Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1200 PM EDT Wed Sep 11 2019 Valid 12Z Sat Sep 14 2019 - 12Z Wed Sep 18 2019 ...Overview and Guidance Evaluation/Preferences... ...15Z update... ...Heavy rain possible from Florida westward to the central Gulf states... A couple of upper-level shortwaves moving eastward across the northern tier states will give way to a more substantial long wave trough settling into the western U.S. during the medium-range period. Meanwhile, an upper-level ridge is forecast to slide off the eastern U.S. as an upper-level low tracks westward across the Gulf of Mexico. This upper low will likely interact with a tropical wave which is forecast to track westward across the Bahamas into Florida during the weekend. The ECMWF has joined the rest of the global models in predicting a surface low developing in the eastern Gulf on Sunday but its track has shifted from a western outlier yesterday to being one of the eastern-most solution. The UKMET is the eastern outlier, showing a track up the east coast of Florida. On the other hand, the GFS and Canadian models show a more consistent track toward the central to eastern Gulf coast before turning northward further inland. The interaction between the upper low and the tropical wave would likely result in a sheared system with limited potentials for rapid intensification. Nevertheless, the threat of heavy rain is becoming more likely from Florida westward to the central Gulf states from the weekend to early next week, possibly spreading northward farther inland into mid-week next week. The WPC morning grid package is a general compromise of the 00Z ECMWF, 00Z EC mean, 06Z GFS, 06Z GEFS, 00Z CMC and its ensemble mean. More weights have been given to the 00Z EC mean for Day 7 to keep a slower solution for the deep trough over western U.S. Kong ...Weather Highlights/Threats... Numerous showers and thunderstorms are expected across Florida and the Gulf Coast states this weekend into early next week in association with the westward moving tropical disturbance. Several cycles of model runs have consistently focused the greatest chance of heavy rainfall along the immediate coastline from Louisiana east to Florida, where several inches of rainfall are possible, spread across multiple days. Moisture from this system may spread farther north into portions of the lower Mississippi Valley by early Mon-Tue. Widespread rains (and perhaps some snow at the highest elevations) are expected to reach the Pacific Northwest by Sat night into Sun as the upper trough begins to amplify. Precipitation should spread inland across the Great Basin and northern Rockies early next week. Southerly flow east of the upper trough may revive monsoonal showers and storms across the Four Corners region, with coverage of precipitation increasing this weekend through early next week. The broad upper ridge across the central and eastern U.S. will result in above average temperatures across a wide area. Highs are forecast to be 5 to 15 degrees for many areas from the Plains to the Eastern Seaboard this weekend into next week. The upper trough reaching the western U.S. Sun-Mon will bring much cooler temperatures, with highs 5 to 15 deg below average expected to overspread the Northwest and northern Great Basin. As mentioned above, with increasing precipitation across the Northwest, temperatures should be cold enough to support snow at the higher peaks. 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