Extended Forecast Discussion NWS Weather Prediction Center College Park MD 210 AM EST Thu Feb 17 2022 Valid 12Z Sun Feb 20 2022 - 12Z Thu Feb 24 2022 ...Developing heavy rainfall threat across portions of the South next week... ...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... Recent model guidance shows fairly good agreement with the large scale pattern through the middle of next week, though with plenty of uncertainty in the smaller scale details, like individual shortwaves as well as the main trough's track and timing. There remains good agreement for a compact upper low likely west of Baja California to begin the period Sunday and weakening as it moves eastward, as well as good consensus 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. For the 12/18Z model cycle, each model suite mainly stayed within their own camps, as shown by the spaghetti plots of individual ensemble members and cluster analysis. For the most part, the ECMWF suite including the deterministic, the ensemble mean, and the bulk of ensemble members showed a slower and deeper solution first with the small Baja upper low and more prominently with the larger scale trough forming and broadening in the West. The CMC and GFS suites were more progressive with these features. These differences are most evident by Wednesday-Thursday, with one reason for the differences in the trough track being additional energy spilling southward near the West Coast--stronger in the GFS runs with less phasing with the initial trough. 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. Given these differences, and considering common model biases, the WPC forecast strove for a middle ground solution with the pattern, not discounting the slower EC or the faster CMC/GFS solutions but leaning in between. The incoming 00Z deterministic ECMWF appears to have sped up somewhat compared to the 12Z run and is fortunately somewhat in line with the WPC forecast trough axis. The forecast blend was based on the 12/18Z (newest available at the time) deterministic model suite through days 3-5, phasing in and increasing the proportion of the GEFS and EC ensemble means by the later days rather than keying on any particular model. ...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 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. Snow could also spread farther east into the northern High Plains Sunday and into the rest of the northern tier states as the first half of the week progresses. The confidence in the axis of heaviest snow and snowfall amounts remains somewhat low. Farther south, moist inflow is forecast to return to the south-central and southeastern U.S., with increasing chances for heavy to possibly excessive rainfall. Models seem to be converging on portions of the Lower Mississippi/Tennessee Valley states as the greatest threat for heavy rainfall next Monday-Wednesday. Multiple rounds of rain and storms over the same area could increase the potential for flooding and flash flooding. There remains plenty of uncertainty in the details that likely are going to take another few days to resolve, but regardless, the pattern itself would support some sort of heavy rainfall and possibly severe weather with thunderstorms for the first half of the week. 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 or more 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. 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, 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