Extended Forecast Discussion NWS Weather Prediction Center College Park MD 254 PM EDT Sat Oct 15 2022 Valid 12Z Tue Oct 18 2022 - 12Z Sat Oct 22 2022 ...Overview... The medium range period begins Tuesday with an amplified/blocky upper level flow pattern consisting of an anomalously strong upper low centered near the Lower Great Lakes with troughing extending through the eastern half of the U.S. and an upper ridge/high over much of the West. This pattern should persist through Wednesday-Thursday while strong shortwaves move through the eastern trough and a southern stream upper low slowly edges eastward across the Pacific toward southern California. Lake effect/enhanced precipitation (including some snow) will be common through midweek across the Great Lakes with lighter amounts later in the week. Through Thursday temperatures will be much below average in the eastern half of the country but warmer than average in the Northwest under the ridge. Then by Friday-Saturday there is a likely trend toward overall deamplification of the upper flow and possibly even some Northwest troughing, which should focus the warmth over the Plains with a trend toward more normal temperatures elsewhere. ...Model and Predictability Assessment... The forecast during the first half of the period continues to show better than average agreement and continuity overall, representative of the typically good predictability of a highly amplified large scale pattern. There are still smaller scale differences involving the wavy front and supporting shortwave energy crossing New England early in the period. Specifics will be important for associated rainfall but will take into the short range time frame to resolve. Meanwhile the models/means continue to develop some spread for the upper low offshore southern California. Over recent runs the GFS/GEFS and CMC have tended to be on the faster side (though with the 12Z GFS slowing down slightly) while recent ECMWF runs have been slower than earlier ones (with the 00Z ECMWF mean between the two extremes). Some of these differences extend through the rest of the period though the 12Z ECMWF does show some acceleration late. The Friday-Saturday period has featured considerable spread with respect to how quickly incoming Pacific flow breaks down the upper ridge over the West. Over the past day the GFS/GEFS have made pronounced trends toward the ECMWF/ECMWF mean idea of lower heights and an eventual trough reaching the West, leading to much better agreement among those solutions. However the 00Z CMC and the 00Z UKMET (through the end of its run) suggested a lingering possibility that the pattern change could still take a little longer to develop. The new 12Z CMC is not as amplified with the shortwave which the 12Z GFS/00Z ECMWF bring to the Northwest by early next Saturday but is much closer to the overall scenario now. As upper heights over the East rise in response to the evolution upstream, latest model runs have been showing potential for some shortwave energy reaching the Southeast to separate from the westerlies and form a separate smaller scale trough/upper low. Such a feature would have lower predictability and thus far the models have been inconsistent with where such a feature would form/track, though the 12Z GFS/ECMWF are not too far from the 06Z GFS/00Z ECMWF. Prefer a conservative approach at this time. A blend of the 00Z/06Z operational models provided a good starting point to resolve lingering detail questions during the first half of the period, with minimal continuity changes. The forecast incorporated an increasing proportion of ensemble means later in the period (06Z GEFS/00Z ECens, as well as 00Z CMCens which was a tad flatter), reaching 60 percent by day 7 Saturday while the 06Z GFS/00Z ECMWF provided the remaining input. This provided a modest nudge toward the developing majority scenario. ...Sensible Weather and Potential Hazards... By Tuesday, a frontal system will have passed across much of the East with a drying high pressure system behind it. However, the front could continue to cause showers and thunderstorms across Maine, the Florida Peninsula, and southern Texas ahead of the frontal passage in those areas. The forecast for Maine continues to be particularly challenging, with model runs varying for exact timing and wave details. Most of the models have been advertising just fast enough progression to push a swath of moderate to heavy rainfall, supported by considerable moisture transport, mostly east into Canada but a minority has been depicting this band farther west over Maine. Now the average of latest 12Z guidance seems to be nudging back a little more to the west, at least increasing the rainfall potential over eastern Maine. It may take until well into the short range period to resolve this part of the forecast satisfactorily. Meanwhile upper level energy and the westerly flow across the relatively warm Great Lakes compared to the cold air temperatures will produce considerable lake effect precipitation downwind of the lakes. Support is highest for notable amounts of rain and/or snow on Tuesday with a surface low pressure system nearby, but at least the flow aloft will be favorable for additional lake effect precipitation through Thursday. Other than those areas, much of the contiguous U.S. should be dry through the end of the week. Exceptions by Friday-Saturday will likely be with an expanding area of precipitation over the Pacific Northwest into northern Rockies with upper level energy and a surface front, and some showers reaching southern California and the Southwest with the southern stream upper low/trough. The amplified western ridge in place for most of the week will cause warmer than average temperatures across the Northwest in particular, with highs commonly 10-20F above average, potentially leading to some daily record highs through Thursday. Conversely, the anomalous upper low/trough in the East should produce rather cold temperatures more common to November and December from the Plains eastward. Below normal temperatures by 10-25F will be widespread and translate into lows in the teens in portions of the north-central U.S. Tuesday and 30s spreading south possibly all the way to the Gulf Coast (Mobile, Alabama for example) by Wednesday-Thursday. The Southeast could also see highs in the 50s and 60s, and record low maximum and minimum temperatures are likely to be set in the east-central U.S. As the upper level pattern relaxes towards the end of the week, temperature anomalies should lessen over both the West and East while highs reach 10-15F above normal over the northern/central Plains by Friday-Saturday. Rausch/Tate 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