631 FXUS02 KWBC 250639 PMDEPD Extended Forecast Discussion NWS Weather Prediction Center College Park MD 238 AM EDT Mon Oct 25 2021 Valid 12Z Thu Oct 28 2021 - 12Z Mon Nov 01 2021 ...Major storm to focus a heavy rainfall/runoff threat from the Midwest/Southeast Thursday to the Mid-Atlantic/Northeast into next weekend... ...Overview and Guidance/Predictability Assessment... A deep cyclone lifting from the Mississippi Valley to the Northeast this weekend will keep much of the medium range quite active. The guidance with this system continues to show good agreement on the large scale evolution/track, increasing confidence a bit, but do offer some uncertainties in the details, which may take until the short range to resolve. A general model blend seemed a good starting point for this system, maintaining good continuity with the previous shift. This system may threaten October low pressure records across parts of the mid-Mississippi to Ohio Valley. Elsewhere, the last couple of model runs show the next trough moving towards the West Coast by the middle of this week may split sending a northern stream shortwave across the northern Rockies and northern Plains into next weekend. The guidance has trended more north and stronger with this system suggesting the Northeast U.S. low may eventually get wrapped up into this system late period. The last few model runs hint at a possible southern stream upper low near California next Sat-Mon, but the ensembles offer plenty of uncertainty on whether this feature even exists at all. This cycle of the WPC medium range progs used a blend of the deterministic models days 3-4 transitioning towards and almost equal weighting of the GFS/ECMWF with the GEFS/ECENS means by day 7. This helps to mitigate the late period differences. ...Weather/Hazards Highlights... The biggest weather threat during the medium range period will be the deep low pressure system working across the eastern third of the U.S. later this week into next weekend bringing widespread moderate to locally heavy rains. Closed upper low development and a slowed frontal system progression will allow rainfall to expand across the Ohio Valley/Appalachians/Southeast later this week then the Mid-Atlantic/Northeast into next weekend as aided by a sustained Atlantic moisture feed. Repeat/duration activity and favored orographic lift should focus a possible flood/runoff threat for parts of the central Appalachians. Out West, shortwave energy progressing towards the West Coast will direct a plume of moisture to support another round of heavy rain/mountain snows to mainly parts of the Pacific Northwest, but also farther inland across the northern Rockies on Thursday into Friday. Conditions should finally dry out by next weekend though as upper ridging builds over the region. For temperatures, the Midwest/South to the East Coast should remain near or more likely below normal through much of the period underneath cloudy and unstable skies. Humidity levels though should allow overnight mins to be more above normal. Behind this, upper ridging shifts across the central U.S. bringing a period of above normal (+5-10F) days before the next shortwave moderates temperatures back towards or even below normal. Out West, temperatures should generally stay within a few degrees on either side of normal. Santorelli Additional 3-7 Day Hazard information can be found on the WPC medium range hazards outlook 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 $$