Preliminary Forecasts
Issued by NWS
Issued by NWS
927 FXUS02 KWNH 160700 PREEPD Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 AM 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... Regarding the eastern upper ridge, there are still notable differences that develop among the dynamical guidance and ECMWF- initialized machine learning (ML) models. In the 12Z/18Z guidance cycle, the UKMET was by far the most eager to shift the upper ridge into the western Atlantic so that solution was not incorporated into the forecast. By mid-late period the dynamical and ML solutions still conflict, as the dynamical models and to some degree the means show the best defined upper high center eventually becoming centered over the far Southeast U.S. or even western Atlantic while the MLs want to keep the high center farther inland (somewhere between the southern Plains and north of the Gulf Coast). The MLs also have been favoring greater southward extent of the upper ridge, reducing the potential influence of easterly waves/moisture along and near the Gulf Coast region. Dynamical and ML guidance generally agrees in principle that energy within the lingering West Coast trough should eject after early Friday as a Northeast Pacific system approaches, with the leading frontal system reaching inland by Sunday. The most common idea is for a shortwave trough to reach the Upper Midwest and vicinity by next Sunday with an associated northern tier surface wave. The new 00Z GFS looks a little fast with the system coming into western North America on Sunday. 12Z/18Z guidance comparisons led to a starting point consisting of the 18Z GFS and 12Z ECMWF/CMC early in the period and then gradually increasing ensemble weight (18Z GEFS and 12Z ECens/CMCens) that reaches 60 percent total by next Sunday with the remainder consisting of the 18Z GFS/12Z ECMWF. ...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 some degree of upper level energy shifting westward 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 dynamical guidance and ML solutions still show important differences for magnitude/placement of heaviest rainfall. During Day 4 the MLs support the general theme of the GFS/GEFS/CMC/ICON which bring heavy rainfall farther north over the eastern half of Texas than the ECMWF/UKMET, with modest expansion of the existing Slight Risk area to give some account for the former cluster. 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. As the moisture/upper energy continue westward, the new Day 5 ERO proposes a Slight Risk area over parts of southwestern Texas along/north of the Rio Grande River. There is still considerable uncertainty over rainfall specifics in this time frame. 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/enhanced moisture. Confidence is in the lower half of the spectrum though. 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 enhanced 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. 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 $$