Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1153 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 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 Plains into the Midwest from Sunday into the middle of next week. ...Model Guidance/Predictability Assessment... Overall there is fairly remarkable model agreement through the entire medium range period (Sun-Thu). For days 3-5, consensus was sufficient to justify a majority deterministic blend (which includes the 06 GFS and the 00z ECMWF/UKMET) as a starting point for the forecast. Models continue to show good agreement that the western U.S. upper low should linger across the northern Great Basin through much of the medium range period, perhaps drifting very slowly southeastward. The main outlier to this was yesterdays 18z GFS run which actually quickly weakened the upper low and ejected the energy eastward into the Plains, but the latest two runs (todays 00z and 06z) keep the upper low strong and locked over the Great Basin. This suggests a possible trend, hopefully, towards that solution which is consistent with the vast majority of models and ensemble means. Farther east, models showed similar timing with respect to shortwave energy traversing the more progressive flow near the U.S. border with Ontario/Quebec days 3-5, with mostly minor amplitude differences. Regardless, a surface cold front should traverse the Great Lakes/Northeast Sun-Tue. Another round of height falls attempts to cross the Great Lakes/Northeast by next Thurs, with models and ensembles showing a range of solutions with respect to the amplitude of this trough and how much of the Southeast ridge would be suppressed. Given the pattern though, would continue to be skeptical of any solutions that weaken the ridge too quickly. The forecast for days 6-7 was based mostly on the EC and GEFS ensemble means, with a more than usual component of deterministic solutions (mainly the ECMWF which shows better run-to-run continuity across the board). 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 north/eastward across the plains/Mississippi Valley by Tue around the western periphery of the southeastern U.S. ridge. ...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 interacts with a frontal boundary across the region. 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. Santorelli/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