Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 AM EST Mon Feb 19 2024 Valid 12Z Thu Feb 22 2024 - 12Z Mon Feb 26 2024 ...Overview... Northern stream upper troughing and a southern stream shortwave interact late in the workweek and develop a central-eastern U.S. surface low/frontal system. This feature should spread rain and northern tier snow to parts of the eastern third of the country as a larger scale upper trough settles over the East. Meanwhile, periods of upper ridging across the west-central U.S. will lead to much above normal temperatures late this week into early next week. Upper and surface lows may approach the West Coast again by the weekend, causing an increase in precipitation chances there by early next week if not before. ...Guidance/Predictability Assessment... In the central to eastern U.S., models are in reasonable agreement that an upper low will close off around James Bay for a time late week, while a shortwave farther south also helps to force a surface low tracking quickly from the south-central U.S. Thursday into the Mid-Atlantic Friday and likely Atlantic Canada by Saturday. GFS runs have been persistently fast and the ECMWF on the slower side, but a middle ground consensus initially close to the CMC and EC mean seems best, though the CMC strays east of consensus by Saturday. Then, models have trended toward a more defined surface low reaching just north of the Great Lakes around Sunday with its associated fronts affecting the U.S., and a blend of models/means seems reasonable with typical model spread for the day 6-7 timeframe. The amplitude of the upper trough looks to relax a bit early next week. In the West, models agree on mean ridging with its axis shifting east slowly into the Intermountain West for the weekend. Shortwaves may be embedded in the northwest flow just east of the ridge axis, with typical spread for these smaller features. The track of the upper low and its associated surface low(s) in the eastern Pacific has been waffling from cycle to cycle with how fast or slow it reaches the West Coast over the weekend/early next week, with the northern stream flow showing a digging trough as well potentially complicating matters. The newer incoming 00Z guidance overall is showing a much faster/eastward trend bringing both northern stream and especially the southern stream troughing into the West compared to the 12/18Z cycle, which would spread QPF into the West on the earlier side. The WPC forecast used a blend of the 12/18Z deterministic guidance favoring the ECMWF early in the period, with gradual transition to about half EC/GEFS ensemble means by the day 6-7 period. ...Weather/Hazards Highlights... Precipitation should increase in coverage and intensity over the Ohio Valley to Great Lakes Thursday under the influence of a couple of low pressure/frontal systems. Precipitation is most likely to be rain except in the far northern tier. Rainfall amounts of an inch or two are likely in the Ohio Valley, with locally higher amounts. Some competing factors are in place regarding whether this rainfall will cause a flash flooding threat. Some instability combined with good dynamical support (left exit region of the jet) may lead to high rainfall rates. The surface low and shortwave helping to force the precipitation should be moving rather quickly, but the west to east track means that rain has the potential to train west to east. Antecedent conditions in terms of soil moisture and streamflows are incredibly dry from the Ohio River northward but closer to normal farther south. The WPC forecast maintains a somewhat broad Marginal Risk across the Ohio Valley to Central Appalachians considering these factors, with the expectation that the area could change over time and possibly embed a Slight Risk at a future time if the positioning of a heavy/training QPF axis becomes clearer. The precipitation should reach the East Thursday-Friday with snow possible in parts of the Northeast. Northern New England has a relatively higher probability of seeing notable snow along with the potential for a period of brisk winds. There is still uncertainty with the snow amounts and placement, so continue to monitor forecasts. Behind this system, parts of the Great Lakes may see mostly light snow from a combination of lake effect activity and one or more waves/frontal systems. The West will see a break from precipitation around Thursday-Friday with a bout of ridging aloft. An eastern Pacific upper low and its surface reflection will meander and gradually drift closer to the West Coast, perhaps spreading precipitation chances to California by late Friday-Saturday depending on how close the lows get to the coast. By Sunday model guidance is more agreeable that precipitation should spread into the Pacific Northwest and California with some moisture spreading farther inland into the Intermountain West as well. Areas from the Plains eastward will see above normal temperatures Thursday with the exception of a cooler Florida Peninsula. Temperatures look to moderate closer to average in the East late week after a cold frontal passage or two, likely reaching below average for highs in the Northeast Saturday. Meanwhile renewed upper ridging in the west-central U.S. will promote temperatures warming even further by the weekend into early next week for much of the Plains and Mississippi Valley. Temperatures 15-30F above normal for late February will be common as forecast highs get into the 80s for much of Texas early next week with 50s reaching as far north as the Dakotas. Meanwhile the western U.S. should be near to slightly above normal until early next week when the upper low approaches and cools temperatures, especially highs. 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 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