Extended Forecast Discussion NWS Weather Prediction Center College Park MD 145 AM EDT Thu Oct 18 2018 Valid 12Z Sun Oct 21 2018 - 12Z Thu Oct 25 2018 ...Pattern Overview... Ensembles continue to advertise a slow release from the meridional flow as the positive/negative height anomaly centers surrounding North America seem to shift/weaken in unison. This will likely lead to a period of decreased relative forecast confidence late next week as a new pattern tries to take hold. Until then, troughing in the east will be reinforced early next week but then lift out as the western ridge gets shunted eastward to the middle of the continent. ...Guidance/Predictability Assessment... Ongoing excellent model/ensemble agreement in the first half of the medium range allowed a deterministic blend to serve as the starting point. Forecast frontal timing/track/position has not changed substantially for a few days as one front escapes through the western Atlantic Sunday and a Canadian system dips through the Lakes Mon/Tue. Also by then, a Pacific front will push into Washington and British Columbia. Thereafter, models and even the ensembles diverge in how the pattern breaks down. The ECMWF ensembles have been uncharacteristically quick to break down the western ridge (prefer the slightly slower timing of the 18Z GFS and 12Z ECMWF/Canadian) but they have been slower slower/deeper with the eastern trough next Wed/Thu than the GEFS. Opted to split the difference given the uncertainty in timing and conflicting signals. Did not prefer any one deterministic model by next Thursday and relied mostly on the ensemble consensus. ...Weather Highlights/Threats... Temperatures will generally be above average in the Northwest but will trend cooler (and east) as ridging weakens and moves east as well. The rest of the lower 48 will see cooler than average temperatures, especially the Northeastern 1/4 of the CONUS (more typical of early November on some days). Precipitation will focus along the Texas coast as a frontal boundary lingers in the Gulf and precipitable water values remain high -- around +2 sigma (between 1.75-2") along an inverted surface trough. 5-day totals will likely exceed several inches in some southern Texas areas. A weak upper low drifting out of California early in the week will spread some generally light rain through the Southwest/Four Corners region through the period. Fracasso 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