Extended Forecast Discussion NWS Weather Prediction Center College Park MD 147 PM EST Tue Jan 16 2024 Valid 12Z Fri Jan 19 2024 - 12Z Tue Jan 23 2024 ...Another Arctic airmass comes into the central U.S. late week, but not as extreme as the first one... ...Multiple rounds of potentially heavy precipitation will affect the West... ...Overview... The large scale pattern will finally become more progressive by late week into early next week as the deep, broad upper trough over the eastern U.S. shifts east into the Atlantic. As the trough shifts east, a ridge will follow behind it, moving from the western to the central U.S. this weekend and to the eastern U.S. early next week. Northwesterly flow ahead of the ridge will bring in another Arctic surface high, producing much below normal temperatures across the central and eastern U.S. late this week into this weekend, especially Friday and Saturday, before gradually moderating. While temperatures are forecast to be generally be 15-30F below average, this second round of cold temperatures should be less extreme than the current cold wave at most locations. Meanwhile, rounds of precipitation are expected for the West Coast, and potential low development along a surface frontal system or two could produce some winter weather in the East, particularly on Friday. Return flow from the Gulf of Mexico will increase precipitation chances across the south-central U.S. for the early part of next week. ...Guidance/Predictability Assessment... Model guidance continues to show good agreement on the large scale patter with expected differences in small scale features. An upper trough will shift over the east early in the period then into the Atlantic late this weekend. Following behind this, a ridge will shift east across the lower 48 and several waves of energy/shortwaves will move towards the West Coast. Models have also been showing a trough/upper low developing near or south of the Southwest U.S. mid-next week, with moist Gulf air moving north into the south-central U.S. ahead of it. At the surface, this pattern will produce a couple of frontal systems moving off the East Coast early in the period and a series of frontal systems pushing into the West through the weekend. There are some timing differences between the models concerning the shortwaves marching into the West, but falls within typical spread expected in the medium range period. These will take additional time to resolve especially since they are tracking from the data-poor Pacific that causes models to struggle. The WPC afternoon forecast was based on a multi-model deterministic blend early in the forecast period. Gradually incorporated the GEFS and EC ensemble mean guidance while phasing out the CMC (strayed a bit farther from the general consensus than other guidance) and UKMET (time-limited) as the period progressed, ending up with an even blend of the 06Z GFS and GEFS mean and 00Z ECMWF and EC mean by Day 7. This smoothed out model differences but maintained some strength in individual systems. ...Weather/Hazards Highlights... The West Coast states can expect rounds of precipitation through this weekend into early next week. By late Friday, a round of precipitation looks to affect California along and ahead of a frontal system. Models indicate that rain rates could be over half an inch per hour as instability may sneak onshore, along with good dynamical support for heavy rain from a strong shortwave aloft. Thus there is a Marginal Risk in the Excessive Rainfall Outlook for portions of northern California on Friday-Friday night. As precipitation generally expands southeastward over the weekend, a similar Marginal Risk is in place for Saturday-Saturday night but with some southward expansion compared to Friday's outlook. These multiple rounds of precipitation should continue into early next week from the West Coast into the Intermountain West. Some locally moderate/heavy totals are possible over favored terrain, depending on upper level and surface details that will be better resolved in coming days. Parts of the interior Northwest could see conditions favorable for freezing rain for multiple days. Eventually by early next week, the pattern should allow for Gulf return flow to produce rain chances for the south-central U.S. on Sunday. Models indicate heavier rain developing in the South by Monday while the precipitation shield expands northeastward Monday-Tuesday, with some possible wintry weather on the northern side. A frontal system with embedded low pressure reaching the East during the latter part of the week may produce some precipitation across the Appalachians and parts of the East Coast Friday. Snow should be the primary precipitation type considering the cold air in place. The central Appalachians will likely see the heaviest snow with upslope flow helping to contribute to amounts. Farther east the most likely axis for notable snow looks to be mainly across the northern Mid-Atlantic into southern parts of the Northeast, but amounts do not look to be too heavy. Expect continued periods of lake effect snow with persistence of cold air and breezy conditions into Saturday. Lake effect snow will likely taper off by Sunday as surface high pressure builds into the East. Arctic high pressure will provide a reinforcing surge of cold air that is forecast to push well into the central and east-central U.S. by Friday and into the East on Saturday. Temperatures 15-30 degrees below normal should be widespread, with localized anomalies more than 30F below normal, and this cold episode will still be notable even if not quite as extreme as the current cold event for most areas. The highest potential for hazardous cold and dangerous wind chills will be across the north-central Plains into parts of the Upper Midwest. Temperatures are forecast to start moderating towards normal Sunday and become near to above average in the central and eastern U.S. by Monday-Tuesday as the surface high shifts into the Atlantic and the upper ridge allows for more southerly flow. From the Rockies westward, near to above normal temperatures are expected, with higher anomalies (generally 10-20F above average) for lows than highs. Dolan/Tate 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 forecast, excessive rainfall outlook, winter weather outlook probabilities, heat indices and Key Messages 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 https://www.wpc.ncep.noaa.gov/#page=ovw