Extended Forecast Discussion NWS Weather Prediction Center College Park MD 250 PM EST Thu Feb 22 2024 Valid 12Z Sun Feb 25 2024 - 12Z Thu Feb 29 2024 ...Overview... An upper ridge traversing the central and eastern U.S. for the first half of next week will spread much above normal temperatures to the Plains and Midwest and eventually the East. Meanwhile northern and southern stream troughing look to track into the West and bring cooler temperatures, lower elevation rain and higher elevation snow, and windy conditions for the West Coast to the Rockies through early-mid next week. Increased precipitation is also likely for the Ohio Valley/Great Lakes by Monday and beyond with a frontal system or two ahead of this trough. ...Guidance/Predictability Assessment... Main forecast challenge through the medium range period (Sun Feb 25 - Thu Feb 29) remains with how a strong northern stream trough dropping south from the northeastern Pacific and closed upper-low in the southern stream off the coast of California progress and evolve together across the CONUS through next week. Ahead of these systems, deterministic guidance is in good agreement that a northern stream system will cross southern Canada/northern tier of the U.S. towards the Northeast early next week as weak ridging in the southern stream proceeds the upper-level energy to the west, transitioning form northwestern Mexico/the Southwest eastward across the Southern Plains towards the Southeast. Initial shortwave energy following the ridge should bring subsequent cyclogenesis/surface system development over the Plains, lifting northeastward towards the Midwest. Smaller-scale details will likely change over time but the general pattern shift towards mean trouging in the West as the upstream energies approach is agreeable across the guidance. Initial evolution of upstream northern/southern stream energies is generally agreeable as the northern stream trough drops across the Pacific Northwest and the southern stream closed low breaks down as energy ejects eastward over the Southwest/California/Baja mid-period. Deterministic solutions begin to diverge a bit more by Wednesday. The ECMWF has been a bit more consistent run-to-run with both the timing and combined phasing of the stream energies, with the GFS notably more inconsistent with both the evolution of the northern and southern stream phasing together over the central U.S., as well as with the timing compared to the ECMWF, with a combined slower eastward progression. The latest CMC depicts lingering southern stream energy over the Southwest/California/Baja, a more split-stream evolution than in the GFS or ECMWF. The GEFS/ECens means notably lack the more subtle details of the stream energies phasing, but the mean trough axis is generally similar to the ECWMF. The more recent runs of the GFS are also a bit more amplified both with the trough and subsequent ridging upstream over the West Coast compared to the ECMWF and ensemble means, and with a deeper, closed low developing as the streams phase together. This translates into Thursday, with the GFS continuing to show a trend towards a deeper and deeper closed low in the 00Z and then 06Z run, while the ECMWF and means remain open and a bit more progressive. Timing wise, both the GFS and ECMWF have trended slower with the eastward progression of the system, regardless of the exact evolution. The axis of mean troughing in both the GEFS and ECens once again tends to align more with the ECMWF, and backs the trend in timing. The updated WPC forecast began with a multi-model blend given good overall agreement on the pattern across the CONUS and with the noted upstream energies through the early to mid-period. The CMC is removed for the later period given the trend towards more stream separation that would be out of phase with the other guidance. A contribution from the ECens and GEFS means is included to replace the CMC and time-limited UKMET with the greatest deterministic contribution remaining from the ECMWF as it aligned most closely with the mean guidance. A split contribution from the 00Z/06Z GFS is used to capture the noted trend towards a more amplified trough/ridge, and deepening closed low, with a higher priority given to the 00Z run as this trend remains an outlier compared to the ECMWF and mean guidance. The incoming 12Z GFS run trended away from a deeper, closed low and towards a weaker, open wave giving more confidence to the reliance on the 00Z ECMWF, and 00Z over the 06Z GFS in the updated forecast blend. ...Weather/Hazards Highlights... As troughing comes into the West early next week, precipitation will spread into the Pacific Northwest and northern Rockies on Sunday and Monday, including snow that could be heavy. Precipitation may begin in California Sunday as well but looks to increase there by Monday. There is still some model spread in rain amounts across coastal California, but given the sensitivity of southern California to additional rainfall after a wet month, a Marginal Risk has been included for the Day 5/Monday Excessive Rainfall Outlook. If wetter models verify this risk may need to be upgraded in future cycles. Snow and some rain will then spread into interior parts of the West Monday-Tuesday. Additional moisture ahead of another system upstream over the Pacific will continue precipitation chances in the Pacific Northwest through Wednesday-Thursday. The highest likelihood for winter-related impacts is in the Cascades, Sierra, and portions of the Northern/Central Rockies, though most of the regional higher elevation mountain ranges will at least see some snow through the period. Widespread strong, gusty winds are also expected across much of the West, particularly for higher elevations, as a couple frontal systems pass through, lingering the longest along the eastern ranges/foothills of the Rockies and immediate High Plains. Farther east, a cold front tracking through the northeastern U.S. could lead to some light showers across portions of the Ohio Valley/Appalachians into the Northeast Sunday, though most locations will likely remain dry. Precipitation chances will begin to increase Tuesday and especially into Wednesday across much of the eastern half of the country as moist southerly flow increases ahead of the approaching upper-level trough and surface low pressure/frontal system from the West. Most precipitation will be in the form of rain given anomalous warmth, but some snow on the backside of the low and cold front for the Upper Midwest/Great Lakes is possible Wednesday, though confidence in this scenario remains rather low. Rainfall amounts could be locally heavy, particularly for the Ohio Valley, with likely multiple rounds of rain continuing through at least midweek. In addition, the moist, unstable airmass as well as increasingly strong winds aloft with the approaching trough will bring the chance for some severe thunderstorms, with the Storm Prediction Center highlighting this threat from the Southern Plains to Middle Mississippi Valley Tuesday and into the Mid-South and Ohio/Tennessee Valleys Wednesday. The central U.S. will see quite mild to warm temperatures early next week underneath an upper ridge. Temperatures of 20-35 degrees above average will expand throughout the Plains and Mississippi Valley, putting actual temperatures into the 80s and even 90s in Texas by Monday-Tuesday while 60s reach as far north as South Dakota and Iowa, with potential for widespread records. The upper trough and cold front pressing eastward should cool temperatures down closer to normal or a few degrees below in the central U.S. by midweek, but temperatures could already start to rebound by Thursday. The well above normal to possibly record-breaking temperatures will get shunted eastward ahead of the upper trough/underneath the upper ridge into the east-central and then eastern U.S. Tuesday-Thursday. 70s should be widespread across the Southeast and could even reach as far north as the Mid-Atlantic. Meanwhile the West is forecast to see near to slightly above normal temperatures into Sunday, but cooling to below normal particularly in terms of highs by Monday-Tuesday as upper troughing comes in. Putnam/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