Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1150 AM EDT Sat Jul 06 2019 Valid 12Z Tue Jul 09 2019 - 12Z Sat Jul 13 2019 ...16Z Update... Overall, no major changes were needed to continuity for this forecast cycle, as the large scale pattern appears well handled by model guidance but with some variability in smaller scales. A multi-model blend was utilized for the medium range forecasts with this reasonably good model agreement. Did eventually eliminate the 00Z CMC from the blend due to its handling of the shortwave in the Great Lakes region on day 5/Thu--the trough was held west a bit, allowing it to amplify on Fri along the Eastern Seaboard unlike other guidance. Models continue to indicate the likelihood of a surface low meandering near the Central Gulf Coast through the medium range period even after a surface front dies in its vicinity by midweek. The intensity of the surface low remains in question, with the 00Z ECMWF much stronger than any other guidance except for a handful of its own ensemble members (00Z and 06Z GEFS members remain weaker, as do the deterministic GFS runs). Yesterday's 12Z ECMWF also had a weaker system. With this run-to-run variability, low confidence remains regarding the intensity of the surface low. For more details, please see the previous discussion below. Tate ...Previous Discussion... ...Overview and Guidance/Predictability Assessment... The latest guidance continues to provide reasonably high confidence in the large scale pattern evolution but there is embedded uncertainty in the medium to smaller scale details. Consensus shows southern tier ridging aloft will shift its emphasis to the southern Rockies/Four Corners region by mid-late week, likely reaching its greatest strength around Thu. As this occurs a shortwave crossing the northern tier will head into the eastern states. Meanwhile most guidance is still suggesting that a current Mississippi Valley impulse aloft may track to the Southeast/Gulf Coast and interact with a wavy front to yield a surface low near the Gulf Coast. An upper low well offshore the Pacific Northwest should track west of Vancouver Island as it weakens and sends some of its energy eastward. Upstream flow will likely feed into a lingering mean trough off the West Coast into the weekend. Comparison among the 12-18Z model/ensemble guidance reveals that the GFS may still be having some fast timing issues in the northern stream--with eventual ejection of the upper low tracking west of Vancouver Island as well as for the shortwave reaching the eastern states. The 18Z GFS was not as extreme as the 12Z run for the eastern shortwave and now the new 00Z GFS has furthered the trend toward consensus. The 00Z GFS still looks fast to eject eastern Pacific upper low energy after early Wed though. By the latter half of the period, the various uncertainties with individual shortwaves from the North Pacific through southern Canada/northern U.S. will limit confidence in surface details across the northern Plains and vicinity. Albeit with typical run-to-run variability, the GFS/ECMWF and their ensemble means (plus the UKMET through the end of its recent runs) have been fairly consistent in showing the potential for a surface low most likely near the Florida Panhandle as an upper level impulse interacts with a wavy front over the region. A consensus blend yields a solution fairly close to continuity. The small scale of this evolution keeps confidence/predictability in specifics fairly low. The approach of a front from the north by Fri-Sat could provide added uncertainty in the forecast late in the period. The manual forecast reflects an operational model blend early and then a model/mean blend that trends to majority weight of the 18Z GEFS/12Z ECMWF means by day 7 Sat. GFS input is 25 percent or lower due to the aforementioned questionable timing of northern stream features after early Wed. ...Weather Highlights/Threats... There is a continued focus for heavy rain potential within an area from the southeastern coast through Florida and possibly to the central Gulf Coast. A wavy front will be positioned over the area by the start of the period early Tue, and while the front should weaken with time it appears likely that a lingering surface low may track in the vicinity of the Florida Panhandle. Exact details of the surface evolution and rainfall emphasis are quite uncertain given the small scale of the feature and typical difficulty that guidance can have in modeling rainfall associated with such systems. Meanwhile a front progressing from the northern Plains into the eastern/southern U.S. should produce periods of organized showers and thunderstorms. Expect best potential for heaviest rainfall to be over/near the Upper Mississippi Valley/Midwest and then portions of the Northeast. Toward Fri-Sat the southern part of the front could begin to interact with the moisture over the Southeast. The upper low offshore the Pacific Northwest may track just far enough east to bring some moisture to the region primarily around Wed-Thu. Expect mostly light to isolated moderate rainfall from coastal Washington/Oregon into the Washington Cascades. Behind the initial northern Plains front, some northern Plains locations should see highs 5-15F below normal Tue into Wed. After midweek the pattern will favor near to above normal temperatures over a majority of the West and by Fri-Sat extending into the northern Plains. Anomalies will likely be higher for morning lows versus afternoon highs. There will also be a band of warmth (mostly single-digit anomalies) ahead of the advancing central-eastern U.S. front. Locations from the southern Plains through Lower Mississippi Valley may see heat index values high enough to be of concern through midweek. 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