Extended Forecast Discussion NWS Weather Prediction Center College Park MD 258 AM EDT Mon Jul 08 2019 Valid 12Z Thu Jul 11 2019 - 12Z Mon Jul 15 2019 ...Overview and Guidance/Predictability Assessment... Models and ensemble means show a fairly strong upper ridge centered over the southern Rockies/Four Corners region through the period, possibly extending into parts of the central-southern Plains at times. Farther west a mean trough will persist over the eastern Pacific, with individual ejecting shortwaves likely becoming embedded within progressive flow across the northern tier states/southern Canada before reaching a shallow mean trough over the Northeast. Some of these shortwaves will have low predictability but overall the most troublesome part of the forecast continues to involve low pressure expected to be over the northern Gulf of Mexico as of the start of the period. Uncertainties over the degree of development and fine details of surrounding flow lead to well below average confidence in the ultimate path of the system and associated sensible weather effects. Consensus and continuity still look good for the shortwave crossing the Great Lakes/Northeast Thu-Fri and associated surface low/trailing front. The 12Z CMC offered an extreme solution regarding how much energy in the southern periphery of the trough might reach the Southeast (affecting the Gulf low). Guidance is starting to maintain better definition beyond Fri for an upstream wave tracking across southern Canada and trailing front crossing the northern U.S.. Thus confidence is now sufficient to continue eastward progress of a front reaching the northern Plains by Fri, crossing the Great Lakes and Northeast/Mid-Atlantic Sat into early Mon. For the eastern Pacific trough the ensemble means have nudged their forecasts slightly eastward over the past day, partially reflecting some operational model solutions that have been bringing one or more stronger shortwaves into the Pacific Northwest. Model runs have been erratic with these shortwaves though, not surprising given the features' small scale. This favors a model/mean blend as the best starting point. Guidance available through the 18Z cycle did not really provide any improvement in confidence for the track of the possible Gulf system. The 12Z CMC (due in part to its aforementioned issue with northern stream energy) and 12Z GFS offered northeastward tracks while recent ECMWF runs and 18Z GFS would have the system crossing the central/east-central Gulf Coast and continue northward from there. These solutions are within an even broader ensemble envelope. A key part of the forecast appears to be exactly how much upper ridging will exist between the Gulf system and the northern stream shortwave late in the week. It is hoped that this can be resolved fairly soon and help to narrow the guidance spread. At the very least the new 00Z guidance seems to be reducing the probability of the northeastward scenario as the GFS/ECMWF have trended westward and the CMC is taking longer before exhibiting its consistent northeast tendency. The manual forecast remains close to yesterday's coordinated track which is a little east of the 18Z GFS. After a model-dominated blend to start day 3 Thu, the forecast transitioned toward half to two-thirds combined inclusion of the 18Z GFS/12Z GEFS mean which were closest to the desired track for the Gulf system. The remainder went to the 12Z ECMWF mean and past two ECMWF runs. The minimal ECMWF input went more toward the old 00Z/07 run which was a little closer to preference than the 12Z version. This blend provided a good representation of the model/ensemble compromise for the eastern Pacific trough and waves/frontal systems crossing southern Canada and the lower 48. ...Weather Highlights/Threats... The heavy rainfall threat continues for some areas along and inland from the Gulf Coast, with specifics depending on the very uncertain path of the system expected to track over a portion of the northern Gulf. Given the diverse guidance spread thus far, it is important to note that a meaningful potential exists over a greater area than represented on any single deterministic model or manual forecast. Latest trends are suggesting there may be a westward shift in highest rainfall potential but further runs will be needed for confirmation. Elsewhere, low pressure and trailing front will bring showers/thunderstorms to the Northeast and Mid-Atlantic late this week with best dynamics aloft supporting highest rainfall totals over parts of the Northeast. Some showers/storms may also accompany another frontal system tracking from the northern Plains into the East late week through the weekend. The trailing part of the front may stall over the northern Plains to provide a longer-term rainfall focus. Locations along and just inland from the Pacific Northwest coast may see one or more periods of scattered light rainfall depending on strength of shortwaves ejecting from the mean trough aloft over the eastern Pacific. Portions of the Rockies may see scattered diurnal convection on one or more days. Most temperature anomalies should be in the single digits through the period. Aided by warm morning lows in particular, areas from the interior West through the northern Plains should be consistently above normal. Frontal progression will provide some temperature variability over the northeastern quadrant of the lower 48. Clouds/rainfall may keep high temperatures a little below normal over some areas along and inland from the Gulf Coast, depending on the track of the possible Gulf system. 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