Extended Forecast Discussion NWS Weather Prediction Center College Park MD 356 PM EST Tue Feb 28 2023 Valid 12Z Fri Mar 03 2023 - 12Z Tue Mar 07 2023 ***Major storm system is expected to produce widespread heavy rain and severe thunderstorms from the interior Gulf Coast states to the Ohio Valley and Mid-Atlantic, as well as snow for the Great Lakes and New England late this week*** ...Synoptic Overview... The main weather headline going into the end of this week will continue to be the potent low pressure system tracking from near the Middle Mississippi Valley east-northeastward, producing widespread strong thunderstorms, heavy rain, and wintry weather. The storm should depart from the East Coast on Saturday with surface high pressure building across the East through Monday. Behind this system, expect the large scale pattern to revert to a variation of the recently familiar West Coast mean trough aloft which will support continuation of unsettled conditions and below normal temperatures over much of the West. The primary Pacific system of note will drop southward from the Gulf of Alaska and reach close to the Pacific Northwest during the weekend and then weaken as it lingers off the coast. Meanwhile leading trough energy may support some surface waviness to the east of the Rockies by early next week. Guidance expects the upper pattern over eastern Canada and the Atlantic to become more blocky with time, as ridging builds back across southern Greenland and an upper low settles over or near the Canadian Maritimes. ...Guidance/Predictability Assessment... Latest guidance from the 00Z/06Z cycles was reasonably well clustered for the system affecting the eastern U.S. Friday-Saturday, with the 00Z CMC and especially UKMET initially somewhat south/slower than the GFS-ECMWF and the three main ensemble means. The new 12Z cycle has added to the spread though, with the GFS/GEFS mean trending faster but the UKMET/CMC still south as of early Friday. By Saturday the 12Z GFS/GEFS is still fairly extreme on the fast side with the Atlantic wave while the UKMET/CMC catch up to the ECMWF/ECMWF mean. Also worth noting, the middle to stronger part of the guidance spread for surface low depth would challenge March sea level pressure records over some locations from just west of the Mississippi Valley into the Ohio Valley. For the upper trough/surface system nearing the West Coast during the weekend, the 12Z GFS is somewhat weaker/faster with the core of the overall system relative to most other guidance. Otherwise differences are generally within typical variability and error ranges for forecasts 5-7 days out in time. More notable differences develop downstream late in the period with various ideas for how leading trough energy may evolve through/around what appears to be a developing flat ridge aloft between West Coast and Atlantic troughing. ECMWF evolution over multiple runs has led to a defined wave crossing the Great Lakes and Northeast/Mid-Atlantic by next Tuesday, while latest GFS runs through 06Z have abandoned that idea after looking more like the ECMWF 24-48 hours ago. The new 12Z GFS has come back to a tempered form of surface waviness versus more pronounced ridging over the East in the 00Z/06Z runs. Just beyond early day 7 the guidance consensus supports the idea of a cold front dropping into the Southeast, so a weak Great Lakes/East Coast wave would at least lead into that scenario. A composite of operational model guidance from the 00Z/06Z cycles represented consensus well during the first half of the period. Then the updated forecast blend incorporated some 06Z GEFS/00Z ECMWF means along with some continued input from the 00Z ECMWF/CMC and a split of 00Z/06Z GFS runs. ...Weather/Hazard Highlights... The storm system that is currently affecting California is progged will likely reach near Arkansas/Missouri by Friday morning with an expansive area of moderate to heavy rainfall extending east across much of the Ohio Valley and northern Mid-Atlantic. The Day 4 (12Z Friday-12Z Saturday) experimental Excessive Rainfall Outlook currently depicts a Marginal Risk area in association with this potential for significant rainfall. Monitor for refinements in this outlook as the forecast transitions into the short-term time frame. Strong to severe thunderstorms are a good possibility across portions of the Deep South and extending to the southeast U.S. coast on Friday where favorable kinematic and instability parameters will exist in the warm sector. Check the latest Storm Prediction Center products for more information on the severe weather threats. Farther north, expect another round of impactful snow from the southern Great Lakes into parts of the Northeast, with some locations possibly getting over a foot of snow accumulation by Saturday. There may also be a transition zone of wintry mix between the rain/snow areas east of the Great Lakes. The expected strength of the surface low will make strong winds another factor to contend with, especially across much of the Midwest and Ohio Valley. A deep upper trough should settle over the West Coast this weekend and persist into next week. This feature and leading/embedded surface low pressure will increase coastal rain and heavy inland snow from the Pacific Northwest into northern California. Highest precipitation totals during the period should be along the central West Coast into the Sierra Nevada, adding to the already very impressive snow pack over the region. Expect lighter snow to spread across the central and northern Rockies. With low confidence in specifics, shortwave energy streaming eastward from the West Coast trough could produce one or more areas of light snow from the northern Plains into the Great Lakes, with scattered light rain possible farther south. In terms of temperatures, much of the southeastern U.S. should have highs of 10-20F above average on Friday in the warm sector of the low pressure system after starting out up to 20-25F above normal relative to morning lows. Readings should return to within a few degrees of climatological averages by the weekend for most areas east of the Rocky Mountains. The southeast U.S./Gulf of Mexico upper ridge will once again exert its influence across much of the south-central U.S. going into early next week with temperatures easily 5-15 degrees above early March levels along with increasing humidity levels near the Gulf Coast. The West will continue to feel like the middle of January through the weekend and into next week with the Intermountain West and Rocky Mountains consistently having highs on the order of 10-20F below normal, which will equate to highs in the 20s and 30s for most areas north of the Utah/Arizona border. Rausch/Hamrick Hazards: - Heavy snow across the upper Midwest through the lower Great Lakes and central New England, as well as the Cascades and the Olympic Mountains, Fri-Sat, Mar 3-Mar 4. - Heavy precipitation across the Midwest through the lower Great Lakes, the central Appalachians and southern New England, Fri, Mar 3. - Heavy rain across portions of coastal northern California into coastal southwestern Oregon, Sat, Mar 4. - Heavy rain across the Midwest, through the northern Mid-Atlantic and across coastal southern New England, Fri, Mar 3. - Heavy snow across the higher elevations of northern California including the Sierra Nevada, Sat-Sun, Mar 4-Mar 5. - Heavy snow along the Sierra Nevada, Tue, Mar 7. - Severe weather across portions of the Southeast, into the Tennessee Valley, the Carolinas, and the southern Appalachians, Fri, Mar 3. - Freezing rain across central and western New York, and into northern Pennsylvania, Fri, Mar 3. - High winds across portions of the middle Mississippi Valley. - Flooding possible across portions of the upper Mississippi Valley. - Flooding likely across portions of the middle Mississippi Valley. - Much below normal temperatures across the central Great Basin, into the northern Rockies, Fri-Sat, Mar 3-Mar 4 and Mon-Tue, Mar 6-Mar 7. - Much below normal temperatures across portions of the southern Great Basin into the Four Corners region, Fri-Sat, Mar 3-Mar 4. - Much below normal temperatures across portions of the northern Plains into the northern Rockies, Mon-Tue, Mar 6-Mar 7. 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