Extended Forecast Discussion NWS Weather Prediction Center College Park MD 238 AM EDT Mon Aug 29 2022 Valid 12Z Thu Sep 01 2022 - 12Z Mon Sep 05 2022 ...Overview... At the start of the medium range period, the flow pattern is expected to still be fairly amplified with strong ridging through the interior West while troughing is forecast over the Great Lakes and Northeast and eastern Pacific. Under the strong Western ridge, temperatures are forecast to be quite warm with daily high records possible mid to late week. By late this week into next weekend, the flow is then forecast to become more zonal and flatten out, allowing shortwave energy to move across the northern CONUS. Meanwhile, across the Southern U.S., a frontal boundary is expected to settle along the Gulf Coast and be the primary focus for scattered to numerous showers and thunderstorms each day. Some of the rainfall could be locally heavy and with several days of potentially repeating rounds, some flash flooding may become possible late this week into this weekend from the Texas Gulf Coast to the Florida Panhandle. ...Guidance Evaluation/Predictability Assessment... The amplified pattern at the start of the period has very good agreement among the various deterministic models this cycle with the strong ridge expected to be centered over the Great Basin through this weekend. A shortwave trough ejecting out of the eastern Pacific troughing will clip the Pacific Northwest and while earlier runs of the GFS/ECMWF/UKMET/CMC had some timing issues, the latest guidance has clustered very well. By day 5 however, the GFS is a bit flatter with the energy as it moves into the northern Plains while the ECMWF and CMC dig a shortwave into the Dakotas. Meanwhile, the GFS is also faster with pushing the trough axis in the eastern Pacific through the Pacific Northwest, especially by days 6-7 when the GFS nearly becomes out of phase with the ECMWF/CMC solutions, which holds the eastern Pacific troughing back. While not entirely discounting the GFS idea, will lean more on the ensemble means which are slower with any energy moving into the Pacific Northwest by day 7. The other area of model differences is across the Gulf Coast with respect to the stationary/residual boundary that interacts with Gulf moisture. The focus for widespread showers and thunderstorms will likely be found initially over the Florida Panhandle and Southern Plains later this week then shift toward the Texas Gulf Coast this weekend into early next week as a strong fetch of moisture transport impinges on that boundary. With anomalously high PWs forecast (near 90th percentile and 2 sigma above normal), forecast models show potential for several inches over the period but the convective/mesoscale nature expected limits confidence and details at this time. ...Weather/Hazards Highlights... The start of Meteorological Fall will feel very fall-like for the eastern U.S., in the wake of the mid-week frontal passage. Canadian high pressure building in its wake will provide seasonable temperatures, less humidity, and dry conditions. Morning lows may dip into the 40s and 50s across the Great Lakes and interior Northeast Thursday and Friday mornings. The frontal boundary is then expected to settle along the Gulf Coast and its interaction with ample instability and moisture will be enough of a lift/focus for scattered to numerous showers and thunderstorms across the Florida Panhandle Thursday into the weekend. Meanwhile, a weakness in the upper ridge over Texas and its interaction with the tail of the frontal boundary will provide focus for more thunderstorms and beneficial rainfall over Texas. Some of this rainfall could be locally heavy with intense rain rates. By this weekend, a surge of Gulf moisture interacting with the front in the area will bring greater chances of widespread precipitation to the Texas Gulf Coast eastward to the Florida Panhandle. Some of this rainfall may be heavy and with repeating days of rainfall, some concern for flash flooding may develop by late in the weekend into early next week. Over the period, rainfall totals could be several inches, particularly along the Texas Gulf Coast. Meanwhile, the rest of the country looks to be mainly dry aside from light rain chances across central parts of the Rockies/Plains and a frontal passage clipping the Northwest. However, the main story for the West to northern Plains will be the warmth. Highs later this week into the weekend are expected to be 10-20F above normal for early September with the potential for daily record highs for portions of the interior West. Some locations could even flirt with monthly temperature records. Those warm temperatures are then expected to spread eastward to the Northeast by Sunday into Monday with highs as much as 10F above normal. Tate/Taylor 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, experimental excessive rainfall outlook, winter weather outlook probabilities and heat indices 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