Extended Forecast Discussion NWS Weather Prediction Center College Park MD 305 PM EDT Thu Jun 22 2023 Valid 12Z Sun Jun 25 2023 - 12Z Thu Jun 29 2023 ...Prolonged heat wave continues across Texas and southeastern New Mexico and will expand northeast next week... ...Overview... The general upper level pattern will remain fairly stagnant through the period with a strong ridge over the south-central U.S. and troughing over the East and West Coasts. The ridge is responsible for excessive heat in the southern Plains, and heat will gradually expand east and north into the Lower Mississippi Valley and parts of the Middle Mississippi Valley and central Plains by mid-next week. Additional showers and thunderstorms are expected in the eastern third of the country as a few surface low pressure systems move across the region. ...Guidance Evaluation/Predictability Assessment... Early in the period, models were in good agreement on the upper level pattern with two shortwave troughs combining over the East to form a deeper trough by late Monday into Tuesday. The resulting trough and the southern ridge are forecast to slowly drift east through the period as throughing strengthens over the West Coast. The GFS was the most progressive with this while the CMC and ECMWF tended to favor slower eastward movement. A model blend of deterministic and ensemble guidance was used to create a middle ground solution with the eastern trough axis over the Northeast and the center of the ridge over east Texas by the end of the period. Another shortwave trough is forecast to move northeast across the West and round the top of the ridge late in the period. Models are in good agreement that the energy associated with this feature will swing through the western trough by late Tuesday, leaving a weakened trough over the West. However, models differ on the exact position, timing, and magnitude of the shortwave as it moves over and east of the ridge. A model blend of deterministic GFS and ECMWF, and ensemble means was used to create a middle of the road solution, which works well for this forecast. ...Weather/Hazards Highlights... At the start of the period Sunday, a low pressure system aloft and at the surface could cause some rain to linger across the Upper Midwest, with locally heavy amounts possible given the good dynamical support. Farther south in the Lower Mississippi Valley and Mid-South, ample moisture and instability ahead of the cold front will support convective development, and some areas may see some heavy rain rates in potential mesoscale convective systems. Meanwhile, there may again be a somewhat focused axis of rainfall across the northern Mid-Atlantic region into the Interior Northeast on Sunday, with some flooding concerns over areas with low Flash Flood Guidance. All these areas have Marginal Risks of excessive rainfall in place for Sunday-Sunday night. Convection looks to overspread much of the East on Monday, with severe weather a possibility too. A large Marginal Risk has been maintained for much of the East as there may be locally heavy rain with these scattered to widespread storms in an unstable environment. Embedded Slight Risk(s) are possible as well in future cycles if model guidance agrees on areas of focus for heavy rain, but as of now, uncertainty is too large to highlight specific areas. By Tuesday and Wednesday, the front should push slowly eastward with drying behind the front, while additional rounds of rain and thunderstorms continue for the Eastern Seaboard ahead of it. The next system moving across the West will also allow scattered showers and storm chances to return across parts of the Northwest and into the northern Rockies and Plains next Monday-Wednesday. In terms of temperatures, there is really no end in sight for the excessive heat that has plagued particularly Texas/southeastern New Mexico in recent days. Temperatures over 100F and heat indices much higher will continue expanding east into the Lower Mississippi Valley and north toward the central Plains next week. The current forecast calls for temperatures exceeding near to just above 100F in Oklahoma/Kansas/Arkansas by Wednesday-Thursday. Several record temperatures could be set or tied, as temperatures nearing or exceeding 110 degrees could again occur in western and southern Texas. The Desert Southwest starts to see temperatures well over 100F next week too, though not really anomalous for this typically hottest time of the year there. Meanwhile the West should see slightly cooler than normal temperatures particularly for highs with the bouts of upper troughing aloft. The East looks to be within a few degrees of average for early summer, with a trend toward below average by around 5-10 degrees for the Great Lakes region. Dolan/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