Extended Forecast Discussion NWS Weather Prediction Center College Park MD 254 AM EDT Thu Jul 11 2019 Valid 12Z Sun Jul 14 2019 - 12Z Thu Jul 18 2019 ...Potential Tropical Cyclone Two to bring heavy rain to the Lower Mississippi Valley this weekend/early next week... ...Overview and Guidance/Predictability Assessment... ...Weather Highlights/Threats... The National Hurricane Center is issuing advisories on Potential Tropical Cyclone Two that may develop soon over the northern Gulf of Mexico. Current forecast shows the system tracking into Louisiana and then continuing northward slowly but steadily in fairly weak flow aloft. Late in the medium range period the southern edge of the westerlies across the northern U.S. may begin to have some influence on the track. Guidance clustering has been improving gradually but there is still meaningful spread (with the UKMET continuing to stray well west of the majority). Models/ensembles have generally had greater than average difficulty in forecasting this system's track thus far, so further adjustments to the most likely track are easily possible over coming days. In the past 12 hours the primary trend of the majority cluster is somewhat eastward to yield a track similar to or a little east of what was favored 24 hours ago. Thus far ECMWF runs have been best in holding onto a defined system through the extended forecast. For this forecast the 12Z ECMWF was closest to the official track through the first half of the period. Then a transition toward a compromise between it and the prior (slower) ECMWF run best represented extrapolation of the track northeastward. Based on current forecast track this system should produce an axis of heavy rainfall from the Lower Mississippi Valley into the southern Mid-Mississippi Valley/Lower Ohio Valley. A weakening front to the north of the system could help to enhance rainfall Sun-Mon and lingering Gulf inflow could support pockets of moderate/heavy rainfall along parts of the Gulf Coast. Please consult latest NHC products for updated information on Potential Tropical Cyclone Two. Meanwhile upper ridging should remain over the southern Rockies. Shortwaves ejecting from an eastern Pacific into northwestern U.S. mean trough will continue around the ridge, crossing the northern U.S./southern Canada and feeding into a broad/weak mean trough over the Northeast. Strong flow to the south of an upper low tracking across the Gulf of Alaska may reach the Pacific Northwest by day 7 Thu and help to push along the leading trough near the West Coast. There is reasonable continuity for an initial ejecting Northwest shortwave that should support southern Canada low pressure and associated warm/cold front crossing the northern U.S.. Thus far guidance has had more difficulty in resolving the next shortwave which should be near the Pacific Northwest as of early day 5 Tue. Recent trends appear to be somewhat faster, which seems plausible given the strength of approaching North Pacific flow. Continue to expect episodes of showers and thunderstorms across the northern states with the waves/fronts, with locations from the northern Plains into Upper Great Lakes seeing the best potential for some locally heavy rainfall. The Pacific Northwest will see periods of scattered and mostly light rainfall with the shortwaves brushing the region early-mid week. By Thu rainfall could become a little more organized over or just north of the region as flow aloft strengthens. Based on combined preferences for Potential Tropical Cyclone Two and features in the northern stream, the latest forecast for days 3-6 Sun-Wed started with half total 12Z ECMWF-00Z/10 ECMWF with the rest consisting of 20 percent 12Z ECMWF mean and lesser input from the 18Z GEFS mean and 12Z/18Z GFS. Increasing detail uncertainty by day 7 Thu led to a more even model/ensemble mean proportion at that time. An area of above normal temperatures will likely expand from the northern Plains into the Northeast during the period with most anomalies in the plus 5-15F range. Clouds and rainfall associated with Potential Tropical Cyclone Two should produce an area of daytime highs as much as 10-15F below normal over the Lower Mississippi Valley Sun-Mon. Some below normal highs may extend farther northeastward after Mon but with lower confidence. Rausch 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