Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1145 AM EDT Fri Jun 14 2019 Valid 12Z Mon Jun 17 2019 - 12Z Fri Jun 21 2019 1545 UTC Update... Relatively progressive flow across the U.S. northern tier is expected to persist through much of the medium range. By late next week a somewhat more amplified flow pattern will take hold aloft as an upper trough amplifies across the Pacific Northwest/northern Rockies with additional wave amplification across the Great Lakes/Northeast by that time. Earlier in the period, a series of upper shortwaves will traverse a persistent frontal boundary extending from the central U.S. to the Mid-Atlantic/Northeast, resulting in several waves of low pressure along the front. Models offered a range of solutions with the timing of these waves, and given their scale predictability is only moderate at best. A blend of the ECMWF and GFS seemed to offer a reasonable solution through days 3-5 (Mon-Wed). The GFS seemed a bit more closely aligned with consensus with respect to the aforementioned frontal waves Tue-Wed, and thus that solution was weighted a bit more relative to the ECMWF. By days 6-7 (Thu-Fri) models agree on a broad upper trough amplification in the Northwest, but differ on the details of the feature and smaller scale waves embedded within it. Farther east, solutions varied on timing and amplitude of the trough amplifying across the Great Lakes/Northeast by that time. Thus, weighting of ECENS/GEFS ensemble means was boosted to comprise a majority of the forecast by days 6-7. Ryan Previous Discussion (issued at 0447 UTC)... ...Overview and Guidance/Predictability Assessment... Latest guidance indicates a persistent southeast Canada trough with confluent flow south in the the MS Valley and Oh Valley to the northeast and mid Atlantic; allowing a frontal system to progress across these areas. Consensus still shows robust strengthening of the Pacific ridge from late weekend into early next week. Teleconnections from an anomalously strong high in the northeast Pacific lead to more energetic troughing over parts of the northwestern-north central U.S. next Tue-Fri 21 Jun. Prefer a blended approach giving more weight to the ensemble means of the 12z ECMWF and 18z GEFS Ensemble Means, with less weighting to the operational 12z GFS and 12z ECMWF. The 12-18z GFS varied as to which northern stream wave amplified, with the 12z run amplifying a wave over the northern Plains Thu 20 Jun and the 18z run weakening the northern Plains wave but amplifying the wave crossing the Great Lakes to southeast Canada by 12z Fri 21 Jun. The 00z run has shifted the Fri morning low 21 Jun south into NY State. Given the large run to run swings and teleconnection signal of a low amplitude Great Lakes trough, using the mean was preferred. Farther west the greatest forecast problem involves the specifics of energy expected to amplify into a trough drifting east from the northern Rockies into the northern Plains next Wed-Fri 21 Jun. The 12z ECMWF was more intense with the 500 mb low with more ECMWF ensemble members maintaining an open wave, so blending with the ECMWF Mean mitigated differences with the trough timing/intensity. Given the ECMWF and GFS indicate an elongated trough with potential for multiple circulation centers over the northern Rockies and northern Plains and the depiction of the 2 models changed considerably from run to run, more weighting was given to the Ensemble Means of the 18z GEFS and 12z ECMWF. ...Sensible Weather Highlights/Threats... The expected pattern evolution at the surface and aloft continues to be favorable for multiple episodes of convection from the southern half of the Plains across the lower-mid MS Valley into the Ohio Valley/Tn Valley/ Appalachians. However there is still considerable uncertainty for some of the day-to-day details. The activity across this area will likely extend into the Northeast/Mid-Atlantic with locally moderate-heavy rainfall possible. Over the West the best concentration of showers/thunderstorms should be over the northern-central Rockies Mon-Tue. During Tue-Thu expect showers/storms to spread into the northern Plains/Upper Mississippi Valley with the system forecast to affect those regions. The Florida peninsula should see diurnal convection through the period. Over the West expect broad coverage of above normal temperatures Mon in CA/OR/interior WA/ID with some pockets of greater than plus 10F anomalies. Readings should then trend cooler as jet energy reaches the Northwest Tue/Wed, leaving interior California/Nevada as the primary area of warmth heading into midweek. Areas across the northern Plains will tend to see below normal highs by as much as 10 degrees Wed 19 Jun through Fri 21 Jun. Petersen 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