Extended Forecast Discussion NWS Weather Prediction Center College Park MD 244 PM EDT Mon Jun 17 2024 Valid 12Z Thu Jun 20 2024 - 12Z Mon Jun 24 2024 ...Record-breaking heat from the Midwest to the East Coast... ...Heavy rainfall threat over the northern Plains/Upper Midwest late week and southern High Plains on Thursday... ...Overview... During the Thursday-Monday period, guidance is showing a steady transition from a strong East Coast upper ridge toward slightly weaker and more elongated upper ridging that settles across the southern tier while flatter mean flow becomes established across the northern states. The initial upper ridge will support a broad area of hazardous heat with potential for daily (and possibly monthly) records extending from the Ohio Valley/Lower Great Lakes into the northern Mid-Atlantic and Northeast, with the pattern evolution helping to moderate temperatures with time especially over northern areas. Meanwhile the northern Plains/Upper Midwest may see a continued threat for heavy rainfall through late this week as ejecting West Coast trough energy develops northern Plains low pressure that will aid the interaction of anomalous moisture and a leading front. Some of this rainfall will extend into the Northeast. A trailing Northeast Pacific system will likely bring a well-defined front into the West by Sunday. 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 over the southern High Plains and vicinity by Thursday. ...Guidance/Predictability Assessment... Latest models and ensembles show overall good agreement on the upper level pattern through the medium range period, but there remain some key differences in the details. There is some uncertainty still in the westward extend of the Eastern ridge early in the period, but a general model blend seemed to provide a good starting point. By the weekend, a shortwave through the northern Plains/Upper Midwest should act to suppress the ridge southward. The 00z CMC was a little different in its evolution of this shortwave into the Great Lakes/Northeast around Sunday due to resolving of details as multiple pieces of energy over central Canada combine in the the overall shortwave trough. The CMC was also notably slower with the next trough into the Northwest early next week, but the 12z run (available after forecast generation time) did trend faster and more in line with the GFS and ECMWF. Todays updated forecast was based on a general blend of the latest deterministic guidance (06z GFS and the 00z ECMWF, CMC, and UKMET) for the first half of the period. By Sunday-Monday, continued to use 60 percent of the deterministic GFS/ECMWF with 40 percent of the ensemble means to help mitigate some of the differences late in the period. Overall, this maintained good agreement with the previous WPC forecast as well. ...Weather/Hazards Highlights... Excessive Rainfall Outlooks for both Day 4 and Day 5 (covering Thursday through Friday night) continue to depict a Slight Risk area covering parts of the northern Plains/Upper Midwest. There has been a persistent signal in the guidance for convection to persist over this region from the short term, with the Days 4-5 period featuring gradual northward progression of a warm front as ejecting West Coast trough energy develops a defined surface wave. In addition to the favorable moisture/synoptic ingredients during the outlook period, prior days of rainfall will lead to wet ground conditions heading into Thursday. Farther south, the Day 4 outlook made minimal change to the Slight Risk area depicted near/east of the Big Bend region of Texas, as guidance continues to show an axis of significant moisture anomalies moving eastward across the region. The outlook reflects a relative majority/intermediate solution among the ongoing spread for rainfall magnitude and south-north coverage. Elsewhere, the Days 4-5 ERO outlooks continue Marginal Risk areas over parts of the southern Rockies/Four Corners region due to the abundant moisture arriving from the east and some forecast instability, along with some model signals for locally enhanced rainfall. Guidance signals are trending more diffuse for rainfall potential along the Southeast coast with a possible wave/moisture arriving around Thursday, so the Day 4 outlook has removed the previous Marginal Risk area. By Friday, the Northeast may merit some monitoring as a front settles over the region. Moisture/instability could support some locally enhanced rainfall rates but antecedent conditions should be neutral to dry by that time and model QPF is not exceptionally high thus far, so the outlook depicts no risk area at this time. From Saturday onward the area of heavy northern tier rainfall should progress eastward into the Great Lakes/New England along with the supporting upper trough and surface system. A broader area of somewhat lighter rainfall will be possible along and ahead of the trailing front between the Plains and East Coast. Moisture lingering over parts of the Rockies may support continued episodes of diurnal convection. The front reaching the Northwest by Sunday- Monday may produce a little light/scattered rainfall. The strong upper ridge initially over the East should produce the greatest temperature anomalies from the Midwest and Ohio Valley/Lower Great Lakes into the northern Mid-Atlantic and Northeast Thursday-Friday, 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. Gradual flattening of the upper pattern should begin to lower temperatures over far northern areas by Friday-Saturday but otherwise a decent area of plus 10-15F anomalies should persist through the weekend. Meanwhile much of the West will trend warmer/hotter late this week into the weekend with highs reaching 10F or more above normal for multiple days. The front reaching the Northwest should bring cooler air to that region by Sunday-Monday, while some of the western heat should reach the northern-central High Plains at that time. Clouds/rainfall will support below normal highs over the southern Rockies/High Plains late this week. Santorelli/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