Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1157 AM EDT Mon Sep 9 2019 Valid 12Z Thu Sep 12 2019 - 12Z Mon Sep 16 2019 16Z Update: There is overall good model agreement across the continental U.S. for the end of the week. Some of the EC ensemble members are supporting a deeper upper trough over the Upper Midwest on Friday, and some of the CMC ensemble members are indicating a stronger trough near the Pacific Northwest. Going into the weekend, both the 00Z and 6Z runs of the GFS begin to differ quite a bit from the model consensus, and then the GFS becomes completely out of phase with the upper level pattern and considered an outlier solution when compared to the ensemble plots, even when compared to GEFS ensemble members. These differences emerge from flow separation in association with an upper level trough tracking east from Asia and across the northern Pacific early in the forecast period. The GFS develops an upper level low that becomes detached from the westerlies, whilst the CMC/ECMWF/ensembles are more progressive. This results in an upper level trough centered near the West Coast, in stark contrast to the ridge depicted by the GFS. Similar to the previous forecast, the WPC medium range forecast is derived primarily from a deterministic blend of the ECMWF/CMC/UKMET through early Saturday, and then more of the GEFS and EC means by Sunday and Monday with no GFS used. Hamrick ...Overview and Guidance Evaluation/Preferences... An upper-level ridge is forecast to remain rather entrenched from the lower Mississippi Valley to the Southeast through much of the medium range. Meanwhile, a stream of relatively vigorous upper shortwaves are forecast to traverse the CONUS northern tier. Farther south, an easterly wave is forecast to approach Florida and the coastal southeastern U.S. by the weekend, traversing the southern periphery of the expansive upper ridge. A blend of the 12Z ECMWF/CMC/18Z GFS was used for the forecast initially during days 3-4 (Thu-Fri). An energetic upper-level shortwave and surface low pressure system crossing the north central U.S. were handled comparably by these solutions (particularly the ECMWF and GFS) and a blend should work quite well. Farther west, a leading shortwave off the Pacific Northwest Thu, and a larger amplifying upper trough in its wake continued to pose the most significant forecast problems this cycle. The problems begin as soon as day 4 when the GFS starts to become faster than consensus with the leading wave, and the differences only grow into day 5 (Sat) as the GFS is much more amplified than other guidance with the shortwave across the northern Rockies. In the wake of the leading wave, the ECMWF/CMC both amplify a fairly deep upper-level trough, bringing significant height falls into the Pacific Northwest by next weekend, while the GFS keeps zonal flow in place - a major difference. Given a relatively amplified flow regime in place farther west across the North Pacific, continue to prefer the deeper trough along the U.S. West Coast by days 6-7 as shown by the ECMWF/CMC. Interestingly, the ECMWF had previously shown a solution more like the GFS but has trended more toward the CMC (which has been consistently more amplified) in recent runs. Thus, the forecast during days 5-7 (Sat-Mon) was based on a blend of the 12Z ECMWF/CMC and gradually increasing ensemble means (ECENS/NAEFS) through time to account for increasing spread. ...Weather Highlights/Threats... Moderate to locally heavy rain will be possible across portions of the Upper Midwest Thu-Thu night in association with a surface low pressure system crossing the region. Farther south, scattered showers and thunderstorms may produce locally heavy rains along the cold front Thu-Fri from the Southern Plains to the Mid-Mississippi Valley. Scattered showers and storms will move to the East Coast states by next weekend along with the cold front. The easterly wave approaching the Southeast and Gulf Coast states by late in the week is expected to increase shower and thunderstorm activity, with locally heavy rains possible. Increasing rain, potentially moderate to heavy, will also be possible across the mountain ranges of the Pacific Northwest by late week as upper troughing progresses into the region. Well above average temperatures are expected to persist across much of the central and eastern U.S. through late this week. High temperatures are forecast to be 5 to 15 degrees F above average across a large area from the Ohio Valley to the lower Mississippi Valley and the Southeast. Several record high temperatures are possible each day, especially from the lower Mississippi Valley to the Southeast where highs may soar well into the 90s, perhaps approaching 100 degrees at some locations. 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 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