Extended Forecast Discussion NWS Weather Prediction Center College Park MD 122 PM EDT Wed Aug 30 2023 Valid 12Z Sat Sep 2 2023 - 12Z Wed Sep 6 2023 ...Heat forecast to increase across the Plains and Midwest this weekend into early next week... ...Wet pattern across the West with flooding possible... 18Z Update: The latest guidance agrees well on the overall weather pattern across the United States going into the weekend. Model differences start becoming apparent across the Gulf of Alaska by Sunday regarding the timing of a shortwave trough and potential closed low that will reach the Pacific Northwest early next week. Additional uncertainties are introduced when this disturbance may phase with the broad upper trough that will initially be over the Intermountain West, and this will have some ramifications on how the trough and associated surface low evolves across the Northern Plains by next Tuesday-Wednesday and the eventual break-down of the upper ridge situated over the Plains. In terms of the fronts/pressures forecast, the 00Z CMC appeared to be too far north with the surface low by the first half of next week when compared to the model consensus, so the latter half of the forecast period focused more on a GFS/ECMWF/ensemble/WPC continuity blend for the evolving low pressure system and the associated fronts. In terms of temperatures, the output from the GFS is probably too high across much of the Midwest states and extending eastward to the Mid-Atlantic, so the NBM and ECMWF served as a better baseline for temperatures. The previous forecast discussion is appended below for reference. /Hamrick ------------------------- ...Overview... An amplified upper-level pattern will be in place this weekend, composed of an upper trough (with an embedded closed low into Saturday) over the West Coast and an upper ridge over the central CONUS. These features should shift gradually eastward next week. This pattern will allow for well above average temperatures to build across the Plains and Upper Midwest, shifting into the East by Monday-Wednesday, while rain chances across parts of the West this holiday weekend could cause flooding concerns. Meanwhile scattered showers and storms could persist across the Gulf Coast region into Florida as a shortwave aloft and a frontal boundary linger there. ...Guidance Evaluation/Predictability Assessment... Model guidance agrees quite well with the synoptic midlatitude pattern at the start of the period Saturday, with an anomalously strong upper high atop the central U.S., a closed upper low over northern California, and a weak shortwave trough moving through the Great Lakes to Northeast. These features are forecast to move eastward into early next week, as greater differences start to arise with an energetic shortwave moving quickly through the eastern Pacific upstream of the upper low. The interaction of this upstream energy with the preexisting low could combine with the trough and/or help push it eastward, with variations among models. The details of this flow do have impacts for positions of surface fronts moving across the West. In terms of the 12/18Z guidance, the timing of the ECMWF and GFS seemed most aligned compared to the UKMET and CMC. The 12Z CMC was deeper than consensus with the trough by Tuesday-Wednesday as it shifts into the north-central CONUS. Another shortwave may come in around midweek to further complicate matters. So the WPC forecast blend favored the EC and GFS with smaller proportions of the UKMET and CMC early, eventually removing those while including some GEFS and EC ensemble means reaching around half by the end of the period amid increasing uncertainty. Something else we have been monitoring is the eventual track of Idalia after it moves into the western Atlantic. Many GFS runs and some CMC runs, as well as some ensemble members from both suites, drifted the remaining low from Idalia southward east of Florida with some even turning it westward into Florida. ECMWF and most of its ensemble members instead took the system east away from the U.S. coast. Now the newer suite of 00Z guidance has transitioned toward this eastward track and are even faster taking it farther out into the Atlantic. So will continue to monitor but it may be most concerning to Bermuda at this point. ...Weather/Hazards Highlights... Rainy conditions are likely across much of the West through this weekend with the amplifying upper trough/embedded closed low settled over the West Coast. The upper low itself will provide forcing for ascent and help cause steepening lapse rates for some instability into the West Coast, while east of the trough strong mean southerly flow will carry monsoonal moisture with precipitable water values well above normal and instability into the interior West. On Saturday, a broad Marginal Risk is planned for areas from Oregon and northern California into the Great Basin and Southwest, with an embedded Slight Risk where the heaviest rainfall looks to focus in the central Great Basin. For the 18Z update, the existing Slight Risk was extended some to the south to include more of northwestern Arizona. The shift of the trough eastward on Sunday should position focused moisture across the Intermountain West, with a Slight Risk proposed a bit farther north compared to Saturday. The 18Z update included a small northward extension of the Slight Risk towards Yellowstone. Burn scars, slot canyons, and urban areas will be particularly vulnerable to flooding. As the trough continues to shift inland, precipitation should expand/push into the northern Plains by Monday and possibly provide some showers in the Midwest by Tuesday. Showers and thunderstorms will also impact portions of the Gulf Coast and Florida as a stalled frontal boundary lingers across the region along with a shortwave aloft. Moisture will pool along the front, producing scattered precipitation chances through the weekend. There is some chance of flash flooding with this activity especially if high rain rates occur in sensitive urban areas or locations that have seen heavy rainfall recently. Thus a Marginal Risk is in place in the ERO for Saturday. While rainfall magnitudes look to decrease into early next week, some scattered showers and storms could continue. Cooler than average high temperatures are forecast for the West Coast into the Great Basin given the troughing aloft. Slightly below normal temperatures are forecast across the Southeast on Saturday before warming to near normal on Sunday. But in the central U.S., the strengthening upper ridge will allow temperatures to rise considerably through the holiday weekend. Highs well into the 90s will expand all the way into the northern Plains and east across the Mississippi Valley, while highs exceeding 100 are likely for the southern half of the Plains. These forecast high temperatures are around 10-20 degrees above average, perhaps locally higher in the Upper Midwest. Some record warm maximum and minimum temperatures could be set there as well. As the upper ridge shifts east with time, temperatures in the Mid-Atlantic and Northeast look to warm to about 10-15 degrees above normal by Monday-Wednesday, for highs in the upper 80s and 90s. Tate/Hamrick 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, excessive rainfall outlook, winter weather outlook probabilities, heat indices and Key Messages are at: 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