Extended Forecast Discussion NWS Weather Prediction Center College Park MD 355 PM EDT Fri Jul 05 2019 Valid 12Z Mon Jul 08 2019 - 12Z Fri Jul 12 2019 ...16Z Update... Overnight model guidance shows a similar overall pattern as in the previous forecast. Regarding the shortwave moving through the northern tier, it seems well-handled on Mon/Tue, but the GFS 00Z and 06Z deterministic runs have the shortwave a little flatter and faster by Wed than other guidance including the GEFS mean, so leaned away from those solutions. Like previous runs, the GFS deterministic runs were on the eastern/faster side of the guidance envelope with the trough in the eastern Pacific and coming into the Pacific Northwest, so went more toward the ECMWF/EC mean with incorporation of the GEFS mean, much like the previous forecast. Another interesting feature to watch is the possibility of a low along or near the Gulf Coast for the second half of the week. It appears energy from an MCV currently in Missouri will propagate southeastward early in the week and possibly join up with a surface front that will gradually die along the central Gulf Coast and the Georgia/South Carolina Atlantic coast. The 00Z ECMWF run is most bullish with the low development, and confidence in the strength of the feature is low. But it does appear there could be a low in the extreme Southeast U.S. or in northern portions of the Gulf of Mexico on Thu/Fri with a fair amount of ensemble support. For more information and sensible weather highlights, please see the previous discussion. Tate ...Previous Discussion... ...Overview and Guidance/Predictability Assessment... Most guidance shows rapid ejection of an upper trough initially just inland from the West Coast. The remaining shortwave should traverse the northern tier states and eventually reach the eastern U.S. while southern tier ridging aloft shifts its emphasis from the southern Plains on Mon to the southern Rockies/Four Corners region by mid-late week. A northern Pacific upper low may approach the Pacific Northwest but with some uncertainty over its ultimate path. One other evolution to be monitored will be diffuse Plains energy that models eventually bring into the extreme Southeast U.S. and eastern Gulf of Mexico and which may interact with waviness along a weakening surface front. Based on the array of guidance available through the 12-18Z cycles, the primary forecast consideration was to underweight the operational GFS runs which became faster/eastward versus consensus (including recent GEFS means) for the upper low nearing the Pacific Northwest. The 12Z GFS was slightly slower than the 18Z run. Upstream details over and north of the North Pacific remain uncertain but at least the new 00Z GFS has trended favorably slower with the upper low. Meanwhile the 12Z ECMWF seemed a bit slow/southwest with the upper low and the new CMC/UKMET/GEFS mean all reinforce that idea. Over the past day the model consensus has become better defined with the ejecting western U.S. shortwave as it reaches the northern Plains/Upper Mississippi Valley by midweek and then settles into the East as the upper ridge strengthens over the southern Rockies. In spite of the favorable 00Z GFS adjustment off the Pacific Northwest it is still on the fast side of the envelope with the northern tier shortwave by Wed. The small scale of features involved lends itself to below average predictability for what may evolve at the surface and aloft over the extreme Southeast and eastern Gulf of Mexico toward mid-late week. The GFS/ECMWF and their means offer fairly similar ideas but other solutions provide alternative scenarios. In light of above considerations, the updated forecast started with a model consensus early in the period--though splitting a portion of the 12Z GFS weight with the 18Z GEFS mean. Then the blend increasingly emphasized the 18Z GEFS/12Z ECMWF means with continued minority input of the 12Z ECMWF. ...Weather Highlights/Threats... A front making steady progress from the northern Plains early in the week to the eastern states by late week will provide a focus for organized showers and thunderstorms of varying intensity through the period. Best potential for heaviest rainfall should be over parts of the northern half of the Plains and Upper Mississippi Valley with a secondary maximum over the Northeast. Some of the northern Plains activity early in the week may be severe. Check the latest SPC outlooks for additional severe info. Another area with heavy rain potential will be from the Southeast coast through parts of Florida and along the eastern/central Gulf Coast. During Mon-Wed there is reasonable consensus toward a wavy front settling over this region. However by late week there is more uncertainty over what may become of potential lingering waviness as the front weakens. The upper low offshore the Pacific Northwest may reach close enough to the region to bring in some moisture, with mostly light to locally moderate rainfall. The western U.S. will likely see a warming trend after starting out below normal Mon-Tue as the initial trough aloft crosses the region. Some of the cool daytime highs will extend into the northern Plains. Best focus for above normal readings later in the week should be over the interior West as the upper pattern evolves toward strong ridging over the southern Rockies/Four Corners and troughing off the Northwest. Farther eastward temperatures will vary in relation to the progress of the northern Plains to eastern U.S. front. Areas from the southern Plains to interior Southeast will see highs up to a few degrees F above normal one or more days. Such anomalies are not exceptional but it will still be hot enough with sufficient humidity to yield fairly high heat index values. 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 - Heavy rain across portions of the Central Plains, the Northern Plains, the Middle Mississippi Valley, the Mid-Atlantic, the Upper Mississippi Valley, and the Southeast, Mon-Tue, Jul 8-Jul 9. - Heavy rain across portions of the Southeast and the Lower Mississippi Valley, Tue-Fri, Jul 9-Jul 12. - Heavy rain across portions of the Mid-Atlantic, the Northeast, and the Great Lakes, Thu, Jul 11. - Severe weather across portions of the Northern Plains, Mon, Jul 8. - Flooding possible across portions of the Central Plains and the Northern Plains. - Flooding occurring or imminent across portions of the Central Plains, the Middle Mississippi Valley, the Lower Mississippi Valley, and the Ohio Valley. - Flooding likely across portions of the Central Plains, the Middle Mississippi Valley, the Upper Mississippi Valley, and the Northern Plains. - Excessive heat across portions of the Southeast, the Lower Mississippi Valley, the Southern Plains, and the Tennessee Valley, Mon-Thu, Jul 8-Jul 11. - Excessive heat across portions of the Central Plains, the Middle Mississippi Valley, the Lower Mississippi Valley, and the Southern Plains, Tue-Wed, Jul 9-Jul 10. 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