Extended Forecast Discussion NWS Weather Prediction Center College Park MD 206 AM EST Tue Jan 07 2020 Valid 12Z Fri Jan 10 2020 - 12Z Tue Jan 14 2020 ...Late week/weekend storm to bring a heavy rain threat to the Ark-La-Tex region/Lower-Mid Mississippi Valley and northeastward, possible severe weather to parts of the South, and wintry precipitation potential to the central Plains/Midwest through Great Lakes/Northeast... ...Active pattern over the Northwest... ...Overview/Guidance Assessment... A mean pattern aloft featuring an amplified western U.S. trough held in place by strong ridges over the east-central Pacific and off the Southeast U.S. will produce significant weather over portions of the lower 48 during the period. Individual shortwaves within the mean flow will support individual events. This pattern will promote a contrast between very warm temperatures over the eastern half of the country (many areas averaging 10-25F above normal for the five-day period) and below normal readings over the northern Plains (up to 20-35F below average one or more days) as well as to a lesser extreme over much of the West. Guidance is still in the process of trying to converge on the ultimate solution for how a shortwave over the Rockies as of early Fri will eject northeastward, with associated low pressure tracking from the southern Plains through the Northeast in the late week-weekend time frame. CMC runs had been the one holdout for a slower/closed feature aloft but the new 00Z run has finally joined the 12Z ECMWF. GFS runs have tended to be on the fast side of the spectrum for this system and that continues to be the case with the 00Z run actually straying ahead of the 18Z run. Meanwhile the 00Z UKMET is a bit slower than the previous run. Slowest solutions may not be ideal given the progressive nature of the southwesterly flow aloft, favoring at least intermediate timing. Latest trends toward a flatter shortwave reaching the East Coast look good given the strong positive height anomalies which multi-day means have persistently forecast just offshore. Upstream the guidance agreement is steadily improving for the robust though weakening system currently expected to tracking into Washington state on Sat. 00Z CMC details aloft leave some question marks but other wise a blend depicts consensus well. Through the weekend the supporting dynamics will drop into the long-term western mean trough position. By days 6-7 Mon-Tue forecast details become more uncertain. In general expect this western energy to eject beyond the Rockies while additional energy aloft and surface system drop southeastward into and beyond the Northwest. Notable timing differences exist for the system reaching the Northwest while models/ensembles become quite varied in how the weekend western energy ejects to the east/northeast. Over the past couple days a number of operational model runs have displayed a general signal for low pressure over east-central North America by early day 7 Tue while the ensemble means (18Z/00Z GEFS being best defined) have focused low pressure over the Great Lakes region--a reasonable starting point given the full guidance spread. Resolving specifics next Mon-Tue will be important for determining the character of another potentially significant rainfall event over some areas over/east of the Mississippi Valley. The updated forecast incorporated a mostly model-based blend among the 12Z and 00Z/06 ECMWF runs (to account for shortwave differences through the period) as well as 18Z GFS/12Z UKMET for days 3-5 Fri-Sun followed by a transition toward a more even model/ensemble mean blend by day 7 Tue. The result provided good continuity with most aspects of the forecast. ...Weather Highlights/Threats... The late week through weekend storm system will produce a wide variety of significant weather across the eastern half of the country. Persistent Gulf inflow ahead of the developing low pressure/frontal system tracking out of the southern Plains will provide the moisture to produce heavy rainfall from the Ark-La-Tex region and Mid-Lower Mississippi Valley northeastward into the Tennessee/Ohio Valleys and possibly beyond. Currently expect the axis of highest rainfall to extend from Arkansas into Ohio, but this is still subject to adjustment given uncertainty in exact details of system evolution/timing. At the same time the Storm Prediction Center is monitoring the potential for severe weather over portions of the South. Check latest SPC products for updated information regarding the severe threat. Meanwhile over the past couple days guidance has been steadily increasing the potential for meaningful cold sector winter weather within an axis from the central Plains/Midwest through the Great Lakes/New England. Continue to expect the front trailing from the surface low to drop into the Gulf of Mexico briefly and then return north as a warm front early next week. Additional ejecting western shortwave energy may support another central-eastern U.S. system which could produce another heavy rainfall event, but with lesser confidence in specifics due to current guidance spread at that time frame. Thus far the best heavy rain signal for this second event continues to be from the Lower Mississippi Valley into Tennessee Valley/southern Appalachians--somewhat southeast of the expected axis of the first event. Some locations that may be affected by either or both events should monitor flash flood/river flood potential due to recent significant rainfall. Over the West, guidance is consistent in highlighting favored terrain from the Pacific Northwest into northwestern California for highest rain/mountain snow totals during the period as multiple systems affect the region. The trajectory of moisture still does not appear to support anomalously high precipitable water values/extreme intensity of precipitation in most cases, but persistence of the rain/snow may lead to several inches liquid over the five-day period. Expect a secondary precip maximum over the northern Rockies. The system reaching the Northwest early in the weekend may be accompanied by a period of brisk winds. Episodes of lighter rain and higher elevation snow will extend farther southeast across the West. From late week into the weekend expect the warm air extending across the eastern half of the country to produce widespread plus 10-30F anomalies and possibly greater especially for morning lows. Daily records for highs/warm lows are possible. Expect warm temperatures to continue into next week. Within the area of colder air to the west, the most extreme anomalies will be over the northern Plains where readings could be as low as 20-35F below normal. Northern parts of the West could see readings drift toward 10F or so below normal by early next week while most of the remainder of the West will see temperatures generally 5-10F or so below normal. Rausch Additional 3-7 Day Hazards information can be found on the WPC medium range hazards chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation, 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/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml