Extended Forecast Discussion NWS Weather Prediction Center College Park MD 256 PM EDT Mon Jun 12 2023 Valid 12Z Thu Jun 15 2023 - 12Z Mon Jun 19 2023 ...Early Season heatwave to persist across much of the south-central U.S. into early next week and beyond... ...Synoptic Overview... A weak omega block pattern across southern Canada and the northern tier of the U.S. is expected to be in place for the middle to end of this week, with a broad upper low situated over southern Quebec, a ridge across Manitoba, and a trough over southwestern Canada and into Montana. This breaks down some going into next weekend with the ridge decaying down and more of a quasi-zonal flow pattern develops across the central states. A stronger trough begins to build southward across the Pacific Northwest and into the northern Rockies by Sunday and next Monday. Meanwhile, the upper ridge over Texas and the western Gulf of Mexico will continue with the ongoing heatwave unabated through the forecast period (and beyond per the latest from the Climate Prediction Center), while another trough and surface low will likely develop across the East Coast region by Sunday. ...Guidance Evaluation/Predictability Assessment... The latest guidance continues to show overall good synoptic-scale agreement through the period for the upper pattern, but some variability in the details. The greatest model differences continue with some weak shortwave energy crossing from this Upper Midwest to the Ohio Valley this weekend, adding to uncertainties in the rainfall forecast across these regions as well. There are hints of some sort of compact upper low crossing into the East early next week but plenty of run to run variability on any sort of feature. There is also timing uncertainties of waves crossing the Northeast ahead of the aforementioned feature, and also placement of a deeper upper low in southwest Canada or the Pacific Northwest at the end of the period. Most of these differences are smaller in scale and may take until the short range period to fully resolve. A general model blend of the deterministic models worked well for the first half of the period, but the latter days transitioned towards the ensemble means, with some contributions from the deterministic ECMWF and GFS just for a little added system definition. This approach fit well with previous shift WPC continuity as well. ...Weather/Hazards Highlights... The main focus for heavy rainfall is expected across portions of the Deep South/Southeast mid-late week along a lingering front, where an axis of 1-2 inch rainfall totals with isolated higher amounts is expected, mainly from the Florida Panhandle to southern Georgia. Most of the showers and storms that develop will be south of the warm front and reach peak intensity during the afternoon hours, and some storms may be strong enough to produce instances of flooding. However, the coverage of this should be fairly isolated in most cases, so this only warrants a Marginal Risk area on the day 4 Excessive Rainfall Outlook at this time. Additional heavy rainfall may also affect the Southeast U.S. going into Sunday and Monday as the next synoptic scale disturbance approaches. Another area to monitor for heavy rainfall potential will be across the central Plains and extending into the Upper Midwest where a few mesoscale convective systems will be possible ahead of a frontal boundary that will intercept increasing moisture advection, and a Marginal Risk area for excessive rainfall is in effect for this region as well. Elsewhere, some showers are likely from the Great Lakes to the Northeast as moisture pivots around the upper level low. The excessive heat continues going through this weekend (and beyond) across much of Texas and into Louisiana and Oklahoma, with widespread 100s across central and southern portions of Texas. Record temperatures and heat indices on the order of 105 to 115 degrees are likely during the hottest parts of the day for many of these areas, and some isolated instances of heat indices up to 120 degrees are possible for parts of Deep South Texas where the greatest combination of high dewpoints and the heat will likely exist. As per the latest forecast from the Climate Prediction Center, this heat should persist beyond day 7 as well. It will also be quite warm across the Northern Plains/Upper Midwest states to close out the week with widespread upper 80s and lower 90s expected before a modest cool down arrives by Saturday, and then another warm-up likely ensues by Sunday and into Monday. The building upper trough across the northwest U.S. will herald the arrival of cooler temperatures by Sunday from California to Washington state, with highs generally as much as 10 degrees below average by Monday. Most of the East Coast region should be near climatological mid-June levels. Santorelli/Hamrick 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