Extended Forecast Discussion NWS Weather Prediction Center College Park MD 157 PM EST Tue Jan 17 2023 Valid 12Z Fri Jan 20 2023 - 12Z Tue Jan 24 2023 ...Overview... Guidance offers an agreeable large scale pattern forecast overall, albeit with significant embedded uncertainties with upper flow/surface details that will affect sensible weather over some areas. From the multi-day mean perspective the ongoing consensus shows positively tilted troughing over the lower 48, between a strong ridge over the eastern Pacific and another ridge to the south or southeast of Florida. This pattern should favor highest precipitation totals over the southeastern quadrant of the country with some moisture spreading elsewhere over the eastern half of the country. Meanwhile shortwave energy dropping into the mean trough will periodically spread rain/mountain snow from the Northwest east/southeast into the Rockies, and keep the most of the West on the chilly side for daytime highs. Within these general themes, the ongoing detail differences for important upper flow specifics keep confidence well below average for resolving the exact character of precipitation coverage/intensity/type over many areas. ...Guidance/Predictability Assessment... Guidance solutions still agree decently for most features up to about early day 4/Saturday. This includes the system departing from the Northeast and the upper low expected to track across Arizona/New Mexico Friday-Saturday. Then models and ensemble members rapidly diverge for the ultimate evolution of a shortwave entering western North America on Saturday (with differences in sharpness/amplitude already showing up by early Saturday), along with what becomes of the Arizona/New Mexico upper low as well as whether northern stream flow interacts with it. These differences lead to a wide array of possibilities for the track and timing of surface low pressure over the eastern half of the country, as well as for the depth/location of upper troughing over the western-central U.S. from the weekend into early next week. Regarding the Saturday shortwave dropping into the West thereafter, recent GFS runs remain the sharpest with the arriving feature, closing off an upper low over the Great Basin while consensus shows an amplified trough. GFS runs and trends from the 00 UTC to the 12 UTC Canadian bring a deep upper low over the Southwest early next week. Teleconnections relative to the East Pacific ridge's core of positive height anomalies still suggest that bringing such low heights into the Southwest could be a little overdone, but latest GEFS/ECMWF runs and recent ECMWF runs agree upon a less extreme trough/implied upper low over a similar area. Farther east, GFS handling of incoming western energy results in sharp digging of southern Canada/Plains flow and phasing with the ejecting Southwest upper low. This results in a south-central Plains through Great Lakes surface system that becomes fairly strong by late Sunday. Recent GEFS mean runs are showing some phasing as well, but ultimately shifts low pressure emphasis to the New England coast by early Monday. The 00 UTC ECMWF and Canadian offered more modest phasing, leading to a Southeast U.S./East Coast track. However, there is an emerging guidance signal in the 12 UTC ECMWF/Canadian toward more phasing. Ensemble members still diverge considerably late in the period for the upper pattern and surface lows. At the surface the GEFS/ECens have been tending to favor a progressive surface system reaching near New England by early Monday. Preference was to continue to incorporate ensemble themes which also yields the best continuity while awaiting more confidence adjustments. However, the trend of the 12 UTC ECMWF/Canadian toward phasing may yield a product continuity shift with future forecast packages that await further confidence building with favorable support from upcoming runs. The WPC medium range forecast was primarily derived from a composite of 00/06 UTC operational models Friday into Saturday. Then as central U.S. phasing became an issue in the GFS by Sunday and with quickly growing forecast spread into/from the West continuing into next week, the WPC forecast quickly incorporated significantly more 06 UTC GEFS/00 UTC ECens mean input. ...Weather/Hazard Highlights... The Northeast will likely see snow lingering through Friday as a band of moisture lingers over the region behind low pressure tracking offshore from New England. The Four Corners states should see some snow and very low elevation rain late this week with the upper low tracking through the region. The trailing upper trough dropping into the West, and the leading cold front, will bring some rain/mountain snow into the Northwest by Saturday and then spreading east/south into the Great Basin/Rockies, possibly reaching into the Southwest as well. Precise coverage and amounts are still uncertain though. Confidence is also below average for the distribution, amounts, and type of precipitation over the eastern half of the country from the weekend into early next week. The best signal at the moment is for some enhanced rainfall across the South/Southeast based on teleconnections relative to the expected large scale mean pattern, and specifically the interaction of the Gulf moisture with a wavy warm front that may reach/pass through the Gulf Coast. There is still a fair degree of guidance spread just for the timing of this front though. Otherwise, there is a very wide envelope for where a significant surface wave may track with obvious precipitation/type implications, including wintry weather threats. Potential low tracks extend as far west as the Midwest or could be confined more to the East Coast, the latter being the scenario currently followed by the most recent manual/gridded forecasts. However, the latest trends from the 12 UTC guidance suite suggest a potentially more westward shifted and stronger low track to certainly monitor for forecast updates. Much of the West and into parts of the High Plains should see below normal temperatures through the period, expect perhaps for far northern areas. Highs will be particularly cool with each day likely to feature max temperatures 5-20F below normal. An isolated location or two could see a record cold high. On the other hand, the eastern U.S. should see near to above normal highs through the period with warmer anomalies for morning lows. The northern Plains will also see above normal temperatures late this week into the weekend but then decline to near or slightly below normal. Rausch/Schichtel 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