Extended Forecast Discussion NWS Weather Prediction Center College Park MD 133 AM EDT Thu Jun 14 2018 Valid 12Z Sun Jun 17 2018 - 12Z Thu Jun 21 2018 ...Heavy rains possible for portions of the northwestern Gulf Coast and portions of the central/northern Rockies/plains... ...Potential for excessive heat from portions of the Midwest to the Mid-Atlantic and Southeast this weekend into early next week... ...Pattern Overview... Progressive flow will be in place during the medium range from the higher latitudes of Canada across Hudson Bay and into the northeastern U.S. Meanwhile, the flow farther south will be rather blocked across most of the contiguous U.S. A broad and anomalous upper ridge is expected to persist from the Mississippi Valley to the Southeast through much of the forecast period while an upper low lingers across the Great Basin, trapped between upper ridging to the west/north (from the eastern Pacific north into western Canada) and to the east. The result will be a persistent quasi-stationary frontal boundary draped from the central Rockies across the central/northern plains from Sun through much of the upcoming week. ...Model Guidance/Predictability Assessment... Model consensus was sufficient during days 3-5 (Sun-Tue) to justify a majority deterministic blend (including the 12Z ECMWF/UKMET and 18Z GFS) as a starting point for the forecast. Models showed good consensus that the western upper low should linger across the northern Great Basin through much of this time period, perhaps drifting slowly southeastward. Some differences exist with respect to the depth of the low, but the aforementioned blend should best represent a consensus solution. Farther east, models showed similar timing with respect to shortwave energy traversing the more progressive flow near the U.S. border with Ontario/Quebec, but with some amplitude differences. The GFS has consistently been a bit more amplified with this wave, likely the result of a slightly weaker upper ridge farther south. The ECMWF and UKMET both had a more limited southern extent for height falls across the Great Lakes/Northeast. Thus, the aforementioned blend is weighted bit more toward keeping the ridge stronger and keeping the height falls farther north. Nonetheless, a surface cold front should traverse the Great Lakes/Northeast Sun-Tue. Also of note during days 3-5 is a broad trough and area of moisture/disturbed weather in the northwestern Gulf of Mexico. Models show good consensus that this feature should drift inland across eastern Texas by Mon and then northward across the plains/Mississippi Valley by Tue - around the western periphery of the southeastern U.S. ridge. By days 6-7 (Wed-Thu) model differences become more significant. The consensus of the vast majority of deterministic/ensemble mean solutions suggests that the western upper low should continue to linger, and may even retrograde a bit to the southwest. The 12Z ECMWF along with the 12Z ECENS/NAEFS and 18Z GEFS means all support this idea. On the other hand, the 12Z and 18Z runs of the GFS made a dramatic shift, quickly weakening the south central/southeastern U.S. ridge and ejecting the upper-level energy from the West eastward into the plains. Given the consensus, the 12Z/18Z GFS runs were rejected. The downstream implication of this preference is that the upper ridge should remain stronger for longer across the south central/southeastern states (relative to the 18Z GFS). Farther north, however, the more progressive flow should continue, with another round of height falls attempting to press Great Lakes/Northeast by next Thu. Ensemble show a range of solutions with respect to the amplitude of this trough and how much the ridge would be suppressed, but given the pattern, would continue to be skeptical of any solutions that weaken the ridge too quickly. Given these considerations, the forecast for days 6-7 was based on a majority ensemble blend including the 12Z ECENS/NAEFS means, but with a significant component of the 12Z ECMWF continued through day 7. ...Weather Highlights/Hazards... Heavy rain is increasingly likely for portions of the western Gulf Coast states as the broad trough/area of disturbed weather move onshore Sun-Mon. Model solutions vary widely as to the specific rainfall totals and the location, but multi-inch rainfall totals appear possible, especially for portions of eastern Texas and western Louisiana. Heavy rains will also be possible from portions of the central/northern Rockies east across the northern plains and Upper Midwest Sun-Mon as moisture leftover from Tropical Storm Bud is spread northward east of the upper low and encounters the southern periphery of the stronger westerlies. By Tue-Wed, models suggest that favorable upper flow will continue to support the transport of deeper moisture from lower latitudes northward into the southern/central Rockies and adjacent High Plains, supporting the potential for additional convection and areas of heavy rainfall, especially in close proximity to a stationary surface frontal boundary. The other significant story during the medium range will be the potentially excessive heat beneath the central/eastern U.S. upper ridge. High temperature anomalies of +10 to +18 deg F are possible Sun-Mon from portions of the mid/upper Mississippi Valley east to portions of the Southeast and Mid-Atlantic. The hot temperatures may persist into the middle of next week for portions of the Southeast and southern Mid-Atlantic region, where upper subsidence along with favorable downslope flow may result in temperatures near or even just surpassing 100 degrees for a few locations. Some areas may near or surpass record high temperatures, especially by Sun-Mon along the East Coast. Widespread record high minimum temperatures are expected from the Midwest to the Eastern Seaboard, increasing the potential for heat-related illnesses. Anomalously warm temperatures may also creep into the Pacific Northwest by early to mid next week, on the eastern side of the upper ridge across the Northeast Pacific. High temperature anomalies of +10 to +20 deg F are possible Mon-Thu, with some locations soaring well into the 90s. Ryan WPC medium range forecasts of 500 mb heights, surface systems, weather grids, quantitative precipitation, and winter weather outlook probabilities can be found at: http://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif http://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif http://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html http://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml http://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4