Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 AM 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... ...Strong to severe storms along with a heavy rain/runoff threat to focus over the central Plains into Middle Mississippi Valley Friday-Saturday... ...Overview... Expect the Friday-Tuesday period to be fairly active in terms of shower and thunderstorm activity from the northern-central 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 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. For early next week the models and ensemble members have been diverging and varying significantly from run to run with respect to embedded details within the general amplifying trend for the Canada into northern/eastern U.S. mean trough. The CMC was the most extreme solution of the 12Z/18Z batch of guidance, using energy in the southwest part of the trough to form an upper low that crosses the Midwest (leading to 500mb height anomalies up to 3-4 standard deviations below climatology over parts of the Ohio Valley). The new 00Z CMC does not have this feature and now drops its main Canada upper low into northern Ontario, which is similar to the 12Z ECMWF. The 00Z GFS is similar as well, after 12Z/18Z runs were a little farther northwest with the deep upper low. Latest GEFS/ECMWF means are hinting at this southward shift of the Canada upper low versus 24 hours ago when the dominant clustering was near northern Hudson Bay (but some GFS runs were already pulling some of the energy southward). Along the Northwest coast there is reasonable agreement on mean troughing with one or more embedded compact upper lows, but 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. The updated forecast started with a blend of 12Z/18Z operational models to reflect the most common ideas of guidance during the first half of the period. Recent behavior of guidance late in the period over central/eastern North America favored a rapid transition to a more conservative approach by day 7 Tuesday with a total of 60 percent total 18Z GEFS/12Z ECens input at that time, while holding on to some components of the 18Z GFS/12Z ECMWF. Low confidence in the 12Z CMC solution favored eliminating it from the forecast after day 5 Sunday. ...Weather/Hazards Highlights... From late this week into the weekend, the central Plains into Middle Mississippi Valley will be the most prominent focus for potentially strong/heavy convection due to the combination of shortwave impulses aloft and a south-central Plains front drifting slowly northeastward. While there is typical uncertainty in details 4-5 days out in time, there seems to be enough clustering in the guidance to favor Slight Risk areas in the Days 4-5 Excessive Rainfall Outlooks that cover the Friday into early Sunday time frame. Some refinement in these areas is likely over the coming days but the pattern appears to favor the overall heavy rainfall threat. 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 focus/rainfall magnitudes are less focused. Thus Day 4 depicts a broad Marginal Risk area from New England into parts of the Southeast, with a lingering Marginal Risk on Day 5 over most of New England. Regarding the latter, there is a fair amount of spread for how much moisture will remain but the area shown captures the best overlap of guidance and the fact that a frontal boundary may persist over the region. 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. 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. Parts of the northern-central Plains will 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. 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, 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