Extended Forecast Discussion NWS Weather Prediction Center College Park MD 256 PM EDT Tue Jul 04 2023 Valid 12Z Fri Jul 07 2023 - 12Z Tue Jul 11 2023 ...Southern tier heat expected to rebuild during the weekend and persist into next week... ...Heavy rain/runoff concerns and possibly a severe weather threat should focus over the south-central Plains and Mississippi Valley Friday-Sunday... ...Overview... Expect the Friday-Tuesday period to be fairly active in terms of shower and thunderstorm activity from much of the Plains to the East Coast. From late week into the weekend a leading front crossing the East will be one feature helping to focus rainfall, while the trailing part of the front along with shortwaves embedded in low amplitude flow aloft should promote multiple episodes of convection that will likely progress from the central Plains into the Middle/Lower Mississippi Valley. Then recent guidance has been suggesting that the mean trough extending south from central Canada may amplify to some degree over the eastern U.S. early next week (with possible southward shift of the embedded upper low in Canada), leading to a rebound of rainfall over the East along/ahead of what may be a complicated reflection of surface waves and fronts. One or more of these fronts may produce lighter rainfall over the central U.S. from late weekend into next week. Farther south, an upper ridge should persist over the southern Rockies/High Plains through the period and strengthen somewhat by early next week, leading to somewhat hotter temperatures over that part of the country. Guidance maintains the idea of mean troughing aloft near the northern half of the West Coast but with varying day-to-day details. ...Guidance Evaluation/Predictability Assessment... Guidance agrees well on the overall theme of the pattern evolution from the Rockies eastward, with a persistent mean ridge axis over the Rockies and cyclonic flow downstream. From Friday into the weekend the cyclonic flow will likely be fairly low in amplitude and contain multiple weak shortwaves that will tend to make resolving surface details more difficult after a well-defined front reaches the East and South late in the week. These differences also impact the QPF position/amplitude. This forecast cycle trended drier across the Ohio Valley and wetter farther south across the Mid-South per the latest guidance, but predictability remains on the lower side for the details, especially in the Ohio Valley. The NBM through 13Z seemed to be lagging a bit on this southern trend compared to the multi-model consensus of QPF farther south. Into early next week, 00/06Z model guidance and the incoming 12Z cycle at least agrees better with a gradual drift south of the Hudson Bay low for more well-defined troughing over the east-central U.S. But differences arise with its centroid position and potential for elongation of the low in some direction, causing trough axis differences. The 06Z GFS went so far as to split the low by Monday-Tuesday while north-south-oriented energy that was more aggressive than in other guidance/other GFS runs tracks across the East Coast. Meanwhile in the West, shortwaves/compact upper lows in multiple streams should lead to mean troughing through much of the period, but with a fair amount of spread for specifics. Part of the difficulty is uncertainty in exact timing/shape of a ridge that builds to its north, along with how upstream energy expected to filter into the mean trough will influence features initially within the trough. Given the above considerations, the WPC forecast utilized a blend of the deterministic guidance particularly favoring the 00Z ECMWF and 06Z GFS for the early part of the forecast period. As the forecast progressed, increased the proportion of the EC and GEFS ensemble means at the expense of the individual models (especially the GFS), reaching about half by the end of the period next Tuesday amid the increasing uncertainty. ...Weather/Hazards Highlights... From late this week into the weekend, southern and central parts of the Plains and Mississippi Valley will be the most prominent focus for potentially strong convection with heavy rainfall due to the combination of shortwave impulses aloft and a south-central Plains front or two meandering across the region. While there is typical uncertainty in details 4-5 days out in time, maintained Slight Risks in the Days 4-5 (Friday-Sunday morning) Excessive Rainfall Outlooks given the pattern should support a heavy rainfall threat with ample instability and above average moisture in place. There has been some shift in the guidance south with the axis of heaviest rains over the past couple of cycles; thus the Slight Risk and the QPF particularly on Day 5 shifted a bit south as well to include some of the Lower Mississippi Valley. Farther east, instability and above climatological moisture along and ahead of a front pushing toward the East Coast will favor some areas of heavy rainfall that could pose a flash flood threat but the position/rainfall magnitudes are less focused. Thus Day 4 depicts a broad Marginal Risk area from New England into the Southeast. Day 5 continues to show considerable spread across the east-central to eastern U.S. in terms of the rainfall axes, with some models favoring the Ohio Valley more than others. At this point there is a large Marginal Risk in place to cover most of the heavy rainfall threats, but this may be able to shrink with time if models converge with their solutions. As upper troughing likely amplifies early next week, the best focus for enhanced rainfall should move from the Mississippi Valley into the East. Details remain uncertain given how guidance has varied thus far. Fronts moving south from Canada may also produce some areas of rainfall over the central U.S. from late weekend into next week, but with amounts averaging somewhat less than those expected over the East. The West is likely to remain persistently dry. With guidance continuing to show the upper ridge over the southern Rockies/High Plains strengthening somewhat from the weekend into next week, expect gradual expansion of hazardous heat across the southern tier during the forecast period. Greatest temperature anomalies of plus 10-12F for highs should be directly underneath the upper high (western Texas and southeastern New Mexico) with broadening coverage of highs at least a few degrees above normal elsewhere. Likewise, expect gradually increasing coverage of maximum heat index values reaching 110F or greater over southern/eastern Texas into the Lower Mississippi Valley. Low temperatures in the upper 70s to 80F will not provide much relief overnight and could exacerbate heat stress as well as cause some records for warm minimum temperatures. Meanwhile parts of the northern-central Plains should see multiple days of below normal highs, with some locations 10-12F below normal Friday-Saturday followed by a gradual trend closer to normal. The Northeast will start out warm late this week ahead of an approaching front and then trend close to normal as well. Tate/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 forecast, excessive rainfall outlook, winter weather outlook probabilities, heat indices and Key Messages 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 https://www.wpc.ncep.noaa.gov/#page=ovw