Extended Forecast Discussion NWS Weather Prediction Center College Park MD 159 AM 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 finally becomes more progressive by late week into early next week, as the deep and broad upper trough that should dominate much of the lower 48 during the short range period will finally get displaced into the East and then the Atlantic over the weekend. This is due to an upper ridge shifting across the western and then central U.S. by late week/weekend and into the eastern U.S. early next week. Ahead of the ridge, mean northwesterly flow will bring in another Arctic surface high, producing much below normal temperatures across the central U.S. and eastern U.S. especially Friday and Saturday before gradually moderating. While temperatures are forecast to be generally be 15-30F below average, at most locations this second round of cold temperatures should be less extreme than the current cold wave. Meanwhile, rounds of precipitation are expected for the West Coast, and in the East possible low development along a surface frontal system or two could produce some winter weather particularly Friday, but with continued uncertainty for details. Return flow from the Gulf of Mexico will increase precipitation chances across the central U.S. for the early part of next week. ...Guidance/Predictability Assessment... Model guidance seems to be handling the large scale pattern well, with good model agreement on most features including the initial East upper trough, surface high pressure diving into the central/eastern U.S. late week, the ridge progressing across the lower 48, and mean troughing coming in behind that. One aspect of the forecast with some potentially impactful model differences is with the exact pattern in the East/western Atlantic on Friday. Models are agreeable that a shortwave should consolidate in the east-central U.S. by Friday as energies from the northwest and north combine within the broad trough, and this strong shortwave pivots east and supports low pressure formation. But a surface low trying to consolidate in the western Atlantic may have a more complex evolution than normal because of another low pressure/frontal system just to the west onshore in the East. Once the coastal low forms it looks to track quickly northeastward, limiting impacts to the East Coast too much, but some snow/wind could produce some issues. After some CMC runs showed the low closest to the coast, the 12Z and now 00Z runs are among the farthest east solutions. Also regarding the CMC, its runs had been persistent in showing troughing lingering across the East to western Atlantic Sunday-Monday while other guidance shifts it away more quickly. The 00Z CMC has now joined with the better consensus for a faster trough. But leaned away from the 12Z CMC at the time of forecast generation. Additional details of shortwaves within the mean trough marching into the West behind the ridge also show some variability in timing, though certainly within typical spread during the mid-late medium range timeframe. These will take additional time to resolve especially since they are tracking from the data-poor Pacific that causes models to struggle. A model/ensemble mean blend seemed best to handle these features. The WPC 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 (and of course the time-limited UKMET) as the period progressed, ending up with an even blend of the 18Z GFS and GEFS mean and 12Z ECMWF and EC mean by Day 7. This smoothed out model differences but maintained some strength in individual systems. ...Weather/Hazards Highlights... 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. Below normal temperatures by 15-30 degrees 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 over most areas. The highest potential for hazardous cold is across the north-central Plains into parts of the Upper Midwest. Temperatures are forecast to start moderating closer to normal Sunday and become near to above average in the central and eastern U.S. by Monday-Tuesday as the surface high shifts away 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. The West Coast states can expect rounds of precipitation for the latter part of this week 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 the Marginal Risk in the Excessive Rainfall Outlook for portions of northern California on Friday-Friday night still looks reasonable. As precipitation generally expands southeastward over the weekend, a similar Marginal Risk is now 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 first for the southern Plains Sunday. Models indicate heavier rain there by Monday while the precipitation shield also 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. 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