Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 AM EDT Thu Aug 15 2024 Valid 12Z Sun Aug 18 2024 - 12Z Thu Aug 22 2024 ...Hazardous heat likely across the southern Plains and central Gulf Coast states into next week... ...Overview... A slow-moving upper pattern will be in place into next week. An upper high atop the southern High Plains will promote hazardous heat over the southern Plains to the central Gulf Coast. An upper low is forecast to meander 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 as the 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... Model guidance continues to show good agreement on the large scale pattern described above, with some model spread/uncertainty in the details. By Wednesday-Thursday, the eastern Pacific upper low shows some model variations in large part due to uncertain upstream energy from Alaska and how it interacts with the existing upper low--merging or pushing the original low east. The 12Z GFS seemed to be an outlier in doing the latter, taking energy into south- central Canada. Other 12/18Z guidance and and the newer 00Z guidance is more in favor of keeping one upper low just offshore of the Pacific Northwest. Farther east, there is also pattern uncertainty with a compact upper low in central Canada and its timing/placement as it could erode the northern part of the Central U.S. ridge. Most 12Z guidance showed the upper low moving east into the Hudson Bay by Thursday, but the 18Z GFS was bullish in having it faster/farther southeast. Did not favor the upper low track in the 18Z GFS, but now the 00Z ECMWF and GFS have come in reasonably similar to each other and not too different from the 18Z GFS in showing a bit of a southeastward dive bringing the upper low toward the Great Lakes. So there is still considerable uncertainty. The evolution of this feature also in part affects the timing of the weakening of the trough in the East. 12/18Z and now 00Z guidance varies on that, with the 00Z CMC throwing a wrench into the pattern by showing a closed low anchored in Quebec late period. The WPC forecast used a multi-model deterministic blend early in the period, reducing the proportion of the GFS runs in particular in favor of the more agreeable ensemble means, reaching half means by Day 7. ...Weather/Hazards Highlights... Great Lakes 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 Marginal Risk of excessive rainfall is in place Day 4/Sunday for the northern Mid-Atlantic into the Northeast, and more limited to the Northeast by Day 5/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). However, the front stalling farther south across Florida into midweek (after an initial front stalls across southern Florida early next week) 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. 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 in place Sunday-Monday for portions of Arizona, Utah, Colorado, and even reaching farther north into the northern Rockies where PWs will be above average. 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. Farther north, the Pacific Northwest can expect rain particularly on Sunday with the eastern Pacific upper low nearby. Some rain is 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, high dewpoints will combine with the hot temperatures to 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. Tate 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