Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 PM EDT Tue Sep 20 2022 Valid 12Z Fri Sep 23 2022 - 12Z Tue Sep 27 2022 ...Overview... The medium range period begins late this week with an upper high over the south-central U.S. promoting warm temperatures well into the 90s, while a cool surface high leads to below normal temperatures in parts of the east-central U.S. The pattern settles into a western ridge and eastern trough overall pattern for the early part of next week, keeping the West generally dry while frontal systems bring some rain to the east-central U.S. to the East Coast. ...Guidance Evaluation/Predictability Assessment... Recent model guidance has remained in good agreement for the large scale pattern, as most impactful systems are on the synoptic scale and thus have greater predictability. Overall a composite blend of the deterministic models (specifically the 06Z GFS and 00Z ECMWF, CMC, and UKMET) worked well for the fronts/pressures forecast, with some inclusion of the GEFS and EC ensemble means by days 6/7 to reduce detail differences between individual models. The 13Z National Blend of Models was used heavily for QPF and temperatures etc. forecasts, though with some enhancements particularly to expand some light QPF amounts. Frontal boundaries tracking across the central and eastern U.S. and the associated QPF early next week show some waffling in timing and placement for some minor differences from continuity. Model guidance is agreeable with the track of Hurricane Fiona near the Canadian Maritimes even as it transitions to a deep extratropical low during the medium range period. WPC and the National Hurricane Center will also continue to monitor the tropics regarding Invest 98L, which is likely to form into a tropical cyclone in the Caribbean per the NHC Tropical Weather Outlook. Broadly, GFS runs indicate a stronger tropical system by early next week and a more northeasterly track compared to the 00Z ECMWF and CMC runs and most 00Z EC ensemble members (GEFS members were more diffuse in placement). Leaned away from these northeasterly GFS runs at this point, though the new 12Z ECMWF is now actually closer to the GFS. There is still ample uncertainty as the track will depend on multiple factors including the exact trough position to the north. Stay tuned for forecast updates over the next several days as this system could eventually be impactful. ...Weather/Hazards Highlights... A shortwave trough tracking across the north-central U.S. will combine with energy from northern Canada to deepen a trough with its axis in the east-central U.S. for the early part of next week. This trough and a couple of frontal systems at the surface should help produce some increasing rainfall chances across the Upper Midwest/Great Lakes and toward the Ohio Valley late week into the weekend, shifting into the Eastern Seaboard by later Sunday into Monday-Tuesday near the frontal system, with some lake enhanced rainfall behind it. The western U.S. should see generally dry conditions during the medium range period after a wet short range timeframe. Showers and storms are possible across the Florida Peninsula and may increase in coverage by around Tuesday and beyond as there is potential for tropical moisture streaming in. A broad cool surface high will lead to below normal temperatures from the northern half of the Mississippi Valley into the Ohio Valley, Great Lakes, and Mid-Atlantic/Northeast on Friday, with cooler than normal temperatures remaining across the East on Saturday. Meanwhile an upper high centered in Texas Friday will produce continued warmth for the Southern Plains and Lower Mississippi Valley through the weekend, with highs in the 90s and perhaps isolated 100s, which are 10-15F above average. As upper ridging develops in the West, warmer than average temperatures by around 5-10F are forecast for the first part of next week, bringing the Desert Southwest temperatures well into the 100s. 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, 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