Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 PM EDT Mon Jul 24 2023 Valid 12Z Thu Jul 27 2023 - 12Z Mon Jul 31 2023 ...Heat continues across the Southwest while above normal temperatures are also likely from the Midwest to the East Coast into late week... ...Overview... The persistent upper high atop the southern Rockies may drift slightly east toward the southern Plains by early next week, bringing dangerous heat in the Desert Southwest closer to normal temperatures (though still hot) as the period progresses. Some ridging will likely stretch into the Midwest and promote much above normal temperatures there and east into the Ohio Valley/Great Lakes and the Mid-Atlantic/Northeast through late in the workweek before moderating. Low amplitude upper flow to the north of the upper high presents some forecast challenges especially on the smaller scales, but the overall tendency should be for northern tier flow to reflect a ridge axis over the High Plains and broad troughing over the Great Lakes/Northeast by the weekend. This mean trough should gradually amplify into the start of next week. This pattern/evolution, which will bring one or more fronts into the northern Plains through Northeast, will favor episodes of potentially heavy convection but confidence remains low in the details. Diurnally driven showers and thunderstorms will be possible over and near the Four Corners states through late week and the weekend with monsoonal moisture incoming under the ridge. ...Guidance Evaluation/Predictability Assessment... Guidance continues to show difficulties with important but very low-predictability shortwave details across the northern tier U.S. and southern Canada, within and south of initially elongated troughing that extends from eastern Canada/Hudson Bay through southwestern Canada into the Pacific Northwest coast mean trough. While shortwave details continue to plague the day-to-day forecast of fronts/waves/QPF as flow aloft transitions from a nearly zonal configuration toward eastern U.S. trough amplification that drives the leading cold front farther southward, latest guidance at least seems to be getting a little closer in a broad sense. This is by way of 00Z ECMWF-based guidance trending a little more amplified with the eastern trough and the 06Z/12Z GFS backtracking from its potentially excessive trough amplitude in the 00Z run. In the new 12Z cycle, the CMC appears to stray a bit to the amplified side of the spread for the trough. Also worth noting, the 12Z GFS/ECMWF have adjusted toward the fast side with ejecting Pacific Northwest trough energy during the weekend. These issues out west maintain some uncertainty for the forecast farther eastward. Meanwhile there are some ongoing detail differences regarding exactly how strongly Atlantic upper ridging builds into the Southeast/Gulf and possibly connects with the Rockies/Plains ridge. The GFS/GEFS still are generally on the weaker side of the spread. Mass field preferences reflecting an operational model blend early transitioning to a model/ensemble mean mix by the latter half of the forecast led to merely typical detail adjustments early-mid period, though by day 7 Sunday reflected the average of guidance that had adjusted a little farther south with the eastern cold front in association with a more amplified upper trough. QPF again broadened amounts across the north-central U.S. and eastern U.S. compared to the NBM using a multi-model and ensemble blend to provide some depiction of the rainfall potential, though more changes are likely yet to come with time. ...Weather/Hazards Highlights... Showers and thunderstorms look likely across parts of the central and eastern U.S. through the latter part of the workweek, with one or more wavy frontal systems reaching the northern tier providing the best focus but with less defined forcing elsewhere. Even with the northern tier frontal systems, there has been considerable uncertainty over the precise east-west timing and southward extent--keeping confidence lower than desired. For the Thursday-Thursday night period covered by the Day 4 Excessive Rainfall Outlook, there are still two areas of interest for relatively better rainfall focus. One exists over the Upper Midwest where the wavy fronts will be approaching, with another extending from the eastern Great Lakes into the far northern Mid-Atlantic into southern-central New England where a compact upper level shortwave may generate locally heavy convection. At this time guidance appears to be providing enough signal to introduce Marginal Risk areas in both cases to reflect the potential events, with the understanding that some adjustments may occur in coming model cycles. By the Day 5 ERO timeframe Friday-Friday night, convection should be more strongly forced with shortwaves aloft and more focused near the cold front, increasing confidence for heavy rain and higher rain rates along with training/repeat activity that could lead to flooding issues. While model QPF solutions are still exhibiting the spread that has been seen over recent days, the overall combination of the pattern (wavy front taking on a fairly east-west orientation) along with favorable moisture anomalies/instability plus locations of relative emphasis in ensemble probabilities seem to favor introducing a Marginal Risk area from locations near the southwestern Great Lakes into the central Appalachians and a majority of New England as a starting point. Expect rain and thunderstorms to continue into the weekend, with best chances likely drifting southward with the cold front. Scattered storms are also possible in the northern half of the Plains and Mississippi Valley each day, again with low confidence in the exact placement and amounts of rainfall. Convection could become a little more organized at times over the northern High Plains due to low level upslope flow. Elsewhere, the Four Corners states are likely to see diurnally driven convection each day this week as monsoonal moisture comes in underneath the southern Rockies upper ridge. The EROs depict a Marginal Risk area on both days. Flow to the south of Rockies/Plains upper ridging could bring an increase of moisture and shortwave energy into parts of the region toward the end of the period, so rainfall amounts may become locally heavier by Sunday-Monday. Florida and parts of the Gulf Coast should see some scattered showers and thunderstorms as well, and Marginal Risks of flash flooding are in place for South Florida for Thursday and Friday as precipitable water values will be well above an already high average and convection could produce heavy rainfall rates, with urban areas particularly sensitive. The persistent upper high centered over the southern Rockies will keep temperatures in the Desert Southwest around 5-10F above average for highs in the 110s through late week. By the weekend into early next week, highs may finally ease to near normal there with some chance for the upper high to drift east. Farther northeast, temperatures approaching 100F are likely across the Plains to the Middle/Upper Mississippi Valley with anomalies for highs of 10-20F above normal, while plus 10-15F anomalies are likely across the Ohio Valley and Mid-Atlantic/Northeast peaking Thursday-Friday. Some record temperatures for highs and warm lows are possible across the East. Temperatures in most of these areas should moderate by the weekend as cold fronts push through the northern tier, though warmer than normal temperatures may continue from the central/south-central Plains into Mississippi Valley depending on exactly where the northern tier cold front ultimately stalls. The Southeast/Gulf Coast could see slightly increasing temperatures into the weekend and early next week under ridging aloft that builds west from the Atlantic and southeast from the Plains. Details of convection and residual cloud cover will add an extra element of uncertainty to the forecast of daytime highs over many areas. Mean troughing near the Pacific Northwest will give that region the most persistent area of potentially below normal temperatures, though still with highs no cooler than a few degrees below average. Cooler trends over the Northeast under upper troughing could nudge highs a little below normal by early next week. 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, 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