Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 AM EDT Mon Sep 26 2022 Valid 12Z Thu Sep 29 2022 - 12Z Mon Oct 03 2022 ...Tropical Storm Ian expected to intensify as it tracks from the Caribbean toward Florida, with likely adverse effects over other parts of the Eastern U.S. as well... ...Overview... The latest forecast offers two primary challenges. The first is the precise track, and to some degree timing, of Tropical Storm Ian which should intensify to hurricane status as it tracks from the northwestern Caribbean into the eastern Gulf of Mexico toward Florida over the coming days. The system should weaken as it progresses inland but still push significant rainfall and strong winds northward over parts of the East. Check the latest National Hurricane Center products for more information on Ian. The other significant issue involves details within an evolving eastern Pacific mean trough and the influence on the evolution of a shortwave reaching the Northwest by early Thursday. ...Guidance Evaluation/Predictability Assessment... Models continue the tendency from recent days to show some east-west oscillation over successive runs along with some moderate timing differences. Over the past 24 hours the average of guidance has been generally nudging the track a little eastward, reversing what had been a slight westward nudge over the prior day. Varying proportions/adjustments among individual models represented the 03Z NHC track for mass fields and associated rainfall. Most solutions indicate an extratropical transition by the latter half of the period, with the system ultimately tracking off the East Coast by early next week. The new 00Z ECMWF suggests a somewhat slower evolution. With the arrival of the 00Z GFS, guidance seems to be making further progress toward a fairly dramatic adjustment from what had been the majority consensus 1-2 days ago. The latest trends suggest much greater potential for Pacific energy to pull off a close low with the remainder of energy progressing along farther north. Then digging upstream flow could start to push the leading edge of height falls toward the West Coast by day 7 Monday. Meanwhile this evolution leads to a slower and more closed depiction of the shortwave energy that starts over the Northwest. As would typically be the case, the ensemble means are still phased with the overall trough. However there has been a trend toward more southwest elongation, reflecting an increasing number of members that pull off a closed low, and keeping higher heights along the West Coast. Based on guidance available through the 18Z cycle, the updated forecast started with a 12Z/18Z operational model blend early in the period and then transitioned toward 60 percent total 18Z GEFS/12Z ECMWF means and the rest 18Z GFS/12Z ECMWF by day 7 Monday. This provided at least modest trending to reflect the increasing proportion of models showing flow separation over the eastern Pacific and slower/closed Rockies into High Plains upper system. New 00Z runs suggest additional trending may be required. ...Weather/Hazards Highlights... Tropical Cyclone Ian, expected to be a hurricane as it tracks over the eastern Gulf of Mexico toward Florida, will produce the most prominent rainfall and wind threats during the period. A front likely to be stalled over Florida before arrival of the system could provide an added focus for rainfall. Strong high pressure tracking across the Great Lakes and Northeast will help to enhance the winds to the north of Ian late this week. Meaningful spread for the exact track/timing of the system persists but there is still a fairly strong signal for a heavy rainfall threat over Florida and other parts of the Southeast during the latter half of the week with some of this moisture extending farther north during the weekend. The experimental Day 4 Excessive Rainfall Outlook depicts Slight/Moderate Risk areas over the far Southeast and parts of Florida, with the Day 5 Slight Risk area encompassing much of the Carolinas and Georgia. Continue to monitor National Hurricane Center products for the latest information regarding this system. The combination of anomalous moisture and the upper trough/low emerging from the Northwest should produce areas of rainfall that should track from the Southwest/Great Basin into the Rockies and High Plains. Recent trend for the upper system to be slower and more closed raises the potential for greater coverage of rainfall and for some of this rain to be locally moderate to heavy. Depending on the depth/track of the upper low, there may also be a slight chance that highest elevations in parts of the Rockies could see a little snow. A majority of the East will see cool temperatures late this week with some highs 10-15F below normal. Northern areas will see the lingering effects of a departing upper trough on Thursday while the Southeast will see below normal highs due to clouds/rainfall from Ian. Readings should get closer to normal by late weekend/early next week. The northern-central High Plains will be quite warm on Thursday with some northern areas seeing highs at least 15F above normal. Thereafter, Plains warmth will moderate and settle over the southern half of the region by Saturday-Monday. The upper trough/low drifting into the Rockies/High Plains will bring moderately below normal highs in its vicinity. Rausch 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