Extended Forecast Discussion NWS Weather Prediction Center College Park MD 151 PM 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 (including the new 12z run) 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. A general model blend should take care of these relatively minor details at this time frame. 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 with some relatively minor timing differences. With time, recent CMC runs have strayed somewhat to the slow side while building more downstream ridging than other guidance. After Saturday, there are some more significant differences with regards to interaction with northern stream energy. Recent GFS runs have been consistent with a stronger southern stream upper low while the ECMWF suggests earlier interaction/elongation of the trough. The new 12z GFS (available after forecast generation time) did trend towards the ECMWF though but this trend will need to continue to be monitored. This will have implications for eventual surface evolution in the Eastern U.S. early next week. There are still a lot of notable differences with the second bit of energy dropping into the Pacific Northwest as well which could influence evolution of the first system exiting the Rockies as well. Recent runs have trended towards more amplified troughing over the West late period. The updated forecast for today started with an operational model composite days 3-5, excluding the UKMET which was even slower than the CMC with the first Southwest closed upper low Thurs-Sat. After that, increased contributions from the ensemble means in an attempt to mix out some of the harder to resolve detail differences late period. Still was able to maintain 60 percent 06z GFS/00z ECMWF with 40 percent ensemble means. This approach maintained good continuity with the previous WPC forecast as well. ...Weather/Hazard Highlights... The system tracking from the Midwest through the Northeast Thursday into Friday should bring some modest 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. Santorelli/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