Extended Forecast Discussion NWS Weather Prediction Center College Park MD 158 AM EST Mon Jan 16 2023 Valid 12Z Thu Jan 19 2023 - 12Z Mon Jan 23 2023 ...Late week system likely to bring meaningful snow to northern parts of the East, especially Thursday-Thursday night... ...Overview... Most models and ensembles continue to show a large scale mean pattern that features a building eastern Pacific ridge and downstream positively tilted trough. In general this regime should favor episodes of moisture spreading through the Northwest and east/southeast into the Rockies (and cool temperatures over most of the West) as shortwaves drop into the mean trough, as well as above average rainfall chances/totals across the Southeast. Individual model/ensemble solutions vary considerably for some details within this pattern, so confidence is much lower for the specifics of coverage and amounts of precipitation within the favored areas as well as for moisture that could extend into other areas at times. Just ahead of the changing mean pattern, a system will track east-northeast from the Midwest Thursday into Friday and bring snow to northern areas along with some rain farther south. ...Guidance/Predictability Assessment... Guidance spread for the leading Northeast system has been fairly stable to slightly improving in latest runs. Recent GFS runs have had the tendency to open the upper low a bit earlier than the model majority, leading to a corresponding earlier shift of emphasis to a leading low tracking off the East Coast from the initial parent low by early Friday. The new 00Z GFS has nudged a bit toward holding onto the parent low a bit longer but is on the fast side with the offshore wave. The remainder of the forecast primarily involves initial shortwave energy dropping into the West to form a closed low likely to track over the Four Corners/southern Rockies region around Friday-Saturday, if/how this feature may interact with northern stream flow, and then significant differences in the character of the eastern Pacific ridge/surrounding energy that should drop into the mean trough--with influence on the first feature. In a broad sense the guidance clusters decently with the leading western feature into Saturday, though new 00Z model runs suggest a somewhat sharper digging/southwestern upper low track versus the 12Z ECMWF/ECMWF mean. With time, recent CMC runs have strayed somewhat to the slow side while building more downstream ridging than other guidance. Significant detail differences for central Pacific evolution arising as early as the short range time frame ultimately lead to the pronounced differences seen over the eastern Pacific/western U.S. from the weekend into next Monday. Based on teleconnections relative to the strong core of positive height anomalies associated with the East Pacific ridge, latest GFS runs may be overdone with the overall depth of the western trough/upper low while the 12Z ECMWF/ECMWF mean could be a underdone--as also suggested by the CMC/CMC mean. Thus prefer an intermediate reflection of this second bundle of amplifying energy. The new 00Z ECMWF has made a dramatic change toward more troughing. Influence of this evolution on the feature emerging from the Rockies, as well as any possible stream interaction, lead to very low confidence on what the surface evolution may be over the eastern U.S. after early Saturday. Latest GFS/GEFS mean runs and the 12Z ECMWF seemed to offer the closest clustering for a coherent system while the 12Z ECMWF mean was on the fast side due to more northern stream dominance. The updated forecast started with an operational model composite for the first half of the period, followed by a transition to 60 percent total 18Z GEFS/12Z ECMWF means and 40 percent 18Z GFS/12Z ECMWF by day 7 Monday. This approach yielded the desired compromise evolution over the West late in the period while keeping the ECMWF mean sufficiently in the minority over the East. ...Weather/Hazard Highlights... The system tracking from the Midwest through the Northeast Thursday into Friday should bring snow to areas from the central/northern Great Lakes through New England. Expect rainfall to the south, with some locally moderate to heavy pockets of rainfall possible but likely not reaching the point of being excessive. With the pattern change developing farther west, the next area of rain/mountain snow crossing the West late this week should be on the moderate side and progress through the Interior West/Rockies. As the supporting dynamics extend beyond the Rockies, expect the South to see a rebound in rainfall by Saturday as a front lingering over the Gulf lifts back northward ahead of possible developing low pressure. This potential system and its moisture may spread farther north over the eastern U.S. through the weekend and the start of next week, with some wintry weather possible over northern areas. Confidence remains low for the details of surface evolution and precipitation coverage/amounts over the East in this time frame. Expect another front to spread some rain/mountain snow into the Northwest and then farther east/southeast Saturday-Monday, but again with decreasing confidence in specifics. The pattern from late this week into early next week will favor persistently cool daytime highs over much of the West, aside from far northern areas and the Pacific Northwest coast, and into parts of the High Plains. Most locations should see highs of 5-15F below normal with some day-to-day variability with the progression of individual features. On the other hand, the eastern U.S. and northern tier should be above normal on most days with somewhat higher anomalies for morning lows relative to daytime highs. Thursday into early Friday will be particularly warm over the East with some anomalies exceeding plus 20F ahead of the Midwest/Northeast system. Most of any daily records should be for warm lows, at least if such readings can hold on through the end of the calendar day. 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