Extended Forecast Discussion NWS Weather Prediction Center College Park MD 159 AM EST Sat Jan 08 2022 Valid 12Z Tue Jan 11 2022 - 12Z Sat Jan 15 2022 ...Overview... The medium range period will begin on Tuesday with split flow across the western U.S. as a southern stream closed low drifts near Baja California through midweek with ridging across the Northwest in a Rex block pattern. The southern stream low is currently expected to push into the Four Corners around Thursday while additional Pacific energy moves toward the West Coast. Meanwhile, shortwave impulses will track through broad troughing in the eastern (especially northeastern) U.S. along with low pressure/frontal systems. ...Guidance/Predictability Assessment... Reasonably good model agreement and predictability exist early in the medium range period with deep upper troughing across the East and ridging across the West undercut by the southern stream upper low. Most guidance for the 12/18Z (yesterday) cycle was aligned and consistent with previous runs with the position and closing off the initial southern stream low feature at least for a period on Tuesday/Wednesday. The main exception in terms of position was the 12Z CMC, which has a center position farther to the southwest compared to the vast majority of other guidance. Then energy tracking eastward across the Pacific and approaching the West Coast Thursday shows considerably more variability with strength, position, and track as well as whether or not the energy closes off (which GFS runs had been persistent with until the 18Z run). Trends in recent guidance indicate that impulse may drift in the eastern Pacific through late week or at least stay limited to the Northwest with less interaction with the initial low, but with low confidence. CMC runs are the exception to this in terms of deterministic guidance, and spaghetti plots show tremendous variability in the details of the western U.S. flow by Friday and Saturday. Meanwhile in the East, initial model agreement is quite good with the trough axis over the Eastern Seaboard early Tuesday, which lifts away and relaxes the amplitude of the overall flow by midweek, though additional shortwaves approach. One upstream shortwave develops some considerable differences among models by Wednesday and Thursday. In terms of the 12Z model cycle, the UKMET and CMC appeared to be outliers in how deep the energy from the shortwave causes the trough axis to dig in the east-central U.S., as confirmed by the 12Z CMC the deepest and the UKMET compared to individual ensemble members' 500mb heights. However, incoming 00Z guidance including the GFS and ECMWF show deeper troughing than their previous runs, even closing off a low across the Atlantic by Friday, so the trend appears to be in a deeper direction. The WPC fronts/pressures forecast was based mainly on a blend of the 12Z/18Z GFS and 12Z ECMWF early on, phasing in the GEFS and EC ensemble means as the period progressed to downplay the specifics of a particular model runs. At the time, did not favor the CMC/UKMET for the reasons above, but given the way 00Z models are trending, perhaps their influence with a deeper trough in the East would have been helpful. ...Weather Highlights... Drier conditions are likely over much of the contiguous U.S. on Tuesday as large surface highs influence the Intermountain West to the central U.S. behind a cold front, and high pressure will push into the East on Wednesday. Showers could linger over Florida on Tuesday after the front passes through, while generally westerly flow could cause lake effect snow downwind of the Great Lakes at times through the week, though in lesser amounts than during the short range period Monday. The Pacific Northwest should see periods of precipitation as fronts approach the area, with higher totals over typical upslope areas of the Olympics and Cascades. Locally moderate to heavy activity may be possible over the Olympics and vicinity around midweek when the area could be on the southeast fringe of an atmospheric river event aimed at Vancouver Island. The upper low initially dropping southward near the California coast may spread moisture into the Southwest by Tuesday-Wednesday. This precipitation should be fairly light but heavier rainfall may develop over the Southern Plains and Lower Mississippi Valley by mid-late week as the upper low tracks/shears northeastward and the surface pattern becomes more favorable for drawing moisture northward from the Gulf of Mexico. It will take additional time to refine the details of rainfall coverage and magnitude though. Cold air will be in place over the Upper Midwest and spreading into the Northeast on Tuesday, with high temperatures 10-30F below average for one of the coldest days of the season thus far. Then the central U.S. will see a pronounced warming trend next week with expanding coverage of max/min temperatures 10-20F above normal Tuesday onward. Some locations in the northern two-thirds of the Plains may see even warmer readings mid-late week. Across the West expect to see daytime highs within 10F on either side of normal through the period, while above normal lows should become more prevalent with time. By the end of the week temperatures are currently forecast to moderate close to normal. Tate 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, 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