Extended Forecast Discussion NWS Weather Prediction Center College Park MD 331 PM EDT Fri Sep 16 2022 Valid 12Z Mon Sep 19 2022 - 12Z Fri Sep 23 2022 ...Moderate to locally heavy rainfall threat likely for parts of the Four Corners into the central 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, then slowly eject eastward into the north-central Great Basin/Rockies by the middle to latter part of next week. Downstream, summertime ridging will build over the central-eastern U.S. with northern stream Canadian shortwave energy dipping across the northern tier and possibly amplifying into the Northeast around Thursday-Friday next week. It is the strength, timing, and evolution of this trough which has strong implications for steering flow of Tropical Storm Fiona beyond day 5, forecast to be near/north of the Bahamas by Thursday. ...Guidance Evaluation/Predictability Assessment... Guidance overall was in good enough agreement for a multi-model blend the first half of the period. After about Wednesday, timing differences become more apparent as the upper low over California ejects into the Great Basin. 06z and 12z runs today of the GFS/GEFS remain on the faster side of the guidance, while the last couple of runs of the ECMWF tends to hang the low back over California longer. The ECENS mean is a hair faster than its deterministic counterpart, but still slower than the GEFS. Opted to lean more on the ensemble means here, which resulted in a solution very close to the previous WPC forecast. 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, with the latest 12z guidance from today, there is increasing confidence that this upper trough should help steer Fiona sufficiently out to see with only minimal impacts to the U.S. East Coast. 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 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 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 area on the Day 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. 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 Hazards: - Heavy rain across portions of the Northeast and California, Mon, Sep 19. - Heavy rain across portions of the Southern Rockies, the Central Rockies, the Central Great Basin, and the Southwest, Tue-Thu, Sep 20-Sep 22. - Excessive heat across portions of the Lower Mississippi Valley, the Southern Plains, and the Tennessee Valley, Mon-Fri, Sep 19-Sep 23. - Excessive heat across portions of the Central Plains, the Lower Mississippi Valley, the Tennessee Valley, the Middle Mississippi Valley, the Southern Plains, and the Ohio Valley, Mon-Wed, Sep 19-Sep 21. - Excessive heat across portions of the Central Plains, the Middle Mississippi Valley, the Southern Plains, and the Ohio Valley, Mon-Tue, Sep 19-Sep 20. - Excessive heat across portions of the Lower Mississippi Valley, the Tennessee Valley, the Middle Mississippi Valley, the Southern Appalachians, the Southeast, and the Ohio Valley, Mon-Thu, Sep 19-Sep 22. 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