Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 AM EDT Mon Jul 22 2024 Valid 12Z Thu Jul 25 2024 - 12Z Mon Jul 29 2024 ...Western U.S. heat to shift into the northern High Plains... ...Showers/storms with locally heavy rainfall expected over the Four Corners and from the western Gulf Coast into parts of the East... ...Overview... Guidance continues to show an evolution toward a more average summertime pattern with flatter flow setting up over the northern tier U.S. underneath and in the wake of an upper low tracking across southern Canada, while moderate mean troughing lingers near the West Coast. Farther east, there is initially good agreement with an upper trough progressing eastward from the Great Lakes (with a leading cold front). However disagreements arise regarding residual trough energy near or offshore the East Coast, as well as for what becomes of energy within an upper weakness over the Plains late this week into the weekend. Initial western ridging should weaken somewhat but linger over the Four Corners, while upper ridging over the Southeast/Gulf coast should oscillate in strength during the period. The forecast pattern will likely favor highest rainfall totals from the western Gulf Coast into the southern Mid- Atlantic as well as multiple days of locally heavy monsoonal convection over and near the Four Corners states. ...Guidance/Predictability Assessment... The primary guidance differences/trends of note on the synoptic scale are over the eastern half of the country into the western Atlantic. Over the past 24-36 hours, guidance clustering has improved for the upper trough initially progressing eastward from the Great Lakes (with the aid of a faster/more open trend in ECMWF runs). However a fair amount of spread develops for the character of trailing shortwave energy that may feed into the trough and linger into next week. While the 12Z ECMWF-initialized machine learning (ML) models vary, they provide a general theme of an intermediate solution between the western ECMWF and farthest offshore GFS. Meanwhile both the dynamical and ML models vary considerably for what becomes of energy within an initial Plains weakness, ranging from some of this energy getting carried eastward/northeastward (with corresponding expansion of rainfall ahead of it) to very little progression but rather more ridging over the East by next Monday. Lack of agreement in both types of models would recommend a conservative blended approach for the time being. Over the West, there is good agreement for the overall pattern with most differences involving smaller scale shortwave impulses that have low predictability several days out in time. The updated forecast based on 12Z/18Z guidance incorporated an operational model composite early in the period and then a transition to nearly half total weight of the 18Z GEFS/12Z ECens means by next Monday. ...Weather/Hazards Highlights... Expect episodes of locally heavy rainfall during the latter half of the week within a broad corridor of enhanced moisture from southern/eastern Texas to parts of the East Coast. On Thursday, the best composite of guidance highlights the region from far southern Virginia through the Carolinas into northern Georgia/southern Appalachians for heaviest rainfall potential. These locations will also have wet ground conditions from recent/forecast rainfall. Thus the Day 4 Excessive Rainfall Outlook depicts a Slight Risk area across this region, representing a southwestward expansion from continuity. Within the broad surrounding Marginal Risk area covering the South and East, the western Gulf Coast region looks to be another area of somewhat higher potential for locally heavy rainfall but with a more diffuse guidance signal at this time precluding a Slight Risk for the time being. Activity over the Northeast in association with an upper trough/cold front appears to represent the lower end of the Marginal Risk range. Southeastward progression of the cold front over the East should suppress the moisture axis near the East Coast by Friday, yielding a band of Marginal Risk from southern/eastern Texas through most of the Carolinas in the Day 5 ERO. The latter region would be on the higher end of that range, with just a little better guidance clustering meriting a Slight Risk area. Beyond early Saturday, northward extent of rainfall over the East becomes increasingly uncertain due to guidance differences in important details aloft. 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. Instances of flash flooding will be possible, especially near steep terrain and burn scars. The Day 4 Excessive Rainfall Outlook valid on Thursday maintains continuity with a Marginal Risk area covering parts of the southern Rockies into the Great Basin, while portions of the Marginal Risk area in the Day 5 ERO shift a bit eastward in concert with progression of highest moisture anomalies and best instability. Embedded Slight Risk upgrades appear likely at some point in future cycles, with specifics depending on how guidance clusters relative to each other and with sensitive burn scar areas/regions with wettest ground conditions. By late week onward, some of this moisture may interact with the front progressing across the northern Plains to produce some areas of focused rainfall. Expect the hazardous heat over the West during the short term to moderate/decrease in coverage late this week while a separate area of hot temperatures will settle over northern half of the Plains. Parts of California and the Southwest may see highs up to 5-10F above normal Thursday-Friday along with the experimental HeatRisk index showing scattered areas of at least major risk of heat- related impacts especially on Thursday. Meanwhile, the northern High Plains should also see the most extreme heat on Thursday with highs 10-20F above normal (with a few daily records possible) and broad coverage of the major category in HeatRisk. After Thursday expect the heat to shift a little east/southeast over the northern half of the Plains and persist into early next week (plus 5-10F and locally higher anomalies). Very warm temperatures may also spread into the Great Lakes and Northeast by Sunday-Monday. In contrast, the generally wet pattern over the South and vicinity will tend to keep highs below normal from southern/eastern Texas into the southern Mid-Atlantic late this week into the weekend. Coolest anomalies with greatest persistence should be over Texas. 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