Extended Forecast Discussion NWS Weather Prediction Center College Park MD 206 PM 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 and the 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 also 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 through day 4. After about Wednesday, timing differences continue to be 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 and to a lesser extent the Canadian remain on the faster side of the guidance, while recent runs of the ECMWF and UKMET hang the low back over California a little longer. The ensemble means generally support their deterministic counterparts, though with maybe slightly less spread. WPC overall prefers the slower side of the envelope of solutions closest to the ECMWF/CMC/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. Likely a result of the issues farther upstream, the GFS is again the fastest to eject the trough out of the Northeast, while the CMC and ECMWF (and the means) are slower. This would have strong implications on how close to the US East Coast Fiona eventually tracks in the late periods. However, recent guidance continues to boast confidence that this upper trough should help steer Fiona sufficiently out to sea 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 are increasing for a well offshore track, there is still some lingering potential Fiona could come close enough to the Northeast 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. Anomalous moisture with some East Pacific 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 to continue 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. 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 the Northeast Wednesday into Thursday. The NBM still appears too light with QPF and so some modest increase in amounts was applied given potential for amplification. The leading Central U.S. heat will get suppressed with this frontal passage with quite cooled Canadian post-frontal high pressure moving in behind. 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. 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