Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 AM EDT Sun Jun 09 2019 Valid 12Z Wed Jun 12 2019 - 12Z Sun Jun 16 2019 ...Heavy rainfall threat near the East Coast Wed-Thu but with greater uncertainty for specifics... ...Overview and Guidance/Predictability Assessment... Latest models and ensemble guidance agree on the general trend toward lower amplitude mean flow aloft over the Lower 48 after a deep trough amplifies into the east-central U.S. and then ejects northeastward while an initial ridge over the West collapses. However compared to the past couple days individual solutions have become noticeably more divergent--not only for details within the overall eastern trough but also for the flow from the eastern Pacific into the western-central U.S. There are multiple detail uncertainties within the larger scale eastern North American trough from midweek onward. Latest GFS runs are deepest with the core of the trough crossing the Great Lakes (leading to some very low standardized height anomalies) plus are quite aggressive with Hudson Bay/Ontario energy that trails the Great Lakes system. GEFS mean runs hint at the GFS scenario but essentially merge the two separate GFS features. By the latter half of the period the end result in the GFS is a much more suppressed surface pattern to the east of the Rockies versus established consensus. Thus far the GEFS mean has been much closer to the other ensemble means and recent ECMWF runs/12Z CMC. The new 00Z CMC holds somewhat more energy on the back side of the trough, and while it is much less extreme than the GFS aloft it is similar in bringing strong surface high pressure into the Great Lakes next weekend. This is in contrast to remaining solutions that show much lower surface pressures. Finally the new 00Z UKMET also shows potential for northwestern Canada flow to play a role in the forecast. Early in the period a multi-model blend (with the 18Z FV3 GFS replacing a portion of the operational run for the GFS component) provides good continuity with an intermediate solution while awaiting better detail agreement. Later in the forecast a model/mean blend best represents the majority (non-GFS/00Z CMC) consensus. Farther westward the models are still finding it a challenge to resolve multiple northern and southern stream impulses that will move into/across the West and likely into the Plains, leading to a general area of low pressure emerging over the central U.S. by the weekend and increasing convective potential. In addition by the latter half of the period guidance diverges for timing of energy on the leading side of a broad upper low that tracks just south of the Aleutians. ECMWF runs have been trending faster with the leading height falls over the past couple days and ECMWF mean runs have been consistent with timing not much slower than the 12Z ECMWF. On the other hand GFS/GEFS mean runs have been stable with holding the upper ridge axis off the Pacific Northwest coast into Sun. The 12Z CMC/CMC mean were a compromise while the new 00Z CMC has adjusted closer to the GFS. For a single deterministic forecast prefer to trend the early-period model blend toward a model/mean composite that keeps incoming Pacific flow fairly flat around early next Sun. The ECMWF/ECMWF mean may be a little less likely at that time but still possible from a multi-day mean perspective. ...Sensible Weather Highlights/Threats... During Wed-Thu there is still the potential for some areas along the East Coast to see heavy rainfall as the amplifying upper trough over the east-central U.S. helps to pull a Southeast surface wave and associated frontal system northward into or near New England. Guidance as a whole appears to have reduced some of the highest amounts previously forecast, though slightly greater amplitude with upper troughing in recent GFS runs versus other guidance may be diverting too much of its rainfall just offshore. Meanwhile a fairly narrow band of enhanced rainfall may extend through the Upper Midwest/UP of Michigan, on the northwest side of Midwest-Great Lakes low pressure. Florida will see periods of rainfall from midweek onward with a combination of diurnal activity and added focus from a front settling into the area. Expect scattered showers/thunderstorms over northern-central parts of the West to be aided by the multiple shortwaves reaching/crossing the region after midweek. Convection should also become more common over the central U.S. with low pressure and frontal system in some form likely providing added focus by late week or weekend. Poor model agreement for shortwave details aloft keeps confidence in specifics low, but at the moment there is some relative agreement for greater rainfall potential over parts of the southern half of the High Plains during early Wed-early Fri and centered over Kansas/Missouri early Fri-early Sun. Expect very warm/hot temperatures over the West on Wed with locally plus 20-25F anomalies over the Pacific Northwest and possible record highs/warm lows. The warmth may extend into Thu over the interior Northwest locations. For the rest of the period above normal temperatures should linger but with only scattered pockets of plus 10-15F anomalies for highs. Below normal temperatures to the east of the Rockies mid-late week will give way to warmer readings from west to east with a large part of the central/eastern U.S. likely to see highs fairly close to normal 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