Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 AM 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... The updated forecast based on 12Z/18Z guidance tilted more toward the operational models relative to minority ensemble mean input early, followed by a gradual increase of the means toward 60 percent by the end of the period next Monday. Low confidence detail differences led to splitting the GFS among 12Z/18Z runs by late in the period. ECMWF-initialized machine learning (ML) models have continued to suggest that the eastern U.S. upper ridge may ultimately remain centered farther west than what some dynamical guidance has been suggesting in recent runs. In the 12Z cycle, the CMC/CMCens were closest to the ML theme so those solutions were given a little more weight in the forecast blend than would ordinarily be the case, though not quite to the point of being in the majority. The new 00Z GFS/GEFS seem to be trending that way as well. Meanwhile latest ECMWF runs are running weaker than most other dynamical and ML guidance for the upper ridge by mid-late period, with more influence from embedded/surrounding upper energy. There is decent agreement in principle but continued detail differences for the northern tier upper trough/surface system associated with initially ejecting West Coast trough energy as well as the Northeast Pacific system that eventually reaches into western North America. The latter actually has reasonable clustering into early Sunday, with both dynamical/ML models diverging thereafter. Most guidance suggests that latest GFS runs may become too strung out/progressive with the incoming energy by next Monday. ...Weather/Hazards Highlights... Excessive Rainfall Outlooks for both Day 4 and Day 5 (covering Thursday through Friday night) have introduced 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. Latest guidance appears to show enough coherence in the location of best convective focus to merit introduction of the Slight Risk area. 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 coverage. The ECMWF/UKMET remain the most suppressed to the south while other dynamical guidance plus ECMWF-initialized ML models show somewhat more northward extent. Elsewhere, the Days 4-5 outlooks have introduced 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 reduced the coverage of the existing Marginal Risk area as a lead-in to eventual elimination if guidance continues the drier trend. 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. 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