Extended Forecast Discussion NWS Weather Prediction Center College Park MD 334 AM EDT Sat Sep 17 2022 Valid 12Z Tue Sep 20 2022 - 12Z Sat Sep 24 2022 ..Tropical Storm Fiona forecast to slowly gain strength while lifting from the Caribbean toward/east of the Bahamas into midweek... ...Moderate to locally heavy rainfall threat from the Four Corners region into the Rockies Tuesday-Thursday... ...Pattern Overview... The upper pattern during the medium range period will feature a very amplified trough/closed low slated to form off the California coast that may slowly eject northeastward over the north-central Great Basin/Rockies by the latter part of next week. Downstream, hot summertime ridging will build over the central-eastern U.S. and ample northern stream Canadian shortwave energy seems slated to dig southeastward across the northern tier to amplify over the Great Lakes Northeast around Thursday-Saturday of next week. This system has some energy connection of the current extremely deep Bering Sea low associated with the extratropical remnants of Typhoon Merbok. The evolution of this trough has strong implications for eventual steering flow of Tropical Storm Fiona beyond day 5, forecast to reach hurricane strength with a track near/east of the Bahamas Wednesday into Thursday. ...Guidance Evaluation/Predictability Assessment... Guidance overall remains in good enough agreement for a multi-model blend midweek. After about Wednesday, timing differences remain more apparent Thursday into next weekend as the upper low over California uncertainly ejects inland across the north-central Great Basin/Rockies. Recent runs of the GFS/GEFS and to a lesser extent the Canadian remain on the faster side of the guidance, while recent runs of the ECMWF/ECMWF ensembles and UKMET hang the low back over California longer. Prefer more into the slower side of the envelope of solutions closest to the ECMWF ensemble mean given the nature of closed systems in separated flow and as per favorable latest run to run guidance trends. Northern stream flow across the northern tier and eventually into the Northeast continues to provide plenty of uncertainties on timing and amplitude. This would have strong implications on how close to the US East Coast Fiona eventually tracks in the late periods. However, recent guidance offers increasing confidence that this upper trough should help steer Fiona sufficiently out to see with only minimal impacts to the U.S. East Coast. A slowed ejection of the Californian closed low also leaves more room for the northern stream upper trough to amplify a bit more downstream than previous forecasts. Important to note though that while odds favor a well offshore track, there is still some lingering potential Fiona could come close enough to the East Coast for some greater impacts. Please refer to the latest NHC forecast for the updated information regarding Fiona. ...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 next week will support cooler temperatures that may lead to some mixed precipitation, or snow for the highest peaks across California and 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/High Plains early in the week and then to the Upper Mississippi Valley by the middle/end of the week as the cold front makes its way east. The lingering onshore rain may be heavy at times across portions of northern California into Oregon into Monday, and burn scars would be particularly vulnerable to flooding concerns and debris flows. The Sierra Nevada could see snow early next week, though the best chances for snow with this activity are in the Northern Rockies Tuesday-Thursday. Moisture with some subtropical/tropical connection is forecast to lift across the Four Corners region and northward early-mid next week ahead of the ejecting western U.S. upper trough, that with upper support may increase chances for moderate to locally heavy or excessive rainfall and thunderstorms. There was enough support in the latest guidance for a small slight risk areas on the Day 4 and 5 experimental WPC Excessive Rainfall Outlook across parts of southwest Colorado and northern New Mexico in the vicinity of the San Juan Mountains. Precipitable water and ARIs are enhanced and the area is also in the ECMWF ensembles maximum probability for 1"+ plus daily rain totals. 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 with actual temperatures well into the 90s into midweek threatening record values for this time of the year. Warmer than normal temperatures will extend to the East Coast into around 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 into the Midwest/Great Lakes then Northheast Wednesday into Thursday. Suspect the NBM is too light with QPF and modestly increases amounts given potential for amplification. Lead heat will then get suppressed with this central to eastern U.S. frontal passage and quite cooled Canadian post-frontal high pressure. Meanwhile, a stalled front is forecast to linger across central to southern Florida and could trigger showers and thunderstorms there as Fiona lifts well offshore the U.S. mainland as per the latest NHC track. 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