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Self introduction and introduction of topic.
Hybrid-moderate to high shear >30 degrees of shear, limited pre-event snow band, but when arctic boundary interacts with lake moisture band intensities and produces short duration/heavy snow squalls (near zero vis) Quick moving and snowfall generally under warning criteria.
Traditional- well established quasi stationary for several hours, single lake effect snow band on 260 flow with strong cloud layer winds 40 knots to advect snow squalls 200-300km downwind of lake Ontario into our forecast area. Interacts with surface boundary to produce significant snowfall rates and near zero vis. Several locations usually receive warning criteria snowfall from SLV to Northern Green Mtns. SW orientation of the ausable river valley helps to funnel lake effect band toward PLB with that area receiving advisory criteria snowfall. Elsewhere, Champlain Valley locations usually receive less than advisory criteria snowfall
Data was gathered from Local Storm data, WFO BUF website, or Wes Cases
Study only used data for SW flow events, didn’t examine due west events that impact southern Green Mtns near Killington
33 events since 1998 across the WFO BTV CWA
Most events concentrated across the SLV/Western Dacks
Advisory and warning criteria snowfall most common across SLV, Northern Dacks, and northern western slopes of the green MTNS -Advisory snow occurred several times at PLB due to low level channeling of the lake effect band in the ausable/Saranac river valleys
16 advisory type events
8 warning type events
24 out of 33 events required a highlight
How far inland will band produce heavy snowfall rates with reduced vis?
Products to issue (mention in zones, then issued nowcast, heard about traffic accidents issued SPS, then received reports of 4 to 5 inches of snow issued lake effect snow advisory
Timing of heaviest snowfall associated with lake effect along the sfc boundary
How quickly the heaviest snow band moves across our forecast area?
The impacts of mountains and valleys (upslope/downslope )flow and enhancement due to funneling of bands
Strong low level winds>20 knots can lead to snow crystal fragmentation
Sustained Surface winds >18 knots can move ice crystals at the sfc, which fractures the crystal and causes compaction and decreases accumulation. Roebber/Schultz
Used 1000-700mb RH/Lapse Rates because it represents deeper layer in the atmosphere and the importance of deep layer moisture and instability.
Many days 1000-850mb lapse rates are steep during the late fall early winter months due to surface heating and cold air aloft.
Also, need moisture favorable snow growth region for development of snow flakes.
Used in cloud shear?
CCL –convective condsensation level ( the level at which condensation occurs due to convection Expected cloud base)
Used CCL because we are treating lake effect band like convections.
CCL ave low level dwpt lifted along mixing ratio line to intersect the temp curve (Convective Condsensation Level) Parcel becmgs saturated
The Ausable and Saranac River Valleys help to funnel lake effect band down into the Champlain valley. This especially impacts PLB. PLB during this event received 4.3 inches of snowfall in 1 hours with near zero vis. This un interrupted band from trrn will transverses across Lake Champlain Valley and be enhance by the Green Mountains in VT
Several embedded 5h vorts along with sfc convergence helps to advect significant lake effect snow band several 100 kms downwind of lake Ontario. -Well established upper level low pres at 85/7/50h limits low level shear and helps to channel band downwind.
Strong 850mb winds of 30 to 40 knots help to transport significant lake effect snow 200-300km downwind of Lake Ontario
Sfc cold frnt helps to advect pre-established lake effect band 200-300km downwind of lake Ontario. Helps hold together band with associated low level moisture convergence
-This is noticeable on radar returns
Also, upper level lift provided by 5h vort helps to hold together lake effect band several hundred km’s down stream of Lake Ontario
Steep low level lapse rates through 850mb
Uni-directional wind field through 3.5 kms
Very strong low level wind field
-Super Adabatic lapse rate in very low levels
 LCL (Lifted Condensation Level) with the CCL (Convective Condensation Level). They often ask "why are the LCL and CCL at different levels in the troposphere? What about the rising process makes them different?" The primary difference has to do with the surface temperature. A LCL occurs when forced lifting occurs. A surface parcel, with its temperature and dewpoint are forced into the vertical by a trigger mechanism such as a front, vort max, dryline bulge, convergence boundary, mountain, and so forth. This air (originally at the surface or lower PBL) cools at the dry adiabatic lapse rate until the temperature equals the dewpoint (temperature lapse rate = 10 degrees C per kilometer, dewpoint lapse rate = 2 degrees C per kilometer (dewpoint lapse rate is the same as the mixing ratio lapse rate.. see laminated skew-T). When the air parcel becomes saturated, the LCL is reached.

The CCL is not found by forced lifting, but by rather a warming of the earth's surface. The air does not rise until the surface temperature warms and reaches a critical value with this process. The CCL is generally higher than the LCL because the AIR MUST FIRST WARM before the air can rise to the CCL (remember air warming causes the relative humidity to decrease and the dewpoint depression to increase, because of this, the air must rise to a higher altitude before becoming saturated). The CCL will be higher than the LCL. The LCL and CCL are found by the same process EXCEPT from the CCL the surface temperature must rise to a critical value (called convective temperature) before a surface parcel will begin the ascent in the vertical due to positive buoyancy. Finding the CCL is the same as the process of finding the LCL when air has warmed to the critical convective temperature. Use the CCL for summertime air mass thunderstorms and thermodynamic daytime heating lifting and the LCL for any dynamical lifting (jet streak, vorticity, frontal, convergence uplift).
-Limited speed shear in cloud layer supports clouds moving at same rate and holding together
-Uni-directional flow between sfc and 0.4 km with speed shear around 20 knots (Straight line in hodograph shows very limited directional shear, but length indicates some speed shear -Some curvature to hodograph between 0.4km to 3.5km indicates some directional sheer, but limited speed shear.
-Ground relative hodograph
-sfc 230 @16 knots with 20 degrees of directional shear below cloud base
-cloud base to echo top 25 degrees of directional shear with limited speed shear (38 to 48knots) following same range ring.
-1km to 3km very little speed shear ( 2 to 3 knots)
More low level moisture depicted by model, therefore increased cape values when compared to KBUF sounding data
Mean wind from cloud based to Echo Top for band direction
<30 knots of 0-3km cumulative shear is needed for significant single band lake effect snow
1-3km cumulative shear used for better representation of shear in cloud and for each layer
>30 knots favorable for multi less organized lake effect events
More CAPE than KBUF sounding because colder
Strong 0-1km winds of 30 to 50 knots helps to transfer heavy lake effect band 300 km downwind of lake Ontario Sfc winds around 20 knots near threshold for crystal fragmentation and less accumulation (higher snow density at the sfc)
Well saturated
Strong low level wind field sometimes breaks down dendrite growth, less accumulation, but warmer BL temps help to aggregates snowflakes to produce large flake size. Max ice crystal growth rate occurs near the level of max upward air motion within the cloud, where the greatest water vapor is delivered. (Auer and White 1982)
Key aspect of snowfall density is the role of in-cloud vertical motions, The max growth rate is expected to occur near the level of max upward air motion within the cloud, where the greatest water vapor is delivery occurs (Auer and White 1982) Some ice crystals will grow relative to their neighbors within the cloud and begin to fall, thus removing the smaller particles, if this occurs aggregation leads to the formation of large snowflakes and relatively low snow density. Ice crystal falling through a cloud of super cooled water droplets will lead to rimed crystals and very high snow densities.
1000-700mb lapse rates large depth to measure instability, deeper layer of instability.
1000-850mb shows strong lapse rates from sfc heating and very cold 85h temps (shows steep lapse rates many times throughout the winter