Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1157 AM EDT Sun Jul 07 2019 Valid 12Z Wed Jul 10 2019 - 12Z Sun Jul 14 2019 ...16Z Update... While the large-scale pattern remains well-handled, the more uncertain parts of the forecast continue to be shortwave impulses moving along the northern stream and the possibility of a low with potential tropical characteristics remaining in the northeastern Gulf of Mexico after a front in the vicinity dies in the short range. Regarding the northern stream, noted that by day 5/Fri the upper low in north central Canada in the 00Z GFS was oriented differently than other guidance, which affected shortwave placement over the weekend, so leaned away from that solution. The 00Z GFS also was stronger with an upper low in the eastern Pacific by the weekend compared to other models. With the Gulf low, the ECMWF has been strongest for the past few days. This remains the case, but GFS runs as well as the UKMET and CMC have consistently been developing a low now, albeit weaker. There is plenty of spread in the position of the low as well especially late in the week, and the 00Z ECMWF is one of the slower and farthest west solutions. Overall leaned toward the GEFS and EC means by the weekend so as not to lock in on any one solution, similar to the previous forecast. Tate ...Previous Discussion... ...Overview and Guidance/Predictability Assessment... Guidance continues to show a fairly typical summertime pattern, as reflected by multi-day means that indicate rather modest mid-level height anomalies over the lower 48. Consensus and continuity for the large scale flow are still good while some significant uncertainties remain for embedded features. Expect upper ridging to be centered over the southern Rockies/Four Corners and into parts of the central/southern Plains for most of the period. Mean troughing should persist over the eastern Pacific while progressive/low amplitude flow will prevail over the northern tier states and southern Canada. A fairly sharp shortwave trough will progress across the northeast quadrant of the lower 48 during the latter half of the week, followed by flat or broadly cyclonic flow. Guidance is continuing to suggest potential for a combination of energy aloft and a surface wave (initially along a weakening southeastern U.S. front) to yield an area of low pressure over or near the northeastern Gulf of Mexico. Looking at the forecast over the eastern Pacific and western U.S., recent runs have settled on an agreeable solution for the weakening upper low west of Vancouver Island as of early Wed. Model differences increase as one or more upstream shortwaves move into the eastern Pacific mean trough, with meaningful spread for the degree of height falls that may occur over the Northwest. To varying degrees and at different time frames recent GFS/CMC/UKMET runs suggest stronger shortwave energy could approach or reach the region than a decent majority of GEFS/ECMWF/CMC ensemble members. For the time being prefer to lean more to the ensemble means given the long-term mean ridging forecast over western North America, but confidence is not especially great. There are still multiple uncertainties regarding evolution of the system over/near the northeastern Gulf of Mexico. For example there are some differences in relative emphasis of mid-level energy--the impulse originating from the Mississippi Valley versus another feature farther east. Then after decent model clustering during Wed-Thu there is rapid east-west divergence for surface low track. For good measure the 00Z UKMET/CMC both track a separate wave northeast from the southeastern coast as well. The manual forecast stays close to continuity that was coordinated yesterday, with a surface low track over the northeastern Gulf through day 6 Sat and a northward day 7 extrapolation based on the motion of recent GFS/ECMWF runs. Elsewhere, clustering is now sufficiently good for a consensus blend to work well for the shortwave progressing eastward from the Upper Mississippi Valley. Uncertain shortwave details within progressive northern U.S./southern Canada flow will continue to provide difficulty in resolving surface features across the northern Plains and vicinity. The updated forecast started with an operational model blend early and then steadily increased 18Z GEFS/12Z ECMWF mean input so that day 7 Sun consisted almost entirely of the means, reflecting late-period preferences along the West Coast and to prevent any one solution from having too much effect on the blend for the possible Gulf feature. ...Weather Highlights/Threats... There is still a heavy rainfall threat over areas from the central Gulf Coast through Florida and the Southeast but with lower than average confidence in determining where heaviest rain will occur. The current most likely track of the possible northeastern Gulf feature would bring highest totals into the east-central Gulf region. A weakening front near the Southeast coast around midweek and then another front settling into the South Fri into the weekend will have the potential to provide added focus for enhanced rainfall. Before/as the front settles into the South, it will produce areas of showers/thunderstorms over the eastern half of the country with highest totals likely over the Upper Great Lakes and Northeast which will see the strongest upper dynamics. One or more waves/fronts over the northern tier may help to produce some areas of showers and thunderstorms over the northern Plains/Upper Midwest late week into the weekend. Areas from the Pacific Northwest coast into the Washington Cascades will see a period of light to locally moderate rainfall early in the period. By Fri-Sun there may be additional rainfall depending on how shortwaves may eject from the eastern Pacific mean trough aloft. Behind the front progressing from the Midwest/Plains into the East/South, the northern-central Plains should see highs 5-10F below normal on Wed. Otherwise a large portion of the lower 48 should see near to above normal temperatures for most of the period. In general anomalies should be greater for morning lows versus daytime highs. The Intermountain West/northern Rockies should see the most consistent warmth for the five-day period. Fairly high heat index values over parts of the southern Plains through Lower Mississippi Valley should trend lower after Wed. 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