Extended Forecast Discussion NWS Weather Prediction Center College Park MD 307 PM EDT Sat Jun 15 2024 Valid 12Z Tue Jun 18 2024 - 12Z Sat Jun 22 2024 ...Record-breaking heat from the Midwest to the East Coast... ...Heavy rainfall threat along the central-western Gulf Coast and parts of the northern tier into central Plains... ...Overview... Guidance continues to show a strong upper ridge building over the East through midweek and then gradually dropping a little southward and elongating to cover the south-central Plains and southern Rockies by the end of the week. This ridge will support a broad area of hot and dry weather next week, with the greatest anomalies and potential for daily (and possibly monthly) records extending from the Ohio Valley/Lower Great Lakes into the northern Mid- Atlantic and Northeast. Between this ridge and mean troughing along or just inland from the West Coast, the area from the northern half of the Plains into Upper Midwest may see multiple episodes of heavy rainfall from a combination of a persistent wavy mean front, anomalous moisture, and upper level impulses. To the south of the upper ridge, upper level energy and abundant deep moisture north of an area of low pressure (see NHC for updates) will bring the potential for a period of heavy rain along and near the western half of the Gulf Coast. ...Guidance/Predictability Assessment... Most aspects of the guidance are fairly agreeable during the first couple days or so of the period, aside from smaller scale detail differences. By mid-late period there is more notable spread among the dynamical guidance and machine learning (ML) models, which were generally farther north with any semi-organized system (with a corresponding northward/northeastward shift of rainfall emphasis). Per coordination with the National Hurricance Center, favored the farther south cluster of solutions with the center/centroid but note that the heavy rainfall threat will likely extend well north. Early-period model consensus favored a blend of 00Z/06Z models for the forecast through midweek, followed by an increasing weight of the 06Z GEFS and 00Z ECens mean with time. This helped to maintain the upper ridge definition in the east. ...Weather/Hazards Highlights... Locations from the central Plains through the Upper Midwest will be one area of focus for heavy rainfall potential during the Days 4-5 Excessive Rainfall period covering Tuesday through Wednesday night. During Day 4 expect convection to expand southward along a cold front anchored by a northern tier into Canada surface wave, with the latest majority guidance cluster favoring some small southward expansion of the existing Slight Risk area again (southward toward central Kansas). By Day 5 the trailing part of the cold front may stall and eventually lift northward while abundant moisture persists over the region. There is more spread for convective details, yielding only a broad Marginal Risk area at this time. The wavy front may remain nearly parallel to upper flow through the rest of the week, maintaining the potential for additional episodes of heavy rainfall from the northern half of the Plains into the Upper Midwest from Thursday onward. In the Gulf, guidance continues to show an area of deep tropical moisture and some degree of upper level energy shifting westward across the western half of the Gulf of Mexico and vicinity during the Days 4-5 ERO period. There are still significant differences for specifics of associated heavy rainfall, but a composite of dynamical/machine learning guidance continues support a Slight Risk area along the western two-thirds of the Louisiana coast into eastern Texas coast on Day 4, and along the entire Texas coast on Day 5. The southern part of the Texas coast has drier antecedent conditions but rain rates may still be high enough to cause some runoff issues. Rainfall along and inland from the western Gulf Coast should trend less heavy later in the week while upstream energy underneath the elongating central-eastern U.S. upper ridge could lead to some rebound in rainfall over Florida/western Gulf region but with low confidence in the details. Total rainfall per the guidance is quite varied, showing a high-end potential but confidence is low. The strong upper ridge building over the East next week will tend to produce the greatest temperature anomalies from the Midwest and Ohio Valley/Lower Great Lakes into the northern Mid-Atlantic and Northeast, with multiple days of readings 10-20F above normal. This would translate to highs well into the 90s, along with lows in the upper 60s to upper 70s providing little overnight heat relief. Daily records will be possible within the above areas, with some isolated monthly records possible as well. With some typical detail differences, the axis of greatest temperature anomalies in the current forecast shows some similarity to the 1994 heat wave that was observed around the same time in June. Some flattening of the northern part of the ridge by the end of the week may make the heat a little less extreme over New England, while the central Plains may trend a little hotter (but with single-digit positive anomalies). In contrast to the eastern heat, lingering upper troughing will support highs 10-20F below normal over parts of the Interior West/northern High Plains on Tuesday with moderately cool readings over the northern-central High Plains Wednesday. Elevation snow will exit the northern Rockies where a heavier/wet snow will accumulate between the end of the short range period into the first part of the medium range. Then rising heights aloft will bring a warmer trend to most of the West, with the region likely seeing highs 5-12F above normal by Friday-Saturday. Moist easterly flow should lead to two or three days of below normal highs over the western Gulf Coast and southern High Plains by mid- late week. Fracasso/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 (QPF), excessive rainfall outlook (ERO), winter weather outlook (WWO) probabilities, heat indices, and Key Messages can be accessed from: 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