Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 PM EDT Sun Jun 16 2024 Valid 12Z Wed Jun 19 2024 - 12Z Sun Jun 23 2024 ...Record-breaking heat from the Midwest to the East Coast... ...Heavy rainfall threat along the western Gulf Coast and parts of the northern tier into central Plains... ...Overview... Latest models/ensembles show a strong upper ridge along the East Coast achieving its greatest strength and northward position around Wednesday. From later in the week through the weekend expect this ridge to drift southward, elongate westward, and weaken a little-- ultimately leaving a broad axis of ridging across the southern tier by next Sunday. This ridge will support a broad area of hot and dry weather during this upcoming 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. This activity should also extend into the Northeast late week into the weekend. 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 Gulf Coast. ...Guidance/Predictability Assessment... Interesting trend in the updated 00/06Z guidance overnight compared to yesterday's 12/18Z runs regarding the eastern upper- ridge. Nearly all of the guidance, with some exception of the 00Z CMC, has trended eastward with the upper-ridge, with the general center anchoring a bit more into the western Atlantic late this week/early weekend as opposed to a more inland position along the East Coast. In tandem, this allows for a wave, with potential tropical characteristics (see NHC for details), to lift northwestward along the western side of the ridge. A comparison of the run-to-run continuity of this overall pattern shows that both the GFS and ECMWF have wavered a bit, heightening the uncertainty. In addition, the 00Z initialized suite of EC machine-learning (ML) models has been more consistent in keeping the inland position of the upper-ridge, again complicating the forecast. As far as temperature impacts, the 12Z ECMWF has remained a bit stronger and maintained a greater northern extent with the ridge as well, which leads to the potential for a higher ceiling for the heat in the Midwest/Mid-Atlantic late this week and into the weekend, while the GFS would suggest a quicker flattening of the northern part of the ridge. It should be noted that the updated 12Z guidance this morning did look to trend a bit weaker with the ridge quicker late week and into the weekend. To the west, the guidance is relatively well clustered around a solution depicting a trough over the west, with eventual northeastward ejection of shortwave energy towards the northern Plains over the weekend, though with lingering uncertainty on exact track/strength/timing. Some combination of westward expansion of the upper-level ridge and/or development of upper-level ridging looks to replace the trough through the southern tier of the West. Another upper-level low should drop southeastward from the northeastern Pacific towards the Pacific Northwest by the end of the weekend. For the updated WPC forecast, used a composite blend of the deterministic models early in the period given good agreement. Began to incorporate a growing percentage of the GEFS/ECens/CMC ensemble means mid- to late period which helped account for the noted uncertainty with upper-ridge placement, as the means tended to a bit more similar to the EC ML guidance. QPF-wise, there is quite a bit of uncertainty with respect to the heavy rainfall potential over coastal and central/south Texas associated with an area of low pressure approaching the region. The GFS and CMC are more aggressive with rainfall totals, with the CMC having the most northerly track that would impact more of north Texas and the GFS covering portions of central in addition to south Texas. On the other hand, the ECMWF/UKMET suggest a more southerly track with still heavy but lower amounts tracking into south Texas. The corresponding ensemble probabilities follow a similar pattern. Opted to include some continuity along with the NBM partially modified by the GFS/ECMWF to account for a potentially broader footprint with localized heavier amounts, but with some restraint on leaning towards the higher end for now. Further east, another area of interest will be with the trough approaching the Southeast, with both the GFS/ECMWF and corresponding ensemble means showing an increase in amounts/coverage, leading to an upward increase in QPF compared to the prior forecast. ...Weather/Hazards Highlights... During the Days 4-5 Excessive Rainfall Outlook period covering Wednesday through Thursday night, latest guidance continues to show an area of deep tropical moisture and upper-energy/surface low (see NHC for additional details) shifting northwestward across the western Gulf of Mexico into the southern High Plains. Operational GFS/ECMWF runs show precipitable water anomalies possibly reaching plus 4 standard deviations or greater. However, as noted, there still remain some significant differences in the magnitude/placement of heaviest rainfall. Have maintained a Slight Risk form the Texas Gulf Coast inland into central and south Texas covering the general overlapping footprint of both deterministic QPF and ensemble exceedance probabilities. Note that the southern part of the Texas coast has drier antecedent conditions but rain rates may still be high enough to cause some runoff issues. Portions of central Texas and southward along the Gulf Coast currently appear most likely to see an outlook upgrade, though the lingering uncertainty and noted antecedent conditions preclude any changes for now. As the moisture/upper-energy continue westward, the updated Day 5 ERO continues a Slight Risk area over parts of southwestern and central Texas along/north of the Rio Grande River. There is still considerable uncertainty over rainfall specifics in this time frame as well. Elsewhere during the Days 4-5 ERO time frame, a persistent wavy frontal boundary interacting with anomalous moisture and shortwave impulses ejecting from the West Coast mean trough will maintain a threat for episodes of heavy convection from parts of the northern/central Plains into Upper Midwest. Thus far the guidance signal is diffuse enough (with somewhat lower amounts/more scatter than in the shorter term) for this activity to maintain only a Marginal Risk each day. Potential remains for an upgrade in risk area closer to the event. Finally, there is a Marginal Risk area along and inland from the Georgia/northern Florida coast in association with possible arrival of a weak wave/surface trough and enhanced moisture, with confidence growing in areal coverage/amounts of at least moderate to locally heavy rainfall. Gradual flattening of the upper pattern by late week into the weekend, with a persistent wavy front near the southern periphery of the westerlies, will support continued northern Plains/Upper Midwest convection and a greater eastward extent into the Great Lakes/Northeast. Confidence in specifics is lower due to the low predictability of important shortwave details. Some of the moisture reaching the southern High Plains by Thursday could ultimately factor into diurnal convection over the Rockies thereafter while moisture embedded within easterly flow may enhance rainfall at times near the Gulf Coast into the weekend. A well-defined frontal system could bring a little scattered light rain to the Pacific Northwest by next weekend. 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). Parts of the Mid-Atlantic could also trend a little hotter late week into the weekend with a transition to more westerly flow. In contrast to the eastern heat, separate areas of clouds/rainfall over the Plains will support highs 5-15F below normal on Wednesday and continuing over the southern High Plains and vicinity through late week. Rising heights aloft over the West will support a warmer trend, with increasing coverage of highs 10-15F above normal Friday-Sunday. 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 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