Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 406 PM EST Fri Mar 01 2019 Valid 00Z Sat Mar 02 2019 - 00Z Tue Mar 05 2019 ...Northern Mid-Atlantic and Northeast... Back to back lows are expected to track up the northeast coast from the Mid-Atlantic through Monday with the second low stronger and tracking more inland. Days 1-2... A shortwave trough moving across the southern Appalachians this afternoon will develop a surface low off the Delmarva coast tonight and track near the Benchmark Saturday morning. This occurs in conjunction with an upper level jet streak lifting out to the New England ahead of the system, while a northern stream shortwave trough crosses the Great Lakes. There is still a split in 12Z precip guidance for Long Island and New England with the GFS farthest inland and the UKMET more out to sea and the ECMWF in between with differences due to interaction with the shortwaves, and strength of the surface low. Snow is expected to develop tonight in the persistent cold air damming wedge near the PA/MD border with a swath on the northern side of the precip shield northeast across coastal New England through Saturday. As the surface low deepens a deformation band will develop NW of the mid-level center which may track along or just inland the SNE coast. This will enhance snowfall, and Day 1 WPC probabilities remain moderate for six inches from NE CT toward Boston, with low probabilities for 8 inches just SW of Boston. Moderate probabilities for 4 inches stretch now from southern coastal ME to the NY/NJ border. This will be highly dependent on the exact track of the surface low, and how much interaction occurs with the northern stream shortwave trough moving in from the west. Flow overrunning the CAD wedge also brings the risk of freezing rain with Day 1 WPC probabilities for a tenth inch from the Potomac Highlands northeast to coastal CT. Days 2-3... A stronger low, currently still well west of CA will pivot over the southeastern CONUS Sunday and track quickly up the northeast coast through Monday. This second system will be more developed with the surface low originating in the southern stream trough in east TX. Timing of phasing with the northern stream longwave trough is key for track with the consensus having it more inland/closer to the New England coast than the first low with more Gulf and Gulf stream moisture to work with. Moderate probabilities for six or more inches are in Days 2.5 and 3 in a swath from the OH/WV border and central Appalachians northeast across PA to interior New England and eastern ME. Southeast of this swath are low probabilities for a tenth inch of ice extending northeast from the Shenandoah Valley to southern New England. ...California across Great Basin to Central Rockies... Days 1-3... Southern stream low pressure undercutting the eastern Pacific ridge that extends into AK will approach the CA coast near the Bay area tonight. Mid and upper level energy from this low will race inland on a 130kt zonal jet with the mid-level shortwave trough axis crossing the central Rockies Saturday evening. The ample Pacific Jet energy will drive above normal moisture across the CA coast, Sierra Nevada, Great Basin and reach the central Rockies all on Day 1. Moderate precipitation is expected with this wave and snow levels will remain fairly high through Day 1, 4000-7000 feet along 40N latitude. The fairly tight moisture corridor will allow a frontal zone to develop and drift south through the period. Forcing from the initial trough and subsequent shortwave troughs through Day 3 will maintain heavy snow threats in this corridor, with impressive totals for Days 1 and 2. In particular, Day 1.5 probabilities are moderate for 18 inches in the southern Sierra Nevada and for the central Rockies. Lighter accumulations are expected in the lower mountains along this swath with moderate to high probabilities for 6 inches. Low development over southern BC Sunday directs northern stream shortwaves from the eastern Pacific ridge across the Intermountain west along the frontal zone through Sunday night before mid-level ridging finally cuts off moisture inflow on Monday. ...Central Plains to Ohio Valley... Days 2-3... A mid/upper level trough ejecting eastward from CA Saturday will ride a 130+kt Pacific jet across the central/southern Rockies Saturday night and across the southern Plains Sunday. At the same time, a cold front will be dropping through the Plains producing a tightening baroclinic gradient. This baroclinic gradient will be acted upon by the aforementioned upper forcing and an increasingly coupled jet structure to produce surface cyclogenesis which will begin late Saturday night over the TX Panhandle. Mid-level confluence will drive Pacific moisture into the southern Plains which will be aided by an open Gulf from high pressure over the eastern Gulf. Increasing isentropic ascent on low-level warm advection and intensifying mid-level frontogenesis will interact with the mid-level impulse and result with a deep DGZ allowing bands of heavy snow to develop and intensity as the push east from eastern CO and cross KS Saturday night. Surface low development over the ArkLaTex Sunday morning focuses snow over southwest MO before quickly spreading east across the lower and central OH Valley Sunday. Moderate to high Day 2 probabilities for 6 inches are over SW MO with lower probabilities over the lower OH Valley due to the speed of the system. Day 2.5 probabilities feature moderate 6 inch probabilities again from eastern KY to SW PA. Isentropic lift atop the cold surface high will likely produce freezing rain over the southern Plains with moderate Day 2 probabilities for a tenth inch of ice over central OK spreading northeast to the MO/AR border. Jackson