Extended Forecast Discussion NWS Weather Prediction Center College Park MD 258 AM EDT Fri Jun 07 2019 Valid 12Z Mon Jun 10 2019 - 12Z Fri Jun 14 2019 ...One or more episodes of excessive rainfall and flash flood/flooding threats possible over parts of the Appalachians/East Coast states next week... ...Overview and Guidance/Predictability Assessment... During Mon-Wed guidance agrees in principle that initially weak Pacific shortwave energy reaching western North America by early Mon will steadily amplify toward the east-central U.S. as a ridge builds into the western part of the continent. This evolution will occur behind a couple leading features (a Southeast U.S./eastern Gulf trough and separate northern stream shortwave). The overall combination will lead to one or more periods of enhanced rainfall over some areas of the East. By Thu-Fri details aloft over much of the Lower 48 become increasingly uncertain but most models/ensembles show a general theme of diffuse shortwave energy moving into the West while the amplified eastern trough that persists into Thu becomes more shallow on Fri. From day 3 Mon into day 5 Wed an operational model blend (using the 18Z GFS/12Z ECMWF and lesser weights of the 12Z UKMET/CMC) represents the synoptic consensus well, while yielding an intermediate solution for important smaller scale differences that are within typical error ranges--and have below average predictability--for the time frame of interest. In particular some of these differences affect the location/intensity/duration of heavy rainfall over the East. Broadly speaking most models and ensemble means have shown fairly stable evolutions over the past 24-36 hours with no pronounced trends evident. During the latter half of the period there are multiple detail questions that arise: whether the sharpening trough energy reaching the Mississippi Valley around midweek closes off a low for some period of time (a solution somewhat in the minority thus far), the evolution of energy over Ontario (the 12Z ECMWF and to a lesser degree ECMWF mean more amplified), and the particulars of multiple pieces of energy that will comprise the overall diffuse shortwave trough moving into the West. Adding 18Z GEFS/12Z ECMWF mean guidance to the blend, reaching nearly half weight relative to the operational models by day 7 Fri, helps to downplay the less confident details within the more agreeable large scale evolution. During this time frame the ECMWF component also includes the prior 00Z run as its eastern solution becomes more compatible with consensus than the 12Z run. ...Sensible Weather Highlights/Threats... Expect highest rainfall totals for the full Mon-Fri period to be over an area encompassing the eastern Great Lakes/Appalachians/East Coast states. Early in the week a north-south front heading into the East and wavy front over the Southeast will provide focus for rainfall. The trailing front and embedded wave(s) will likely stall over the Southeast Tue-Wed and finally be dislodged as the upper trough amplifying into the East brings another surface low/frontal system into and through the region. Over the central third of the U.S. there will likely be areas of showers/storms with some moderate to heavy amounts but on a more localized scale. Low level upslope flow may enhance rainfall over the southern High Plains early in the week and a frontal system/low pressure may bring rainfall to portions of the northern half of the Plains and points east from Mon night or Tue through midweek. The shortwave energy moving into the West late in the week may promote scattered convection with some activity possibly reaching the Plains as well. The upper ridge building into the West early-mid week will bring very warm to hot temperatures to the region, with fairly broad coverage of plus 10F and greater anomalies Mon-Wed. Readings will begin to moderate later in the week as heights aloft decline. Expect highest temperatures Tue-Wed with locations along the Medford-Portland corridor seeing the best potential for highs 20F or more above normal. Daily records for highs/warm lows will be possible over the West Coast states. Meanwhile the pattern will tend to favor below normal temperatures over most of the central U.S. aside from a possible trend toward normal by Fri. Clouds/rainfall should produce a decent area of minus 10-20F anomalies for highs over the southern High Plains on Mon. The East will see above normal mins early in the week but otherwise should see near to below normal min/max readings as well. 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