365 FXUS02 KWBC 231547 PMDEPD Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1046 AM EST Wed Jan 23 2019 Valid 12Z Sat Jan 26 2019 - 12Z Wed Jan 30 2019 1530 UTC Update... Models showed relatively good consensus during the early portion of the medium range, with a significant decrease in predictability at smaller scales after Mon. A multi-model blend of deterministic guidance (ECMWF/GFS/UKMET) served as a forecast starting point during days 3-5 (Sat-Mon). This blend should resolve a low pressure system crossing the Great Lakes/Northeast during this period, and a new surface low developing across the Central U.S. in response to another shortwave digging across the Rockies/northern Plains. After Mon, while broad consensus exists that ridging will persist off the West Coast, with troughing from eastern Canada into the eastern U.S., models have shown a high degree of variability with respect to individual shortwaves traversing the eastern trough. The same holds true with the position of the upper vortex (near Hudson Bay or farther south across the Great Lakes), with significant implications on the weather impacts across the eastern half of the CONUS. Therefore, made much heavier use of ECENS/GEFS means during days 6-7 (Tue-Wed) due to significantly increased spread at medium/smaller scales. Ryan Previous Discussion (issued at 07 UTC)... ...Overview... Amplified and expansive mean troughing aloft should persist over the eastern half of the continent, between a West Coast/eastern Pacific mean ridge and a center of strong positive height anomalies over the North Atlantic. This pattern will favor periodic surges of cold air pushing south from Canada across the central/eastern U.S. The Rockies may see some of this cold air at times as well. Guidance continues to show some retrogression of the overall mean ridge which teleconnections suggest could eventually lead to some positively tilted troughing over the West. Exactly how this occurs remains quite uncertain. Also uncertain is how energy in separate streams may produce one or more low pressure systems across the Gulf of Mexico/western Atlantic with potential effects along adjacent coasts. A system tracking across the Midwest/Great Lakes/Northeast during the first half of next week may produce meaningful precipitation and exert some influence on any Atlantic features. ...Guidance Evaluation/Preferences... The overall mean pattern has good consistency/continuity but there are a number of embedded features that complicate the forecast with models and ensembles exhibiting various ideas and day-to-day adjustments. As is typically the case the solution envelope widens with time, favoring a consensus approach early in the period followed by increasing emphasis on the ensemble means. At the start of the period early Sat the blend excluded the 12Z CMC due to being a fast/weak extreme with the weak wave forecast to be over the Ohio Valley at that time. Then from Sun onward the 18Z GFS traced a wider arc with an upper low which other guidance kept over Canada instead of brushing New England by Mon--favoring the 12Z cycle for the forecast blend's GFS component. The new 00Z GFS is not as extreme as the 18Z version though by Mon it still shows more eastward elongation aloft versus most other solutions. This upper low should support a system tracking from Lake Superior into Quebec. Gulf of Alaska shortwave energy rounding the mean ridge and reaching western Canada by early Sun. As it drops south/southeast it will serve to flatten eastern U.S. flow temporarily. Over recent days there has been significant uncertainty over how eastern U.S. troughing and northern Mexico/Gulf shortwave energy may in some fashion support Gulf/western Atlantic low pressure--mainly in the Sun-Tue time frame. Yesterday's GFS/ECMWF runs began suggesting mid-latitude CONUS flow would be sufficiently progressive to deflect any developing low pressure well away from the coast. The 12Z UKMET and weaker/eastward CMC held onto the idea from earlier GEFS/ECMWF mean runs but 12Z/18Z means have reflected the GFS/ECMWF trend. Instead there is now somewhat of a delay with possible Atlantic development. The 12Z ECMWF scenario of Atlantic and Midwest systems merging over New England has low confidence but the ECMWF mean hints at it and for the purposes of the manual forecast represents the ongoing potential for an Atlantic system. As for the northern Plains into Northeast system late weekend into next week, the 12Z GFS was closer to the majority cluster versus the farther south 18Z GFS. New guidance still shows a moderate amount of north-south spread. Spaghetti plots and operational model comparisons become increasingly diverse over the eastern Pacific/western U.S. toward midweek with a variety of possible outcomes for the next bundle of energy heading into/around the mean ridge. This is in addition to the 18Z GFS that is in the minority for how much westward elongation occurs with the preceding shortwave. Teleconnection-favored flow suggests some potential for shortwave energy to reach the West but confidence in how this occurs is low. ...Weather Highlights/Hazards... The northern Plains through the Northeast will see one or more periods of snow during the period. Two systems will provide synoptic focus. The first should develop over Lake Superior by Sun and then track into eastern Canada, while the second will likely enter the northern Plains by late in the weekend and then progress across the Ohio Valley or lower Great Lakes into New England. There may be a band of significant snowfall just north of the low track but there is still a decent amount of spread among model/ensemble guidance for exactly how much liquid this system will produce. Trailing cold cyclonic flow should promote areas of lake effect snow. Southern stream energy will produce a brief period of enhanced rainfall primarily over southern Texas. Then waviness progressing across the southern half of the Gulf may produce a period of locally heavy rainfall over parts of the Florida Peninsula. Trends have lowered the potential for meaningful precipitation along Southeast/Mid-Atlantic coast compared to previous days but some combination of Atlantic low pressure and the early week Midwest system may still bring snow into the Northeast. Farther west expect an area of snow to drop southward through the Rockies, primarily in the Sun-Mon time frame. The Midwest/Upper Mississippi Valley into western Great Lakes region continues to be the most likely area for coldest temperature anomalies with readings 15-25F below normal most likely on Sat and then over a larger area extending south/southeast with another cold surge Tue-Wed. The High Plains should see a brief period of well above normal temperatures (by 10-25F) especially on Sun before colder air drops southward and brings temperatures to 5-15F below normal over the Rockies/High Plains. The West Coast states will remain on the warm side with best potential for plus 10F or greater anomalies during the first half of the forecast period. Rausch WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation and winter weather outlook probabilities are found 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 $$