Extended Forecast Discussion NWS Weather Prediction Center College Park MD 258 AM EDT Mon Oct 17 2022 Valid 12Z Thu Oct 20 2022 - 12Z Mon Oct 24 2022 ...Overview... One more day of amplified upper troughing is forecast for the eastern U.S. as the medium range period begins on Thursday, causing cooler than normal temperatures in the East and warm temperatures underneath ridging in the West, with dry conditions for most areas. Then a pronounced pattern change is expected for late week as a trough in the West develops and persists into early next week with some movement eastward. The energy aloft and a surface frontal system or two will lead to increasing precipitation chances across the West with higher elevation snow, while temperatures cool there as warmer than average temperatures shift toward the central U.S. into the Northeast. ...Model and Predictability Assessment... Model agreement remains quite good at the beginning of the period for eastern troughing extending from a southeastern Canada upper low, ridging across the West, and a southern stream upper low offshore California. There are only minor differences with embedded shortwaves and aside from any influence on lake effect precipitation, impacts on sensible weather should be minimal given the otherwise dry pattern. By day 4/Friday models show the bulk of the eastern troughing is likely to retreat northeastward, but with some energy lingering in the southern stream that is more uncertain. ECMWF runs have been persistent with showing a closed upper low forming by Saturday, though with some position oscillations, as have some EC ensemble members, while other guidance has been weaker. The weak and fairly small scale nature of this energy leads to low predictability in the medium range time frame, so the WPC forecast continues to take a conservative approach with some weak troughing and its relation to a wavy front in the western Atlantic, not as strong as the ECMWF. Meanwhile in the West, energy begins to suppress the initial western ridge by Friday but additional, stronger energy is forecast to dig into the Northwest by Saturday to deepen troughing. Model guidance is agreeable about this, but the more uncertain factor is with the possibility of phasing the trough with the southern stream low. Through the 12/18Z model cycle, GFS runs showed more phasing of the features compared to the ECMWF and CMC that maintain separation, while ensemble members show considerable variability. By Sunday and Monday this leads to one trough trekking more slowly across the west-central U.S. in the GFS, while the EC/CMC have the northern stream trough tracking across the north-central U.S. faster while the southern stream low lingers near Baja California. The 00Z GFS remains more phased while the 00Z ECMWF sticks with stream separation, and the 00Z CMC switches to more phasing. These uncertainties in the upper levels will affect the frontal position and the timing of rain beginning in the central U.S., so the forecast is likely to be refined with time. Another round of energy may approach the West by Monday but with considerable variability. The ECWMF including the 00Z run is on the stronger side of the guidance with this energy. The WPC forecast for fronts/pressures was based on a blend of the deterministic 12Z/18Z models to resolve lingering detail questions during the first part of the period, and then incorporated GEFS and EC ensemble means for the latter part of the period and quickly increased their proportions as operational models diverged with various details. This approach maintained good continuity with the previous forecast aside from typical nudges reflecting latest trends. ...Sensible Weather and Potential Hazards... The contiguous U.S. should overall be rather dry on Thursday, with the exception of lingering light lake effect rain/snow due to upper level energy and the westerly flow across the relatively warm Great Lakes compared to the cold air temperatures. South Florida may see a few rain showers as well. By late in the week through next weekend, the upper trough arrival/amplification across the West and an associated surface front pushing across the region should produce a notable increase in precipitation coverage and intensity. The best focus will be over favored windward terrain over the Pacific Northwest on Friday and then extending southeastward into the northern/central Rockies and Wasatch over the weekend, with higher elevation snow and lower elevation rain. Confidence in a meaningful precipitation event is becoming fairly good, but exact amounts of rain/snow and timing will still need to be refined with time. Precipitation may also be possible across the Southwest Friday and especially Saturday depending on the track of the southern stream low and its eventual possible phasing. By early next week, rain chances may increase for the central U.S. as Gulf moisture streams in ahead of a cold front. Meanwhile some rain may be possible along or near the East Coast this weekend into Monday, but with low confidence on how much rain could fall onshore. The amplified western ridge in place for the early part of the period 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 for Thursday. Below normal temperatures by 10-20F will be widespread and translate into lows in the 30s spreading south possibly all the way to the Gulf Coast (Mobile, Alabama for example), which could set low temperature records across the Southeast. As the upper level pattern relaxes around Friday and then transitions to a western trough/eastern ridge pattern, above normal temperatures should get pushed into the central U.S., with the north-central Plains seeing highs of 10-20F above average. Temperatures are also expected to gradually warm in the Midwest to Northeast early next week. Meanwhile below average temperatures particularly for highs are likely across the West given the trough aloft. 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