Extended Forecast Discussion NWS Weather Prediction Center College Park MD 150 PM EST Thu Jan 9 2025 Valid 12Z Sun Jan 12 2025 - 12Z Thu Jan 16 2025 ... Arctic airmass to reinforce over the lower 48 next week... ...Pattern Overview and Guidance/Predictability Assessment... The overall pattern throughout the medium range period (Sunday Jan 12 - Thursday Jan 16) is generally well handled across the guidance with differences limited to the usual less predictable specifics such as individual shortwave timing. Following a more active pattern with southwesterly upper flow across the central- eastern U.S., a northern-stream upper-low will shift southward from south-central Canada to southeastern Canada/the Great Lakes/the northeastern U.S. helping to reinforce broad mean upper- level troughing and renew a very chilly Winter pattern. Precipitation chances are likely across the Great Lakes, with a trend for higher QPF specifically for favorable lake-effect regions following the initial incoming system and persistent northwesterly flow thereafter. To the west, shortwave energy moving southward along the periphery of the expanding upper-trough to the east is expected to cut off over coastal California under the development of a strengthening northern stream ridge building over the Pacific Northwest/southwestern Canada. This pattern is expected to remain in place through early next week before things gradually begin to change mid-week. Differences in the guidance also increase by the end of the period, with the ECMWF/CMC favoring additional energies helping to reinforce the upper-trough over the central/eastern U.S., and the ECMWF/GFS indicating some eventual eastward progress of the western cutoff low while the CMC remains further offshore. These trends would favor colder temperatures for the eastern U.S. with more persistent, stronger troughing and an uptick in precipitation chances over the Four Corners region with eastward movement of the upper-low. The GFS is also more aggressive in bringing an upper-trough over the northeastern Pacific southward to the Pacific Northwest while the CMC and ECMWF remain further north, which would impact potential precipitation chances for the region. The updated WPC forecast beings with a composite blend of the 00/06Z deterministic guidance given good agreement. A contribution from the ensemble means is gradually increased in the mid- to late period as differences increase, and mostly in expense of the GFS/CMC over the ECMWF given the noted outlier nature of these models concerning certain parts of the forecast. ...Weather/Hazards Highlights... Low pressure system translation and upper support should favor light to moderate organized snows expected to slowly work over the Midwest and Great Lakes early next week, and also leading to the triggering of additional potentially heavy lake effect snows. In the wake of this system, another reinforcing blast of Arctic air is forecast to spill into the north-central U.S. and spread to the East and Southeast next week. Hazardous cold temperatures are expected from the northern Plains to the Gulf and Southeast Coasts, with temperature anomalies potentially reaching 10-20 degrees below normal and even more frigid wind chills. Some moderate to locally heavy showers will be possible late this weekend into early next week along the immediate Gulf Coast and Florida as a cold front passes through the region. Some Gulf moisture may return far enough northward to bring showers to southern Texas by the middle of next week. Following some light to moderate snow showers for portions of the northern Rockies Sunday, the West will be mostly dry into early next week as upper-level ridging builds over the region. Eastward progression of an upper- level cut-off low over coastal California may bring renewed precipitation chances to the Four Corners region mid-week. Putnam 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 (QPF), excessive rainfall outlook (ERO), winter weather outlook (WWO) probabilities, heat indices, and Key Messages can be accessed from: 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