Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 AM EDT Wed Nov 03 2021 Valid 12Z Sat Nov 06 2021 - 12Z Wed Nov 10 2021 ...Overview... The medium range period will be characterized by an upper ridge with its axis gradually shifting from the central U.S. over the weekend toward the East by Tuesday-Wednesday, separating weekend troughing in the East and rounds of troughing in the eastern Pacific/West. A series of fronts and shortwaves coming into the West Coast will keep precipitation chances high there, while a front and developing low pressure system off the East Coast will spread some rain across Florida and up the Southeast coast this weekend. ...Guidance/Predictability Assessment... Model guidance begins the medium range period with reasonably good agreement this weekend, indicating a shortwave over the Midwest/Great Lakes and southern stream troughing over the Southeast beginning to combine in the Eastern Seaboard, shortwave energy tracking through the Northwest, and upper ridging in the central U.S. The main exception is the 12Z CMC in the eastern Pacific/West, as it has less initial energy with the Pacific Northwest shortwave but more energy dropping southward around Sunday in the eastern Pacific, causing a mid-upper trough to dig along the West Coast by Monday. This is not supported by other guidance including almost all GEFS/EC/CMC ensemble members. The new 00Z CMC appears to be more in line with consensus, another indication that leaning away from the 12Z is a good move. Other than that, a multi-model deterministic blend favoring the 12Z ECMWF and UKMET and 18Z GFS was used for the beginning of the period. At the surface, this maintains a low track from the Florida Peninsula northeastward into the western Atlantic. As the workweek begins, there are some differences with placement of shortwaves among guidance, but nothing too out of the ordinary for that time range. For the most part, deterministic and ensemble guidance indicate deeper troughing and likely a closed low just offshore of the Pacific Northwest Tuesday, with troughing/energy making its way quickly eastward Wednesday. The 18Z GFS and 12Z ECMWF were both quite strong with this upper closed low and the associated surface low, and this updated forecast showed an increase in its strength, though not to the level of 957 or 965 mb respectively at the surface Tuesday at 12Z. Other than that, the main change to the previous forecast issuance was pushing a cold front more quickly through the central U.S., per the recent model guidance indicating a faster movement of incoming troughing for the western/central U.S. and upper ridging pushing eastward. The latter part of the medium range forecast utilized a fairly even blend of the 18Z GFS and GEFS mean and 12Z ECMWF and EC mean. ...Weather/Hazards Highlights... As a surface low pressure and frontal system develops near/over Florida Saturday and tracks northeastward into the western Atlantic, it will spread rain across Florida and into the coastal Carolinas and possibly the Mid-Atlantic region on the backside of the low over the weekend. Current forecasts do not show rainfall reaching very far inland, given a well offshore track of the low, but small shifts of the low track could lead to significantly more or less rainfall in coastal areas, especially for the Carolinas. Abundant moisture ahead of frontal systems and upper-level shortwaves edging in from the Pacific will result in periods of enhanced precipitation this weekend into early next week near the West Coast. Through the period, the Pacific Northwest and northern California can expect the heaviest precipitation, with normal terrain enhancements and higher elevation snow. Precipitation is likely to spread into the Intermountain West and Rockies as well, with amounts there increasing by next Monday into Wednesday. Behind the low pressure system in the western Atlantic, a cold surface high to the west will bring northerly winds and high temperatures that are 10-20F below normal to the Southeast through the weekend. Lows could be 5-10F below normal farther north in this pattern as well, with temperatures in the mid to low 30s for much of the Ohio Valley Saturday morning, which could result in the first frost or freeze of the season for some places. Meanwhile, a persistent upper-level ridge over the west-central U.S. should lead to temperatures that are above average by 10-20 degrees over the Plains into the Midwest through early next week, with highs 5-10F above normal stretching toward the East Coast by Tuesday-Wednesday. Periods of unsettled weather will keep highs somewhat below normal over the West Coast on most days, eventually entering into the Great Basin and Rockies by midweek. 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, 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/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml