Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 PM EDT Mon Aug 5 2024 Valid 12Z Thu Aug 08 2024 - 12Z Mon Aug 12 2024 ...Heavy rain and flash flooding threat associated with Debby expected to spread up the East Coast toward New England late this week... ...Excessive Heat continues for portions of Texas and central Gulf Coast Thursday into this weekend... ...Overview... The National Hurricane Center forecast track for Debby takes it up the Eastern Seaboard Thursday into this weekend where there is the threat for heavy rainfall along with gusty winds. Latest model guidance has trended toward greater frontal interaction of tropical moisture associated with Debby leading to increasing heavy rain potentials up toward new England. Upper ridging centered near the Four Corners will maintain monsoonal flow over the Southwest through this weekend with heat concerns shifting south ahead of the ridge, over Texas to the central Gulf Coast (areas west of Debby's cloud cover). ...Guidance/Predictability Assessment... Focus areas in the synoptic pattern latter half of the week area the track of Debby and the associated eastern CONUS trough along with troughing over the northern tier of states. The latest NHC track continues to favor the ECMWF and a weaker eastern CONUS trough that allows Debby to track up the Eastern Seaboard late this week after stalling near the Southeast coast. That said, situations like this with weak steering flow aloft means model guidance typically struggles. The outlier with the Debby track remains the GFS which has a stronger eastern trough and keeps Debby below it, causing meandering over the Southeast, while the 00Z consensus is for a slow trek up the eastern Seaboard. However, the GFS retrogrades more than its ensemble mean and the 12Z Graphcast AI run off the GFS initialization is in line with the non-GFS 00Z consensus. The model preference was hedged in the direction of the ECMWF, the 12Z UKMET and then increasingly in the ECENS and GEFS means. ...Weather/Hazards Highlights... Slow motion to Debby raises the threat for extreme rainfall to continue in the Southeast Thursday. A Day 4 Moderate Risk for excessive rainfall remains for eastern SC into southern NC. Given the ejection north of Debby/perhaps its remnants, an expansive Slight Risk is introduced over much of the Eastern Seaboard for the Day 5 ERO. A surge of tropical moisture ahead of Debby into a frontal boundary over the Northeast favors a PRE setup with additional rainfall stretched northeastward across the Mid- Atlantic and Northeast. Monsoonal moisture on the western side of the ridge will keep a wet and unsettled pattern in place from the Four Corners region northward into the Rockies/High Plains through much of the extended period. Marginal Risks for excessive rain are in effect for parts of the Southwest/Four Corners (especially the sensitive slot canyons in southern Utah) through the northern High Plains Thursday with a Slight Risk maintained for Day 4 over northern NM/southern CO where higher QPF is noted. Will need to monitor the need for a Slight introduction on Day 5 with continued anomalous moisture influx over the West. Above seasonal normal temperatures across the Southwest and southern Plains on Wednesday are forecast to trend toward normal into the weekend as the ridge suppresses south under troughing along the Canadian border. Hazardous heat is likely for Texas and Lower Mississippi Valley/central Gulf Coast late this week into the weekend. Experimental HeatRisk values are generally major for much of this region with extreme for the central Gulf Coast area. Much of the central U.S. will cool off in the wake of a cold front midweek. Kong/Jackson 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