Extended Forecast Discussion NWS Weather Prediction Center College Park MD 505 PM EDT Tue May 17 2022 Valid 12Z Fri May 20 2022 - 12Z Tue May 24 2022 ...Record Heat from the South to the Mid-Atlantic/Northeast Friday and Saturday... ...Heavy Rainfall and Severe Weather Threat to focus over the East-Central U.S. Friday and Saturday... ...Heavy Snow Threat for the Central Rockies/High Plains Friday... ...Overview... The overall pattern across the lower 48 will amplify again by the start of the medium range period on Friday with a digging trough over the West/north-central U.S. and rising heights in the East. This yields cooler temperatures and lowering snow levels in the West and a continued hot regime for the South and East late this week. A deepening low pressure system tracking from the northern tier/Upper Great Lakes into Canada will push a strong cold front eastward, with increasing chances for heavy rainfall and severe weather along and ahead of it. Lead southern stream impulses will add to convective rainfall potential across the South. The main front/trough will reach the East this weekend as the guidance continues to be very uncertain with details upstream across the Northwest into early next week. A northward surge of tropical moisture will also increase rain/thunderstorm chances across Florida late this week and into the weekend. ...Guidance Evaluation/Preferences... Overall, guidance continues to show a more open shortwave across the northern Plains/upper Mississippi Valley on Friday with the models also showing decent agreement (with some minor timing differences) on amplified troughing sliding into the East by late weekend. However, the 00 UTC ECMWF in particular also places more emphasis on a southern stream impulses ejecting underneath across the South along with heavier convective rainfall across the South/Southeast this weekend into Monday. The 12 UTC GFS/ECMWF support this emphasis. By Sunday, the most prominent source of uncertainty continues to be flow evolution across the West. The GFS and many GEFS members continue to be much stronger/more pronounced with energy on the backside of the trailing trough into the West this weekend, which eventually results in another fairly amplified shortwave into the south-central U.S. early next week. On the other extreme, recent runs of the ECMWF are rather flat/more ridgy with the flow over the West late period. This all seems to stem from differences in how quickly energy from the North Pacific moves into western North America. The ECMWF is much faster to break down the East Pacific ridge which allows the energy to slide inland, while the GFS tends to maintain a more blocky ridge allowing for more amplification downstream across the West. The ensemble means themselves seem to agree with their deterministic counterparts, although there are a fair number of members showing both possibilities. This aspect of the forecast remains very uncertain, with WPC progs suggesting a solution a bit closer to the GFS/GEFS. The WPC medium range product suite was primarily derived from a general blend of the deterministic models through day 5 along with the NBM and a transition towards the ensemble means days 6-7 along with the GFS/ECMWF for a little more definition to individual systems. ...Weather/Threat Highlights... The relentless heat across the Southern Plains/Lower Mississippi Valley should finally break into the weekend as record heat spreads into the East. Numerous daily records could be met or exceeded still Friday across Texas/Louisiana and the Southeast, with record values reaching the Upper Ohio Valley/Appalachians/Mid-Atlantic/Northeast Friday into the weekend. A strong cold front will spread for rain and thunderstorms to spread through the Great Lakes/Ohio Valley/Mississippi Valley on Friday and into the East on Saturday. Heavy rainfall as well as severe weather is possible along this boundary given above normal moisture and instability present. Flooding and flash flooding could be an issue as well, particularly in places that may be sensitive to additional rainfall given wet antecedent conditions. Lead southern stream impulse energy may also focus heavy rainfall areas underneath over the South. Behind this front, unseasonably cool temperatures will move from the Rockies into the north-central U.S., lowering snow levels across the Rockies. There is a threat of heavy snow over the central Rockies into Friday and even some snow into the adjacent High Plains. Additional shortwave digging into the south-central U.S. into next week may re-kindle convection/rainfall, albeit with less certainty at these longer time frames. The Northwest will be fairly dry late this week but may see some moisture return during the weekend or early next week, with uncertain flow details keeping confidence low for specifics of timing/coverage/amounts of any precipitation. The southern half of the West should see a warming trend expand from west to east Saturday-Monday with California seeing the warmest high temperature anomalies of plus 10-15F. Elsewhere, guidance agrees that tropical moisture should surge northward across the Florida peninsula late this week into the weekend, bringing a round of heavy rainfall to portions of the Sunshine State. Schichtel/Santorelli 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 Central/Southern Plains, the Lower Mississippi Valley, the Tennessee Valley, the Great Lakes, the Middle Mississippi Valley, and the Ohio Valley, Fri-Sat, May 20-May 21. - Heavy rain across portions of the Southeast and the Lower Mississippi Valley, Sun-Mon, May 22-May 23. - Heavy rain across portions of the Southeast, Fri, May 20. - Heavy rain across portions of the Mid-Atlantic, Sun, May 22. - Heavy snow across portions of the Central Rockies and the Central Plains, Fri, May 20. - Severe weather across portions of the Central Plains, the Lower Mississippi Valley, the Southern Plains, the Middle Mississippi Valley, the Great Lakes, and the Ohio Valley, Fri, May 20. - Flooding possible across portions of the Northern Plains. - Flooding occurring or imminent across portions of the Lower Mississippi Valley, the Upper Mississippi Valley, and the Northern Plains. - Flooding likely across portions of the Northern Plains. - Excessive heat across portions of the Central Gulf Coast, the Lower Mississippi Valley, and the Southern Plains, Fri-Sat, May 20-May 21. - Excessive heat across portions of the Lower Great Lakes, the Northeast, the Central Appalachians, and the Mid-Atlantic, Fri-Sun, May 20-May 22. - Much below normal temperatures across portions of the Central Rockies, the Central Plains, the Central Great Basin, the Northern Plains, and the Northern Rockies, Fri-Sat, May 20-May 21. - Enhanced wildfire risk across portions of the Southern Rockies and the Southwest, Fri, May 20. WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation, experimental excessive rainfall outlook, 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/#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