Extended Forecast Discussion NWS Weather Prediction Center College Park MD 201 AM 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... For multiple forecast cycles, operational model guidance has been agreeable that a southern stream upper low in the eastern Pacific offshore of California embedded in a positively tilted upper trough will phase with strong northern stream energy/troughing as it all tracks into the western to central U.S. for the first half of next week. Upper ridging is forecast downstream of this in the southern stream, tracking from the Southwest into the central to eastern U.S. at the same time. Deterministic models diverge somewhat more notably with the timing of the trough by Wednesday-Thursday as it tracks east. The 18Z and now 00Z GFS runs are on the slow side in its track and even show an upper low closing off. The ECMWF was more of a middle ground and aligned with ensemble means, which was favored, while the CMC runs are a bit faster than consensus. However, despite the relatively good agreement in most operational models, confidence for the northern and southern stream troughs to phase as they shift eastward is not necessarily high. Cluster analysis and spaghettis of height lines show some ensemble members have the northern stream troughing outpacing the southern stream low, rushing the northern trough forward and holding back the southern low/trough. Also notable is that most (four) 12Z ECMWF-based AI models have notable stream separation, with only one available closer to the operational ECMWF. The 12Z GFS run of the AI Graphcast also switched to showing some stream separation after more phased previous runs. A pattern like this can be quite tenuous and the energy that ends up forming the northern stream troughing is still over Siberia at this point, so it is possible that more stream separation will be what verifies. For now, kept with an operational model blend favoring the 12Z models and gradually increased the means to half the blend by Day 7, which maintained good continuity with the previous forecast for Days 3-6 as well. But continue to monitor forecasts as there is a chance they change quite a bit. ...Weather/Hazards Highlights... As troughing comes into the West early next week, precipitation will spread into the Pacific Northwest and northern Rockies on Sunday, 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, went ahead and introduced a Marginal Risk in 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 should continue to increase in interior parts of the West into Monday-Tuesday. The highest likelihood for winter-related impacts is in the Cascades Sunday-Tuesday, but the snow will likely cause at least minor impacts to much of the higher elevation areas of the Sierra Nevada into the Intermountain West and Rockies. Gusty winds are forecast for the Interior West behind a couple of cold fronts on Monday-Tuesday as well. Farther east, a cold front tracking through the northeastern U.S. could spread some light to moderate precipitation from the Ohio Valley/Great Lakes into the northern/central Appalachians and the Northeast on Sunday. Then ahead of the approaching upper trough and cold front, precipitation chances will increase late Monday into Tuesday in the Midwest to the Ohio Valley and Great Lakes. Most precipitation will be in the form of rain given the anomalous warmth, but some snow on the backside of the low and cold front in the north-central U.S. is possible, though with low confidence. Rainfall amounts could be locally heavy in the Ohio Valley, with likely multiple rounds of rain continuing through at least midweek. 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. 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