Extended Forecast Discussion NWS Weather Prediction Center College Park MD 307 AM EDT Tue May 17 2022 Valid 12Z Fri May 20 2022 - 12Z Tue May 24 2022 ...Relentless record-breaking heat continues through Friday across the South with anomalous heat into the East this weekend... ...Heavy rainfall and severe weather likely along a cold front from the Midwest to the East Friday and Saturday... ...Overview... The 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. The front/trough will eventually 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, although the 00z UKMET came in showing a compact closed low again, so some uncertainty remains with that particular feature. Regardless, the models show good agreement (with some minor timing differences) on amplified troughing sliding into the East by the weekend. After Saturday/day 4, 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 Northwest this weekend, which eventually results in another fairly amplified shortwave into the Intermountain West/Rockies by 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. Yesterday 12z CMC was more like the ECMWF, although todays 00z run is more amplified like the 18z/00z GFS. The ensemble means themselves seem to agree with their deterministic counterparts, although there are a fair number of members showing both possibilities. In other words, this aspect of the forecast remains very uncertain. The WPC forecast for tonight used a general blend of the deterministic models through day 5, with a quicker than usual transition towards the ensemble means days 6-7 (with smaller parts of the both the ECMWF and GFS for a little more definition to individual systems). This resulted is a good middle ground compromise for the Western U.S. flow, which also maintained good continuity with the previous WPC forecast through day 6. ...Weather/Threat Highlights... The relentless heat across the Southern Plains/Lower Mississippi Valley should finally break after Friday, though much warmer than normal temperatures continue across the East into Saturday. Numerous daily records could be met or exceeded still on Friday across Texas/Louisiana and the Southeast on Friday, with a handful of record values on Saturday in the East. 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. Behind this front, unseasonably cool temperatures will move from the Rockies into the north-central U.S., lowering snow levels across the Rockies and even allowing for some light snow in northern parts of the High Plains. 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. Santorelli/Rausch 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