Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 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 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 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 onshore rain may be heavy at times across portions of northern California Sunday, 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 connection from the tropical eastern Pacific is forecast to lift across the Four Corners states 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. A frontal system and a shortwave trough rounding/digging on the northern side of the upper ridge should cause a round of rainfall with main focus from the mid-upper Ohio Valley through the Northeast Monday. The models suggest there could be instances where moderate to heavy rainfall will fall, particularly over New York and parts of New England where higher amounts may lead to isolated flash flooding. A second and stronger cold front is forecast to advance across the northern tier which would bring another round of showers. A stalled front is forecast to linger across central to southern Florida and could trigger showers and thunderstorms there. With the ridging building into the central U.S., warmer than average temperatures by 10-20F are forecast for the Central Plains and Middle Mississippi Valley through Wednesday before getting suppressed by the front, with actual temperatures well into the 90s threatening record values for this time of the year. Warmer than normal temperatures will extend to the East Coast into later next week, though with slightly lesser anomalies. 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