Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 PM EDT Sun Sep 25 2022 Valid 12Z Wed Sep 28 2022 - 12Z Sun Oct 2 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... 19Z Update: The degree of model spread with future Hurricane Ian over the eastern Gulf has improved, and the latest GFS is now slightly east of its earlier runs, and the ECMWF/UKMET continuing to favor a solution closer to the Florida Peninsula. The past two runs of the CMC have been relatively close to the NHC track, and this helped serve as a proxy for QPF placement as the storm lifts farther north, so some of the CMC was incorporated with the NBM, along with some previous WPC continuity. In coordination with WFO Tampa Bay, a small Moderate Risk area was introduced for the Day 4 period in the excessive rainfall outlook for the central western Florida Coast, with many of the local rivers and creeks in this area already near or above their banks. Even if the core of the hurricane and its heaviest rains pass to the west, there is still concern for training convection in the eastern feeder bands that could produce several inches of rain over a relatively short period of time. Elsewhere across the continental U.S., rainfall totals for Thursday and Friday were raised above NBM across the Four Corners region to better depict some lingering monsoonal moisture over the region, which will likely fuel scattered showers and storms during the afternoon and evening hours, similar to the forecast process yesterday. Going into the beginning of October this weekend, there is a growing model signal for heavy rain near the Southeast and Mid-Atlantic coasts with deep onshore flow east of the weakening tropical system. The WPC fronts/pressures forecast was primarily derived from a CMC/ECMWF/GFS consensus through the end of the week, and then removing the CMC for next weekend owing to significant differences becoming more apparent over the western U.S., and incorporating more of the ensemble means by this time. The previous forecast discussion is appended below for reference. /Hamrick ------------------- ...Overview... Latest models continue to show a rapid transition from an amplified Rockies-High Plains upper ridge/eastern U.S. trough toward less amplified mean flow during the mid-late week time frame, as a shortwave trough nearing the Northwest coast as of early Wednesday moves inland. Another holdover from prior cycles is increasing uncertainty in flow details from the Pacific into North America after late Friday, with some influence on the ultimate evolution of the initial Northwest shortwave as it continues into the central part of the continent. The 03Z National Hurricane Center advisory for Tropical Storm Ian has the system intensifying to hurricane status fairly quickly and then tracking northward over the eastern Gulf of Mexico, reaching close to Florida by late Thursday. There is persistent timing/longitude spread as Ian reaches near Florida. Latest trends suggest the departing eastern U.S. upper trough may play a minimal role in Ian's track and evolution beyond Florida, but the issues with flow to the west could have some influence on the forecast by next weekend. Away from the heavy rainfall threat over parts of the Southeast and perhaps farther northward, the Northwest should see a brief period of rain around midweek and the Southwest into the Great Basin and Rockies may see one or more days of rainfall. ...Guidance Evaluation/Predictability Assessment... Individual models continue to vary with the exact track and timing of Ian as the system progresses northward over the eastern Gulf of Mexico toward Florida mid-late week. The overall average of guidance has adjusted noticeably westward over the past day. GFS runs remain on the western side of the envelope while other models are shuffling around farther east than the GFS to varying degrees. At least recent ECMWF trends toward a flatter/more quickly departing eastern U.S. upper trough have brought that model into agreement with other solutions for that feature, increasing confidence that the trough should have minimal influence on Ian's forecast. By late in the period the added uncertainty will be how the weakening system will become incorporated into the westerlies. Most models suggest this will happen by around Sunday, with specifics somewhat dependent on flow details upstream. As for this upstream flow, clustering is good for the upper trough moving into the Northwest during Wednesday-Thursday. Then details over the Pacific from Friday into the weekend have been an ongoing problem. Latest GFS/ECMWF and ensemble mean consensus shows a phased upper trough (though possibly containing an embedded upper low for a brief time) approaching the West Coast by next Sunday. However recent CMC runs and the new 00Z UKMET pull off a farther southwest closed low, ultimately impacting the pattern farther east. This includes the CMC/00Z UKMET straying slower than the majority with the upper trough/low crossing the Rockies into the Plains. Prefer the majority GFS/ECMWF and ensemble mean cluster until there are compelling trends away from that scenario. The updated forecast started with the 18Z GFS/12Z ECMWF and lesser input from the UKMET/CMC for days 3-4 Wednesday-Thursday. Then the blend phased out the CMC and ultimately transitioned to a half GFS/ECMWF and half GEFS/ECens solution by day 7 Sunday. Subsequent adjustments were made to reflect the 03Z National Hurricane Center advisory track for Ian. ...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. 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 far Southeast mid-late week. Thus the experimental Days 4-5 Excessive Rainfall Outlooks depict Slight Risk areas over this region, with some improvement in detail clustering needed to improve confidence before introducing any embedded Moderate Risk areas. Continue to monitor National Hurricane Center products for the latest information regarding this system. From Friday through the weekend Ian should continue northward, spreading wind and areas of locally heavy rainfall over other portions of the East. Strong high pressure tracking across the Great Lakes and Northeast will help to enhance the winds to the north of Ian. Rapid weakening/departure of the eastern U.S. upper trough and the approach of high pressure should bring a drier trend to the lee of the Great Lakes after some lingering light rainfall on Wednesday. The Pacific Northwest will see a period of light to isolated moderate rainfall around Wednesday-early Thursday with the frontal system forecast to push into the region. The Northwest start another episode of light rain with a front next weekend. The combination of anomalous moisture and some shortwave energy (including the southern periphery of the shortwave crossing the Northwest) should produce scattered rainfall from the Southwest/Great Basin into the Rockies, with the greatest coverage likely to be around Thursday. Some of this activity could be locally moderate or heavy. The upper trough initially over the East will support chilly temperatures, with highs up to 10-15F below normal over and near the Great Lakes/Ohio Valley east into the Appalachians Wednesday-Thursday. Then clouds/rain associated with Ian will keep daytime highs similarly cool versus normal over the Southeast Thursday-Friday. Eastern U.S. temperatures should moderate closer to normal by next Sunday. Warmth over the Great Basin and northern Rockies/High Plains on Wednesday (up to plus 15F or so anomalies for highs) should become less pronounced as it moves farther into the Plains Friday-Sunday, with most areas seeing only single-digit positive anomalies. The front pushing through the Northwest Wednesday-Thursday will bring a brief period of slightly below normal highs, followed by some oscillation corresponding to upper ridge/trough progression. 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