Extended Forecast Discussion NWS Weather Prediction Center College Park MD 341 PM EST Wed Jan 13 2021 Valid 12Z Sat Jan 16 2021 - 12Z Wed Jan 20 2021 ...Overview and Guidance/Predictability Assessment... A weakening low pressure system over the Great Lakes on Saturday will likely give way to a new low pressure system forming on a triple-point near the northern Mid-Atlantic coast. Model consensus this morning had again trended slightly slower regarding this system as a whole although the 06Z and 12Z GFS have sped it up. The timing of the track of this coastal low would favor rain possibly heavy at times to overspread coastal New England during the day on Saturday and possibly farther inland. In any event, the precipitation should change back to or remain as wet snow especially interior New England on Sunday before taper off. Some light snow can be expected to linger across the lower Great lakes down into the western slopes of the central Appalachians into early next week. Meanwhile, models are in rather good agreement that a couple of shortwaves will dip into the southwestern U.S. through the medium-range period. The first wave on Sunday should be more subtle and should produce light rainfall amounts near the western Gulf Coast. Thereafter, the guidance appears to indicate a more robust amplification to a second wave dipping into the southwestern U.S. early next week. This would likely result in a low pressure wave forming along the western Gulf Coast with decent rainfall expanding eastward across the central Gulf States toward the Southeast by the middle of next week. Run to run variability on this scenario is high, so there is a lot to be worked out still with regards to timing and intensity. Therefore, prefer a more modest approach towards the ensemble means in the day 6-7 time frame on this system until details can be further resolved. In addition, some light precipitation is possible in parts of the Desert Southwest as the upper trough amplifies. The WPC forecasts were derived from a blend of the 06Z GFS/GEFS with the 00Z ECMWF/EC mean. This maintains good continuity with the previous forecast. The CMC was not included at all with this package because its synoptic evolution was noticeably different, especially northern stream energy through the Ohio Valley mid-period, and also with the cut off low scenario early next week. ...Weather/Threats Highlights... Deepening low pressure through New England on Saturday will bring heavy precipitation to much of the region. Latest guidance suggests heavy rainfall along the coast, with the better potential for accumulating snowfall across interior portions from upstate New York through northern Maine. This of course, is highly dependent on the exact low track and so lingering uncertainties could mean heavy rainfall for areas farther inland as well. Gusty winds will likely be associated with this system too. Lingering mainly light lake effect snowfall on the backside of this system will continue through the weekend. Shortwave energy moving through the West will bring some light rain or mountain snows through the period. As troughing deepens over the Plains this weekend, precipitation should expand into parts of Texas and the lower Mississippi Valley. Greater moisture transport ahead of amplifying energy into northern Mexico early to mid next week would increase the potential for moderate to heavy rainfall into parts of the central Gulf Coast. Above normal temperatures should persist most of the period across California and the Desert Southwest, while initial above normal temperatures on Saturday in the Northeast, get replaced by more typical January like values Sunday and beyond. Near or below normal temperatures can be expected for parts of the Southeast this weekend, and the Rockies/southern High Plains next Tuesday and Wednesday. Another round of above normal temperatures should also move into the Northern Plains by next week, with daytime highs 10 to 15 degrees above normal. Kong/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 Hazards: - Heavy precipitation across portions of the Northeast, Sat, Jan 16. - Heavy rain across portions of the Southeast, the Lower Mississippi Valley, and the Southern Plains, Tue-Wed, Jan 19-Jan 20. - Flooding occurring or imminent across portions of the Southeast, the Pacific Northwest, and the Northern Great Basin. - Flooding likely across portions of the Pacific Northwest. 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