Major Lake Effect Snow Outbreak...
"Lake Effect Outbreak of December 5-9th"
A multi-day extended northwest, cyclonic flow will cause a large lake effect snow outbreak over much of the Pennsylvania/Maryland snowbelts. Additionally the long fetch may cause several coatings to a light accumulation downwind of the Appalachians. A weak shortwave (Alberta Clipper) is currently moving well south of the Middle Atlantic. This is causing a large area of light to moderate snowfall over West Virginia, southern Virginia, and parts of North Carolina. This will cause some weak upper level ridging over the northern Middle Atlantic with an area of subsidence. This will bring a quick end to the lake effect snow showers for the first half of Saturday night despite a steady northwest flow. Lows Saturday night won't drop too much with some stratocumulus cloud cover and 5-10mph winds mixing the surface. The clipper will move offshore by late Saturday and redevelop as a relatively strong 1000mb coastal low. The low will undergo rapid cyclongenesis, but several hundred miles well off the coast. The low will deepen to 976mb by late Sunday and begin to retrograde towards Nova Scotia and the Gulf of Maine. The 968mb low will track northwest into Nova Scotia scraping parts of the northern half of Maine with a moderate snowfall. This retrograding, cutoff low will set the stage for a dominant cyclonic flow over the entire Northeast. 700mb RH values indicate increasing humidity values aloft by Sunday morning under a 310 degree trajectory. Organized snow bands will begin to develop downwind of Lake Erie across northwestern Pennsylvania. GFS bufkit indicates convective temperatures being reached by mid morning for most areas on Sunday allowing for daytime instability to fuel cellular snow showers over the rest of western and central Pennsylvania. Across the Laurel Highlands, moderate orographic lift, upslope snow will begin with daytime accumulations around 1-4in. Initially the bands over northwestern Pennsylvania will be relatively light, but increase in intensity with better dendritic growth and excellent Omega values later in the day. MOS guidance indicates highs below freezing for many areas with mid to upper 30s elsewhere for the entire northern Middle Atlantic. But looking at the H85 thermals at nearly -10C, MOS/MAV may be running a bit too warmer. I would tad highs lower just a degree or two especially considering any evaporational cooling from snow showers and widespread stratocumulus cloud cover.
By late Sunday afternoon, a tightning pressure gradient and strong atmospheric mixing will promote increasingly gusty winds. Winds at the surface may gust upwards of 30-35mph during the day with advisory criteria winds along the ridgetops of nearly 50-55mph especially in the Laurel Highlands and across western Maryland. Increasing moisture from the Great Lakes will continue to feed and strengthen the lake effect snow bands over northwestern Pennsylvania. By Sunday evening the flow will migrate to a 320 trajectory promoting a clear Huron-Erie connection allowing for long-extensive bands well inland across the Alleghanies. With gusty winds and falling temperatures by Sunday evening, near blizzard conditions will occur across the snow belts with blowing and drifting snow as visibility drops to near zero. Snowfall rates will increase to nearly 1-2in per hour in the heaviest bands. Monday will feature the passing a weak vortex of energy just north of Pennsylvania. This will increase the instability allowing for the heaviest of snow bands to fall Monday. Guidance also suggests widespread flurries and snow showers over much of Pennsylvania, western Maryland, and northern Maryland near the Mason-Dixon line. As the vortex passes by, high resolution models indicate a dominant lake effect band forming from Meadville to Du Bois to Philipsburg to State College and possibly as far south as Harrisburg. This common 322 streamer is most likely at some point Monday or Monday night and may cause light snow accumulations even into the Lower Susquehanna Valley of central Pennsylvania. The setup reminds me a bit of November 2008 when a vortex of energy passed through Pennsylvania allowing for a long lake effect snow streamer to reach well across Pennsylvania. Link. Exact placement and development of this band still remains highly uncertain. Monday evening will continue the upslope moderate to heavy snow over the Laurel Highlands with bands over northwestern Pennsylvania stretching southeast. Also this northwest flow will favor lake effect streamers from Ontario reaching southward across the Finger Lakes Region down into northeastern Pennsylvania. Some areas in northern Susquehanna and Wayne Counties may see 3-5in by mid week. Upslope snows will also occur in the Poconos. I have found often the Mt. Holly radar is not sensitive enough to pick up on the accumulating snows over the Poconos in lake effect situations when in fact they can receive 1-3in. H85s will drop to nearly -13C by Tuesday and Wednesday with advecting drier air. Lake effect snows will decrease in areal coverage, but continue to add light snow accumulations.
Total snow accumulations will be highly variable as typical with lake effect snow. Areas across the plateau of southern Erie county and Crawford county will likely see the highest accumulations, 12-24in, with isolated 30in areas. Across the Laurel Highlands and western Maryland upslope totals will be on the order of 6in to possibly over one foot especially for favored areas in southern Somerset County near Laurel Summit and Mt. Davis. Northeastern Pennsylvania will see 1-4in with the Poconos having similar amounts. Across the Pittsburgh metro area, midweek cumulative totals will be also 1-4in. For the ridge and valley region into east central Pennsylvania accumulations will not be widespread, but patchy coatings to two inches+ are possible in isolated areas. By Wednesday high pressure will take control slowly bringing an end to the extended snows, high winds, and widespread stratocumulus deck. Most of the week will be typical of a northwest flow with highs only reaching above freezing across Maryland, Delaware, and extreme southern Pennsylvania.
A quick look at the progression of the banding... High resolution NMM and NAM models show two dominant bands for late Sunday one from Erie County to Forest County to Clearfield County. The second band a bit farther south into Butler County stretching into the Laurel Highlands. Both bands show Huron-Erie connections. Then it appears a dominant band forms towards Monday or Monday night along rt 322/22 possibly stretching as far south as east-central Pennsylvania. These bands will the heaviest producing for accumulations over northwestern Pennsylvania. The Laurel Highlands will generally see upsloping snows with embedded heavier bands. Also a few streamers may stretch from Ontario into northeastern Pennsylvania.
"Current Lake Erie Wind Direction and Speed"
(Courtesy of NOAA)
"Current Lake Erie Water Temperature"
(Courtesy of NOAA)
"Lake Effect Snow Impacts"
1. Steady due northwest 310-320 degree flow
2. 20-25F land/water temperature contrast along lake shore
3. Extended four to five days of organized bands
4. Possibility for a few bands to extend into Ridge and Valley Region of Pennsylvania with C-2in of snow
5. Heaviest accumulations in higher elevations of southern Crawford and Erie counties up to 25-28in of snow.
(Courtesy of NOAA)
(Courtesy of NOAA)
"Selected City Accumulations"
Erie- 10-15in with higher amounts southeast of the city (12-24in)
Meadville- 10-15in of snow
Bradford- 8-14in of snow
Butler- 3-5in of snow
Pittsburgh- 1-4in of snow
Latrobe- 2-5in of snow
Indiana- 3-6in of snow
Johnstown- 9-15in of snow
Somerset- 8-14in of snow
Altoona- 2-4in of snow
Du Bois/Clearfield- 3-6in of snow with locally higher amounts
Philipsburg- 3-5in of snow
State College- 1-4in of snow
Lock Haven- 2-4in of snow
Williamsport- Locally 1-3in of snow
Mt. Pocono- 1-4in of snow
Selinsgrove- Locally 1-3in of snow
Harrisburg- C-2in of snow
Hagerstown- Locally C-2in of snow
Cumberland- 1-3in of snow
Frostburg- 4-8in of snow
McHenry- 6-12in of snow
Oakland- 7-14in of snow
*This will be a long duration snow event. These forecasted snow totals are a cumulative snow total forecast over the four to five day period. Also areas downwind of the forecasted 1-3in of snow will see spotty coatings to two inches.
"Here northeast of Harrisburg 2010-2011 winter statistics"
Current Snow Cover- Trace
Monthly Total (November)- Trace
Monthly Total (December)- 0.5in
Seasonal Total- 0.5in
Winter Weather Advisories- 0
Winter Storm Warnings- 0
Ice Storm Warnings- 0
Blizzard Warnings- 0
Freezing Rain Advisories- 0
Winter Storm Watches- 0
Lowest High Temperature- 30.5F
Lowest Low Temperature- 15.3F
Wind Chill Advisories- 0
Wind Chill Warnings- 0
(Snow Storms Stats)
First accumulating snow- December 10 0.50in
"Local Harrisburg Radar"
(Courtesy of WGAL)