Extended Forecast Discussion NWS Weather Prediction Center College Park MD 156 PM EST Sun Feb 12 2023 Valid 12Z Wed Feb 15 2023 - 12Z Sun Feb 19 2023 ...Mid-late week Four Corners to Great Lakes system to bring a threat for heavy snow to the central High Plains through Upper Great Lakes and heavy rainfall/severe weather farther southeast... 19Z Update: 19Z Update: The 12Z model guidance suite continues to have above average agreement through the end of the week on all three storm systems of interest and the overall flow pattern. The first storm system exiting the Upper Midwest has excellent depiction in the guidance and a general deterministic blend works for this event. With the second and more impactful storm developing over the Plains mid-late Wednesday and beyond, the 12Z UKMET and to a lesser degree the ECMWF is slower than the well clustered ensemble means and GFS/CMC, and the 12Z ECMWF run has trended slower compared to its 00Z run. Similar to yesterday, the GFS remains on the stronger side of the guidance. The prospects for severe weather from Texas to the Tennessee River Valley will likely be the main weather headline going into the middle to end of the week, and the snowstorm from southern Colorado to the central Plains and eventually the Upper Midwest is also expected to be noteworthy. For the third system dropping south along the West Coast with the well defined upper low, the GFS drops the low south faster than the other guidance, but still reasonable agreement for a day 7 forecast. The WPC fronts/pressures forecast was primarily derived from a multi-deterministic blend through Thursday, and then increasing the ensemble means to about a third by Sunday. The previous forecast discussion is appended below for reference. /Hamrick --------------------- ...Overview... Behind a system departing from the Great Lakes early in the forecast period on Wednesday, the primary features of note will be an upper low tracking out of the Four Corners and producing a significant storm system on a path from the southern High Plains through the Great Lakes mid-late week and then a trough/embedded upper low approaching the West Coast late in the week. Expect the emerging Four Corners system to produce a band of heavy snow to the northwest of its track and areas of significant rainfall plus some severe thunderstorms to the southeast in the warm sector. Precise details of the West Coast system remain uncertain, with precipitation amounts very sensitive to the exact track. Teleconnections relative to the strong ridge building over the east-central Pacific support the idea of this trough settling over the West toward the end of the period and possibly beyond, while eastern U.S. upper heights rebound after trough passage and as a ridge centered over the Bahamas on Friday shifts into the southeastern Gulf of Mexico. ...Guidance/Predictability Assessment... Latest model consensus looks good for the system expected to be over the Upper Great Lakes as of early Wednesday. The Four Corners upper low as of early Wednesday has been consistent with only modest spread at that time, but solutions continue to diverge and vary between runs farther east/northeast due to differences in how northern stream flow interacts with the ejecting Four Corners low. Through the 12Z/18Z cycles, recent ECMWF/CMC runs and the GEFS/ECMWF ensemble means have generally been the most consistent with this system. The GFS has been waffling between faster and slower timing, though the latest runs have adjusted closer to the majority cluster. The 12Z and new 00Z UKMET runs are on the slow side. The new 00Z CMC offers the one surprise so far, changing to a much weaker/elongated and faster surface wave that bears some resemblance to the 12Z CMC mean. Preference based on 12Z/18Z guidance was with the 18Z GFS and 12Z ECMWF/CMC. For the upper trough/low nearing the West Coast by late in the week, guidance continues to show spread that may be fairly typical for a days 5-7 forecast but the exact path of the upper low will be very important for determining precipitation coverage/amounts over the southwestern U.S. The ensemble means have been remarkably consistent so far, bringing the upper low close to the central or south-central California coast by early Saturday with an upper trough axis reaching into the Southwest by Sunday. Most operational models have been trying to close off/track the low a little farther offshore, though the 12Z ECMWF adjusted closer to the means than other models or prior ECMWF runs. CMC runs have been close to the means into Friday before keeping the upper low more offshore. The GFS had been a western extreme with the upper low but latest runs, and in particular the 18Z version, have nudged closer to the guidance average. Even a slightly more west solution like the new 00Z GFS is plausible as it still leads into the teleconnection-favored western U.S. upper trough around and after the end of the forecast period. An average of the 18Z GFS and 12Z ECMWF/CMC best represented consensus/continuity into day 5 Friday followed by a transition to the GFS/ECMWF and their ensemble means for the rest of the period. ...Weather/Hazard Highlights... The system exiting the Great Lakes on Wednesday may produce some lingering light snow or rain early in the day. The system emerging from the Four Corners region/southern High Plains on Wednesday and then tracking northeastward should produce a band of potentially heavy snow from the central Rockies/High Plains into the Upper Great Lakes. This axis has been fairly consistent over recent forecasts but there is enough uncertainty in details of this storm to temper confidence in snow totals, especially from the Midwest northeastward. Meanwhile leading Gulf inflow ahead of the system's trailing front should promote areas of heavy rainfall for some locations from the Mississippi Valley into the Appalachians. The days 4-5 experimental Excessive Rainfall Outlooks depict at least a Marginal Risk area for each day with the day 5 outlook (12Z Thursday-12Z Friday) likely introducing an embedded Slight Risk area over and surrounding northwestern Georgia--representing the best overlap of relatively higher rainfall totals from this event and lingering soil moisture from the event currently in progress over the Southeast. Rainfall along the East Coast should be somewhat lighter due to expected acceleration of the upper trough/leading cold front. The Storm Prediction Center is also monitoring the potential for severe weather from parts of the southern Plains east/northeast into the Lower Ohio Valley south to the Gulf Coast. Check latest SPC products for the latest information on severe threats. Also, this system may be strong enough to produce some areas of brisk to strong winds. High pressure will settle in over the Interior West as this system continues to the east of the Rockies/Plains. The upper trough/low nearing the West Coast late this week, with some uncertainty over its track/progression thereafter, may bring an increase of precipitation to that region late in the week. Most precipitation that does fall should be in the light to moderate range and near the immediate coast, while sufficient progression could start to bring a little moisture into the Southwest by Sunday. Specifics are still very sensitive to the exact upper low track so confidence remains somewhat below average. The closed upper low tracking through the Four Corners region on Wednesday will support a broad area of highs 10-25F below normal across much of the southern two-thirds or more of the West during Wednesday-Thursday, with some of this cold air reaching the Plains as well. Expect some moderation over the West from Friday into the weekend but may areas may still see moderately below normal highs. Warm flow ahead of the Four Corners through Great Lakes system and its trailing cold front will bring well above normal temperatures to the eastern half of the country Wednesday-Friday. Some readings may reach at least 20-25F above normal for both high and low temperatures. Widespread record warm lows are possible mid-late week (if they hold through the end of the calendar day), while some record highs could be possible on multiple days from parts of the Ohio Valley into the Northeast and over the Florida Peninsula on Thursday. Frontal passage will bring a brief period of cooler temperatures late week into early weekend but the northern half of the East should rebound to above normal readings by Sunday. Rausch 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