Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 AM EDT Thu Jul 06 2023 Valid 12Z Sun Jul 09 2023 - 12Z Thu Jul 13 2023 ...Heat expected to rebuild from the Southwest to the central Gulf Coast this weekend into next week... ...Heavy rain/runoff concerns likely to focus over portions of the south-central Mississippi/Tennessee Valley region and Mid-Atlantic/Northeast Sunday into Monday... ...Overview... Most guidance shows the large scale flow settling into a fairly persistent pattern from late weekend through next week. This configuration will consist of an eastern U.S. mean trough anchored by a deep upper low tracking from the western shore of Hudson Bay into northern Ontario, a strengthening Rockies/High Plains upper high that should drift a bit westward to around the Arizona/New Mexico border along with a ridge extending to its north-northwest, and mean troughing offshore the Pacific Northwest. Shortwaves within the upper trough east of the Rockies and associated waves/fronts will likely provide a focus for multiple episodes of showers and thunderstorms with areas of heavy rainfall possible. Meanwhile the upper ridge will support an expanding area of hazardous heat between the Southwest U.S. and central Gulf Coast region, likely persisting into the week two time frame per Climate Prediction Center forecasts. ...Guidance Evaluation/Predictability Assessment... A 12Z/18Z operational model composite early in the period followed by phasing in up to 30-40 percent total 18Z GEFS/12Z ECMWF mean input by days 6-7 Wednesday-Thursday continues to represent the consensus ideas of the large scale pattern evolution while reducing the influence of smaller scale details that have lower predictability and confidence. The models have still not come into agreement regarding regionally important shortwave specifics within the overall eastern U.S. upper trough Sunday-Tuesday, leading to some ongoing spread and run-to-run variability for surface wave/front and QPF details from the Mississippi Valley to the East Coast. Relative discrepancies of the CMC and UKMET (more and less amplified versus consensus, respectively) cancel out among the minority components of their solutions over the East. At least their QPF patterns over the Northeast early in the period have trended closer to the GFS-GEFS/ECMWF-ECens that have held up well over the past 24 hours aside from a little east-west variability. Farther west there is decent clustering for the northern Ontario upper low, with typical spread in track for forecasts several days out in time. Specifics of position/embedded waves for the leading cold front that drops into the northern tier by Monday and continues south/east thereafter will depend on lower predictability shortwave details so expect some continued variability with that aspect of the forecast. The mean trough off the Pacific Northwest coast should contain multiple embedded features, with the axis of the overall trough likely becoming aligned somewhat farther west late in the period in response to an upper ridge that amplifies near 160W longitude. ...Weather/Hazards Highlights... During the Days 4-5 period covering Sunday through Monday night, there will be two areas of greatest focus for heavy rainfall potential. One should extend from around Oklahoma eastward into the Tennessee Valley and southeast toward the east-central Gulf Coast, which will likely be the continuation of a multi-day period of convection starting out over the central Plains in the shorter term. Guidance has on average shifted southward with this area of rainfall, so the Day 4 Excessive Rainfall Outlook has made some corresponding adjustments for the Slight Risk area compared to the previous Day 5 outlook. By Day 5 there are signals for a possible band of convection extending back northwest into the central Plains within an axis of favorable moisture and instability, to the north of a front that begins lifting north as a warm front. However guidance is sufficiently divergent to recommend only a Marginal Risk over that area for now. The other significant region of focus will be the northern Mid-Atlantic into the Northeast on Day 4, with good continuity for the Slight Risk area from recent outlooks. Most guidance suggests anomalous moisture will persist over the Northeast into Day 5 but the full range of guidance provides mixed messages for rainfall specifics, favoring a Marginal Risk as a starting point. Continue to expect some bands of rainfall across the far northern tier with an initial weakening front and a stronger front pushing south from Canada. Some east-west training of this activity could be possible but relatively modest moisture anomalies and moderately dry antecedent conditions still favor no risk area for now. By Tuesday-Thursday the East should see at least a brief drier trend while more showers/thunderstorms develop over parts of the central U.S. into the Mississippi Valley/Great Lakes as the front reaching the northern tier early in the week continues pushing south and east. Some of this rainfall could be locally heavy, with location and magnitude details gradually becoming better defined over the coming days. Most of the West should remain dry through the period. Strengthening of the initial southern Rockies/High Plains ridge and a slight westward drift with time will likely expand coverage of hazardous heat across the southern tier during the forecast period. Expect the greatest temperature anomalies of plus 10-15F for highs to be near the upper high center (western Texas and parts of New Mexico) with a broadening area of highs at least a few degrees above normal elsewhere, including into the Southwest by the start of next week. Likewise, expect more coverage of maximum heat index values reaching 110F or greater over southern/eastern Texas into the Lower Mississippi Valley, while lower humidity will lead to maximum heat index values over the Southwest near or slightly below the expected air temperatures reaching up to 110-115F or so from late weekend onward. Low temperatures in the upper 70s to low 80s will not provide much relief overnight and could exacerbate heat stress as well as yield some daily records for warm minimum temperatures. There may be a few daily records for high temperatures as well. Climate Prediction Center forecasts indicate this heat should extend beyond next Thursday. The central Plains should see temperatures rebound to near normal for a couple days or so after a cool Sunday (highs up to 5-10F or so below normal). A cold front dropping south from Canada should bring temperatures back down to 5-10F below normal over the northern into central Plains by next Wednesday-Thursday. Parts of the Interior Northwest/far northern Rockies should see highs up to 10-15F above normal Sunday-Monday and then a moderating trend. Much of the eastern half of the lower 48 should see near to somewhat below normal readings Sunday-Monday and more near-normal readings thereafter. Rausch 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