Extended Forecast Discussion NWS Weather Prediction Center College Park MD 159 PM EST Sat Jan 22 2022 Valid 12Z Tue Jan 25 2022 - 12Z Sat Jan 29 2022 1900 UTC Update... The latest guidance for today continues to show relatively good agreement on the large scale pattern during the medium range, but continued significant uncertainties in the details, especially concerning western Atlantic low pressure next weekend. The past several runs of both the GFS and ECMWF have shown very inconsistent surface low positions. The 00z/12z ECMWF both show a more offshore surface low, but yesterdays 12z ECMWF was much farther to the southwest hugging the Southeast U.S. coast. Meanwhile, the GFS run had been fairly consistent showing a slower/deeper solution near the Southeast coast, except for today's 06z run which slingshot the low well into New England (12z solution back near the Southeast). The CMC is also farther north like the 06z GFS run. Needless to say, it appears an ensemble mean solution seems to be the best way to mitigate these uncertainties and conveniently, the means do show a very middle ground solution. Otherwise, no major changes to the overnight forecast was needed and so the previous discussion still stands. Todays WPC progs used a deterministic model blend for the first half of the period (sans the UKMET which was much quicker off the bat with the first southern stream shortwave), and trended towards a 50/50 blend of the ECMWF/GFS and the ensemble means (with a little more weighting towards the ECMWF/ECENS). See previous discussion below for more details in the pattern and weather threats. Santorelli Previous Discussion issued at 0700 UTC... ...Overview... Guidance continues to show a strong eastern Pacific ridge helping to keep a broad mean trough across a majority of the lower 48 through Wednesday. The Pacific ridge will likely drift into the West by Thursday-Friday, followed by some weakening depending on very uncertain details of upstream shortwave energy. Meanwhile within the lower 48 mean trough, one shortwave will rotate through the eastern half of the country during the first half of the period and then another shortwave dropping into the central U.S. late in the week may reach near the East Coast by early Saturday. There is a decent signal that this latter shortwave will support a developing East Coast/western Atlantic storm by next weekend but with low confidence in details. The pattern through the period will generally favor below to well below normal temperatures over the eastern half of the country and mild temperatures from the West into the northern/central Plains. Precipitation coverage and intensity across the lower 48 will tend to be low aside from an area of moisture crossing the East on Tuesday and then possibly over parts of the South and East by the end of the week/next weekend depending on details of upper flow and surface development. ...Guidance Evaluation/Predictability Assessment... At the start of the period, guidance still varies somewhat on the fine-scale details of initial northern Gulf of Mexico low pressure quickly tracking into the Atlantic as supporting dynamics shear out within accelerating flow aloft. A blend approach maintains the prevailing theme of the evolution. Then continuity and agreement are decent with a Canadian system dropping into the Great Lakes by late in the week and then weakening. The rest of the late-period forecast becomes more uncertain as models and ensembles diverge considerably for eastern Pacific/western U.S. details. With some possible influence from these issues, the evolution over and near the East by late Friday-Saturday also displays a fair degree of uncertainty. In contrast to previous trend, over the past day guidance has trended stronger with the upper ridge reaching the West late in the week and increasingly varied with how energy within the shortwave approaching from the west may separate (as illustrated most simply by the 12Z and new 00Z ECMWF runs). By day 7 Saturday the end result over the West is a chaotic ensemble spaghetti plot with anything between a shortwave or lingering ridge a possibility. The 12Z ECMWF mean's shortwave did not have much agreement among other 12Z/18Z models and means but the new 00Z CMC has come in with a similar feature and the new ECMWF has a fairly strong shortwave. There is a somewhat better suggestion that energy rounding the ridge may support a wave/front dropping into the northern tier on Saturday. The most common theme farther east is for low pressure to be organizing over the western Atlantic by early Saturday. The 18Z/00Z GFS runs are closest to the relatively higher density of ensemble surface lows at that time while recent CMC runs have been somewhat in the neighborhood. The 12Z GFS/ECMWF were on the southwestern edge of the spread. The new 00Z ECMWF is closer to the majority cluster in principle. Based on latest guidance comparisons, the updated forecast reflected a blend of 12Z/18Z operational models early in the period and then transitioned toward greater weight of the GEFS/ECens/CMCens due to the uncertainty of upper details over the West toward the end of the period and to yield the highest probability track for western Atlantic low pressure by Saturday. Manual depiction of this low will likely trend deeper as confidence in strength and track improves. ...Weather Highlights/Threats... The combination of a northern Gulf of Mexico wave quickly moving into the Atlantic and a cold front crossing the East will spread an area of precipitation across the East on Tuesday. Most amounts will be in the light to moderate range, with some proportion of the precipitation possibly being in the form of snow from the Ohio Valley/central Appalachians into the Northeast. A period of lake effect snow will be possible behind the front. A trailing wave/front may spread areas of light snow from the northern Plains into the Great Lakes and Northeast from about midweek into the weekend. Supporting dynamics could be amplified enough to support some precipitation farther south as well but with low confidence over coverage and amounts. By next Saturday the coverage/intensity/type of precipitation along the East Coast are likewise uncertain at this time and will require close monitoring. Parts of the High Plains may see light snow on Tuesday. Much of the West should be dry during the period. Some moisture could eventually reach the Pacific Northwest by next Saturday. A surge of cold air will initially be over the Upper Mississippi Valley and Midwest on Tuesday, with some areas seeing temperatures 20-25F below normal from Tuesday into early Wednesday. This cold will then spread toward/into the South and East with time, bringing a day or so of readings that are 10-20F above normal. By Friday-Saturday the Mid-Atlantic and Northeast may moderate some while parts of the Southeast may see highs at least 10F below normal. After less extreme cold passes through the central/southern Plains early in the period, most of the Plains will see a warming trend with some areas seeing temperatures at least 10F above normal. The West will see moderately above normal temperatures next week with some localized plus 10-15F anomalies for highs. Rausch 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