Extended Forecast Discussion NWS Weather Prediction Center College Park MD 313 AM 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 (though still hot) temperatures 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 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... Model guidance has remained consistent and agreeable with the southwestern to south-central U.S. upper high through this week, though by the weekend into early next week there is some slight divergence in solutions with its centroid as western Atlantic ridging may stretch west and influence/combine with it, particularly shown by the ECMWF and CMC runs. The newer 00Z ECMWF looks more in line with other guidance with drifting the high east compared to its 12Z run. Farther north, the low amplitude flow continues to present an incredibly challenging forecast where shortwave details and the depth of associated troughing are both uncertain, leading to differences in frontal positions and QPF. Within a trend toward more troughiness in the Northeast as the period progresses, in terms of the 12Z/18Z guidance, the GFS runs seemed most amplified with troughing over the Northeast by Friday-Saturday and many (but not all) ensemble members that were amplified were from the GEFS as well. A 12Z ECMWF/CMC type of solution seemed more in line with the ensemble means and so this was more preferred. The 12Z UKMET became out of phase in areas of southern Canada and the northern U.S. and was not favored. With the incoming 00Z model cycle, the ECMWF was even less amplified than the previous run while the GFS is perhaps more amplified. This is casting even more doubt on the already uncertain timing of the cold front dropping south into the north-central to northeastern U.S. that could produce heavy QPF. Upstream there are model differences too with timing of additional energies/lows in the eastern Pacific to Northwest. The WPC forecast began with a blend of deterministic and ensemble guidance early in the period and with time shifted the balance to over half ensemble means to provide an intermediate solution. For QPF, once 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 is a lot of uncertainty over the precise east-west timing and southward extent--keeping confidence low. This is particularly true on Thursday for the Day 4 ERO, where there is considerable uncertainty with placement of heavy rainfall within the general area of the northern two-thirds of the Plains and Mississippi Valley into the Appalachians and Mid-Atlantic/Northeast ahead of frontal boundaries. There does seem to be some emerging focus in the model guidance for rain in and around the Lower Great Lakes region, but given the uncertainty with amounts and placement, continued to hold off on introducing any Marginal Risk area. By the Day 5 ERO timeframe Friday, 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 that could lead to flooding issues. Training/repeat activity is also likely to become more of a factor with the front taking on a west-east orientation. Thus a Marginal Risk is in place across much of the Ohio Valley to Northeast for Friday. However there is the possibility that some guidance shows where the front is slower and the front and thunderstorms are focused in Canada until Saturday, so this will continue to be monitored. Rain and thunderstorms look 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 placement and amounts of rainfall. 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. 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 centered in the Mid-Mississippi Valley, with low confidence. The Southeast could see slightly increasing temperatures into the weekend and early next week underneath some potential ridging influence from the west and east. 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 only notable area of potentially below normal temperatures, though still with highs no cooler than a few degrees below average. 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