Extended Forecast Discussion NWS Weather Prediction Center College Park MD 309 PM EST Thu Feb 17 2022 Valid 12Z Sun Feb 20 2022 - 12Z Thu Feb 24 2022 ...Increasing confidence in a southern U.S. heavy rainfall event with northern tier heavy snows... ...Overview... As next week begins, quasi-zonal flow across much of the U.S. will be transitioning to a more amplified pattern as upper-level energy spills into the West to create deep troughing, while ridging builds in the East. The trough will slowly track eastward along with a strong frontal system, behind which arctic high pressure should produce well below normal temperatures in the central U.S., with warmer than average temperatures in the East. Light to moderate precipitation is likely in the West underneath the trough and spreading into the central and eastern U.S., with the possibility of northern tier snow and southern U.S. rain and thunderstorms. A multi-day rainy period for parts of the Lower Mississippi and Tennessee Valleys could lead to flooding and flash flooding there. ...Guidance Evaluation/Predictability Assessment... Through the 00z/06z cycle, model guidance continues to show very good agreement with the large scale pattern into the middle of next week, though with some lingering uncertainty in the smaller scale details. A compact upper low likely west of Baja California to begin the period Sunday should weaken as it moves eastward, with good consensus as well on the large scale for energy to spill into the West east of an amplified eastern Pacific ridge by next Sunday-Monday, gradually deepening and shifting slowly eastward. 00z/06z models showed somewhat better agreement on Western trough timing, with the 00z ECMWF adjusting westward with its axis, more in line with the GFS. However, the 12z ECMWF again slowed down a little bit with the axis, while the 12z CMC is faster than its previous run. So, questions on timing continue to loom late period. Additionally, the evolution of upstream energy is rather uncertain stemming from high latitudes and rounding the Pacific ridge, leading to low confidence in the specifics of the embedded energies within the main trough and its eventual movement, especially by day 7. The past few runs of the GFS has been quite sharp and fast with the next shortwave into the Pacific Northwest next Thursday, but the 12z run looks more reasonable and in line with the ECMWF/CMC. The WPC blend for todays progs featured a multi deterministic model solution for days 3-5 amidst above average agreement. For days 6 and 7, the ensemble means were phased into the blend with increasing proportions to help account for the uncertainties in the Western U.S. trough timing and mitigate the smaller scale details. The resulting solution provides a middle ground solution for the trough and maintains good continuity with the overnight WPC package as well. ...Weather Highlights/Threats... Precipitation will be increasing in the Pacific Northwest and Northern Rockies over the weekend with the upper trough coming in. Coastal and valley rain and moderate to heavy mountain snow are likely over the Pacific Northwest spreading into California, the Great Basin, and the Rockies and persisting there into the middle of next week. As the western trough deepens, it should tap into Gulf of Mexico moisture to fuel a heavy to possibly excessive rainfall event from portions of the lower Mississippi Valley into the Tennessee Valley during the early to middle part of next week. A multi-day event over roughly the same areas is possible as additional energy gets reloaded into the trough out West. This likely increases the threat for flooding and flash flooding into at least next Wednesday. SPC continues to mention the threat for severe weather within this pattern as well across parts of the South. On the north side of this system, confidence is increasing that a swath of moderate to heavy snowfall will be present stretching from the northern Plains into the Upper Midwest/Great Lakes Monday and Tuesday, and eventually affecting parts of far northern New England as well, though the exact axis of heavy snows remains somewhat uncertain. Temperatures as next week begins are expected to be above normal for parts of the West into the Plains, with the largest anomalies of highs 20+ degrees above average over Nebraska. As the pattern amplifies, the south-central U.S. to the East Coast will further warm through the first half of next week, with widespread highs 10-20F and lows 20-30F above normal. Meanwhile, a cold Arctic high is forecast to expand across the north-central U.S., leading to well below average temperatures initially across the northern High Plains and spreading southward (as far south as Texas) through midweek. Temperatures in some places could be 30-40 degrees below normal, and highs in the single digits to even below 0F are expected for Montana, the Dakotas, and northern Minnesota Monday and Tuesday, with cold temperatures lingering through Wednesday and Thursday as well. Santorelli/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 Hazards: - Heavy rain across portions of the Lower/Middle Mississippi Valley, the Ohio/Tennessee Valley, the Southern Appalachians, and the Southeast, Mon-Thu, Feb 21-Feb 24. - Heavy snow across portions of the Pacific Northwest, the Northern Rockies, and the Northern Great Basin, Sun, Feb 20. - Heavy snow across portions of the Great Lakes, the Upper Mississippi Valley, and the Northern Plains, Mon-Tue, Feb 21-Feb 22. - Heavy snow across portions of the Northeast, Tue, Feb 22. - Flooding possible across portions of the Northeast, the Central Appalachians, the Middle Mississippi Valley, the Mid-Atlantic, the Great Lakes, and the Ohio Valley. - Flooding likely across portions of the Great Lakes and the Ohio Valley. - Much below normal temperatures across portions of the Great Plains, the Central Great Basin, the Northern/Central Rockies, and the Upper/Middle Mississippi Valley, Mon-Thu, Feb 21-Feb 24. - Enhanced wildfire risk across portions of the Southern Rockies and the Southern Plains, Sun, Feb 20. - Heavy precipitation across portions of mainland Alaska, Mon, Feb 21. WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation, 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/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml