Extended Forecast Discussion NWS Weather Prediction Center College Park MD 248 PM EDT Tue Jun 20 2023 Valid 12Z Fri Jun 23 2023 - 12Z Tue Jun 27 2023 ...Prolonged heat wave continues across Texas and southeastern New Mexico throughout this week with some expansion across the Southern Plains and Lower Mississippi Valley early next week... ...Heavy rain and flooding are possible across the north-central U.S. late this week with a low pressure system... ...Overview... The medium range period will begin Friday with an upper low finally lifting north out of the Tennessee Valley, though with some remaining showers and thunderstorms across the East with a low pressure/frontal system at the surface. Farther west, another upper low is forecast to press across the West into the northern Plains through the weekend and then into the Great Lakes region, spreading rain with it that could be heavy and cause flash flooding concerns particularly in the northern Plains/Upper Midwest. Meanwhile, a strong upper high will continue to meander around the Rio Grande through the week and into next week, causing persistent heat across much of Texas and southeastern New Mexico, and high heat indices are likely to expand further into the Southern Plains and the Lower Mississippi Valley as well by early next week. ...Guidance Evaluation/Predictability Assessment... Model guidance indicates the persistent blocky pattern that has been in place across the CONUS on and off for several weeks will initially start off the forecast period on Friday (6/23). A previously cutoff upper-level low will be lingering over the Southeast/Tennessee Valley while slowly being absorbed into the northern stream bringing an end to a brief Rex Block pattern, with high pressure to the north shifting eastward over the Atlantic. To the west, an upper-level high looks to be maintained over northern Mexico into portions of the Southwest/Texas, with ridging extending northward over the Plains, as mean troughing remains over the West. Through the forecast period, the upper-level high/ridging over Texas will briefly expand northwestward as a piece of shortwave energy ejects from the western trough northeastward, breaking down the northward extent of the ridging over the Plains, before reinforcing upper-level troughing over the northeast following the absorption of the low over the Tennessee Valley. Meanwhile, mid- to late period, additional upper-level energy over the Gulf of Alaska flows southward, reinforcing the mean troughing over the West Coast as the upper-level high/ridging over the southern CONUS shifts back eastward. Most large-scale, and even smaller-scale, details are relatively consistent across the guidance at the start of the period, with decent run-to-run consistency within the GFS/ECMWF indicating a good handle on the forecast. Two aspects where the model solutions diverge relatively quickly into the period is the 00Z UKMET depicting a much weaker shortwave traversing the northern Plains, and the 00Z ECMWF/UKMET trending weaker with the upper-level energy that shifts southward from the Gulf of Alaska off the West Coast. In contrast, the GFS has actually trended stronger with this feature over the last few runs, with a deeper closed low in both the most recent 00Z/06Z and updated 12Z runs, which is also depicted in the 00Z CMC. The 00Z ECens and 06Z GEFS means are notably less detailed, but do both signal the potential for a bit deeper closed low at different times in the mid- to late forecast period, giving credence to lean towards this solution for now despite the difference between the 06Z GFS/00Z CMC and 00Z ECMWF. The updated WPC forecast blend is initially based off a combination of the 00Z ECMWF/CMC and 06Z GFS, with the 00Z UKMET not included given the differences with respect to the shortwave over the northern tier and a similar weaker trough along the West Coast that is already accounted for by the inclusion of the 00Z ECMWF. A contribution from the 00Z ECens mean is introduced mid-period as the 00Z ECMWF contribution is reduced given the weaker representation of the western upper-level low/trough compared to the other guidance that the 00Z ECens mean trends more similarly towards. Greater contributions of both the 00Z ECens and 00Z GEFS mean are included towards the end of the period as the deterministic guidance begins to diverge more with increasing lead time, especially with respect to the northern tier shortwave re-amplifying troughing over the Great Lakes into the East Coast, with the means providing a compromised solution. ...Weather/Hazards Highlights... As the upper low meandering over the Southeast/Tennessee Valley in the short range weakens and lifts north, it will still provide some support for rain and thunderstorms but likely with reduced widespread amounts compared to the short term. The upper and surface lows lifting north should allow for rain chances to spread north into the Mid-Atlantic and Northeast as well. For Friday, a Marginal Risk of excessive rainfall is in place across the Southeast and Mid-Atlantic for lingering convection over what is likely to be wet ground after multiple days of rain in some areas. The instability in place could support localized heavy rain rates. By Saturday for the Day 5 ERO, there may be a somewhat focused axis of rainfall across the northern Mid-Atlantic region into the Interior Northeast, so a Marginal Risk is in place for those regions, and areas of low Flash Flood Guidance could be of particular concern. Scattered thunderstorms may remain farther south too. A wet and stormy pattern looks to persist across the East through early next week as a trough re-amplifies over the region and another frontal system moves in. Farther west, support starts to become favorable for heavy rain in the north-central U.S. as a coupled upper jet (right entrance/left exit region combination) sets up around Montana on Friday and into the northern Plains and Upper Midwest by Saturday, while at the surface a low pressure/frontal system consolidates. Broad Slight Risks of excessive rainfall are in place for this potential, across the northern High Plains on Friday and for the northern Plains/Upper Midwest on Saturday. Ensemble guidance indicates moderate to high probabilities for areal average rainfall exceeding an inch with deterministic QPF indicating locally heavy amounts in the 2-4" range each day are possible. Shower and storm chances will likely also extend southward across the Missouri and Mississippi Valleys this weekend ahead of the frontal system. As the system picks up speed and moves east early next week, areas west of the Great Lakes should dry out as scattered shower and storm chances return from the Northwest into the Northern Rockies/adjacent High Plains as another upper-level trough approaches from the west. Meanwhile, the excessive heat will continue through late week, with widespread 100s across portions of southern New Mexico and western and southern Texas, eventually expanding into central and eastern Texas early next week. Highs in the mid- to upper 90s are also expected to expand across the Southern Plains and into the Lower Mississippi Valley this weekend into early next week, with humidity contributing to heat indices between 105-110 degrees, locally as high as 115. Many daily record high temperatures are possible, and some monthly records may even be challenged. The Desert Southwest starts to see temperatures well over 100F by early next week too, though not really anomalous for this typically hottest time of the year there. As mean upper ridging expands farther north, highs in the 90s are likely across much of the Plains and Mississippi Valley, while 80s and low 90s in the Upper Midwest/Great Lakes region late this week will be 10-15F above normal. Meanwhile the West should see cooler than normal temperatures particularly for highs with the bouts of upper troughing aloft, while cooler than average temperatures will also be present in the southeastern U.S. late this week given the cloudiness/rain chances and the upper low. Putnam/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, experimental excessive rainfall outlook, winter weather outlook probabilities and heat indices 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