Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 AM EDT Sun Jul 21 2024 Valid 12Z Wed Jul 24 2024 - 12Z Sun Jul 28 2024 ***Western U.S. heat wave to become more pronounced over the northern High Plains while moderating elsewhere, and showers/storms with heavy rainfall expected from the Four Corners to portions of the East*** ...General Overview... Latest guidance continues to show a steady transition toward a more typical summertime mean pattern after starting out fairly amplified at the start of the period early Wednesday. Expect the strong ridge initially over the West (producing hazardous heat from the West into the northern High Plains) to become weaker and more suppressed as a Pacific upper low tracks across southwestern Canada and residual troughing sets up along the West Coast. Monsoonal moisture will contribute to daily episodes of showers/storms over the Four Corners states into the Great Basin. Some of this moisture may eventually interact with a front pushing into the northern Plains as Canadian dynamics continue eastward. Meanwhile, one or more wavy fronts will be on the leading side of Great Lakes into southern Plains mean troughing aloft, leading to multiple days of rain/thunderstorms with areas of heavy rainfall from the southern Plains into the Mid-Atlantic and parts of New England. The Great Lakes and Northeast should eventually trend drier late week as the northern part of the trough moves eastward, but a more persistent upper level weakness over the Plains may continue to generate episodes of rainfall farther south. Atlantic upper ridging should build into the Southeast for a time, peaking in strength around Wednesday-Thursday. Southeast ridging may rebuild by next Sunday. ...Guidance/Predictability Assessment... The most prominent forecast issue continues to involve the mid-late week trough crossing the Great Lakes and Northeast. Through the 12Z/18Z cycles, the array of guidance remained similar to the past couple days with the ECMWF/ECens mean on the slow side and GFS runs fairly fast and open with the UKMET/CMC in the middle. The CMCens mean tilted to the slower side as well but ECMWF-initialized machine learning (ML) models have consistently been in the middle to faster part of the spread--at least favoring somewhat of a lean away from the slow 12Z ECMWF. The new 00Z ECMWF has arrived with a significantly faster/open trend. Farther west, as the western Canadian upper low continues eastward and possibly opens up later next weekend, the 18Z GFS strayed a bit south/southeast of consensus with the upper low, bringing greater height falls across the northern tier U.S. Thus the 18Z GFS could be overdone with the southward extent of the leading cold front. The new 00Z GFS looks better in that regard. The overall array of guidance supported an intermediate solution among 12Z/18Z operational models early in the period and then a transition toward approximately half models/half means late in the period. ...Weather/Hazards Highlights... Dangerous heat over the West/northern Rockies should extend at least into midweek with high temperatures reaching the 90s and 100s and warm overnight lows providing only limited relief. After decent coverage of plus 5-15F anomalies for highs on Wednesday, some cooling will likely move in from the West Coast (bringing highs within a few degrees on either side of normal) as moderate upper troughing sets up near the coast. Meanwhile, forecasts continue to show the most anomalous heat shifting into Montana/northern High Plains during mid-late week. In particular during Wednesday-Thursday the experimental HeatRisk index shows fairly broad coverage of major risk of heat-related impacts. A few daily records will be possible as well. The prolonged period of dry heat over the West extending into this week could also result in enhanced wildfire danger. In contrast, the cloudy and wet pattern over portions of the South and East will tend to keep highs below normal from Texas east-northeastward, with greatest coverage of negative anomalies Wednesday-Friday (and coolest versus normal over Texas/Louisiana). Southern and eastern Texas may remain below normal into the weekend. Monsoonal moisture will continue to support episodes of diurnally favored convection over the Four Corners region into the Great Basin, with some shifting of coverage over the course of the period based on pattern evolution. Isolated to scattered instances of flash flooding will be possible, especially near steep terrain and burn scars. The Day 4 Excessive Rainfall Outlook valid on Wednesday has expanded the Marginal Risk area northwestward based on guidance signals and favorable combination of anomalous moisture and instability (especially in the GFS) and proximity of a wavy front. The Day 5 ERO for Thursday reflects guidance consensus of moisture pushing eastward given southwesterly mean flow aloft. Potential remains for embedded Slight Risk upgrades entering the short range period depending on how guidance clusters relative to each other and with sensitive burn scar areas/regions with wettest ground conditions. Toward late week through the weekend, some of this moisture may interact with the front progressing across the northern Plains to produce some areas of focused rainfall. The upper trough extending from the Great Lakes into southern Plains and one or more leading wavy fronts will support a broad corridor of locally heavy shower and thunderstorm potential from the southern Plains northeastward next week. The best guidance signals in terms of coherence and rainfall magnitude (with ample moisture and instability) persist over southern/southeastern Texas on Wednesday. Thus the Day 4 ERO made little change to the Slight Risk area depicted over this region. The surrounding Marginal Risk area extends northeastward through the Mid-Atlantic and southern New England, with some embedded Slight Risk areas possibly emerging as guidance refines the most likely regions for heavy rainfall potential in the shorter term. The Carolinas and southern Virginia may be one such area of focus, given wet ground from recent rainfall. The Day 5 ERO valid on Thursday depicts a broad Marginal Risk area from southern/eastern Texas through the South and into the Northeast, with persistence of the corridor of enhanced moisture from Day 4 and potential interaction of the Great Lakes system (with its separate Marginal Risk area on Day 4) with this moisture over the Mid-Atlantic/Northeast. Again there will likely be some embedded Slight Risk areas to emerge once guidance signals become more coherent. Expect the Great Lakes/Northeast to trend drier late week into the weekend while showers and thunderstorms of varying intensity will likely persist into the weekend over the southern tier. 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