Extended Forecast Discussion NWS Weather Prediction Center College Park MD 457 PM EDT Mon Oct 23 2023 Valid 12Z Thu Oct 26 2023 - 12Z Mon Oct 30 2023 ...A heavy to excessive rainfall threat across parts of the Upper Midwest on Thursday... ...Heavy snow increasingly likely for parts of the Northern Plains on Thursday... ...Overview... The medium range period will begin Thursday with shortwave energy lifting northeastward through the central U.S., which could contribute to a heavy rainfall threat across parts of the Upper Midwest along and ahead of a strong cold front. A separate shortwave ejecting from the Northwest U.S. will produce an early season snowstorm across parts of the Northern Rockies into Northern Plains on Thursday as well, and help to develop a wave along the Upper Midwest front. Upstream energy dropping down the east side of strong Northeast Pacific/Alaska ridging will further amplify the mean trough aloft over the West by the weekend into early next week. At the same time a strong upper ridge should persist over the Southeast quadrant of the U.S. and Gulf of Mexico through the period. This pattern should support a multi-day period of enhanced rainfall over parts of the central U.S., with a pronounced contrast between unseasonably cold conditions over the West plus gradually increasing coverage over the Plains and very warm weather farther eastward. ...Guidance Evaluation/Predictability Assessment... The updated forecast based on 00Z/06Z guidance maintained the approach of blending the latest operational runs early in the period and mixed in some of the GEFS/ECens means later on to account for decreasing confidence in exact details within an overall agreeable large scale pattern. At the start of the period early Thursday, the GFS continues to be a bit slower and more closed with the shortwave crossing the Northwest. Then the GFS becomes a little weaker and more suppressed with the associated surface wave that develops over the Upper Midwest and then tracks into eastern Canada. By the latter half of the period the primary issues involve how closed the energy digging into the West may become and exact progression of northern stream flow--i.e., how much phasing exists between the two streams. Although not a perfect match, a more closed western low (per latest GFS/CMC runs) generally goes along with a slightly more progressive northern stream and faster surface front progression over the Northeast U.S. later in the period. Thus far guidance has been split with no clear majority/trend becoming evident yet. The updated blend kept the forecast close to continuity, reflecting an intermediate solution with some hint of separation aloft over the West but not quite a closed low along with somewhat slower Northeast frontal progression than the GFS/GEFS/CMC. The new 12Z ECMWF has adjusted a little toward the middle of the prior spread so the envelope seems to be narrowing some. ...Weather/Hazards Highlights... Highly anomalous moisture, with tropical connections to what is currently Tropical Storm Norma, will interact with a frontal boundary through the north-central U.S to fuel a heavy to possibly excessive rainfall threat on Thursday across much of the Upper Midwest/Great Lakes. There was enough model agreement to introduce a Slight Risk area from northeast Iowa into central Wisconsin on the Day 4/Thursday Excessive Rainfall Outlook, with latest guidance reasonably supportive of maintaining this outlook area without change for now. There has been a little more variability on where the axis of heavy rainfall may cross northern Michigan (mostly in the U.P.) so this will be one location of ongoing evaluation with regard to Slight versus Marginal Risk. Despite dry antecedent conditions across this region, slow moving storms along the frontal boundary with plenty of moisture and instability to work with, could pose a flash flood risk. On the north and west side of this system, heavy snow threats continue to increase across parts of the north-central Rockies into the Northern Plains on Thursday. This would be the first accumulating snow event of the season for the lower elevations of eastern Montana/North Dakota where several inches of snow could fall. By the Friday-Friday night time frame covered in the Day 5 ERO, guidance is showing decent instability and increasingly anomalous moisture ahead of the wavy front slowly advancing over the southern Plains, with some signal for localized enhanced totals (12Z UKMET being heaviest so far). The best potential for some fairly intense rain rates should be offset just to the east of highest totals in prior days but the pattern and ingredients seem to favor introduction of a Marginal Risk area from central Texas into northwestern Arkansas and far southwestern Missouri. Elsewhere, showers will continue across parts of the southern/central Plains on Thursday along a weakening frontal boundary before the stronger front approaches on Friday. Beyond Friday, this strong front should continue to serve as a focus for significant rainfall over parts of the eastern half of the Plains into at least the Mississippi Valley. It will take some additional time to determine how much overlap may occur (yielding the highest totals) over the multiple days of this longer term event. Lighter rain should extend into the Great Lakes/Northeast from the weekend into early next week. Out West, additional precipitation/higher elevation snows will move eastward across the Region this coming weekend followed by a drier trend as the moisture contributes to the enhanced rainfall area farther east. Potential exists for some snow in the northern fringe of the precipitation shield across the northern tier. Specifics remain very uncertain but require monitoring. Renewed and amplified troughing across the West will continue a prolonged period of below normal temperatures, with daytime highs 15-30 degrees below normal (locally colder over Montana late this week) to spread across much of the interior West and the Northern/Central Plains. By next Monday, much below normal temperatures could sink as far south as the southern Rockies and northwestern Texas. A few daily records are possible within this cold air mass. Meanwhile, well above normal temperatures across the Central U.S. and the Midwest will shift east and settle this weekend across the East/Southeast as upper ridging builds over this region. Expect a broad area of highs 10-20 degrees above normal with some morning lows 20-25 degrees above normal. Daily records are possible, with record warm lows likely to be more numerous than record highs. Rausch/Santorelli 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 Hazards: - Heavy rain across portions of the Central/Southern Plains and the Middle/Lower Mississippi Valley, Sat, Oct 28. - Heavy rain across portions of the Middle/Lower Mississippi Valley, and the Southern Plains, Fri, Oct 27. - Heavy rain across portions of the Great Lakes and the Upper Mississippi Valley, Thu, Oct 26. - Heavy snow across portions of the Northern Plains, Thu, Oct 26. - Frost/freeze across portions of the Central/Southern Plains and the Upper/Middle Mississippi Valley, Sun-Mon, Oct 29-Oct 30. 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