief time around Tue-Wed) will initially support a fairly deep western U.S. trough. One significant bundle of energy will pass through/eject from the core of the trough early in the period, bringing some precipitation through the Northwest/northern Rockies as well as supporting southern Canada low pressure with a trailing front crossing the Northern Plains. Trailing flow should promote residual Northwest troughing into Thu or Fri (and continued precip chances over the northern Rockies and vicinity), followed by gradual progression and/or weakening as a modest ridge builds into the Pacific Northwest. Expect any heavier rainfall with the Northern Plains front to be fairly localized into midweek given modest low level moisture advection and relatively high heights aloft. After midweek the northern part of the Plains front should continue eastward while the trailing part of the front stalls over the Central Plains and possibly Midwest. This evolution along with moderately improving low level flow of moisture may increase the potential for higher rainfall amounts between the northern half of the High Plains (where easterly upslope flow north of the front may enhance activity) and the Midwest/Upper Mississippi Valley later this week into the weekend. The shortwave energy beginning to exit the Northwest during this time may also play a meaningful role in generating rainfall. Over recent days guidance has varied with specifics of energy aloft and exact southeastward progression of the surface front. Therefore it is a little early to pinpoint the most favored areas for highest rainfall totals during Thu-Sat. 12Z model changes for some aspects of northern U.S./southern Canada flow add to the uncertainty in the forecast. Ahead of the front approaching the Northern Plains early in the period, areas from the northern half to two-thirds of the Plains into the Upper Mississippi Valley will see well above normal max/min temperatures Tue-Wed with plus 10-20F anomalies. Some locations within this area could see record warm lows on Wed if rainfall does not generate further cooling later in the day. After Wed expect the warmth to focus over the Great Lakes and Northeast with plus 5-15F anomalies. Cool air over the Northwest on Tue (highs up to 10-15F below normal) will move east into the Northern Plains with time. Anomalies may moderate a bit but northern High Plains rainfall could still keep highs 10F or so below normal over some locations. Meanwhile the Pacific Northwest will see a warming trend by Fri-Sat as the modest ridge aloft builds into the region. Some locations could see highs at least 10F above normal by Sat. Finally, the upper low over the Carolinas may lead to highs up to 10-15F below normal in its vicinity through midweek. Gradual weakening of the low should allow temperatures to toward closer to normal by Fri-Sat. ...Guidance/Predictability Assessment... Based on the 00Z/06Z guidance the updated forecast started with a model/mean blend that placed primary emphasis on operational runs early in the period and tilted slightly more to the 06Z GEFS/00Z ECMWF means by day 7 Sat. The blend somewhat underweighted the 06Z GFS versus average as it was on the amplified side of the spread for the upper trough initially over the West and showed more of a northwestward drift versus most other guidance for the eastern upper low, especially in the Wed-Thu time frame. It also excluded the 00Z CMC as that model was on the eastern side of the spread for the upper low. Various aspects of Canadian and northern tier U.S. flow remain in flux. Through the 00Z/06Z cycles guidance had developed a 12-24 hour oscillation regarding amplitude of flow aloft emerging from the Northwest, with the 00Z/06Z cycles on average somewhat less amplified than yesterday--thus leading to somewhat less southeastward progression of the leading cold front. This follows an opposite trend observed yesterday. There are also important detail differences for troughing that lingers over the Northwest for a time. Some 12Z guidance has raised the uncertainty to another level, altering details of the northwestern Canada ridge with wide-ranging ramifications across Canada and even into the lower 48 by late week. These include aspects of Northwest U.S. trough energy and ejection/progression plus the leading surface front--and in the case of the UKMET, keeping enough of an upper ridge north of the eastern U.S. upper low to bring the low's path close to the latest GFS runs. ECMWF changes do not appear to have a dramatic effect on the eastern upper low but the 12Z run is significantly deeper with the associated East Coast surface wave. 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