Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 AM EST Mon Feb 25 2019 Valid 12Z Thu Feb 28 2019 - 12Z Mon Mar 04 2019 ...Pattern Overview... For the period as a whole expect progressive and broadly cyclonic mean flow aloft over the Lower 48. As an upper ridge initially extending from the Gulf of Alaska into the Arctic drifts into northwestern Canada, a deep south-central Canada upper low will descend close to or across the Upper Great Lakes region during the weekend and then continue rapidly eastward. Flow around this low will likely interact with energy from a Pacific Northwest coast upper low that opens up as it ejects inland. The combination of these features should produce an Ohio Valley/Great Lakes into Northeast storm system by late Fri or Fri night into the weekend. Meanwhile another Pacific system will likely reach the West Coast by the weekend. Anticipate highest precipitation totals over favored terrain from California into the central Rockies and from the Gulf Coast region into the Northeast, while much below normal temperatures persist over portions of the central U.S. ...Guidance Evaluation/Preferences... There are still some details that remain to be resolved but in principle the guidance is gradually improving in its clustering compared to some previous days. There is good agreement on the overall evolution of ejecting Pacific Northwest upper low energy, the southern Canada upper low that may brush the Upper Great Lakes, and their interaction that should produce a storm system over the northeastern quadrant of the country by late Fri into the weekend. However medium-smaller scale detail and track/timing issues (which have lower predictability) lead to ongoing uncertainty for the precise character of the storm system. GFS and some GEFS runs have been a bit on the faster and amplified side of the envelope with the ejecting western energy, tending to result in some of the stronger solutions for the downstream surface low. ECMWF runs have been inconsistent while the past couple CMC runs have been fairly weak. The 00Z UKMET is much weaker than its prior run. Continuity provides fairly high confidence in the storm's existence and to some degree the track. However dependence of the storm's evolution on lower predictability subtle details as well as the observed inconsistency/spread of operational model runs for depth favor an intermediate solution for a single deterministic forecast. For the system forecast to approach California by the start of the weekend, solutions have maintained moderate spread for strength and track--northern solutions tending to adjust back down toward majority clustering over subsequent runs. On the other hand there has been a persistent spread for timing. The CMC has backed off a bit from its 12Z run but remains in the faster portion of the spread (along with recent UKMET runs) while the past couple of ECMWF runs are slower than earlier runs and now cluster more with GFS/GEFS runs that have generally been in the slower half of the spectrum. Behavior of guidance over recent days favors maintaining a compromise approach as the best starting point. Guidance agrees that supporting energy aloft will rapidly shear out to the east as it becomes embedded within the progressive mean flow. Based on system considerations the updated forecast started with an operational model blend for days 3-4 Thu-Fri and then adjusted toward a blend of 18Z GFS/12Z ECMWF runs and their ensemble means. ...Weather Highlights/Hazards... During Thu-Fri the initial flow of moisture over the West will bring areas of rain and higher elevation snow to about the northern 2/3 of the region with a brief drying trend progressing from west to east. The next Pacific system should start to bring moisture into California by the start of the weekend with rain/mountain snow then spreading eastward into the Rockies. For the 5-day period expect an area of enhanced precipitation from northern-central California/southwest Oregon into the north-central/central Rockies with highest totals likely over windward terrain of the Sierra Nevada range. Late this week diffuse shortwave energy may assist in producing rainfall of varying intensity over the southern half of the East while the northern fringe of the moisture shield could contain a little light snow from the Mid Mississippi Valley into the northern Mid-Atlantic. The developing storm system expected to reach the Northeast during the weekend should initially spread snow across parts of the northern Plains late this week with precipitation coverage then expanding over the eastern states. Best snowfall potential will extend from the Midwest through the Great Lakes into New England. Pockets of locally moderate-heavy rainfall may be possible over the South but thus far with no coherent signal in the guidance for precise location/magnitude. This system may produce strong winds over some areas even if not to the magnitude of the system currently affecting the Great Lakes/Northeast. Expect a period of lake effect snow in the wake of this system while lingering moisture along southern latitudes may produce some rain near the Gulf Coast into early next week. Behind the aforementioned developing storm, a strong surge of very cold air will spread south and east across the central/eastern U.S. from late week onward. The forecast has been consistent in focusing coldest anomalies (at least 30-40F below normal) over the northern Plains while high temperatures of at least 10F below normal may reach the Gulf/East coasts by next Mon. Expect the best potential for daily records to be for cold daytime highs over the northern-central Plains and Midwest during the weekend. The mean pattern will tend to keep the southern 2/3 of the West warmer than average for morning lows but near to slightly below normal for highs. Northern areas should remain below normal. The South and parts of the East will see above normal temperatures late week/early weekend ahead of the developing storm system before the trend toward colder readings. 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