Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 AM EDT Tue Mar 12 2024 Valid 12Z Fri Mar 15 2024 - 12Z Tue Mar 19 2024 ...Heavy snows possible across parts of the southern Rockies into Friday... ...Multi-day heavy rainfall event likely for parts of the South late this week and weekend... ...Overview... The general flow over the CONUS will be settling into a much more amplified regime during the medium range period. By the start of the period on Friday, a sharp/amplified upper ridge off the West Coast will support a deep Southwest upper low that will likely meander in place into early next week. This feature would promote a multi-day period of heavy rainfall potential across parts of the Southern U.S. late this week and into the weekend with possibly significant snowfall ongoing through Friday across parts of the southern Rockies. A leading Plains through Northeast system may also produce some enhanced rainfall near its track late this week. The East Pacific into western Canada upper ridge will help to amplify downstream troughing into the eastern half to two-thirds of the lower 48 by this coming weekend into early next week, with a colder trend behind a northern tier to East Coast front. ...Guidance/Predictability Assessment... Latest model guidance continues to show fairly good agreement on the large scale forecast, but a lot of differences in the details. There remain some minor differences in timing and strength of an initial wave tracking from the Midwest through the Northeast late this week (the GFS is still a tad fast), but latest clustering does suggest this could become a fairly vigorous (but progressive) wave off the New England coast by early Friday. For the Southwest upper low, there is very good agreement that this low will generally hang around through at least Sunday-Monday. After this, the biggest uncertainty becomes any kind of phasing between this and an amplifying trough over the north-central U.S.. The models have shown some run to run variability in showing the northern stream phasing with the southern stream low and dragging it eastward by around Monday, or a faster progression of the trough leaving the upper low behind. The ECMWF has been most consistent in showing the two phasing, while the past few runs of the GFS have supported that idea as well. However, the ensembles are more supportive of the upper low hanging back through at least Tuesday as the northern stream trough speeds eastward, and this has support from most of the EC-initialized ML models as well as the CMC. Given the support from the ensemble means, the WPC forecast also leaned more on holding the low back, and interestingly enough, both of the 00z runs of the GFS and ECMWF tonight (which was available after forecast generation time) also jumped back into this camp as well. Bottom line, there is still a lot of uncertainty in how, if at all, the two streams will interact and still plenty of time for the forecast to change, one way or the other. The WPC forecast blend for tonight used a composite blend of the deterministic solutions days 3-5 where large scale agreement was good. By day 6 and especially 7, the blend phased out the GFS and ECMWF and quickly increased the contributions from the ensemble means. Overall, this maintained good continuity with the previous WPC forecast as well. ...Weather/Hazards Highlights... A long-duration winter storm will be coming to an end across parts of the southern Rockies on Friday, with the some possibly significant storm total accumulations across the higher terrain. Farther east, a Midwest through Northeast wave and frontal system will support some enhanced precipitation near its path into Friday. On Day 4/Friday, favorable moisture/instability in the warm sector, a subset of operational models with locally significant QPF, and fairly wet antecedent conditions/FFGs, seem to favor holding on to the marginal risk into parts of the Northeast. Some moisture reaching far northern New England could also produce a little snow. Meanwhile the moisture streaming northward ahead of the stagnant upper low over the Southwest and interacting with the front trailing from the aforementioned northern wave will support what should be a multi-day heavy rainfall threat across the South, which will start towards the end of the short range period. The cold front may get hung up for a period of time supporting repeat/training thunderstorms across parts of the Lower Mississippi Valley into the central Gulf Coast states, but there is plenty of uncertainty in the exact placement of the highest QPF still. The days 4 and 5 Excessive Rainfall Outlooks both depict slight risk areas across this region, and there is potential for an upgrade as the event gets closer in time especially considering possible overlap in the heaviest QPF. Some heavy rainfall potential may extend northward along the boundary into the southern/central Appalachians as well. The heavy rainfall threat may linger across the Southeast through the weekend. The system tracking through southeastern Canada Saturday-Monday and trailing front may produce areas of rain and perhaps cold sector snow over parts of the Great Lakes/Northeast. Mean ridging aloft over the Eastern U.S. will continue to support an expansive area of above normal temperatures, with the core of the warmest anomalies over the Mid-Atlantic and Ohio Valley on Friday where high temperatures could reach 15-20 degrees above normal. Temperatures across the East will trend cooler with time after Friday, but should generally remain above normal through the weekend. Over the West, moderately below average highs and near/above average lows will linger over the Great Basin/Southwest Friday-Saturday, before shifting slightly east into the Plains on Sunday. Temperatures along the immediate West Coast should increase by Friday and through the weekend as strong ridging builds in aloft, with the Northwest seeing the best potential for some highs reaching 10-15 degrees above normal. Santorelli 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