Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 AM EDT Sat Jun 08 2019 Valid 12Z Tue Jun 11 2019 - 12Z Sat Jun 15 2019 ...Heavy rainfall and flash flood/flooding threats possible over parts of the Appalachians/East Coast states Tue-Thu... ...Overview and Guidance/Predictability Assessment... From Tue into Thu expect a strong ridge moving into the West to promote downstream trough amplification into the east-central U.S. Incoming Pacific energy (in both northern and southern streams) will rapidly erode the western ridge and produce a diffuse trough over the West by Fri-Sat while northeastward progression of the eastern trough should ultimately yield flat mean flow over the East by the start of next Sat. The forecast maintains better than average continuity as guidance displays similar and consistent ideas for evolution of mean flow during the period, providing good confidence in the large scale evolution. However detail uncertainties become gradually more noticeable with time so some specifics will take added time to be resolved. The relative agreement of models/means still favors a consensus approach with a blend consisting of mostly operational guidance early and then trending toward about half models/half means to tone down lower confidence details. During the first half of the period the primary consideration was to remove the 12Z UKMET from the blend after day 3 Tue as it became slower than other solutions for the trough amplifying into the Mississippi Valley. The new 00Z UKMET looks more reasonable and remaining 00Z guidance conforms well to established consensus. By Thu-Fri there is still the unresolved question of whether the amplifying trough energy may at least briefly close off an embedded low center. Latest operational models seem to be hinting at increased potential for some separation and a possible upper low (actual or implied, depending on the height contour used) whose most likely track would be from the Great Lakes into eastern Canada. The diffuse upper trough moving into the West has good agreement/continuity in principle. Confidence is rather low in specifics though. Operational runs show separate pieces of energy within different streams plus meaningful differences in amplitude and timing of each feature. Shortwaves of this scale tend to have low predictability several days out in time--favoring a blended/mean approach until details become more refined. ...Sensible Weather Highlights/Threats... Specifics are not fully resolved yet but there is still a decent signal for a heavy rainfall threat over areas from the Southeast/southern Mid-Atlantic northeastward. The trailing part of a frontal system reaching the East Coast on Tue will stall into Wed and then a rainfall-focusing wave over the Southeast will lift northeastward near the coast in advance of the complex surface system associated with the upper trough amplifying into the east-central U.S. by early Thu. Best potential for highest totals currently exists over the Carolinas. The aforementioned trailing system may bring some locally enhanced activity from the northern Plains into the Great Lakes. Moisture will persist over Florida into the weekend but the rest of the eastern states should see a drying trend by Fri-Sat. Shortwave energy moving into the West will bring scattered showers/thunderstorms of varying intensity primarily into the northern/central parts of the region from Wed or Thu into the weekend. Currently expect highest totals over and near the northern half of the Rockies. Shower/storm potential should expand into the Plains during the late week/weekend time frame but with low confidence for timing/location of greatest rainfall. The forecast remains on track for the brief episode of very warm to hot conditions over the West mainly Tue-Wed in association with the upper ridge moving in from the Pacific. Most guidance continues to show a broad area of plus 10-20F anomalies with locations between and including Portland/Medford having the best potential to see highs 20-25F or so above normal. Daily records for highs/warm lows will be possible over the West Coast states as temperatures reach levels that will likely be the hottest temperatures so far this year. Readings should moderate later in the week but somewhat above normal readings may persist over the Northwest into Sat. On the other hand the pattern will favor below normal temperatures from the Plains into the East for multiple days. The central U.S. will be first to see a moderating trend late in the week and then rapidly flattening flow aloft will support temperatures closer to normal over the East next weekend. 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