Extended Forecast Discussion NWS Weather Prediction Center College Park MD 306 PM EDT Thu Jun 20 2024 Valid 12Z Sun Jun 23 2024 - 12Z Thu Jun 27 2024 ...Heat wave persists for the Mid-Atlantic Sunday with elevated heat over the South/Plains through much of next week... ...Heavy rainfall threat over parts of New England into Sunday... ...Overview... Guidance shows the upper pattern settling into a stable regime featuring a persistent upper high anchored over the southern Rockies and vicinity, with some ridging extending to the north, while the Northeast Pacific/West Coast and eastern North America see varying degrees of mean troughing. Individual shortwaves within the westerlies crossing southern Canada/northern tier U.S. will support a series of progressive southern Canada surface systems and trailing fronts extending into the northern half to two-thirds of the lower 48. A leading low pressure system affecting the Northeast on Sunday may produce some areas of heavy rainfall, while the trailing front and another one behind it may produce episodes of convection over the central/eastern U.S. Expect hazardous heat to extend into Sunday over the Mid-Atlantic, with high heat indices likely across the South/Plains and Southwest for most of next week. There may be a rebound of some heat into the Mid-Atlantic around midweek. ...Guidance/Predictability Assessment... 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. A couple of shortwaves near the Canadian side of the Great Lakes or so to start the period may combine as they move toward the Northeast, but with detail uncertainty. Models are still having a tough time determining when there may be a low or two embedded within the upper trough, affecting the surface evolution. The new 12Z ECMWF becomes slow compared to other models with this trough by Monday-Tuesday. Then upstream, latest guidance is holding onto recent trends toward progressive Pacific energy evolving into a southern Canada/northern tier U.S. shortwave that eventually feeds into the eastern North America mean trough. Remaining upper low energy near/offshore British Columbia may combine with incoming North Pacific energy. This looks to form troughing and an embedded upper low nearing the Northwest by next Thursday with an increasingly better defined leading surface system. 00/06Z model guidance was reasonably agreeable in the timing of this, but newer 12Z models split with GFS/GEFS farther east than the consensus with the ECMWF/CMC farther west. The updated WPC forecast was based on a blend of the 00/06Z GFS and 00Z ECMWF with lesser proportions of the 00Z UKMET and CMC early in the period. Lessened the percentage of the deterministic runs in favor of the GEFS and EC means as the period progressed, reaching half means in the blend by Days 6-7. ...Weather/Hazards Highlights... 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. Will introduce Marginal Risks across much of Arizona through the Mogollon Rim into western New Mexico where the best moisture transport is for slow-moving convection. Any burn scars in the region would be particularly vulnerable. Parts of the Southeast/Florida may also see episodes of diurnally enhanced convection, but ground conditions are on the drier side with FFG values quite high, so favored no risk areas for the time being. Beyond early Tuesday, a central into eastern U.S. front will help to focus areas of showers and thunderstorms while portions of the South and the Rockies should continue to see diurnally favored convection. A front on Sunday and then a late- period front reaching the Northwest may produce some light rainfall each time. The Mid-Atlantic should continue to have temperatures 10-15F above normal into Sunday with some daily records possible for max/warm min temperatures. The front progressing into the East will briefly suppress the heat southward next Monday but the hot temperatures should return northward toward midweek ahead of another front. Meanwhile a broad area of temperatures generally 5-15F above normal will extend from the South into the Plains as well as California into the Interior West. Best potential for maximum heat index values of 105-110F or so will be from the southeastern U.S. into the southern half or so of the Plains as well as over the Desert Southwest. Near to slightly below normal highs over the Pacific Northwest should begin to expand southeastward by next Thursday. Rausch/Tate 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