Extended Forecast Discussion NWS Weather Prediction Center College Park MD 250 AM EDT Fri Aug 11 2023 Valid 12Z Mon Aug 14 2023 - 12Z Fri Aug 18 2023 ...Severe weather and flash flooding possible across parts of the Upper Midwest to Eastern U.S. early next week... ...Heat wave to prevail across the South into the Southwest... ...Hazardous heat likely over the Pacific Northwest next week... ...Overview... Upper ridging remains locked in place through the period across the Southeast and Southern Plains, with extension of the ridging up into the Northwest as well. This supports dual heat waves - the longstanding one across the South and a second one building into the Pacific Northwest this weekend and lasting through much of next week. To the north, broad cyclonic mean flow will persist over the northern half or so of the lower 48 east of the Rockies, including two individual systems to move through the flow. The first one should be the most robust, moving from the Upper Midwest, across the Great Lakes, and into the Northeast the first half of next week, possibly generating episodes of strong to severe convection and heavy rainfall. The next system drops in to the northern Plains mid next week, with greater uncertainties on strength and how this may try to break down the ridge across the Northwest. ...Guidance Evaluation/Predictability Assessment... Main area of concern remains with the vigorous and unseasonably strong shortwave/embedded low tracking through the Upper Midwest, Great Lakes, and Northeast. Timing remains uncertain, with the GFS continuing to be farther east/faster than the non-NCEP guidance (which is a typical bias for the GFS suite). CMC, ECMWF, and UKMET all support something quite a bit slower which is the preference from WPC at this time. These differences do have notable implications on the timing and placement of QPF/sensible weather. Elsewhere, additional energy may drop in from west-central Canada later next week. Guidance is still very unsure whether this is a weaker shortwave embedded within the main cyclonic flow across central-eastern Canada (GFS/CMC), or a more robust separate shortwave (ECMWF) which may try to break down the Western U.S. ridging more. There is not a whole lot of support at this time for the stronger ECMWF from the ensemble means, especially with the new 00z run which came in even stronger and with a compact closed low over the northern Plains next Thursday. Upcoming model trends will need to be watched with this feature to see if the other guidance starts to trend stronger. The WPC blend for days 3 and 4 used a multi-model deterministic model blend, leaning most heavily on the 12z/Aug 10 ECMWF for the initial strong system through the Great Lakes/Northeast which was most in the middle of the guidance spread. By day 5, replaced the UKMET with the ensemble means as spread begins to grow with regards to details within the broad cyclonic flow. Gradually increased the ensemble means (with less emphasis on the ECMWF late period) amidst growing uncertainties with additional features. This approach also works for the southern U.S. upper ridge which shows much more consistency and agreement and also fits well with the previous WPC forecast. ...Weather/Hazards Highlights... The most prominent focus for active weather during the period will be from the Midwest/Great Lakes into the Northeast next Monday-Tuesday. Strong cold front sweeping through the region should support areas of strong to severe thunderstorms and heavy rainfall along and ahead of the boundary. This is reflected in the Day 4 WPC Excessive Rainfall Outlook highlighting slight risks across the Upper Great Lakes/Michigan, near and along the track of the surface low, with a second risk area across the central Appalachians. Plenty of uncertainty in the details still, especially with regards to timing, so these areas area likely to be modified with time. The Storm Prediction Center also highlights a risk for severe thunderstorms across parts of the Ohio Valley/central Appalachians on Monday as well. This system should race quickly through the Northeast on Tuesday, but given anomalous moisture and sensitivity in some regions of the Northeast, heavy rainfall and/or flash flooding cannot be ruled out. A marginal risk was added across this region to the Day 5 excessive rainfall outlook. Some risk for heavy rains near an area of low pressure along the boundary for eastern North Carolina as well on Tuesday. Showers and storms should accompany this cold front into the Southeast/Florida as well. The broad troughing across the Great Lakes should continue unsettled weather into the latter part of next week as well for this region. Elsewhere, monsoonal moisture will continue into the Southwest/Four Corners next week, peaking Monday into Tuesday. As such, marginal risks were carried on both days in the Excessive Rainfall Outlooks. Oppressive heat will continue through at least next week across much of the southern tier of the country. It will be most persistent from southern and southeast Texas into Louisiana, with some moderation expected across the Southeast by later next week. Numerous daily record highs and warm overnight lows are likely. The combination of heat and humidity will also result in extremely dangerous heat indices in excess of 110F in many locations. Heat should also build back into the Southwest later next week as well. Meanwhile, a second area of heat is expected to build into parts of the Northwest this weekend and through much of next week, with daytime highs 10 to 20 degrees above normal through Thursday. High temperatures in the 90s to near (or above) 100 are likely across much of Washington, Oregon, and the interior Valley of California. Shortwave energy into the Northwest around Friday of next week may begin to moderate temperatures some. See key messages issued by WPC for additional information and graphics related to these two heat waves. Santorelli 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