Extended Forecast Discussion NWS Weather Prediction Center College Park MD 253 PM EDT Thu Jun 11 2020 Valid 12Z Sun Jun 14 2020 - 12Z Thu Jun 18 2020 ...Heavy rain threat over parts of the Mid-Atlantic/central Appalachians and eastern Carolinas... ...Overview... Most guidance expects the flow aloft to settle into a persistent pattern during the Sun-Thu time frame. An upper low forming and then remaining over the East will increase the potential for significant rainfall totals in its vicinity. At the same time mean troughing should be anchored over British Columbia and northwestern U.S., leading to periods of unsettled weather from the Northwest into the Northern Plains. Embedded features will likely support two separate frontal systems crossing this part of the country. In-between expect a ridge aloft to prevail over northern Mexico and portions of the Plains, leading to very warm to hot conditions over the central U.S. for most of the period. ...Guidance/Predictability Assessment... The 00Z/06Z operational models and ensemble means came into better agreement for the eastern U.S. upper low versus what was seen in some previous cycles. Overall the ECMWF/ECMWF mean have been significantly more consistent over the past couple days, keeping the upper low center inland for multiple days. Some GFS/GEFS mean runs had been more progressive. Through at least Tue the 00Z UKMET/CMC were close to consensus though the CMC became a tad faster than consensus late in the period. Within the Northwest upper trough there is good agreement on ejection of a leading upper low/shortwave from the Northwest Sun-Tue. Guidance differs more for the specifics of upstream flow Tue-Thu. The majority cluster showed a Northeast Pacific upper low tracking inland close to the U.S.-Canadian border and an upper trough lingering over the Northwest through Thu--one question being whether the lingering trough will be from the upper low itself or the trailing flow after the upper low ejects. The 00Z CMC kept the upper low center offshore British Columbia but its mean compared better to the other models/means. Based on the guidance comparison the updated forecast started with an operational model blend for days 3-4 Sun-Mon. The days 5-7 Tue-Thu part of the forecast eliminated the CMC and gradually increased 06Z GEFS/00Z ECMWF mean input such that the models and means had equal weight by day 7 Thu. 12Z guidance as a whole nudges confidence down a bit over the East. The 12Z ECMWF/CMC have adjusted somewhat southeast of their previous runs for the upper low, though the GFS/UKMET and GEFS/CMC means have stayed close to the 00Z/06Z consensus. The 12Z CMC adjusted favorably with the mid-late period pattern over the eastern Pacific/western North America while the GFS is in the minority with its stalled upper low near the Pacific Northwest coast next Thu. ...Weather/Hazard Highlights... Some guidance is still showing meaningful spread and variability but there is gradually increasing potential for a heavy rainfall event somewhere within an area extending from the central Appalachians through Mid-Atlantic to the eastern Carolinas. Typical guidance error for forecasts several days out in time--as well as difficulty in resolving important smaller-scale details that will determine where heavier rainfall bands as well as training/repeat activity set up--keep the confidence in specifics fairly low at this time. Localized maximum five-day totals of 5-10 inches may be possible depending on the exact path of the upper low. At the same time the clouds/rainfall will keep high temperatures around 5-12F below normal over portions of the East. Readings should drift closer to normal by next Wed-Thu. Mean troughing aloft over the Northwest and embedded features ejecting downstream will lead to an unsettled pattern from the Northwest through the Northern Plains. A leading front should reach the Northern Plains Sun-Mon and then dissipate by Tue, followed by another Pacific front that comes ashore early in the week and continues across the northern tier. The best signals for highest five-day totals exist over the Olympics and northern Washington Cascades. Some guidance shows potential for locally heavy convection over/near the Northern Plains. Convection could contain pockets of intense rainfall but confidence in the heavy rainfall totals is lower given that activity may be fairly progressive and a surface high pressure axis from the Great Lakes into the Southern Plains/Gulf Coast should minimize any low level flow of Gulf moisture into the Plains at least into midweek. The Northwest will likely see below normal highs for most of the period. Coolest readings should be over and near the northern Rockies on Sun and to some degree Mon-Tue with some minus 10-15F anomalies. A majority of the Plains will see above normal temperatures Sun-Thu, with central areas most likely to see highs reach up to 10-15F above normal. Across northern areas the emphasis for warmth/heat should gradually adjust eastward with time, reaching the Upper Mississippi Valley by Wed-Thu and possibly extending through parts of the Great Lakes and New England as well--with some anomalies reaching plus 10-15F as well. The above normal temperatures over the central Plains in particular will be notable for their persistence but should remain below daily record values. 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