Extended Forecast Discussion NWS Weather Prediction Center College Park MD 440 PM EDT Fri Jun 12 2020 Valid 12Z Mon Jun 15 2020 - 12Z Fri Jun 19 2020 ...Heavy rain possible over parts of the Mid-Atlantic/Carolinas and Appalachians next week... ...Overview and Weather/Hazard Highlights... Persistent upper-level ridging across Alaska and western Canada will favor broad cyclonic flow across the northwestern U.S. along with ridging across the central U.S. (though flattening somewhat by later in the week) and across the North Atlantic Ocean. In-between the central U.S. and Atlantic ridges, guidance shows general consensus that shortwave energy will separate from the westerlies in the form of a cutoff mid/upper-level low across the central-southern Appalachians/Southeast/Mid-Atlantic early next week. This feature may linger over the region through much of the week and has the potential to bring one or more days of heavy rainfall to parts of the Mid-Atlantic/Carolinas and perhaps central and/or southern Appalachians. There is still some spread and variability with important details so confidence remains lower than desired for the specifics of coverage/intensity/duration of any heavy rain that occurs. Farther west a series of shortwaves will eject from the broad Pacific Northwest trough and traverse the northern periphery of the central U.S. ridge, reflecting at the surface as two defined fronts. The first front should weaken across the Northern Plains Mon-Tue. The second front will enter the Northwest early in the week and continue eastward thereafter. East of the Rockies the supporting dynamics with the latter front will likely be stronger, leading to greater southeastward progression into the Plains/Midwest/Great Lakes later in the week. With relatively strong upper ridging in place across the Plains at least through midweek, moisture advection northward into these fronts should be relatively modest and some degree of capping inversion is also likely to be present. Thus QPF amounts with each of these systems across the north central U.S. should remain light-moderate in most cases through midweek. Localized/isolated areas of heavy rainfall will be a possibility though. Late in the week guidance suggests the second front could decelerate or stall over the Plains/Midwest while solutions diverge over how much low level moisture may ultimately interact with the front. Solutions on the heavier side of the rainfall spectrum are currently in the minority but this area will require monitoring. The Northwest will see at least one episode of precipitation, early in the week and with best focus over favored terrain. Specifics of flow aloft will affect precip coverage and amounts later in the week. The central U.S. upper ridge will support very warm to hot conditions from the Central Plains into Upper Midwest early-mid week in particular, with some areas seeing highs and/or morning lows 10-15F or so above normal. During the latter half of the week expect such anomalies to progress across the Great Lakes/New England. Meanwhile cool conditions over the Northwest/northern Rockies early in the week (highs 5-15F below normal) will modify somewhat as the second northern tier front brings a cooling trend into the Plains during the latter half of the week. Clouds and rainfall near the eastern upper low may keep highs up to 10-15F below normal at some locations early in the week. Weakening/possible departure of the upper low should lead to a warming trend by late week. ...Guidance/Predictability Assessment... The 00Z/06Z models and ensembles on average provided a more centered location for the eastern U.S. upper low relative to the various solutions from 24-36 hours ago. A consensus approach looked reasonable given the spread/variability seen thus far and typical extended-range verification. Among the 12Z runs the CMC becomes a little fast with ejecting the feature late in the week. The 12Z GEFS mean/ECMWF wobble a bit to the southern side of the spread into midweek but are within the realm of possibility given multi-day trends. The west/northwest drift of the 12Z ECMWF by Thu presents more of a question mark though. Complexity across the northern tier has increased somewhat, at least partially due to the brief closing of an upper high over northwestern Canada. First there are still detail differences over specifics of initial Northeast Pacific upper low/trough energy that flows inland. Then 00Z/06Z guidance showed increased potential for some combination of this energy, leading shortwave energy north of Montana on Mon, and amplifying central Canadian flow to produce a more amplified southern Canada/northern tier U.S. shortwave Wed-Fri. This trend led to greater southeastward progression of the leading cold front. GFS runs were on the deep side of the spread with the upper trough so preference was to underweight that aspect of the GFS. As was the case yesterday, models and individual ensemble members vary over trough amplitude and form (open wave or compact closed low) over the Northwest late next week. The ensemble means provided a good template to resolve the spread for this part of the forecast. Based on consensus for the eastern upper low and preference to lean away from the more extreme details of northern tier flow, the updated forecast started with an operational model consensus for about the first half of the period and then transitioned toward a half model/half mean weight by day 7 Fri. Opposing specifics in the operational runs over the Northwest yielded the desired solution near the means late in the week. Rausch/Ryan 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 Hazards: - Heavy rain across portions of the Central Plains and the Middle Mississippi Valley, Thu-Fri, Jun 18-Jun 19. - Heavy rain across portions of the Southeast, the Central/Southern Appalachians, and the Mid-Atlantic, Mon-Wed, Jun 15-Jun 17. - Flooding occurring or imminent across portions of the Mississippi Valley, the Northern Plains, the Southeast, the Great Lakes, and the Ohio Valley. - Flooding likely across portions of the Upper/Middle Mississippi Valley, the Northern Plains, and the Northern Rockies. - Much above normal temperatures across portions of the Northern/Central Plains, the Upper/Middle Mississippi Valley, and the Great Lakes, Mon-Thu, Jun 15-Jun 18. 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