Extended Forecast Discussion NWS Weather Prediction Center College Park MD 416 PM EDT Fri May 31 2019 Valid 12Z Mon Jun 03 2019 - 12Z Fri Jun 07 2019 ...16Z Update... The overnight model guidance suite remained in fairly good agreement regarding the main forecast features. Firstly, the upper low centered in southeastern Canada on day 3/Mon will gradually move north throughout the forecast period. The ECMWF suite brings the low farther north than the 00Z GFS/GEFS suite by day 6-7/Thu-Fri, but the 06Z GFS run trended north. Thus, leaned a little more toward the northern solutions. Details of shortwave energy traversing southern Canada in the northern stream throughout the period are yet to be resolved, and a multi-model blend worked adequately there. Secondly, a trough in the southern stream will remain practically stationary centered across southern California through the short and early medium range, ejecting slowly eastward into the Four Corners region midweek and the Plains late in the week, causing potentially widespread rain and thunderstorms along and ahead of it. The 00Z UKMET and CMC both seemed too fast with this feature by day 5/Wed compared to the model consensus, so excluded those from the forecast blend. Troughing is expected to come into the Pacific Northwest by day 6/7, but with uncertainty as to the orientation and strength of the height falls, went toward the 00Z ECMWF/GEFS means there. For more details, see the previous discussion below. Tate ...Overview and Weather/Threats Highlights... It remains the case that an active flow pattern is expected to continue next week with a strong North Pacific upper-level jet splitting as it approaches the West Coast of North America. An upper-level low will linger over southern California and the desert Southwest, only very slowly moving eastward. Meanwhile, a hot subtropical ridge will migrate westward and reposition over the south central U.S. and the Gulf of Mexico. A persistent and amplified upper low over southeastern Canada to the lee of a mean upper ridge will cause incoming northern stream shortwaves to periodically amplify as they dig into the Great Lakes and Northeast, traversing the broader anticyclonic flow. These digging waves along with the westward migration of the subtropical ridge allows heights to fall for much of the Great Lakes/Ohio Valley and the Mid-Atlantic/Northeast. Relatively slow-progressing southern stream shortwaves will periodically eject eastward from the upper low, reaching the central U.S. and potentially interacting with amplifying northern stream energy. These waves along with return flow across a persistent baroclinic zone may continue to support areas of increased convective potential from the Southern/Central Plains into the Mid-Mississippi Valley and lower Ohio Valleys. This offers a risk of episodes of locally heavy rainfall/flash flooding as well as strong to severe thunderstorms/MCS activity. This may be especially evident if the slow moving closed low over the unsettled Southwest ejects into the central U.S. later next week in response to amplified approach of kicker Pacific energy upstream across the West Coast. Kicker trough/height falls may themselves support the spread of modest to moderate precipitation across the Northwest quarter of nation later next week. ...Guidance/Uncertainty Assessment... The WPC medium range product suite was mainly derived from a composite blend of well clustered 18/00 UTC GFS/FV3/GEFS mean and 12 UTC ECMWF/ECMWF ensemble mean mass field guidance. This bolsters forecast confidence in the overall pattern evolution. The main heavy convective rainfall focus is less predictable within this pattern, but there remains a good signal supporting a threat from the south-central Plains to the Mid-Mississippi/Lower Ohio Valleys. Schichtel 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 Hazards: - Heavy rain across portions of the Central/Southern Plains and the Middle/Lower Mississippi Valley, Thu-Fri, Jun 6-Jun 7. - Heavy rain across portions of the Middle/Lower Mississippi Valley, the Ohio Valley, and the Tennessee Valley, Fri, Jun 7. - Flooding possible across portions of the Central Plains, the Upper/Middle Mississippi Valley, and the Great Lakes. - Flooding occurring or imminent across portions of the Plains, the Mississippi Valley, the Northern/Central Great Basin, the Great Lakes, and the Ohio Valley. - Flooding likely across portions of the Northern/Central Plains, the Northern Rockies, the Upper/Middle Mississippi Valley, and the Northern Great Basin. 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