Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 PM EDT Sun Jul 23 2023 Valid 12Z Wed Jul 26 2023 - 12Z Sun Jul 30 2023 ...Dangerous heat will continue across the Southwest while hot temperatures are also likely from the Midwest to the East Coast... ...Overview... Recent guidance has been most consistent and agreeable with the strong upper ridge/high anchored over the southern Rockies, keeping extremely hot temperatures across the Desert Southwest, and an Atlantic upper ridge building west across Florida and the Southeast by late week. Some ridging will likely to stretch into the northern half of the Plains and Mississippi Valley for a time, promoting much above normal temperatures there and east toward the Ohio Valley/Great Lakes and the Mid-Atlantic/Northeast especially Wednesday-Friday. Mean upper troughing near the Pacific Northwest should promote the only area of persistent below normal temperatures over the lower 48. This trough will be part of a larger scale Rex block type of pattern covering northwestern North America and the northeastern Pacific, with guidance having considerable difficulty over day-to-day details of generally low amplitude flow along the U.S.-Canadian border. The overall tendency should be for northern tier flow to reflect a ridge axis over the High Plains and broad troughing over the Great Lakes/Northeast by next weekend. This pattern/evolution, which will bring one or more fronts into the northern Plains through Northeast, will favor episodes of potentially heavy convection but confidence remains low in the details. Diurnally driven showers and thunderstorms will be possible over and near the Four Corners states through late week and the weekend with monsoonal moisture incoming under the ridge. ...Guidance Evaluation/Predictability Assessment... Operational models and ensemble members continue to be very chaotic with important but low predictability medium/small-scale details from the northeastern Pacific across much of southern Canada and the northern tier of the U.S. One element that seems to have persisted over the past 12-24 hours is the idea that the upper low forecast to reach Alberta by early day 3 Wednesday should shear out fairly soon thereafter. This has tempered some of the oscillations with eastward progression of the front into the Great Lakes/Northeast toward the end of the week. However in the past day guidance has diverged a lot more regarding what will happen with one or more pieces of upstream energy in the Vancouver Island/Pacific Northwest mean trough and the overall amplitude of shortwaves crossing the northern tier. Among the more extreme details among recent operational model runs, the 00Z GFS becomes more amplified with Great Lakes troughing late in the week (with very minimal ensemble support, with the 06Z GFS considerably improved) and the 00Z ECMWF's evolution aloft just north of the Plains by next weekend leading to surface low pressure tracking into the Great Lakes by day 7 Sunday (whereas most other model/ensemble means show varying degrees of high pressure from south-central Canada into the Great Lakes at that time). Needless to say, the wide range of possibilities for shortwave details in the generally low amplitude flow along the Canadian border and associated surface low/frontal details over the northern half of the lower 48 lead to some significant QPF differences among operational model runs and to some degree in ensemble-based probabilities. The WPC QPF continues to show somewhat broader coverage of light amounts in the north-central U.S. and eastern U.S. compared to the NBM using a multi-model and ensemble blend to provide some depiction of the rainfall potential, but expect these rainfall forecasts to change considerably with time when the models eventually start to converge. For the mass fields the WPC forecast started with a blend of the 00Z/06Z operational models for the first part of the period. Then the forecast rapidly transitioned to more 06Z GEFS and 00Z ECens/CMCens mean emphasis as confidence decreased for individual model solutions. By next weekend, guidance comparisons favored 50-75 percent ensemble weight with an eventual phasing out of the 00Z ECMWF. ...Weather/Hazards Highlights... Showers and thunderstorms look likely across parts of the central and eastern U.S. through the latter part of the workweek, with one or more wavy frontal systems reaching the northern tier providing the best focus but with less defined forcing elsewhere. Even with the northern tier frontal systems, there is a lot of uncertainty over the precise east-west timing and southward extent--keeping confidence low. For the Day 4 Excessive Rainfall Outlook covering Wednesday/Wednesday night, low pressure and associated frontal system initially have enough clustering in guidance along with favorable moisture and instability to produce a threat for locally intense rainfall mainly over eastern North Dakota and northern Minnesota, so a Marginal Risk remains in place for this activity. Areas farther south across the Mid-Mississippi and Ohio Valleys into the Appalachians could also see convection with heavy rainfall amounts given the instability in place, but EROs continue to depict no excessive rainfall areas at this time due to the uncertainty in the placement of storms. For Day 5/Thursday, likewise there is considerable uncertainty with placement of heavy rainfall within the general area of the northern two-thirds of the Plains and Mississippi Valley into the Appalachians and Mid-Atlantic/Northeast ahead of frontal boundaries. Some of the new 12Z guidance is starting to signal a relatively better potential along and just south of the Canadian border, including over/west of the Upper Great Lakes and parts of the Northeast. For now will keep any risk areas out of the ERO given the poor guidance behavior up to this point. Depending on remaining 12Z guidance and upcoming 18Z GFS/GEFS runs, those aforementioned areas may be the best candidate for introduction of an area upon further evaluation in the next issuance cycle. By later in the week the main cold front and low pressure system pushing southeast look to provide a clearer focus for convection generally from the Midwest to Great Lakes region and the Northeast. Training/repeat activity could eventually become more of a factor for a time if flattening flow aloft allows some of the front to take on a more east-west orientation. The trailing end of the front and possibly another wave could lead to lighter/less expansive convection over the northern half of the Plains next weekend. Meanwhile over the West, the Four Corners states are likely to see diurnally driven convection each day this week as monsoonal moisture comes in underneath the southern Rockies upper ridge. The EROs depict a Marginal Risk area on both days. Florida and parts of the Gulf Coast should see some scattered showers and thunderstorms as well, and a Marginal Risk of flash flooding is in place for South Florida for Thursday as precipitable water values will be well above an already high average and convection could produce heavy rainfall rates, with urban areas particularly sensitive. Latest guidance continues to show this increase of moisture by Thursday. The relentless upper high centered over the southern Rockies will keep temperatures in the Desert Southwest around 5-10F above average for highs in the 110s. Though these anomalies are a little less extreme than in the shorter term, the heat could continue to be hazardous through the week, but with perhaps some possible easing by next weekend. Highs around 100F are forecast to stretch into much of Texas and farther north into parts of the Plains and Mississippi Valley. The largest anomalies for highs of 10-20F above normal will be located across the northern Plains to Upper Midwest/Great Lakes for Wednesday-Thursday, while plus 10-15F anomalies are likely across the Ohio Valley and Mid-Atlantic/Northeast peaking Thursday-Friday. A handful of record highs may be possible in the south-central U.S. into Wednesday and over Florida underneath ridging, but the heat in the East may tie/set more widespread records for highs and warm minimum temperatures. The cold front forecast to progress from the northern Plains around midweek into the Northeast by Saturday should erode the northern tier heat area from west to east, keeping the event fairly short-lived over the northern third of the country. The heat may persist longer to the south, most likely over the central Plains/Middle Mississippi Valley and parts of the Southeast. Details of convection and residual cloud cover will add an extra element of uncertainty to the forecast of daytime highs over some areas. Mean troughing near the Pacific Northwest will give that region the only notable area of potentially below normal temperatures, though still with highs no cooler than a few degrees below average. Rausch/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 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