Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 AM EDT Fri Aug 16 2024 Valid 12Z Mon Aug 19 2024 - 12Z Fri Aug 23 2024 ...Hazardous heat likely across the southern Plains and central Gulf Coast states into next week... ...Overview... Guidance still agrees that a slow-moving upper pattern will be in place next week. An upper high centered over the southern High Plains will promote hazardous heat over the southern Plains to the central Gulf Coast. An upper low/trough is forecast to meander just off the West Coast, leading to a cooler and wetter pattern for the Northwest. Meanwhile, an upper trough will take hold from the Great Lakes and southward and slowly move east and be periodically reinforced by shortwave energies as next week progresses. This trough and a surface low pressure/frontal system will promote scattered to widespread thunderstorms with the potential for heavy rain across the Northeast U.S. into early next week. Hurricane Ernesto is forecast to track north over the western Atlantic and remain offshore of the U.S. East Coast. ...Guidance/Predictability Assessment... Models and ensembles continue to offer a reasonably similar flow pattern evolution over the coming week. Forecast spread and cycle to cycle continuity has now improved, generally bolstering forecast confidence to above normal levels. A composite of the 18 UTC GFS and 12 UTC ECMWF/UKMET/Canadian models along with the 01 UTC National Blend of Models seems to provide a solid mid- larger scale forecast basis and resultant smaller scale detail that seems consistent with local system predictability valid for Monday into Wednesday. Added to the mix the 12 UTC ECMWF/Canadian ensemble means as uncertainties grow through longer frames. These ensembles offer a slightly more amplified and less progressive pattern than the 18 UTC GEFS mean. Upstream flow amplitude and a slow to dislodge pattern history seems in line with this plan. Blocky patterns tend to linger. ...Weather/Hazards Highlights... It remains that case that Great Lakes/Eastern U.S. upper troughing and a surface low pressure/frontal system will provide forcing for rain and thunderstorms across the eastern U.S. into early next week. Instability and above normal moisture will be in place and could lead to locally heavy rain. A WPC Marginal Risk of excessive rainfall remains in place over the Northeast Day 4/Monday. While widespread heavy rain is not particularly likely, embedded heavy totals could cause flooding concerns especially considering wet antecedent conditions in much of the region and the multiple days in a row of rain. Rain is expected to taper off in the Northeast Tuesday-Wednesday as surface high pressure builds in behind the cold front(s) and in the wake of Ernesto lifting well offhore. However, trailing fronts slated to stall farther south across Florida will maintain several days of thunderstorm chances there. Flooding issues are a possibility if heavy rain falls in urban areas. This front moves slowly as it bends back to the west atop the central Gulf Coast and back north into the Plains. Scattered storms are possible near the front as impulses "ride the ridge" aloft, but with generally low confidence in placement still at this time. Monsoonal convection will affect the Four Corners states northward into early next week with moist southerly flow west of the upper ridge axis. Marginal Risks are slowly spread Monday-Tuesday over portions of Arizona, Utah, Southeast California, southern Nevada and Colorado. Storms with embedded heavy rain could cause isolated flooding, particularly in vulnerable areas like the slot canyons of southern Utah. Monsoonal convection looks to gradually reduce in scope into midweek. Some rain is also forecast to move across the northern tier as the week progresses. Upper ridging over the south-central U.S. will maintain above normal temperatures by 5-15 degrees from the High Plains east into the central Gulf Coast. Highs well into the 100s in much of Texas could set daily records. From eastern Texas into the Lower Mississippi Valley, hot temperatures and high dewpoints should produce heat indices likely above 110 degrees. Low temperatures in the upper 70s to low 80s will exacerbate heat stress and could set records for warm minimum temperatures. Experimental HeatRisk continues to show Major to Extreme impacts from this lengthy heat wave. North of this region, most areas should see near to slightly below normal highs, with some below normal lows as well for the Ohio Valley, while the Intermountain West can expect above average lows. The Pacific Northwest into northern California should see temperatures a bit below normal in response to the low aloft. Schichtel 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