Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1200 PM EDT Fri Oct 26 2018 Valid 12Z Mon Oct 29 2018 - 12Z Fri Nov 02 2018 ...Pattern Overview... From the large scale perspective a moderately progressive flow pattern across the U.S. early in the medium range period will become increasingly amplified and slower by the middle of next week. Guidance agrees that the upper ridge which has been prevalent off the West Coast will rebuild with a broad and amplified mean trough setting up over the central U.S. This flow pattern will set the stage for transport of chilly air southward into the central U.S. while wet conditions develop on the eastern side of the evolving trough, especially from the lower Mississippi Valley to the Ohio Valley. ...Guidance/Predictability Assessment... For about the first half of the period there is reasonable consensus among the guidance regarding significant features, with some typical detail/timing differences that are best resolved by an operational model blend. Guidance has not fully locked in though, as the latest cycle of guidance seems to be furthering the recent faster trend for the upper trough crossing southern Canada and northern U.S. east of the Rockies--along with the leading Canadian surface low pressure and trailing front crossing the central U.S. This trend provides added push for the initial East Coast upper trough and Northeast low pressure tracking into the Canadian Maritimes. During the mid-late week time frame, the evolution of shortwave energy reaching the vicinity of the Rockies/Plains by Wed becomes a major uncertainty in the forecast. Specifics of this energy--whose scale lends itself to moderate to lower predictability by 6-7 days out in time--will determine the timing/track/strength of a surface wave that may lift northeastward from the southern Plains/Mississippi Valley. As a whole the 00Z/06Z operational model solutions display somewhat faster timing of such a wave than the ensemble means. Recent faster trends for some systems provide support for including a component of the operational model ideas, but individual ensemble members fall within a very broad envelope for possible surface wave location with minimal clustering by days 6-7 Thu-Fri. Thus it is also prudent to incorporate half or more weight of the 06Z GEFS/00Z ECMWF means that are similar to each other in principle. As the axis of the upstream ridge aloft nears the West Coast by day 7 Fri the other significant late-period uncertainty emerges, regarding the character of approaching northern Pacific shortwave energy. Ensemble spaghetti plots become fairly chaotic by that time and individual model runs have varied depictions of timing/amplitude of the shortwave energy. At the very least, strength of the ridge leading into Fri would seem to favor leaning away from the more amplified side of the envelope for the shortwave. A blend of the ensemble means and modest weight of any individual model run would provide a good starting point for maintaining a fairly strong ridge until there is overwhelming model/ensemble agreement that argues otherwise. ...Weather Highlights/Threats... The system departing from New England after early next week will bring mostly light precipitation to portions of the Northeast. Then the focus will be on increasingly wet conditions expected to develop over portions of the eastern half of the lower 48 mid-late week as mean troughing aloft becomes established over the central U.S. This pattern will favor northward transport of Gulf of Mexico moisture and instability where it will interact with a surface frontal boundary and one or more embedded waves. Overall the latest model/ensemble guidance highlights best potential for highest rainfall totals from the lower half of the Mississippi Valley into the Ohio Valley with multi-inch rainfall totals possible over some areas. Locations from the Pacific Northwest through the northern and central/south-central Rockies will see multiple rounds of precipitation (rain and high elevation snow) as weakening shortwaves/frontal systems traverse the northern periphery of the Pacific ridge. Within this area there is a fairly good guidance signal toward highest amounts being over favored terrain from the Pacific Northwest into northern Rockies. Some focused activity is likely farther southeast but with somewhat lower 5-day totals. Pattern evolution aloft will lead to an area of above average temperatures progressing from the west-central U.S. early in the week to the eastern states by mid-late week. Eastern U.S. warmth will be confined more to min temperatures as low level moist flow becomes established. Some anomalies may reach into the plus 10-20F range for min/max readings early in the week and for mins over the East during the latter half of the week. The West will see a period of modestly below normal readings before the approaching ridge aloft brings a return of above normal temperatures to the West Coast States. Below normal temperatures with some minus 10-15F anomalies will extend from the central-southern High Plains into east-central U.S. mid-late week. Rausch/Ryan WPC medium range 500 mb heights, surface systems, weather grids, quantitative precipitation, winter weather outlook probabilities and heat indexes are found 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