Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 AM EDT Sun Oct 10 2021 Valid 12Z Wed Oct 13 2021 - 12Z Sun Oct 17 2021 ...Strong northern Plains storm to bring heavy rainfall to the region on Wednesday along with some High Plains snow... ...Heavy rain threat over parts of the southern Plains Wednesday-Thursday... ...Overview... Guidance continues to show a very active pattern over the central U.S. mid-late week. A strong storm system tracking northeastward from Nebraska/South Dakota into Canada will bring significant precipitation to the northern Plains and vicinity, while incoming Pacific shortwave energy and a flow of eastern Pacific tropical moisture/energy are likely to interact with the trailing front to produce areas of heavy rainfall over the southern Plains. The amplified pattern consisting of mean troughing aloft over the west-central U.S. and a ridge from the Gulf of Mexico into the eastern U.S. will lead to a pronounced temperature contrast with much below normal readings over the West and unseasonable warmth over the East. By the weekend precipitation coverage/intensity over the lower 48 will steadily decrease and temperatures will moderate as an eastern Pacific upper trough encourages a ridge to build into the West, pushing the western/central U.S. trough into the East. ...Guidance Evaluation/Preferences... Latest models and ensemble means are fairly well clustered for the strong storm tracking northeastward from Nebraska/South Dakota early Wednesday onward, at least with respect to typical errors for three or four days out in time. Some GFS runs over recent days may have been a bit fast/east but overall trends from other guidance have been gradually deeper, toward a depth that the GFS advertised with earlier lead time. 00Z models maintain good continuity and agreement for depth and track. Meanwhile agreement is improving for the Pacific shortwave energy entering the West by early Wednesday and emerging into the Plains by Friday, though some modest spread is still noted late in the week. On the other hand models continue to show more spread and variability than desired for the specifics of an eastern Pacific tropical system currently expected to form later today per the latest Tropical Weather Outlook from the National Hurricane Center. There has been some narrowing of the envelope for the track of this system (northeastward progression as it passes southeast of the southern tip of Baja California) but the CMC is the persistent fast extreme and the 00Z UKMET is fairly fast as well. The 12Z ECMWF was not far from recent GFS runs but now the new 00Z ECMWF has trended faster. The precise timing of this system will be important for resolving the effects over the southern Plains. The depiction on the manual forecast maintains a solution reflecting yesterday's WPC-NHC coordination. Beyond early Friday the 18Z GFS/12Z ECMWF and their ensemble means (plus the 12Z CMC aside from its slower eastern U.S. wave that went away in the new 00Z run) provide reasonable agreement in principle as the pattern adjusts to a trough-ridge-trough configuration, favoring a model/mean blend to tone down smaller scale shortwave uncertainties by days 6-7. ...Weather/Hazards Highlights... The strong storm tracking northeastward across the northern Plains Wednesday into early Thursday will bring an area of rainfall that should be heaviest over the Dakotas, along with possibly some meaningful snow over parts of the northern High Plains, and strong/gusty winds. Parts of the southern Plains will see the potential for heavy rainfall around Wednesday-Thursday due to approaching western U.S. shortwave energy and eastern Pacific tropical moisture/shortwave energy interacting with the trailing front over the region. The general signal for this event has persisted over the past couple days but the uncertainty regarding the timing of the Pacific tropical system keeps confidence lower than desired for resolving the finer details at this time. Some of this moisture may continue into the eastern U.S. late week into the weekend as the front becomes more progressive but rainfall totals should be less extreme. Flow on the eastern fringe of Pacific mean troughing aloft will bring periods of light to perhaps moderate precipitation to the northern Pacific Northwest through the period. Light rain and high elevation snow may extend from the Northwest down into the central Rockies with the upper shortwave crossing the West during Wednesday-Thursday. The West will see a broad area of highs 15-30F below normal through at least Thursday followed by steady moderation toward mostly single-digit anomalies with only pockets of colder readings during the weekend. Morning lows will be less extreme but there will still be multiple days with lows of 10-20F below normal. A few daily records for lows/cold highs will be possible. Meanwhile at least the northern two-thirds of the East will see highs of 10-20F above normal mid-late week with some cooling that starts over the Mississippi Valley. A much broader area will see plus 10-20F or so anomalies for lows. Numerous record warm lows will be possible from the western Gulf Coast or Lower Mississippi Valley northward/northeastward. Progression of the Plains cold front into the East should bring a cooling trend during the weekend with highs dropping to near or slightly below normal. Rausch 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