Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 AM EST Thu Jan 19 2023 Valid 12Z Sun Jan 22 2023 - 12Z Thu Jan 26 2023 ...Heavy snow threat for the interior Northeast Sunday into Monday... ...Overview... The established mean pattern extending into next week will feature a strong eastern Pacific ridge and a sequence of shortwaves dropping into the West then ejecting beyond the Rockies to produce individual systems generating various types of significant weather. These shortwaves/systems will likely produce some locally focused rain/mountain snow from the Northwest east and southeast into the Rockies, and then heaviest rainfall potential over the Southeast and winter weather potential from the Plains into the Northeast, with more moderate rainfall totals between these two areas. Confidence is steadily increasing for the leading system forecast to reach New England by early Monday given improvement in guidance clustering. However there are still important differences for the energy dropping into the West Sunday-Monday, ultimately causing continued forecast challenges for precipitation specifics over the West and the system that may affect the central-eastern U.S. about three days after the first one. Guidance suggests that subsequent energy dropping into the West will be a lot weaker, yielding a broader mean trough by next Thursday. ...Guidance/Predictability Assessment... For the leading system reaching New England by Monday, following a majority model/ensemble cluster has held up well continuity-wise over the past day or so since guidance fully adjusted to the currently depicted phasing aloft over the central U.S. during the weekend. Latest UKMET/CMC runs have adjusted closer to the majority scenario for the surface low track after some earlier runs were on the south/southeast side of the spread previously. The 00Z CMC does stray a bit on the slow side by late Monday though, as most guidance shows a fairly strong system reaching near Nova Scotia at that time. Overall an operational model composite with greater emphasis on the 12Z GFS/ECMWF provided a reasonable starting point. Models/ensembles have been diverse and inconsistent for days with the trailing shortwave digging into the West, highlighted by pronounced reversals in the ECMWF and GFS runs over the past day or so. As of now the GFS is on the open and progressive side of the spread through Monday, in contrast to a couple days ago when it was a deep/southwestern extreme with its upper low. On the other hand the ECMWF has made the opposite adjustment from being progressive to joining a slower cluster that is now close to the UKMET/CMC. These differences lead to corresponding timing and evolution differences across the central-eastern U.S. through Thursday. The 12Z/18 GFS was somewhat closer to other guidance (versus the faster 18Z run, and now the 00Z run) so preferred that run for what GFS input was included in the forecast blend. Even with some of the differences along the way, the 12Z ECens/CMCens and 18Z GEFS means along with the 12Z GFS/ECMWF/CMC ended up with remarkably similar surface low positions near New England by early day 7 Thursday. Including components of those three operational models helped to add some definition relative to the ensemble means along with intermediate timing. The resulting blend provided reasonable continuity for the upper system and surface reflection through the period. ...Weather/Hazard Highlights... The forecast remains on track with the general theme of two significant systems affecting the eastern half of the country during the period, with greater confidence for details of the first one than the second. The first system will likely track from the Mississippi Valley/Gulf Coast into New England Sunday-Monday, bringing the potential for some wintry weather from the Midwest into the central-northern Appalachians and interior New England. The Appalachians/New England area has the best potential for meaningful to heavy snow totals. Expect rain to the south of the snow area, with heaviest rainfall most likely over the Southeast. Currently expect the next system to develop over the southern Plains/Gulf Coast around Tuesday and then reach near New England by Thursday, but some changes are possible in the details. Depending on exactly how this system evolves, there may be similarities in precipitation distribution/amounts, including the best heavy rainfall potential over the Southeast and a band of wintry weather from the Plains northeastward into New England. The exact rain-snow line will depend on finer details of system strength/track that are still not resolved well. Both of these systems may be strong enough to produce enhanced winds over the Northeast. Across the West, rain and mountain snow of mostly moderate amounts (but perhaps locally enhanced) should extend south-southeast across the Rockies and nearby areas with upper trough/low support and lower level upslope flow. Southwestward extent of the precipitation will depend on exact track/amplitude of the weekend/early next week feature whose details remain uncertain. Most areas from the northern three-fourths of the West into the central-southern High Plains should remain chilly through the period with highs generally 5-15F below normal but possibly up to several degrees colder around Monday underneath or in the wake of an upper trough/low over the Four Corners states. Far northern parts of the West should be within a few degrees on either side of normal, while highs at locations along the central-northern West Coast may drift a few degrees above normal as the eastern Pacific upper ridge extends a tad eastward. Meanwhile the eastern U.S. will tend to see above normal lows, occasionally by more than 10F, while highs should be near to moderately above normal. The northern Plains and vicinity will tend to be above normal into midweek and then trend colder by Thursday after passage of a cold front dropping south from Canada. 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