Extended Forecast Discussion NWS Weather Prediction Center College Park MD 258 AM EDT Fri Sep 16 2022 Valid 12Z Mon Sep 19 2022 - 12Z Fri Sep 23 2022 ...Pattern Overview... Tropical Storm Fiona is forecast by the National Hurricane Center (NHC) to gradually reach Hurricane strength on a track from the Greater Antilles to the southern Bahamas over the next 5 days. The system will then contend with a large-scale upper level pattern over the lower 48 to feature a trough/closed low slated to form off the California coast then slowly eject over California and the north-central Great Basin/Rockies, summertime ridging building over the central-eastern U.S. and less certain northern stream Canadian shortwave trough energies dipping across the U.S. northern tier overtop the ridge. ...Guidance Evaluation/Predictability Assessment... The WPC medium range product suite was primarily derived from a composite blend of reasonably well clustered guidance from the latest GFS/ECMWF/UKMET/Canadian along with compatible guidance from the 01 UTC National Blend of Models (NBM) for Monday/Tuesday. Supportive GEFS/ECMWF ensemble means and the NBM were mainly used for mid-later next week in a period of gradually growing forecast spread leading into any late period U.S. threat from Fiona. ...Weather/Hazards Highlights... The expected slow 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 spread 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 by Tuesday-Thursday. Tropical moisture from building Tropical Depression Thirteen-E in the far eastern Pacific is forecast to stream into the Four Corners states as early as Monday, but especially by Tuesday-Wednesday, increasing chances for showers and thunderstorms there while potentially also providing some connection for the precipitation activity into the north-central Great Basin/Rockies. 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. However at this time the exact location of where there concerns may be is uncertain. A second and strong 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. The western end of this front pushing south and dissipating across the Gulf should lead to drier conditions for the Gulf Coast after the weekend. 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. Warmer than normal temperatures will extend to the East Coast into later next week, though with slightly lesser anomalies. Meanwhile, a looming question for later next week concerns a potential threat for the Bahamas then possibly Florida/Southeast dependent on the future strength and track of current Tropical Storm Fiona that is now approaching the Antilles. Please refer to the latest NHC forecast for updated info. 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