Extended Forecast Discussion NWS Weather Prediction Center College Park MD 208 AM EST Tue Dec 07 2021 Valid 12Z Fri Dec 10 2021 - 12Z Tue Dec 14 2021 ...Winter weather is likely for the northern Plains to Midwest/Upper Great Lakes behind a surface low Friday, while heavy rain is expected in the east-central U.S. along its associated cold front... ...Heavy precipitation is forecast to spread southward along the West Coast late this week into early next week... ...Overview... A generally progressive and active weather pattern is forecast to occur through early next week, as a mid-upper trough pushes from the Intermountain West early Friday toward the East by Sunday, while a surface low and potent cold front track ahead of it. A swath of wintry weather is likely to spread from the Central Rockies into northern portions of the Plains to the Midwest/Upper Great Lakes on Friday behind the low, while heavy rain is possible late this week across the east-central U.S. along and ahead of the cold front with some timing uncertainties. Meanwhile, slow-moving and reloading troughing in the eastern Pacific will give persistent moist inflow to the West Coast, with heavy mountain snows likely that should spread southward as the period progresses. ...Guidance/Predictability Assessment... The medium range period begins Friday with quite good model agreement with the overall large-scale pattern, despite some minor differences in shortwave energy within the main trough with its axis tracking through western and central parts of the U.S. through the end of the week. The track of the surface low pressure system moving quickly from the central High Plains into the Upper Great Lakes has also been in good agreement for a few model cycles now, though the timing of the cold front extending southward from the low remains somewhat variable. A multi-model deterministic blend was able to be used early in the period. By Sunday, differences begin to arise in the East that amplify Monday-Tuesday, particularly with whether or not northern and southern streams will separate depending primarily on distribution of energy within the trough. This is highly uncertain and has had quite a bit of run-to-run variability within and between model guidance. With the 12/18Z (yesterday) model cycle, which the WPC forecast is based on, trends seemed to be for more separation of the streams and a southern stream low that is slower to exit the Southeast. This was shown by the 12Z ECMWF, CMC, and UKMET (through its period), with the CMC and EC ensemble means also showing stream separation. The GFS and GEFS runs appeared to be basically alone in their more phased and faster track of the trough eastward into the Atlantic. Considering typical biases of the model suites, the WPC forecast was based on an intermediate solution but slightly favoring the slower side of the guidance--mainly a blend of the EC and CMC ensemble means with some component of the in between deterministic CMC, and not favoring the 12Z ECMWF that was slower than basically every ensemble member but also not favoring the fast GFS runs. However, the incoming 00Z deterministic models including the CMC and ECMWF are showing faster movement of a more phased trough offshore, indicating high uncertainty continues. Expect forecast changes with this feature as models still vary. Upper troughing is forecast to drop southward from Alaska across the eastern Pacific over the weekend and into early next week. While model guidance agrees upon the existence of this feature and its slow drift southward, there is plenty of variety in how amplified it could be at any given time, which affects the moist onshore flow axis into the West Coast and the extent/amounts/type of precipitation. At this point, a similar model blend previously described worked for this feature as well--with deterministic models early on quickly transitioning to a blend heavy on the EC and CMC ensemble means. In between these troughs, model agreement is reasonably good for upper ridging to build in across the central U.S. ...Weather/Hazards Highlights... The surface low pressure system tracking northeastward into the Midwest/Great Lakes Friday to early Saturday will likely spread snow on the backside from the Central Rockies into northern portions of the Plains and into the Midwest, with moderate to locally heavy snow possible. The trailing cold front associated with this low will track through the eastern half of the U.S. and spread widespread rain with embedded thunderstorms along and ahead of it. There is potential for heavy rain amounts with quick-hitting high rainfall rates that could cause flooding/flash flooding issues for portions of the Lower Mississippi, Tennessee, and Ohio Valleys on Friday into early Sunday. Then, how much rain falls along the Eastern Seaboard will depend on the timing of the upper trough and cold front exiting into the Atlantic, which remains uncertain. Meanwhile, persistent onshore flow is likely initially in the Pacific Northwest and northern Rockies and then could track south into California by early next week. Enhanced totals are likely along favored terrain, with heavy mountain snow. Ahead of the trough and cold front, mild to warm temperatures are forecast to be in place in the central U.S. on Friday, with highs currently forecast to rise into the 80s from portions of Texas to the Lower Mississippi Valley, which could approach or set daily records as the temperatures should be 15-30F above normal. These warm temperatures will push into the Ohio/Tennessee Valleys and Eastern Seaboard on Saturday, while temperatures moderate to near normal behind. The western U.S. can expect below average temperatures especially Friday and Saturday, with the Central Great Basin in particular seeing temperatures 10-20F below normal. Then, as ridging builds across the lower 48 by early next week, temperatures should return to 10-20F above normal across much of the central U.S. by Sunday and lasting/warming further though Tuesday. Tate 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