Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 AM EDT Wed May 18 2022 Valid 12Z Sat May 21 2022 - 12Z Wed May 25 2022 ...Record Heat in parts of the East on Saturday ... ...Heavy Rainfall and Severe Weather Threat to focus over the East-Central U.S. this weekend... ...Overview... A rather amplified pattern will be in place by the start of the medium range period with a digging trough over the West/north-central U.S. and strong ridging in the East. This yields cooler temperatures across the Rockies and central U.S. and record heat in the East into the weekend. A strong cold front across the Midwest and East this weekend will result in increased chances for heavy rainfall and severe weather along and ahead of it. A northward surge of tropical moisture will also increase rain/thunderstorm chances across Florida this weekend as well. By Sunday into Monday, another shortwave should enter the West Coast with very uncertain details as it pushes east and also with additional energy upstream around next Wednesday. ...Guidance Evaluation/Preferences... The models are showing increasing agreement on the shortwave into the upper Mississippi Valley on Saturday and amplified troughing tracking into the East on Sunday. A general model blend seems to handle this fine. As early as Sunday-Monday however, details regarding flow over the West become highly uncertain. Models agree the next shortwave should cross through the Northwest on Sunday, but the ECMWF/ECENS mean continue to be quite a bit weaker and faster compared the GFS and CMC which favor amplification of the shortwave over the West and thus a much slower progression (and consequently a better tapping of Gulf moisture and precip across the central U.S. early next week). 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, with deterministic models also showing plenty of run to run discontinuities. Given the uncertainty, the WPC forecast for tonight stayed close to continuity which seems to side slightly more towards the slower/stronger GFS/CMC/GEFS mean. ...Weather/Threat Highlights... The relentless heat across the Southern Plains/Lower Mississippi Valley should finally break by this weekend though record heat continues into Saturday across the East. Numerous daily records could be met or exceeded on Saturday across the Appalachians, Mid-Atlantic, and Northeast with a few lingering records across far southern regions in Texas/Louisiana. A strong cold front will spread rain and thunderstorms through the Mississippi Valley/Ohio Valley on Saturday and into the East on Sunday. 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 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