Extended Forecast Discussion NWS Weather Prediction Center College Park MD 335 PM EST Fri Jan 27 2023 Valid 12Z Mon Jan 30 2023 - 12Z Fri Feb 03 2023 ...Dangerously cold temperatures and wind chills are forecast across the Northern Plains and Upper Midwest through early next week... ...Overview... A mean trough aloft from a Hudson Bay closed low through the southwestern U.S. early in the week should shift somewhat to the east by the latter part of the week as the Hudson Bay low tracks eastward and a southwestern U.S. shortwave/possible embedded upper low ejects out of the region. The initial pattern will lead to colder than average temperatures that could be most hazardous for the north-central U.S. during the first half of the week. Meanwhile a ridge/upper high over Cuba during much of next week should keep the Southeast warmer than normal. Between these upper-level features and temperature regimes, expect active weather as shortwave energy and the combination of a couple wavy fronts along with periods of Gulf inflow support a couple of significant precipitation events east of the Rockies with rainfall to the south/southeast and wintry weather in the colder air. Some locally enhanced rain and higher elevation snow are possible in the Southwest early in the week with the upper feature crossing the region, while the West Coast may see increasing precipitation chances by later next week with the approach of an eastern Pacific upper trough. ...Guidance/Predictability Assessment... Regarding the upper trough dropping through the southwestern U.S. during the first half of the week, latest models and ensembles seem to be converging some compared to recent runs. Operational models still generally show a closed low tracking down the southern California coast early in the week with the overall shortwave pivoting eastward thereafter, though with a hint of slightly less flow separation than some solutions had been showing. At the same time the ensemble means do not quite show a closed low yet but have been trending closer to the operational model scenario for early next week. In the new 12Z cycle, the CMC strays from the majority as it depicts a slower/southward upper low by Wednesday-Thursday. Timing differences persist for the ejection of this feature as it opens up, with latest GFS runs faster than ECMWF/CMC runs per historical tendencies. Potential for upstream shortwave energy to affect the evolution of this feature by mid-late week also complicates the forecast. An operational model composite trending to a compromise between model/ensemble ideas later in the period provides a reasonable starting point and decent continuity for depicting this system. Upstream there is still a lot of model and ensemble spread/variability for how approaching Pacific trough energy may erode the ridge which consensus says will move into the West after midweek. Options range from a progressive shortwave per the 00Z CMC mean and operational 12Z/26 ECMWF, to a trough of varying amplitude nearing the West Coast in latest GFS/GEFS mean/CMC runs, to the 00Z ECMWF keeping a strong ridge along/offshore the West Coast (with the ECMWF mean between the GFS cluster and 00Z ECMWF. At the very least the 00Z ECMWF looks extreme in how it pulls off its Pacific upper low, and teleconnections relative to the core of negative height anomalies between the Canadian Maritimes and Greenland offer support for some shortwave energy to reach the West Coast (perhaps a combination of a day 7 shortwave and another 2-3 days later). Thus having at least weak shortwave energy reach the West Coast seems reasonable as a starting point while waiting for added clustering before shifting toward a more amplified trough. The new 12Z ECMWF has made progress in adjusting away from the 00Z run. Farther east there are still important differences in the handling of northern Plains through Northeast shortwave energy, affecting the position of surface front(s) and high pressure to the north. As has been the case for a number of runs, the GFS is an amplified and fast extreme with this shortwave, leading to stronger high pressure and farther south frontal position near the East Coast by day 5 Wednesday. However there has been a multi-day trend toward a more suppressed midweek pattern in the East so the GFS cannot be fully discounted--and in fact the new 12Z ECMWF has adjusted closer to the GFS scenario. The 00Z CMC was a pronounced exception to the overall trend with a wave tracking just south of New England but the new 12Z run did adjust southward. Corresponding to the mass field trend, precipitation for the Northeast has been trending lighter and the updated forecast tried to compromise between the fairly dry operational model trends (again minus the 00Z CMC) and the seemingly lagging ensembles that still had broad coverage. Based on the above considerations, the updated forecast started with a 00Z/06Z operational model blend early in the period followed by gradual incorporation of the 06Z GEFS/00Z ECMWF means while phasing out the 00Z CMC and reducing 06Z input by Wednesday. Due to West Coast forecast issues, after midweek the blend transitioned to a split of the 00Z and 12Z/26 runs for ECMWF input while eventually tilting the proportion of the means a little more to the GEFS. ...Weather/Hazard Highlights... Very cold conditions over the north-central U.S. during the weekend (now in the short range time frame) will continue into the next work week as well, in the wake of a strong cold front producing a surge of arctic air. Highs in the single digits and even below zero are possible in the Dakotas into Minnesota while bitterly cold lows combined with wind will produce dangerous wind chills. Much below normal temperatures should extend across much of the central U.S. through midweek especially, as well as across much of the Interior West with temperatures 15-25F below normal through Monday-Tuesday with gradual moderation thereafter. A few isolated record cold highs may be possible early in the week. Meanwhile the far Southeast and Florida Peninsula should see the most consistent above average temperatures with highs 10-15F above average and plus 10-20F anomalies for lows. A few record highs are possible Monday-Wednesday. While the western/central U.S. moderates later in the week, the East will trend somewhat colder with New England possibly seeing highs 10-20F below normal by next Friday. The possible upper low embedded within the California/Southwest upper trough could produce some enhanced precipitation across southern California continuing into Monday and across the Southwest Monday-Tuesday. Then this feature and the overall trough and other shortwaves will provide support for frontal boundaries with waves of low pressure through the south-central to southeastern U.S., which combined with Gulf moisture streaming in could lead to multiple rounds of precipitation. On the southern side locally heavy rain and localized flooding are possible, especially at times when there is sufficient instability for higher rain rates. With colder than normal air in place on the northern side, winter weather could also be a threat. A leading northern stream wave may produce some snow over the Northeast on Monday. Then there is potential for snow across parts of the southern half of the Plains and Mississippi Valley into the Ohio/Tennessee Valleys and the Mid-Atlantic at times, but with fairly low confidence in specifics so far. This is also a pattern that could support freezing rain with ice accumulations with the cold air in place at the surface and potential for warm moist air to flow overtop it. The placement of heaviest rain, freezing rain, and snow all remain in question given small-scale details like surface frontal positions and late week shortwave specifics remain uncertain. Recent trends have been for drier conditions over the Northeast after Monday, while periods of lake effect are possible to the lee of the northern/eastern Great Lakes. The West Coast may start to see some precipitation by the latter half of the week but with uncertainty over the southward extent and amounts. Currently the best potential for at least some rain/mountain snow is over the Pacific Northwest with moisture possibly extending as far south as northern California. Rausch/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 Hazards: - Heavy precipitation across portions of the Middle Mississippi Valley, the Lower Mississippi Valley, the Ohio Valley, and the Tennessee Valley, Tue, Jan 31. - Heavy rain across portions of the Southern Appalachians, the Lower Mississippi Valley, the Southern Plains, and the Tennessee Valley, Wed, Feb 1. - Heavy snow across portions of the Central Rockies, the Central Great Basin, and the Southwest, Mon, Jan 30. - Flooding occurring or imminent across portions of the Lower Mississippi Valley. - Flooding likely across portions of the Southeast. - Much below normal temperatures across portions of the Plains, the Central Great Basin, the Northern/Central Rockies, Middle Mississippi Valley, California, the Northern Great Basin, and the Pacific Northwest, Mon-Wed, Jan 30-Feb 1. - Much below normal temperatures across portions of the Northern/Central Plains, the Upper/Middle Mississippi Valley, the Great Lakes, and the Ohio Valley, Mon-Thu, Jan 30-Feb 2. 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