Extended Forecast Discussion NWS Weather Prediction Center College Park MD 241 PM EDT Sun Sep 18 2022 Valid 12Z Wed Sep 21 2022 - 12Z Sun Sep 25 2022 ...Fiona to gain Hurricane strength lifting from the Caribbean to just east of the Bahamas toward Bermuda... ...Heavy rainfall threat for the Four Corners region into Wednesday-Thursday... ...Pattern Overview... The overall upper pattern mid-later this week will feature a steady ejection of an amplified trough/closed low from California to over the north-central Great Basin/Rockies to the north-central U.S. as a hot summertime ridging becomes suppressed from the central U.S. to the South. Meanwhile, ample northern stream trough energies will dig southeastward from Canada through the Upper Midwest to the East. This scenario should act to steer Hurricane Fiona well away from the U.S. East Coast with a forecast track from east of the Bahamas towards/just west of Bermuda. ...Guidance Evaluation/Predictability Assessment... Most recent models and ensembles continue to offer a more closely aligned pattern evolution at medium range time scale over the lower 48, bolstering forecast confidence despite some lingering local embedded system timing/strength differences. The most notable differences arise after day 5 with amplitude/speed of the departing trough out of the Northeast (likely influenced by interaction with/absorbtion of Fiona) and also the next shortwave into the Pacific Northwest. For the latter, the 00z ECMWF was the quickest with this wave, but the latest suite of 12z models appear to have trended towards that more fast solution (with the exception of the GFS). The WPC medium range product suite was primarily derived from a blend of the latest 00z/06z deterministic guidance through day 5, with much more inclusion of the ensemble means for the late periods which helps mitigate some differences noted above. This also maintains good WPC product continuity. ...Weather/Hazards Highlights... The expected slower inland ejection of an initially quite amplified upper level trough from California through the cooling north-central Great Basin/Rockies mid-late week will support cooler temperatures that may lead to some snow for higher elevations of the north-central Great Basin/Rockies. The daily temperatures are forecast to be 10 to 15 degrees F below seasonal average for mid September for the West with the cool airmass spreading to the Rockies/Plains early in the week and then to the Upper Mississippi Valley/Midwest by the later week/weekend as the cold front works downstream. Anomalous moisture with some East Pacific subtropical/tropical connection is forecast to lift across the Four Corners region and northward into mid-late week ahead of the ejecting upper trough, that with upper support may increase chances for moderate to locally heavy or excessive rainfall and thunderstorms. There remains support in the latest guidance to continue slight risk areas on the Day 4 and 5 experimental WPC Excessive Rainfall Outlook across parts of southwest Colorado into northern New Mexico. Farther to the north, additional organized activity including some potential for local convective downpours/runoff issues are expected to develop with system approach from northern California/Oregon midweek through the north Great Basin/Rockies then north-central U.S. later week into the weekend given favorable system focus. With lead ridging building into the central U.S. into the start of this period, warmer than average temperatures by 10-20F+ are forecast for the Central Plains and Middle Mississippi Valley through Wednesday. Warmer than normal temperatures will linger into the Southeast on Thursday, though with slightly lesser anomalies. A strong cold front is forecast to then advance across the northern tier from Canada which would bring showers/thunderstorms with some locally enhanced downpours through the Great Lakes to the Northeast Wednesday into Thursday. Leading heat will quickly get suppressed with this frontal surge/passage as quite cooled Canadian post-frontal high pressure settles in. Meanwhile, a stalled front is forecast to linger across Florida and could trigger showers and thunderstorms there as Fiona lifts well offshore the U.S. mainland from near/east of the Bahamas toward Bermuda Wednesday/Thursday as per the latest NHC track. Santorelli/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, 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