Extended Forecast Discussion NWS Weather Prediction Center College Park MD 159 AM EST Tue Jan 17 2023 Valid 12Z Fri Jan 20 2023 - 12Z Tue Jan 24 2023 ...Overview... In general, latest guidance is showing little change in both the agreeable large scale pattern forecast but also 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... As was been the case lately, the guidance agrees decently for important 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, latest GFS runs have generally been the sharpest with the arriving feature. The new 00Z run has become more extreme by Sunday, closing off an upper low over the Great Basin while consensus has a mere trough. Prior GFS runs closed off the low over the northern Rockies at that time. Either way, these GFS runs eventually 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 runs and the 12Z ECMWF both agree upon a less extreme trough/implied upper low over a similar area. Latest CMC runs and the 12Z ECMWF divert the trough somewhat eastward while the old 00Z/16 ECMWF was more like latest GFS runs. Farther east, the 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. GEFS mean runs are showing some phasing as well but ultimately shifts low pressure emphasis to the New England coast by early Monday. The 00Z/16 ECMWF had more modest phasing, leading to a Southeast U.S./East Coast track. Non-phased solutions with a faster-moving northern stream shortwave (CMC/UKMET runs and 12Z ECMWF) tend to yield more upper ridging near the East Coast by early Monday and a slower and/or westward surface low. Not surprisingly given the model spread, individual ensemble members diverge considerably late in the period for the upper pattern and surface lows. GEFS members tended to cluster somewhat closer to the amplified Southwest trough while the ECens/CMCens members had greater spread aloft. At the surface the GEFS/ECens have been tending to favor a progressive surface system reaching near New England by early Monday. Preference is to incorporate these ensemble themes which also yields the best continuity while awaiting more confidence adjustments. The updated forecast blend started with a composite of 12Z/18Z operational models Friday into Saturday (though with the ECMWF split between the last two runs). Then as central U.S. phasing became an issue in the GFS by Sunday, the forecast started to incorporate more 18Z GEFS/12Z ECens mean input and then phase out the CMC due to its pattern straying from the GEFS/ECens. By days 6-7 Monday-Tuesday, the blend could still incorporate the 18Z GFS in tempered form while using only the 00Z/16 ECMWF for that model's contribution since its pattern compared a lot better to the means whose total weight reached 50-60 percent. ...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. 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 manual/gridded forecasts. 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 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