Extended Forecast Discussion NWS Weather Prediction Center College Park MD 520 PM EDT Fri May 15 2020 Valid 12Z Mon May 18 2020 - 12Z Fri May 22 2020 ...Heavy precipitation likely for interior northern California, the northern Great Basin and the northern Rockies/Montana Monday night through Wednesday night... ...Areas of heavy rainfall for parts of the Northeast Monday night into Tuesday... ...Overview and Weather/Hazard Highlights... Two lows interior of each coasts and an Omega High over the central CONUS in between is expected to dominate the flow pattern through the middle of next week. In the West, a deep trough axis swings ashore into CA Monday night, closing into a low over the Great Basin Tuesday before slowly lifting over the northern Rockies Wednesday and the Canadian Prairies later in the week. Locally heavy rainfall and high elevation snow is expected to continue as this system lifts from northern California across the northern Rockies. Low level upslope flow associated with a developing low pressure system will likely serve to enhance precipitation across eastern slopes of the Rockies in Montana in the Wednesday timeframe. Meanwhile in the East, the trough currently ejecting east out of the Pacific Northwest today will amplify and close over the Dakotas Saturday before strengthening further as it crosses the Great Lakes Sunday before turning southeast to the Mid-Atlantic through Tuesday before stalling in place/merely wobbling through at least the middle of next week. This slow to move upper low, combined with associated surface low pressure development, is likely to continue a heavy rain threat for the northern Mid-Atlantic Monday night into Tuesday and a wet pattern for the Northeast then into late next week. This low is expected to direct the potential sub-tropical low coming from the Bahamas on a path east of the Southeast Coast Monday, pushing out to sea off the Mid-Atlantic coast Tuesday. Will need to continue to monitor the risk of tropically sourced air being wrapped into the system and enhancing rain over the northern Mid-Atlantic into Tuesday. For temperatures across the CONUS, the systems forecast to affect the western and eastern states will each support a period of below normal max temps with areas under the lows at least 10-15F below normal through Thursday. In between, expect an area of warmth to drift from the Rockies and across the Plains. Greatest anomalies for daytime highs should be plus 15-20F or so over the northern-central High Plains through Tuesday and across the northern Plains Wednesday. ...Guidance/Predictability Assessment... Continuity and clustering over the West continue to be better than average through much of the period, with the core of the upper trough (likely to be a closed low for a decent part of the period) tracking from California into the northern Rockies/Montana Tuesday into Thursday. On the contrary, the Eastern U.S. system continues to show run to run variability for both track and depth, although the majority of the guidance does support deep troughing and a slow moving upper level closed low. The GFS continues to be the deepest and farthest west with the low in the East while the 00Z ECMWF/CMC were preferred into Day 5 given their similarity in track and depth. The UKMET continues to be an outlier with The 00Z ECENS/NAEFS means were a good compromise for Days 5-7. Notes on models available after the forecast was done this morning. The 12Z UKMET has finally come around on not allowing a southern low to get caught under the Omega High and stall over Texas through the period. The 12Z CMC is much less like the 12Z ECMWF now (much more open and advective with the eastern low). A compromise between the 12Z ECMWF/GFS now looks best for the eastern low with the ECMWF a little farther southwest than the GFS. Jackson 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 precipitation across portions of California, Mon, May 18. - Heavy rain across portions of the Northern Rockies, and the Northern Great Basin, Mon-Wed, May 18-May 20. - Heavy rain across portions of the Northeast, the Central Appalachians, and the Mid-Atlantic, Mon-Wed, May 18-May 20. - Flooding possible across portions of the Southern Plains, the Northern Rockies, the Middle Mississippi Valley, the Northern Great Basin, the Upper Mississippi Valley, and the Great Lakes. - Flooding occurring or imminent across portions of the Middle Mississippi Valley, the Lower Mississippi Valley, and the Northern Plains. - Flooding likely across portions of the Central Plains, the Middle Mississippi Valley, and the Southern Plains. - Much above normal temperatures across portions of the Northern/Central Plains, the Rockies, and the Upper Mississippi Valley, Mon-Thu, May 18-May 21. - Enhanced wildfire risk across portions of the the Southern Rockies, the Central Rockies, the Central Great Basin, and the Southwest, Mon-Tue, May 18-May 19. - Heavy precipitation across portions of mainland Alaska, Tue, May 19 and Thu, May 21. 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