Extended Forecast Discussion NWS Weather Prediction Center College Park MD 157 AM EST Tue Jan 31 2023 Valid 12Z Fri Feb 03 2023 - 12Z Tue Feb 07 2023 ...Dangerously cold temperatures and wind chills are forecast across the north-central U.S. late week spreading into the Northeast for the weekend... ...Overview... The overall upper-level pattern during the medium range period looks to be fairly wavy and progressive, with northern stream troughing extending from a Hudson Bay closed low shifting from the Midwest/Great Lakes into the Northeast late this week and producing significantly cold temperatures and wind chills. Rain and thunderstorms may linger into Friday for the far Southeast/Florida Peninsula with a southern stream wave. Meanwhile, a couple of rounds of upper troughing coming into the West will lead to a wetter pattern there, but nothing too extreme. While the first trough moves across the rest of the lower 48 this weekend with little in the way of precipitation (aside from possible Great Lakes snow) until it reaches the Eastern Seaboard, the second trough is more likely to produce precipitation across the central U.S. by around next Monday-Tuesday, but with uncertainty in the details at this point. ...Guidance/Predictability Assessment... Late this week, model guidance remains in reasonably good agreement with the track of the northern and southern stream troughs in the eastern U.S., with just minor differences in the timing of phasing of those features. Meanwhile most 12/18Z models reached a general consensus with the axis position and the depth of the troughing coming into the West day 3/Friday and into the central U.S. over the weekend. However, the 12Z CMC seemed to be a deep and slow exception when compared to other deterministic models as well as ensemble members. The CMC also showed more stream separation, putting most energy in the southern tier. Meanwhile the 12Z ECMWF indicated some stream separation, while GFS runs and the UKMET kept the trough phased. By late Sunday into Monday when the trough reaches the East, this creates some notable differences in the ECMWF with a faster northern stream trough and lingering energy in the Gulf as opposed to a phased trough steadily tracking east in the GFS runs. The GFS runs are more similar to the ensemble means so their solution was favored. Then with the second trough digging into the West early next week, models are in agreement with the existence of this feature with some typical variations for the latter medium range timeframe. The ECMWF was perhaps an outlier with additional energy digging in on the western side of the trough by Monday-Tuesday, eliminating the ridge across the Northwest that other models/ensembles show. Given the above considerations, the WPC forecast was based on a blend of the 12/18Z GFS and 12Z ECMWF and UKMET early in the period, increasing the proportion of the GEFS and EC ensemble means as the period progressed and limiting the amount of 12Z ECMWF. Good continuity was maintained with the previous forecast, and the incoming 00Z models seem to be consistent as well--the 00Z CMC especially seemed to come into better alignment with the first couple of features, though so far it is the most aggressive with closing off a low within the second trough in the West. ...Weather/Hazard Highlights... By the beginning of the medium range period Friday, rain and thunderstorms may be ongoing across portions of the Southeast before the upper troughing and the surface low pressure system and frontal boundary track offshore. Most areas across the East will be dry by Saturday, but some moisture return by Sunday could lead to precipitation across the Eastern Seaboard, but with uncertainty for how much falls onshore. Farther north, light snow may expand in coverage across the Midwest/Great Lakes/Northeast by the weekend with a clipper system, adding to some light lake effect amounts. Meanwhile in the West, rounds of light to moderate precipitation can be expected given the couple of upper troughs moving through. The Pacific Northwest into northern California will be the most likely to see lower elevation rain and higher elevation snow, while the northern Rockies and other areas of the Intermountain West could see some light to moderate snow as well. Precipitation amounts will be more typical of normal cool season events and nothing like those in the parade of atmospheric river events about a month ago. Then the second upper trough in particular may provide some forcing for precipitation to develop across the central U.S. by Monday-Tuesday, but the specifics remain uncertain at this point. Very cold conditions are likely for the north-central to northeastern tier of the U.S. late this week underneath cold upper troughing. On Friday highs in the single digits are forecast for the Upper Midwest into northern New England, and bitterly cold lows combined with wind will produce dangerous wind chills especially Saturday morning. Several low maximum and minimum temperatures could be set especially in the Northeast before a moderating trend by Sunday. But other than that, temperatures are expected to trend warmer than average across most areas, including the central U.S. and spreading into the East early next week. The West could cool down especially in terms of high temperatures underneath a longwave trough by Monday-Tuesday though. 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, experimental excessive rainfall outlook, 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/#page=ero https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml