Extended Forecast Discussion NWS Weather Prediction Center College Park MD 210 PM EST Sat Jan 13 2024 Valid 12Z Tue Jan 16 2024 - 12Z Sat Jan 20 2024 ...Hazardous and record-breaking cold from an Arctic airmass will spread across the Northwest into the central and east-central U.S. next week... ...Potential for an impactful storm system near the East Coast next Tuesday but still with low confidence... ...Overview... Latest guidance maintains a deep and broad mean trough aloft over much of the lower 48 through much of the week, ahead of the upstream mean ridge over the eastern Pacific tracking into the western U.S. late week into the weekend. This upper pattern and Arctic high pressure filtering in at the surface will support very cold temperatures over areas from the Northwest through much of the Plains/Mississippi Valley into Tuesday, with numerous daily records for cold lows/daytime highs. By Wednesday-Thursday there should be some moderation and eastward shift of the cold temperatures before another surge of cold air pushes into the central U.S. and eventually the East late week into Saturday. At most locations this second round of cold temperatures should be less extreme than the the first. Precipitation-wise, meaningful snow is still possible in parts of the Mid-Atlantic/Northeast on Tuesday, though with continued uncertainty in amounts. The East may see another episode of precipitation late in the week but again with considerable uncertainty for details. Meanwhile the northern half or so of the West should see increasing precipitation chances Tuesday and beyond, first with a system affecting the Pacific Northwest and then from moisture on the leading side of an approaching upper trough. ...Guidance/Predictability Assessment... Latest models/means agree well on the large scale evolution during the period. During Tuesday-Wednesday expect a leading deep trough crossing the East during Tuesday-Wednesday, supporting low pressure that rapidly tracks northeastward from near the Southeast Coast, while a gradually improving consensus shows a complex upstream interaction of elongating western Canada into Pacific shortwave energy to the north of a piece of mid-latitude energy that breaks off from the east side of a mid-latitude Pacific upper low--bringing surface low pressure into Vancouver Island or a little south. After Wednesday, guidance has been very consistent with the combination of this energy and southern Canada energy amplifying into the East as the West Coast ridge rebuilds and drifts inland while an eastern Pacific trough approaches the West Coast. Typical differences in upper trough details over the East late in the week lead to meaningful spread for surface low/frontal details and associated precipitation. Near the East Coast on Tuesday, the past 24 hours of GFS runs and GEFS/ECens/CMCens means have gravitated more offshore toward the ECMWF that had first signaled the possibility. The 00Z ECMWF-initialized machine learning models also fall in line with the GFS/ECMWF/ensemble mean cluster and yesterday's machine learning runs seemed to provide an early lead on the faster trends seen in latest guidance. While trends would suggest this cluster as the most probable at the moment, there are still lingering farther west tracks. This includes the CMC (now closer to consensus from 24-36 hours ago) and the UKMET that has consistently been a western extreme. From the short-range realm, the Canadian RegGEM and NAM also have a fairly western track. Finally there is the occasional historical tendency for guidance sometimes to back off from an East Coast system in the early-middle part of the medium range period before returning westward in the shorter term. Preference is to stay close to continuity near the GFS/ECMWF/ensemble mean cluster but with modest confidence. Guidance is steadily improving its agreement for the evolution affecting the Pacific Northwest into midweek. After the 12Z/12 GFS finally latched onto the majority evolution aloft, solutions since then have still varied with the north-south track of low pressure with the 00Z machine learning models also showing this spread. However the new 12Z runs have narrowed the spread, depicting a fairly compact but vigorous system reaching Vancouver Island or northern Washington. The 00Z CMC represented a southern extreme and the 00Z ECMWF was on the deep edge of the envelope. A 00Z/06Z model composite reflected a good intermediate solution for this part of the forecast. With good large scale agreement for the rebuilding upper ridge that moves into the West as well as the trough nearing the West Coast (aside from low-predictability details), primary focus for uncertainty after midweek is on how incoming western energy and southern Canada dynamics evolve as the amplify into the East along with the resulting surface evolution. The 00Z CMC was slowest with the upper trough especially around Thursday-Friday and thus was phased out of the forecast blend. The new 12Z CMC compares better to consensus. Otherwise confidence is low for specifics. Dynamical models/ensembles and machine learning models suggest potential for defined low pressure to be anywhere from the Southeast into Upper Ohio Valley or farther east over the western Atlantic as of early Friday. By Saturday the ensemble means and machine learning models generally show faster progression of low pressure than seen in latest GFS/ECMWF runs. Thus preferred to tilt the updated forecast somewhat more toward the GEFS/ECens/CMCens means (with some lingering GFS/ECMWF input for definition) by next Saturday. ...Weather/Hazards Highlights... Very cold air originally stemming from the Arctic will be atop the northern Great Basin and Rockies and southeastward through the central U.S. on Tuesday, though starting to moderate a bit from the short range period. Still, widespread temperatures of 20 to 40 degrees below normal are forecast, with low temperatures in the -10 to -20 range across the northern half of the Plains and into the Upper Midwest, and locally even colder in higher elevations of the Rockies. High temperatures at least look to be above 0F in most areas by Tuesday. Any wind chills would be even colder and thus more dangerous. The cold air diving farther south into the southern half of the Plains and Mississippi Valley will not be as cold in terms of absolute temperatures, but temperature anomalies of 20 to 40 degrees below average are likely to set widespread records across the South into Tuesday. By Wednesday temperatures finally look to moderate in the Rockies to High Plains, though chilly temperatures (albeit in moderated form) will shift into the eastern half of the country before warming closer to normal on Thursday. Locations over the Lower Mississippi Valley and western Gulf Coast may see record lows Wednesday morning. Reinforcing cold air should reach the northern Plains by Thursday and expand to include most areas between the Rockies and Appalachians Friday and farther east by Saturday. Expect less extreme temperature anomalies than those forecast earlier in the period, but Friday and Saturday may still see temperatures 10-25F or so below normal. A low pressure system developing along a front in the western Atlantic will track quickly northward Tuesday into Wednesday. Snow could spread across the Mid-Atlantic and Northeast to the west of the low track, but snow amounts and placement/coverage will be very sensitive to this storm track and strength. The general model trend over recent cycles has been for a farther east/more offshore low track, plus with a faster speed that would also lessen totals. Therefore the latest probabilistic guidance for 0.25 inch liquid in the form of snow has declined to only 10-30 percent along the northern Mid-Atlantic/New England coast. However there is still a lingering minority scenario maintaining a far enough west track to produce greater snowfall than the currently preferred solution so continue to monitor forecasts for any additional changes. Farther south, moisture may focus along and ahead of the front that moves across Florida on Tuesday. There is perhaps a nonzero chance of heavy rainfall, but does not look impactful enough for an area in the Excessive Rainfall Outlook Tuesday. Back to the north, expect continued periods of lake effect snow with persistence of cold air and breezy conditions. Then a front and possible low pressure system reaching the East late in the week may produce some precipitation. This system could initially spread some light snow across the northern-central Plains by Thursday, with increasing uncertainty for coverage and amounts of precipitation over the East late in the week. Currently expect most of this activity to be snow over and north of the Ohio Valley/northern Mid-Atlantic with perhaps some rain over the Southeast. Expect moisture to increase over the Northwest around Tuesday, with steadily increasing confidence that fairly vigorous low pressure tracking toward or near Vancouver Island by Wednesday will help to enhance amounts. This event will likely see snow confined to higher elevations than in the shorter term, but inland areas where cold air persists longer could still see some lower elevation winter weather issues. Expect this system to spread some enhanced snow into the northern-central Rockies as well. Precipitation is likely for the northern half or so of the West into Wednesday-Thursday, while coastal areas should generally see the better precipitation focus late week into the weekend as a Pacific upper trough approaches. In terms of the Days 4-5 (Tuesday-Wednesday) Excessive Rainfall Outlooks, both days maintain no risk areas for the time being but there may eventually be enough of a signal to merit a Marginal Risk area somewhere over the Pacific Northwest. Overall the combined magnitude/duration of moisture anomalies from most guidance tempers confidence in the highest end of the QPF spread, but improving consensus for supporting low pressure does raise the potential for a brief period of enhanced focus which may be split between the Day 4 and Day 5 periods. Emerging themes from latest model runs seem to suggest a greater rainfall potential for western Washington south into parts of Oregon for Day 5/Wednesday versus broader north-south coverage with lesser totals on Day 4. 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 WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation forecast, excessive rainfall outlook, winter weather outlook probabilities, heat indices and Key Messages 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 https://www.wpc.ncep.noaa.gov/#page=ovw