Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 PM EDT Fri Oct 14 2022 Valid 12Z Mon Oct 17 2022 - 12Z Fri Oct 21 2022 ...Overview... Guidance continues to show amplified/blocky upper level flow for most of the period, with troughing in the eastern U.S. held in place by strong ridges over the Northwest U.S./western Canada and the Atlantic. The Northwest ridge will be part of a Rex Block configuration, with a short-term upper low weakening over the southern Rockies on Monday and another upper low forming offshore California by midweek as upstream eastern Pacific trough energy splits. Around the end of next week the models/means suggest the start of a transition to a new pattern, with differences in how this will occur. The established pattern for most of the period will support well below normal temperatures over the eastern half of the country as well as the southern Rockies/High Plains, along with very warm temperatures over the Northwest. The weakening southern Rockies upper low should help produce notable rainfall for portions of New Mexico and Texas Monday and perhaps lingering in southern Texas Tuesday. Meanwhile the energy aloft and westerly flow over the Great Lakes will lead to multiple days of lake effect precipitation (including some snow) while a developing low pressure system and leading wavy front may bring meaningful precipitation to the Northeast Monday-Tuesday. ...Model and Predictability Assessment... Overall the guidance has settled into a reasonable cluster for the early-mid week evolution over the East, with an upper low likely tracking eastward from the Upper Great Lakes and then lifting northward over western Quebec. Associated low pressure over southeastern Canada should anchor a wavy cold front that lifts through the Northeast. There is still some spread for exact details of frontal timing/waviness, which will important for rainfall coverage and amounts, but current differences are generally within typical guidance spread and error ranges for 3-5 days out in time. Farther upstream there has been a consistent signal for northern stream Pacific energy to round the western ridge and then drop into the eastern U.S. mean trough, supporting a frontal system that pushes into the lower 48 to the east of the Rockies around mid-late week. However there has been a fair degree of spread over the details of this energy and associated surface system over recent days, reflecting the typically lower predictability for resolving specifics of such shortwaves. There are still noticeable differences in the new 12Z run but the models do seem to be trending a little closer together. After midweek larger scale pattern issues start to arise, as ECMWF/ECMWF mean runs have consistently been most aggressive to flatten the Northwest U.S./western Canada upper ridge. In contrast, the past couple days of GFS/GEFS runs have actually trended stronger with the ridge. The CMC/CMC mean offer a compromise that tilts somewhat in the GFS/GEFS direction. The 00Z ECMWF was on the quick side to split its east-central Pacific flow around midweek and may be somewhat aggressive to break down the ridge, with consensus particularly emphatic about maintaining broader cyclonic flow over the East into Friday versus the ECMWF. Teleconnections relative to the core of positive height anomalies over the Pacific in the D+8 multi-day means support eventual troughing near/inland from the West Coast--which most guidance shows by next weekend. Thus an intermediate solution close to continuity appears best at this time. Farther south, the GEFS mean and CMC/CMCens are fastest with the next upper low approaching the Southwest while the ECMWF/ECMWF mean are slowest. A compromise currently looks good for resolving this spread. The updated forecast maintained the approach of using a 12Z/18Z operational model composite for about the first half of the period followed by a transition to a blend of models and GEFS/ECMWF means by days 6-7 Thursday-Friday. This yielded the desired intermediate solution similar to continuity with the increasing spread late in the forecast. ...Sensible Weather and Potential Hazards... Heavy rainfall is likely to remain across New Mexico and much of the southwestern half of Texas on Monday, aided by a weakening closed low crossing the Southwest and a surface front pushing through and south of southern Texas. Isolated flash floods are possible with the highest rainfall amounts in southern Texas, but the drier soil moisture may preclude widespread issues. Also the continued scatter in guidance placement of the highest QPF within this general area keeps confidence low enough not to introduce any Slight Risk area in the Day 4 experimental Excessive Rainfall Outlook. The initial clouds/rain and the cooler airmass behind the front will keep high temperatures 10-20F below normal through the first half of the week for the Southern Rockies/Southern Plains, gradually warming to near normal by late in the week. Within the persistent eastern upper trough, Canadian energy digging into a Great Lakes upper low by Monday will develop a potent surface wave and frontal system that could produce areas of moderate to heavy rainfall over the Great Lakes and Northeast on Monday and Tuesday. Important timing differences in the guidance are becoming less pronounced but are still enough to maintain some uncertainty in coverage/amounts of enhanced rain. Latest models are generally suggesting that heaviest rainfall potential along the front may be on Tuesday with considerable moisture transport ahead of the front. However it will be a very close call as to whether this band of heavy rain will be over parts of Maine or just to the east, depending on fine-scale upper shortwave/frontal wave details that may not be fully resolved until the short range. Well below average temperatures (minus 10-20F anomalies, locally greater over some areas Monday-Tuesday) will spread through the eastern half of the country behind this system. Records for low maximum temperatures may be most common, though a handful of record low minimum temperatures may be set as well. These temperatures will provide an early preview of November/December. Areas of lake effect rain/snow will be widespread Monday-Tuesday but should gradually decrease in coverage/intensity after midweek. High pressure building over the Midwest and into the Southeast in the wake of a cold front will support a broad area of dry weather. The Northwest upper ridge persisting into at least Thursday will keep temperatures over the region well above normal through the period, with each day featuring some plus 10-20F anomalies. Scattered record highs are possible. Some of this warmth will gradually spread into the northern-central Plains by Wednesday-Thursday. Expect the West to remain mainly dry through the period until some showers may reach the southwestern U.S. later in the week as the next upper low approaches. The far northern parts of the Northwest could begin to see a little precipitation by the end of the week depending on how quickly the upper ridge weakens. 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