Extended Forecast Discussion NWS Weather Prediction Center College Park MD 251 AM EDT Mon May 20 2024 Valid 12Z Thu May 23 2024 - 12Z Mon May 27 2024 ...Overview... Multiple progressive upper shortwaves/possible closed lows and surface low pressure/frontal systems will traverse the West to northern/central Plains and Midwest during the medium range period. By Thursday, a potent shortwave and deep surface low will be exiting the Upper Great Lakes while its attendant cold front will push across the East and partway into the South beyond midweek, with rain and storms along and ahead of it and cooler temperatures behind. There is some recurring threat for heavy rainfall along the western side of this front from parts of Texas into the Tennessee Valley and parts of the Mid-Atlantic as the boundary stalls and lifts back north as a warm front. Additional energy/closed lows look to traverse the northern tier late week and next weekend, but with more uncertainty as strong upper ridging builds over Mexico into South Texas with a continued threat for hazardous heat. ...Guidance/Predictability Assessment... The greatest uncertainty during the medium range period lies with a pair of upper lows through the northern tier. The first will be in place over the Northwest by the start of the period on Thursday but by Friday begins to show some timing uncertainties. The past few runs of the GFS have been faster with this system while the remainder of the deterministic solutions and ensembles are slower through the Northern Plains and Upper Midwest. What is interesting is that while most of the deterministic and ensemble guidance shows good agreement on the progression of this feature, did notice the ECMWF-initialized ML models keep this as a deep closed low farther south across the Upper Midwest/Great Lakes rather than lift it into south-central Canada around Saturday. Prefer a blend towards the deterministic/ensemble solutions for now. The next upper low drops into the Northwest next weekend and with a lot of spread in the timing and progression of this system both with the deterministic, ensembles, and ML/AI models. The 12z 5/19 run of the CMC was a much weaker outlier but the new 00z run did trend closer to the GFS and ECMWF which show some deepening closed low development over the Northern Plains Sunday-Monday. Significant timing uncertainties and how far south some of the energy may reach and also, whether some sort of southern stream energy out of the Southwest may try to combine or stay separate with the northern feature. Given enough agreement, was able to use a blend of the deterministic solutions (anchored by the ECMWF) for the first half of the period. After this, increased the ensemble means to try and cut down on the noise, while also maintaining some of the deterministic GFS and ECMWF for added system definition. Overall, good continuity with the previous WPC forecast. ...Weather/Hazards Highlights... Main focus for late week convection will be from parts of the southern Plains into the Lower Ohio Valley as the western part of a frontal boundary stalls/lifts north ahead of the next upper low into the region. The Day 4/Thursday Excessive Rainfall Outlook continues to depict a fairly large slight risk area from northeast Texas through Arkansas where ample moisture/instability should be present within a very recently wet region. Heavy rainfall potential across this region is also possible at the end of the short range period/Wednesday. By Friday, some heavy rain/flood potential will be possible farther north and east across the Lower Ohio Valley/Appalachians/southern Mid-Atlantic where a marginal risk is in place for the Day 5 ERO. The Upper Low over the Northwest/northern Rockies on Thursday may also bring some heavier rain to parts of the northern High Plains, but with an overall lack of instability and anomalous moisture, combined with dryer antecedent conditions, not anticipating a flash flood risk at this time, despite some 1-2 inch QPF totals for central-eastern Montana. Showers and storms should progress with the accompanying cold front late week into the weekend through the Midwest. An additional heavy rainfall threat may develop farther south across the Middle Mississippi Valley region late this weekend or early next week depending on still very uncertain upper level dynamics and stream energies. Expect South Texas to see multiple days of hazardous heat during the period with highs persistently running 10-15F above normal with max heat index values possibly reaching at least 110F. Highs near 100 degrees could stretch farther north across the southern High Plains later in the period as an upper high begins to build more into the region. Some daily records for highs/warm lows will be possible. Above normal highs will also track east into the Northeast/Mid- Atlantic on Thursday but should moderate thereafter as the cold front pushes through the region. The forecast pattern will favor below average highs over the Northwest to northern Plains for most of next week but relatively normal temperatures everywhere else. 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 forecast (QPF), excessive rainfall outlook (ERO), winter weather outlook (WWO) probabilities, heat indices, and Key Messages can be accessed from: 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 https://www.wpc.ncep.noaa.gov/#page=ovw