Extended Forecast Discussion NWS Weather Prediction Center College Park MD 321 AM EDT Mon Sep 17 2018 Valid 12Z Thu Sep 20 2018 - 12Z Mon Sep 24 2018 ...Pattern Overview/Guidance/Predictability Assessment... While the extratropical remains of Florence continue to accelerate into the Canadian Maritimes, the focus shifts toward a band of height falls sweeping through the north-central U.S. on Thursday morning. An accompanying wave of low pressure should lift from the Middle Mississippi Valley toward the Upper Great Lakes on Thursday evening where considerable deepening is likely to take place. Further strengthening is expected while the system moves toward western Quebec on Friday. The attendant cold front should make considerable progression eastward but should struggle to get south of the Mid-Atlantic region given the stout upper ridge toward the southeastern U.S. Back toward western North America, a pair of upper troughs are primed to affect the region during the period. This wave train of systems should traverse from the Gulf of Alaska while a broad positive height anomaly sits upstream. The initial closed low is forecast to sweep through the Pacific Northwest early Friday before adjoining the northern stream flow across western Canada. On its heels will be a more large-scale trough which should juxtapose itself somewhere along the Pacific/western U.S. interface during the Day 5-7, September 22-24 period. Elsewhere, an area of disturbed weather across the subtropical Pacific should lift northward toward the southern Baja California peninsula. While tropical development remains possible, the bigger threat will be the associated moisture plume pushing into the Four Corners region. This should augment the potential for heavy rainfall early in the forecast. The models initially exhibit reasonable agreement amongst one another while eventual divergence in the solutions exist beyond Day 4/Friday. Initially, the uncertainty looms around the deepening wave of low pressure exiting the Great Lakes toward eastern Canada. Multi run comparisons have shown a tendency for the ECMWF to be decidedly weaker than the rest of the guidance. Given the dynamic nature of the approaching shortwave, it does appear to be too weak as other models favor deepening to possibly below 990-mb while moving through Quebec. Fortunately, most of the spread does not significantly impact the continental U.S. but it is worth noting. Back to the western U.S., models have exhibited a deepening trend with the upper low approaching the Washington coast on Friday morning. This has resulted in quite the uptick in precipitation potential over western Washington up into British Columbia. While the 18Z GFS/12Z ECMWF initially agreed on timing, the 00Z GFS made a decided westward shift which suggests some timing uncertainties are in play. Shifting focus to the longwave trough during the end of the forecast, the 12Z CMC/CMC ensemble mean appeared to be quick outliers. However, the 00Z GFS actually made a large adjustment on Day 7/September 24 toward that particular solution. Ensemble spaghetti plots show plenty of spread with this scenario lying on the far eastern edge of the pack. Would imagine future operational models will waffle back and forth the next couple of days. Through Day 4/Friday, was able to utilize a combination of operational solutions led by the 18Z GFS/12Z ECMWF with some contributions from the 12Z UKMET mixed in. Thereafter, building spread with the eastern Canada wave coupled with issues regarding the western U.S. trough evolution supported inclusion of ensemble means. Gradually raised percentages from the 12Z ECMWF/NAEFS ensemble means from Day 5/Saturday onward with confidence roughly around average throughout this period. ...Weather Highlights/Hazards... A pair of heavy rainfall threats are in the picture through perhaps Saturday. The first area should concentrate near the dynamic area of low pressure sweeping through the Great Lakes. Secondly, tropical moisture pumping out of the Pacific should enhance the threat for heavy precipitation over the Four Corners region but particularly into the Southern High Plains. The latter area is more model dependent as there are plenty of solutions showing lower QPF values. Eventually the boundary stalling out of the lower Mid-Atlantic could prove to keep conditions wet toward the latter half of the weekend into the following week. Across the Pacific Northwest, the compact upper low should spread a burst of showers to western Washington up into British Columbia on Friday with additional precipitation likely during the following days in response to the next trough. Throughout the forecast, the most persistent area of below average temperatures will reside over the Upper Intermountain West eastward across the Northern Rockies and into the Dakotas. On Thursday, such departures could reach the 10 to 15 degree range as highs struggle to escape the 50s. To the south of the migratory boundary, warm conditions will prevail across the Lower/Middle Mississippi Valleys into the Ohio/Tennessee Valleys. Toward the weekend, anomalies should decrease a bit but that is also an artifact of heavy ensemble usage. However, it should generally remain on the cool side across the north-central U.S. given the influence of Canadian air masses. Rubin-Oster 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