Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 AM EDT Tue Sep 12 2023 Valid 12Z Fri Sep 15 2023 - 12Z Tue Sep 19 2023 ...Potential for Hurricane Lee to bring rainfall and strong winds to parts of New England... ...Heavy rainfall threat for the south-central Rockies/Plains likely to taper off after late this week... ...Overview... Guidance continues to show a strong upper ridge building into the West while troughing amplifies into the east-central U.S. during the Friday-Sunday time frame. The western ridge should gradually weaken and drift eastward as North Pacific into Canada flow drifts southward with time, possibly bringing the southern periphery of a mean trough into the far northwestern U.S. early next week. At the same time a weak upper low may approach California. Some troughing should linger over the East but in weaker fashion by next Tuesday. Meanwhile solutions display typical spread but are gradually coming closer together for the speed and track of Hurricane Lee that should progress northward over the western Atlantic this week and reach a position close to the southern tip of Nova Scotia by late Saturday per the 03Z National Hurricane Center advisory. This track would bring strong winds to New England and significant rainfall to parts of Maine. Initial western U.S./Plains shortwave energy along with easterly low level flow over parts of the southern Plains will help to generate areas of rain and thunderstorms over parts of the central-southern Rockies/Plains late this week followed by a drier trend. This energy and northern stream flow feeding into the east-central U.S. trough, along with a leading front, may generate some areas of rainfall over the central/eastern U.S. The central Rockies into south-central Plains will see cool daytime highs late this week while areas from the Pacific Northwest into northern High Plains will see above normal highs through the weekend followed by a cooling trend from west to east. ...Guidance Evaluation/Predictability Assessment... Regarding Hurricane Lee, model and ensemble guidance continues to show some meaningful differences for timing and track but overall the solution envelope is gradually narrowing. For the forecast based on 12Z/18Z guidance, the 12Z ECMWF/UKMET were generally the closest to the past couple National Hurricane Center advisories. Models will likely continue to shuffle around some in coming runs. Farther west there are still some meaningful detail differences with exactly how the amplifying east-central U.S. trough will amplify. Lately there has been a fair number of solutions suggesting an initial southern Canada upper low may wobble into the Great Lakes, and the latest GEFS mean/12Z CMCens are hinting at that idea or with a low track just a bit northward. The ECMWF/ECMWF mean through 12Z keep the trough open and more progressive. At this time prefer an intermediate idea of showing an upper low but with a track slightly north of the Great Lakes. The new 00Z ECMWF shows some trends toward the other guidance. For Pacific flow progressing into Canada and northern tier U.S., the 18Z/00Z GFS runs are extreme versus other solutions for upper energy continuing along the U.S.-Canadian border by next Monday-Tuesday. The 00Z GFS strays west of consensus for the upper low approaching California early next week. In order to come up with the best starting point for features of interest, the updated forecast started with a blend of 12Z ECMWF/GFS/UKMET early in the period and then transitioned toward a late-period blend of 60 percent total weight of the 18Z GEFS and 12Z ECens/CMCens means with 40 percent input from the 12Z ECMWF/GFS. ...Weather/Hazards Highlights... The Day 4 Excessive Rainfall Outlook that covers Friday-Friday night maintains a Marginal Risk area over parts of the central/southern Plains, with only some adjustments to reflect the latest array of plausible guidance. Initial western U.S./Plains shortwave energy and easterly low level upslope flow over some areas, along with a front dropping into the central Plains, should contribute to rain and thunderstorms from the central Plains into parts of Texas. Signals continue to be fairly diffuse with respect to exactly where the best focus of highest rainfall totals will be, so confidence remains to low to depict any embedded Slight Risk area. The Day 5 ERO (Saturday-Saturday night) depicts a Slight Risk area over Downeast Maine and vicinity, along with a Marginal Risk to the west, reflecting the potential for heavy rainfall in association with Lee based on the current forecast track. Currently no other risk areas are depicted in the Day 5 outlook. Areas of interest for monitoring include southern Texas (some lingering moisture and instability but inconsistent guidance signals for rainfall) along with the Tennessee Valley and surrounding areas (wide range of guidance QPFs and mixed signals for moisture anomalies over the region). Some rain/thunderstorms should continue over parts of the southern Rockies/Plains and perhaps the South beyond early Saturday but likely with less coverage and lighter totals. Meanwhile a front settling over the Florida Peninsula may produce one or more days of rain over that region. Over New England, expect any lingering rain from the midweek system to depart on Thursday. Continue to monitor National Hurricane Center products regarding the track of Hurricane Lee and potential effects on New England and the Canadian Maritimes by late week and the weekend. Uncertainty with details for the upper trough evolving over the eastern half of the country during the weekend and early next week keeps confidence low for what the surface pattern may look like along with any areas of rainfall, though into Friday or so a front reaching the Upper Midwest could produce some areas of mostly light rain. A wavy cold front progressing from the Midwest through the East may produce some areas of rainfall of varying intensity over the course of the period. The Florida Peninsula should be fairly wet through the period due to a persistent front over the area. A little measurable rainfall could reach the northern Pacific Northwest early next week as a front moves in. Clouds and rainfall from the central Rockies into south-central Plains will keep daytime highs up to 10-15F or so below normal on Friday before a warm trend to or slightly above normal. The eastern U.S. should see near to somewhat below normal highs on most days. Hot weather with highs up to 5-10F above normal may persist over far southern Texas through Friday before a cooler trend. The upper ridge building into the West will support highs 10F or more above normal over the Pacific Northwest from late week through the weekend, with a broader area of morning lows 5-15F above normal across the West. Some of this warmth should extend into the northern High Plains Saturday onward while the Pacific Northwest will likely trend cooler early next week, reaching near normal by next Tuesday as a Pacific front approaches/arrives. 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 forecast, excessive rainfall outlook, winter weather outlook probabilities, heat indices and Key Messages 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 https://www.wpc.ncep.noaa.gov/#page=ovw