Extended Forecast Discussion NWS Weather Prediction Center College Park MD 255 AM EDT Fri Jun 21 2024 Valid 12Z Mon Jun 24 2024 - 12Z Fri Jun 28 2024 ...Hazardous heat possible much of next week for the South/Plains... ...Overview... An upper level ridge will remain anchored over much of the Southern half of the nation through the week, leading to multiple days of above normal to possibly hazardous heat. To the north, individual shortwaves within the westerlies crossing southern Canada and the northern tier of the U.S. to the Northeast will support a series of progressive surface lows and trailing fronts across the northern half to two thirds of the lower 48. A deeper upper low will build off the Northwest coast, moving into the region sometime later in the week. ...Guidance/Predictability Assessment... The latest dynamical and machine learning (ML) guidance agrees fairly well for the overall pattern but displays typical spread and run- to-run variability for the details. Latest runs of the ECMWF remain on the slightly slower side for an initial shortwave through the Northeast Monday-Tuesday, while the GFS is slightly stronger with the next one near the Great Lakes at the same time. The greatest region of uncertainty was out west with a building trough off the Northwest Coast. The 18z/Jun 20 GFS/GEFS guidance was quite a bit faster than the ECMWF/CMC to bring it inland. The ensembles and ML models generally supported something in the middle. Trends in the new 00z guidance for tonight (available after forecast generation time) are favorable, with the ECMWF/CMC a bit faster, and the GFS slower. WPC forecasts for tonight favored a majority deterministic model blend for the first half of the period amidst relatively good agreement. Trended away from the fast 18z GFS and more towards the ensemble means late period for the Western U.S. system and elsewhere. Overall, this maintained good continuity with the previous WPC forecast as well. ...Weather/Hazards Highlights... The greatest focus for possible impactful rainfall/convection will be along a frontal boundary as it shifts eastward from the Plains to the East Coast Monday-Wednesday. There is a marginal risk highlighted on the Day 4 Excessive Rainfall Outlook (Monday- Monday night) from far eastern South Dakota to western Michigan where copious moisture and instability should be present in an area that has seen heavy rainfall in past days and may see more in the short range period. The threat for excessive rain will shift south and east on Tuesday with a broad marginal risk depicted from the south- central Plains, across the Middle Mississippi Valley, into the Upper Great Lakes on the Day 5 ERO where a typical summertime convective pattern should emerge along and ahead of the front. Convection and at least locally heavy rainfall should accompany the front as it pushes into the East by Wednesday as well. The southeastern Canada surface low and trailing front affecting the Northeast on Sunday and perhaps to a lesser degree into Monday will have the potential to produce some areas of locally heavy rainfall across the region. There is continued spread in the guidance for exactly how this system will evolve and thus for rainfall details. The Day 4 Excessive Rainfall Outlook covering Sunday-Sunday night maintains a Slight Risk area over parts of northern-central New England with modest adjustments based on latest model/ensemble guidance and first-guess fields. The surrounding Marginal Risk that extends as far south as the central Appalachians is also close to continuity and reflects the combination of potential for locally heavy activity and terrain sensitivity. The Day 4 ERO maintains a Marginal Risk area over far southern Texas where moist flow to the north of a second possible western Gulf surface low may produce some enhanced rainfall. It is more likely than not that impactful rain will stay south out of Texas, but a low-end chance makes maintaining a Marginal for far South Texas reasonable. In the day 5 ERO period (Monday-Monday night), guidance continues to suggest an area of possible convection diving southeast over the Upper Midwest/western Great Lakes ahead of an advancing warm front. A Marginal Risk remains a good starting point, as downsides for confidence in flash flooding are relatively fast movement and model spread with placement, but upsides are high rainfall rates and wet ground conditions. Anomalous moisture and sufficient instability should persist over the Four Corners states during the Days 4-5 period and potentially lead to some locally heavy rainfall, particularly over burn scars. Marginal Risks are in the outlook across parts of Arizona and New Mexico where the best moisture transport is for slow- moving convection. Parts of the Southeast/Florida may also see episodes of diurnally enhanced convection, but overall dry conditions/high FFGs precluded any excessive rainfall risk areas at this time. A late- period front reaching the Northwest may produce some light rainfall Wednesday into Thursday. For the first half of the week, above normal temperatures will stretch from the West to the East, though the focus will be across the central Plains states into the Southeast where daytime highs 10-15F above normal will be present, and the combo heat/humidity could lead to heat indices near 110F for some, and widespread major to extreme heat risk. Hazardous heat is also possible across the Southwest where heat indices 105-110F are likely. Temperatures should moderate back towards normal for the East and West states as upper troughing moves through both regions. 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