Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 AM EDT Tue Jul 23 2024 Valid 12Z Fri Jul 26 2024 - 12Z Tue Jul 30 2024 ...Hazardous heat to shift into the northern Plains... ...Showers/storms with locally heavy rainfall expected over the Four Corners and from the western Gulf Coast into the southern Mid-Atlantic/Southeast... ...Overview... Models/ensembles still show the large scale flow evolving toward a more typical summertime pattern as progressive flow becomes more prominent across the northern tier U.S. and southern Canada with time, while moderate mean troughing should persist near the West Coast. Southwestern U.S. upper ridging should persist for most of the period, while a separate Southeast/Gulf Coast upper ridge may strengthen and merge with the Southwest ridge by next Tuesday. There are continued uncertainties with specifics of initial energy over the Plains as well as northern stream energy that may settle near or offshore the East Coast. This pattern will likely bring the greatest potential for anomalous heat to the northern-central Plains. Areas of enhanced rainfall will be possible over parts of the southern tier, along an ahead of a wavy northern tier front, and with monsoonal convection over parts of the West. ...Guidance/Predictability Assessment... As has been the case recently, the most prominent guidance differences appear over the eastern half of the lower 48 into the western Atlantic. 12Z/18Z runs diverge with respect to the eventual strength and path of initial upper level energy over the Plains, with some influence from timing differences for upper ridging that cross the Midwest on its way into the East. These differences then affect the longitude of a separate shortwave that could settle along or offshore the East Coast. 24 hours ago the dynamical and machine learning (ML) models were evenly distributed between the ECMWF scenario (weak Plains energy that does not lift northeastward due to a slower Midwest Ridge, plus an upper trough settling near the East Coast) and the GFS (stronger Plains energy that ejects northeastward plus farther eastward shift of features downstream). Now 12Z MLs are 4-to-1 in favor of some variation of the GFS/UKMET (CMC a somewhat weak compromise for the Plains energy, the 00Z UKMET perhaps too strong), and the new 00Z ECMWF has indeed come in with a trend toward what has become the dynamical/ML majority. Along the West Coast, the majority of guidance maintains mean troughing but with some differences in strength that have low predictability. There are sporadic model runs that have been showing a little more split to the flow. Guidance comparisons for the 12Z/18Z cycles led to an operational model composite early, trending toward a model/mean mix later, and incorporating a little less 12Z ECMWF input than usual due to its less favorable comparisons. ...Weather/Hazards Highlights... Guidance continues to show the potential for episodes of locally heavy rainfall within a corridor of enhanced moisture from southern/eastern Texas to the southern Mid-Atlantic/Southeast coast late this week. Thus the Days 4-5 Excessive Rainfall Outlooks covering the Friday through Saturday night period depict a broad Marginal Risk area across the southern tier. Within the Day 4 Marginal Risk, the region encompassing eastern North Carolina and northeastern South Carolina has shown continuity in the guidance for relatively greater heavy rain potential as an eastern U.S. cold front approaches from the north, while wet ground conditions prevail as well. Thus a Slight Risk area has been introduced for this region. Some enhanced activity may continue along the Southeast coast into Day 5 with upgrade potential if guidance signals persist in future runs. The western Gulf Coast looks to be another area of interest for potentially organized heavy rainfall on Day 5, but signals are currently too diffuse for a Slight Risk. Finally, it is worth noting that diverging guidance for flow details aloft over the East by Day 5 leads to below average confidence for the northern periphery of meaningful rainfall. For now the Marginal Risk area leans to the conservative side (representing the most common overlap) with some northward expansion possible depending on trends of future runs. 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 Days 4-5 EROs both depict Marginal Risk areas, with Day 5 representing a trimming of the westward extent of potentially heavy convection as the best moisture/instability shift eastward somewhat. Embedded Slight Risk upgrades will be possible 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 Saturday-Saturday night the guidance suggests that a wavy front moving into the northern Plains may start to produce areas of heavier rainfall given potential interaction of shortwave energy and deep moisture being carried along in southwesterly flow aloft. The Day 5 ERO introduces a Marginal Risk centered over the eastern half of North Dakota into northern Minnesota, as ground conditions are still on the wet side over this region. Beyond early Sunday, northern tier convection should gradually continue eastward while areas of rainfall may expand across the east-central U.S. depending on the details of initial Plains energy aloft that may lift northeastward. Diurnally favored convection should continue across parts of the southern tier while western U.S. monsoonal activity should become more confined to southern areas. Expect the northern and central Plains to see the most persistent anomalous heat from late this week into next week, with best potential for some highs 10-15F above normal to shift from the Dakotas on Friday into the central Plains by Sunday-Tuesday. These anomalies would yield highs in the upper 90s to low 100s. Some of this heat should spread eastward across the Great Lakes/Northeast by Sunday-Tuesday, with highs 5-10F above normal (upper 80s to around 90). Much of the West will likely see near to slightly below normal highs through the period, with the exception of the Southwest which should see some lingering above normal readings through Friday and possibly Saturday. The wet pattern across the southern tier late this week into the weekend will keep most areas below normal for highs, especially southern/eastern Texas. The rest of the southern tier should trend closer to normal by early next week while southern Texas may remain a bit on the cool side. 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