Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 AM EST Mon Feb 13 2023 Valid 12Z Thu Feb 16 2023 - 12Z Mon Feb 20 2023 ...Midwest-Great Lakes snow event around Thursday becoming more uncertain but heavy rainfall/severe weather threats persist farther south... ...Overview... Ongoing question marks regarding how northern stream flow will interact with an ejecting Four Corners upper low have actually created more forecast spread over the past day for the system forecast to affect the eastern half of the country Thursday into Friday. Uncertainty over the character of this system is currently leading to the greatest spread for snowfall in the cold sector, while there is somewhat better agreement and continuity for the heavy rainfall/severe potential between the Gulf Coast and Ohio Valley. The supporting upper trough will pass through the East quickly late this week. Then the strong upper ridge building over the east-central Pacific should support a longer-term mean trough setting up over the West, a combination of northern stream flow rounding the ridge and eventually (with uncertain timing) an upper low dropping southward offshore California. Upper heights over the East should rise to above normal levels, also consistent with teleconnections relative to the upstream pattern. ...Guidance/Predictability Assessment... Over the past 24-36 hours the guidance spread has actually increased notably for the system crossing the eastern U.S. during the latter half of the week. Ever since the 00Z/12 run, the CMC has been on the weak/progressive side of the envelope for the surface low, while latest GFS/GEFS mean runs are on the fast side. The 12Z UKMET was close to the 12Z ECMWF in principle but the new 00Z version becomes a southern extreme near the East Coast. Thus by early Friday the latest guidance for the main surface low ranges from the 00Z UKMET in northeastern Pennsylvania to the GFS/GEFS well north of Maine. In general the ECMWF runs have been the most stable for track and timing so far, though the guidance spread and general weaker trend in the guidance over the past couple days significantly tempers confidence in any specific solution now. A solution close to an average of the 18Z GFS/12Z ECMWF runs provided the best starting point relative to the 12Z/18Z guidance. For the upper trough/low just off California, the mostly offshore models and previously along-coast ensemble means each appear to have adjusted a little toward the other. Latest CMC runs maintain their status as the slow/westward extreme later in the period, while the more-favored 18Z/00Z GFS runs and the 12Z ECMWF end up fitting fairly well within the ensemble mean trough axis into day 7 Monday. A blend of these models, and some ensemble mean input late in the period, also provide a reasonable representation of northern stream energy that enters the West during the latter half of the period. ...Weather/Hazard Highlights... Recently increasing model spread for the character of the system tracking northeastward from the Mississippi Valley Thursday onward is lowering the confidence in details of a potential band of snow to the northwest of the low track. The relatively greater potential for meaningful snow still extends from the Midwest into the Great Lakes but the average of guidance has shown recent trends to be somewhat weaker/southward, suggesting potential for lighter/southward adjustments to the snow axis. There is better agreement and continuity in the idea that leading Gulf inflow ahead of the system's trailing front should promote areas of heavy rainfall for some locations from the central/east-central Gulf Coast northward. The day 4 experimental Excessive Rainfall Outlook (to be issued soon after issuance of this discussion) will depict a slightly expanded Slight Risk area relative to continuity over and a little west of the southern Appalachians. Rainfall along the East Coast should be somewhat lighter due to expected acceleration of the upper trough/leading cold front. The Storm Prediction Center continues to monitor the potential for severe weather from the Gulf Coast into Ohio Valley on Thursday. Check latest SPC products for the latest information on severe threats. This system may be strong enough to produce some areas of brisk winds but recent weaker trends for the system are at least decreasing the potential for higher-end wind speeds from some earlier forecasts. High pressure will settle in over the Interior West during the latter half of the week as this system crosses the East. The upper trough/low forecast to track offshore California late this week late this week through the weekend may bring an increase of precipitation to areas along the immediate coast. Thus far there has been enough spread and variability with the upper low track to keep confidence fairly low for determining how much moisture tracks along the coast versus staying just offshore. Trends have been slower for the eventual arrival of the upper low into the Southwest but eventually it should bring at least some moisture into parts of the Southwest/southern Rockies by the start of next week, with precipitation possibly starting to develop over parts of the Plains/east-central U.S. around next Monday or so as well. Meanwhile, northern stream flow rounding the upstream Pacific ridge should bring an increase of rain and mountain snow to the Northwest/northern Rockies, especially by Sunday-Monday. The Four Corners states and parts of the Plains will be quite chilly on Thursday with highs 10-20F below normal. Temperatures will moderate thereafter but the Interior West and Rockies may remain modestly below normal (mostly single digit anomalies) into early next week. Meanwhile expect unseasonably warm air across the East Thursday into early Friday ahead of the cold front advancing across the region. Some temperatures could reach up to 20-30F above normal, with widespread record warm lows (if they hold through the end of the calendar day) but record highs tending to be more confined to the Northeast and the Florida Peninsula. Frontal passage will bring a brief period of near to below normal temperatures Friday-Saturday (especially over the South) but then expect temperatures to rebound to above normal levels over the central-eastern U.S. by Sunday-Monday as supported by the large scale pattern evolution. 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