Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1157 AM EDT Mon Sep 17 2018 Valid 12Z Thu Sep 20 2018 - 12Z Mon Sep 24 2018 ...Tropical moisture forecast to lift into portions of the Southwest later this week... ...Pattern Overview... Upper ridging near the Aleutians and in the Southeast CONUS will favor positively tilted and reinforced troughing centered near the Pacific Northwest coast this weekend. A lead storm system will lift into Southeastern Canada late this week and settle its cold front into the Mid-Atlantic and Ohio Valley as a western front pushes through the interior. The National Hurricane Center is monitoring an area of disturbed weather south of Baja California that has the potential for tropical development. This system (named or not) will lift northward and spread rainfall into portions of AZ/NM later this week. ...Guidance/Predictability Assessment... The overall pattern remains synoptically stuck as the anomaly centers meander in the same general area. Agreement among the latest ensembles remain at least average with expected timing/amplitude differences. Followed a deterministic blend to start, with a preference toward the 00Z parallel GFS vs the operational 00Z/06Z runs due to a seemingly overdone sfc low across southern Canada on Friday. By late this weekend into next week, the ensembles clustered slower to lower heights across the interior west (eastern WA/OR into ID) vs the deterministic ECMWF/Canadian/GFS but the predictability of the northern/southern emphasis in heights falls due to the positive tilt of the trough is low. Thus, opted to split the difference between the deterministic models and the ensemble means by next Monday. To the south, any potential tropical development will be short-lived but the moisture associated with the system will traverse NW Mexico and push through AZ/NM later this week. Fracasso ...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