Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 PM EST Wed Jan 25 2023 Valid 12Z Sat Jan 28 2023 - 12Z Wed Feb 01 2023 ***Coldest weather so far this month expected across the Dakotas and the Upper Midwest starting this weekend*** ...Synoptic Overview... Guidance agrees on a large scale pattern featuring a positively tilted mean trough over the lower 48, between an amplified eastern Pacific into Alaska upper ridge and another upper ridge expected to meander over the Bahamas and Cuba. This pattern will support persistently cold temperatures from the northern/central Rockies into the Upper Midwest, while the West will trend colder (with a period of terrain-enhanced precipitation) as a shortwave dropping into the western side of the mean trough pushes a cold front southward through the region. The Bahamas/Cuba upper ridge will tend to keep the Southeast on the warmer side of normal, especially after the weekend. Expect the southeastern quadrant of the country to see the highest rainfall totals during the period as shortwave energy interacts with Gulf moisture and one or more surface fronts becoming parallel to the west-southwesterly mean flow aloft east of the Rockies. ...Guidance/Predictability Assessment... The primary forecast issue involves the vigorous shortwave dropping southward along and near the West Coast during the period, with varying ideas for timing and amplitude. In the 00Z/06Z guidance the UKMET leaned to the slower/westward amplified side toward the end of its run on Monday while GFS runs leaned to the more amplified side (while also carrying a more persistent upper low) by days 5-7 Monday-Wednesday. Other models have been showing potential for an upper low but one that would be more transient and with a farther northeast track, if it forms at all. Latest GEFS/ECens/CMCens means lean much more toward the 00Z ECMWF/CMC and new 12Z ECMWF/CMC. The GFS is acceptable as part of a blend though. While the overall clustering of guidance is better than average for a days 6-7 forecast at the moment, GFS runs over the past couple days have been highly variable with how progressive this western shortwave will be and even the ECMWF was a lot faster a couple days ago after stabilizing upon the current scenario starting with the 00Z/24 run. The model variability and full ensemble spread offer the potential for some aspects of the forecast to go astray again for a time in the near future. There are also uncertainties with how northern stream flow could influence eastern U.S. surface waviness toward next Wednesday. One other note is that the 00Z GFS raised an added question mark, regarding evolution of the Alaska upper ridge and nearby shortwave energy late in the period. That run become more closed with the Alaska ridge and ultimately allowed more Pacific flow to reach the Northwest U.S. by midweek. Thus far the 00Z GFS scenario is very much in the minority in that regard. The updated forecast maintained the general approach of an operational model composite early (00Z ECMWF, 06Z GFS, 00Z CMC, 00Z UKMET from more to less weight) and then a trend to a model/ensemble mean mix. This yielded good continuity for the overall pattern albeit with some typical run-to-run adjustments for the corresponding surface reflection. ...Weather/Hazard Highlights... Behind a cold front anchored by a departing eastern Canada surface low, strong high pressure building into areas from the north-central U.S. into northern Rockies will bring much below normal temperatures from the weekend through at least the first half of next week. There may be one or more days with highs as much as 30-40F below normal from the central High Plains into Montana, with Sunday-Monday expected to be the coldest days. This would be the coldest weather since Christmas for this region, with locations from eastern Montana to northern Minnesota likely remaining below zero for highs Saturday or Sunday through Monday. Some of this cold weather should also reach into the Northwest/northern Intermountain West with some locations 15-25F below normal by late weekend into the next work week. Southern parts of the West will trend colder as well, with decent coverage of highs 10-20F below normal by Monday-Wednesday. Upper trough passage combined with low level upslope flow will produce moderate to locally heavy terrain enhanced snows across north-central parts of the Intermountain West/northern Rockies/western High Plains Saturday and Sunday. While some uncertainty remains with the evolution of the upper-level pattern over the western U.S., rain and mountain snow prospects should generally increase with time in California and the Southwest as the Pacific cold front drops southward, with at least some higher elevation/interior snowfall and rain for the southern California coast and Desert Southwest. There may be some localized terrain enhancement but this event should have a lot less moisture to work with than many recent precipitation episodes. Weak shortwave energy initially ejecting from the southwestern U.S. will likely support one or more surface waves from the southern Plains/Gulf Coast into the East during the weekend into early next week. Gulf moisture return may support a corridor of moderate to heavy rainfall over portions of the central Gulf Coast states and vicinity Saturday and Sunday, with more modest rains spreading northward over the east-central U.S. into/ahead of a wavy front. The Day 5 (12Z Sunday-12Z Monday) Excessive Rainfall Outlook maintains a Slight Risk area from southern Louisiana to the far western Florida Panhandle, where multiple rounds of heavy rainfall will be possible. Somewhat lesser probabilities of heavy rainfall exist over a larger portion of the Southeast. Another wave near the central Gulf Coast will increase rain chances again later in the period by Tuesday or Wednesday, with potential sensitivity for areas where the two episodes of rainfall could overlap. Meanwhile winter weather will be possible farther north, with locations from the Midwest into New England generally in the most favored axis for snowfall. Sufficient persistence of cold air by late in the period could increase the possibility for a transition zone of freezing rain/sleet between the rain and snow areas but any specifics have low confidence at this time. Rausch/Hamrick 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