Extended Forecast Discussion NWS Weather Prediction Center College Park MD 246 PM EDT Sun Jul 02 2023 Valid 12Z Wed Jul 05 2023 - 12Z Sun Jul 09 2023 ...Major southern tier heat wave to ease this week, but may start to rebuild over the southern High Plains and vicinity next weekend... ...Overview... A weak shortwave will depart from the Northeast on Wednesday and take its associated frontal system with it, aside from the trailing part of the front that will linger over the southern Mid-Atlantic. Farther west an upper trough extending from Canada into the northern Plains as of Wednesday will progress eastward with time, pushing a central U.S. front toward the East Coast and southern states. Progression of this front will produce a broad area of showers/thunderstorms over the central-eastern states over the course of the period. In addition, persistence of the western part of the front over the Plains and its eventual return northeast as a warm front may help to generate more locally heavy convection over the central Plains/Missouri Valley late this week into the weekend. Meanwhile consensus maintains the idea of a West Coast upper trough (with possible embedded low) settling over the West Coast from Thursday onward in response to an eastern Pacific ridge. An upper high over the southern Rockies/High Plains through the period could begin to strengthen slightly next weekend, with a somewhat better defined Interior West/Rockies ridge extending to the north-northwest into Canada late week into the weekend as the West Coast trough becomes established. ...Guidance Evaluation/Predictability Assessment... Model guidance is reasonably well clustered through the early to mid-forecast period next week with decent run-to-run continuity also evident in the ECMWF/GFS, indicative of an above average level of predictability. A frontal system and associated upper-level wave will stall/linger along the East Coast as another shortwave upstream over south-central Canada drives a cold front southeastward across the Plains/Midwest. This stronger front should pass through to the southern tier supported by additional subtle shortwave energy in the mean flow, keeping precipitation chances up along the boundary. Weak upper-ridging in the southern stream should remain in place over portions of the south-central/southwest CONUS with a hint this may build northward towards the end of the period. One notable area of guidance disagreement and much lower predictability compared to the rest of the overall pattern is with the distribution/evolution of upstream energy over western Canada and the northeastern Pacific beginning to impinge on the Pacific Northwest at the end of the forecast period. Mean ridging looks to develop in the northern stream over western Canada as an upper-level low cuts off and retrogrades west a bit, but the details are still unclear and this may have an impact on above average temperatures expected over the Pacific Northwest through next weekend. There are also somewhat subtle but likely influential differences with the potential for another northern stream shortwave to drive a frontal system southward into the north-central CONUS which was also noted in the prior forecast. Given the relatively good agreement in the early to mid-forecast period a general model blend of the 00Z ECMWF/CMC/UKMET and 06Z GFS is used for this update and retains continuity with the prior forecast. A contribution from the 00Z GEFS/ECens means is introduced mid period as the forecast time-limited UKMET is removed, and increased through the end of the period as more subtle details with regard to shortwave energy lose predictability. A larger contribution from the 00Z ECMWF is favored over the GFS towards the end of the forecast period as well given better run-to-run continuity. Overall, this update led to very minimal changes in the overall WPC medium-range product suite, once again indicative of above average predictability and run-to-run consistency. However, mesoscale driven summertime rainfall pattern leads to less confidence in associated QPF forecast despite a good handle on the overall synoptic pattern. ...Weather/Hazards Highlights... The wavy cold front progressing from the Upper Midwest/central Plains into the eastern/southern U.S. will likely produce multiple episodes of showers and thunderstorms, with some locally heavy. The Days 4-5 Excessive Rainfall Outlooks covering the Wednesday through Thursday night time frame continue to depict a broad Marginal Risk area from the western Great Lakes through the Mid/Upper Mississippi Valley and central/south-central Plains (Day 4) and Ohio Valley south-southwest to the Gulf Coast (Day 5). At this time guidance signals are not yet coherent enough for any Slight Risk areas but one or more could eventually arise if short range models begin to cluster better. Frontal wave specifics will play a role in how much rain falls over the Upper Midwest and vicinity around midweek. By Thursday-Thursday night, guidance is signaling potential for locally heavy convection to develop over the central Plains as the trailing part of the front stalls to the south and west of the region. Differences in the model guidance preclude anything more than a Marginal Risk for now, but some of the guidance is signaling the potential for more widespread areal coverage of heavy rain totals supportive of a Slight Risk. Meanwhile the Days 4-5 outlooks have a Marginal Risk area mainly over the Carolinas, where especially on Day 4 latest solutions show a decent combination of instability, anomalous moisture, and corresponding locally heavy QPF, along and ahead of a stalling surface front. The front may weaken some by Day 5 but the moisture and instability should persist. Beyond Thursday night, expect locally heavy rainfall to continue eastward toward/through the East Coast into the weekend while the central Plains into the middle/lower Missouri valley will see additional heavy rainfall. Excessive heat across the southern tier should become less widespread for a time this week as upper ridging returns closer to climatological strength and latitude. The best potential for maximum heat index values to reach 105-110F, locally up to 115F, mid-late week should be more confined to the far Southeast/Florida and central Gulf Coast through southern Texas. Upper ridging over the southern Rockies/High Plains will keep temperatures 5-10 degrees above normal, with the potential this may build further northward next weekend. The greatest anomalies will be over the Pacific Northwest with highs up to 15-20F above normal Wednesday (with some daily record highs possible) followed by gradual moderation as upper troughing looks to dig in from the north. Meanwhile, to the north of the Plains cold front expect unseasonably cool highs 10-20F below normal over the northern-central High Plains especially on Wednesday, with gradual moderation leading to single-digit negative anomalies by the weekend. The leading warm sector will bring above normal temperatures from the Midwest into Northeast mid-late week, followed by near or slightly below normal readings after frontal passage. Putnam/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, experimental 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