Extended Forecast Discussion NWS Weather Prediction Center College Park MD 245 AM EDT Fri May 15 2020 Valid 12Z Mon May 18 2020 - 12Z Fri May 22 2020 ...Heavy precipitation possible from northern California into the northern Rockies/Montana... ...Areas of heavy to significant rainfall increasingly likely over parts of the East... ...Overview and Weather/Hazard Highlights... The guidance in the medium range period continues to show an evolution towards a very amplified pattern aloft. In the West, a deep trough and embedded closed low will progress from the eastern Pacific into the Western U.S. early next week, and lift slowly northward into southwest Canada by the end of the week. This system will likely bring heavy rainfall to portions of the Western U.S., with possibly some high elevation snows from northern California into the northern Rockies. Low level upslope flow associated with a developing low pressure system will likely serve to enhance precipitation across western Montana and northern Idaho next Wednesday into Thursday. Meanwhile in the East, another deep trough and closed low will drop southward from the Midwest on Monday to the Southern Appalachians on Tuesday. Through mid-week, the guidance shows this low may meander/remain nearly stationary across the Appalachians/Mid-Atlantic region before beginning to weaken as it lifts slowly northward again by the end of the week. This slow to move upper trough, combined with associated surface low pressure development, continues to present an increasing likelihood of periods of heavy to significant rainfall across portions of the Mid-Atlantic and Appalachians through next week. Exact placement of heaviest rainfall is subject to change given behavior of the guidance over recent runs. Meanwhile, the National Hurricane Center continues to monitor near-future potential development of low pressure along a weak frontal boundary over the Bahamas. The track of the low should remain offshore the East Coast but the possibility of some moisture from this system interacting with the amplifying flow over the interior eastern U.S. will have to be monitored. For temperatures across the CONUS, the systems forecast to affect the western and eastern states will each support a period of cool highs with some locations at least 10-15F below normal for one or more days. In between, expect an area of warmth to drift from the Rockies into the Plains. Greatest anomalies for daytime highs should be plus 15-20F or so over the northern-central High Plains during Mon-Tue. ...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, although the majority of the guidance does support deep troughing and a slow moving upper level closed low. The past few runs of the GFS and the ECMWF have shown a clear southward shift with the core of the upper low (the ECMWF most dramatic), which is also supported by the ensemble means. The latest runs of the CMC/UKMET are farther north/east with the low and thus were not included in tonight's blend. Across the board, the latest WPC progs were based on a blend of the GFS/ECMWF with their respective ensemble means with more deterministic model weighting days 3-5, transitioning to more ensemble means days 6-7. This resulted in fairly good continuity with the previous shift, with some shifting of the surface low/front across the East needed given recent model trends. Santorelli 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