Extended Forecast Discussion NWS Weather Prediction Center College Park MD 145 PM EDT Sat Oct 9 2021 Valid 12Z Tue Oct 12 2021 - 12Z Sat Oct 16 2021 ...First significant winter storm of the season for parts of the central and northern Rockies continuing into the middle of next week... ...Heavy rain and severe thunderstorms possible across portions of the southern/central Plains Tuesday into Wednesday... 18Z Update: Excellent deterministic model agreement exists for the middle of the week owing to the rather amplified pattern in place, even on smaller shortwave features for Tuesday and Wednesday. Even by next Friday, above normal model agreement is noted, although more significant differences become apparent across the northeast Pacific with the next trough approaching the coast. The 12Z CMC is displaced west with the trough axis and more amplified with the low west of British Columbia compared to the model consensus, and stronger with a downstream ridge over the Pacific Northwest by next Saturday. The GFS is noted as a more progressive solution by the end of the forecast period across the eastern U.S. with the cold front, and slightly ahead of its GEFS ensemble mean. A well clustered GFS/ECMWF/UKMET blend was used as a starting point for Tuesday through Thursday, and then ECMWF/GFS/ECENS/GEFS for Friday and Saturday. The previous discussion follows below for reference. /Hamrick ...Overview... Behind a Great Lakes system that rapidly weakens as the supporting dynamics flow around a Gulf of Mexico/Eastern U.S. upper ridge, a Southwest/southern Great Basin upper low will track northeastward through the northern Plains and into Canada. This upper low will support strong low pressure expected to progress from the central Rockies through the northern Plains during Tuesday-Thursday, bringing significant mountain snow to parts of the Rockies as well as areas of heavy rainfall to portions of the northern Plains. The trailing cold front may also serve as a focus for locally heavy and strong to severe convection over the southern half of the Plains. The amplified upper pattern will bring a pronounced temperature contrast with well below normal readings over the West and multiple warm days over the East. Features should become a little more progressive by next weekend as an upper trough over the northeastern Pacific helps to build a western ridge that in turn will encourage an eastward displacement of the mid-late week trough. ...Guidance Evaluation/Preferences... At least in terms of the track and timing of the ejecting upper low, the ECMWF mean continued to provide the most reasonable individual solution relative to the full array of 12Z/18Z guidance. GFS runs have continued to be a bit on the fast side of the spread (especially late Tuesday into Wednesday) while the 12Z ECMWF leans to the slow side. From about late Wednesday onward, latest guidance including the new 00Z GFS/UKMET/CMC reinforce the idea of somewhat faster progression than the 12Z ECMWF. The new 00Z ECMWF has indeed adjusted a bit faster. Employing a 12Z/18Z model blend provided the desired intermediate timing while providing more detail than the ECMWF mean at the surface. Guidance agrees fairly well that weak shortwave energy arriving from the Pacific by Wednesday will drop into the western U.S. mean trough and likely promote some waviness along the trailing front over the southern half of the Plains late in the week. An added complication after midweek will be what happens with the energy aloft and moisture associated with a tropical system that is likely to develop in the eastern Pacific (high formation chance in the next two days per the latest Tropical Weather Outlook from the National Hurricane Center). There is general agreement on a corridor for this energy/moisture to travel between the Gulf of Mexico ridge and western U.S. trough but there are still significant track and timing differences for the expected tropical system. CMC runs are particularly fast with it. Toward the end of the period the primary issues are how quickly height falls reach into the Pacific Northwest/western Canada ahead of a northeastern Pacific trough and progression of the remaining upper trough across the central U.S. The 18Z GEFS/12Z ECMWF means are quite similar and provide reasonable support for the 18Z GFS/12Z ECMWF timing of the upper trough reaching the east-central U.S. by Saturday. The 12Z GFS was somewhat faster while multiple CMC runs have been slower than consensus. The new 00Z ECMWF has adjusted slower, closer to the 00Z CMC. Meanwhile GFS runs thus far have been more eager than most other guidance to bring height falls into western North America. These considerations led to adjusting the early-period operational model blend toward two-thirds total ensemble weight and more 12Z ECMWF than 18Z GEFS for the rest by day 7 Saturday. ...Weather/Hazards Highlights... The cold storm system developing over the Interior West/Rockies on Tuesday and then tracking northeastward will lower snow levels in the mountains with potential for heavy snow accumulations in the higher terrain of the northern-central Rockies, specifically from Colorado through Wyoming into far southern Montana. This would be the first significant winter storm of the season for this area. Away from the mountains, an axis of heavy rainfall is possible around and to the north of the surface low. Some mixing of snow may not be out of the question during the colder overnight hours. A fairly broad area of strong/gusty winds will also progress from the Interior West/Rockies into the Plains Tuesday through Thursday. Across the central and southern Plains, moderate to locally heavy rainfall is possible and the Storm Prediction Center is monitoring a threat for severe weather on Tuesday and/or Wednesday depending on exact timing of the front that will extend from the northern Plains low. Around mid-late week there continues to be a potential for eastern Pacific tropical moisture to stream northeastward and interact with the front to produce areas of heavy rainfall over parts of Texas and possibly extending northeastward. However the combined uncertainties over timing of the front, wave details, and timing/track of the likely tropical system keep confidence fairly low for resolving important specifics of location and magnitude of heaviest rainfall. The cold front and accompanying rainfall should extend farther into the East by next weekend. Elsewhere, the Pacific Northwest may see one or more episodes of mostly light precipitation, first from weak Pacific energy that continues into the West and then flow on the leading edge of a Northeast Pacific upper trough. The Great Lakes and vicinity may see some rainfall on Tuesday from the weakening system initially over the region. The West will see a broad area of highs 15-30F below normal through midweek followed by gradual moderation toward more single-digit anomalies by Saturday. Morning lows will be less extreme but there will still be multiple days with lows of 10-20F below normal. On the other hand the East will be quite warm through late week. At least the northern half of the region will see highs of 10F or more above normal on one or more days while 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 northeastward. Eventual progression of the Plains cold front should bring a cooling trend from west to east by late Friday/Saturday. 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