Many times throughout the winter season, you will hear many of us on the blog refer to heavy snowfall rates/quick accumulation as snowfall as "dynamic snowfall". Understanding this process is actually pretty essential for winter weather forecasting around the New York Metro area, as it occurs often; usually at least once or twice a winter. Though the New York Metro area is by no means a winter wonderland, we are in a prime location for reasonably cold air during the winter season, and a favorable juxtaposition of a strong tropical moisture fetch and deepening low pressure systems. The result, with the deeper low pressure systems, or sometimes those along frontal boundaries or with plenty of moisture, can often be heavy precipitation driven by strong lift throughout the atmosphere. Often you will hear meteorologists or weather enthusiasts refer to "Omega" or "Omega forcing". Without getting too complicated, omega forcing generally is involved with synoptic scale rising or sinking of air parcels. The official definition actually states that: cold air advection in the low levels of the atmosphere causes sinking while warm air advection in the low levels of the atmosphere causes rising air. PDVA causes rising air while NDVA causes sinking air. PDVA means higher values of positive vorticity are being advected and that vorticity is increasing with height. A vort max will usually have vorticity increasing with height since winds in the upper levels are generally stronger than low-level winds. The omega equation takes into account the magnitude of sinking or rising due to thermal advection and couples that with the magnitude of sinking or rising due to PDVA or NDVA and determines if the resultant vertical motion is upwards or downwards. Upward motion implies a greater chance for: increasing RH through a large depth of the atmosphere, clouds, and precipitation. The closeness of the air to saturation and the amount of uplift can be used to forecast clouds, forecast precipitation, and forecast precipitation amounts.

Believe it or not, there is a reason for this post. A good example of favorable omega/uvv (upward vertical velocity) values was displayed this afternoon on the 18z NAM forecast for, coincidentally, the New York Metro area. The image to the left shows the 18z NAM valid at 48 hours at 700mb (H7). Relative humidity is shown, with the maroon colored lines overlaying displaying the omega values (-5,-10,-15). The red circle is meant to highlight the H7/700mb low which is closed off, displaying the strength of the individual system as its own entity. The blue highlight shows the areas of enhanced omega forcing...dictating heavier precipitation. The area of widespread omega along the eastern seaboard is on the northwest side of the low pressure system, within the cold conveyor belt region of the precipitation. Additionally, you can see the moisture fetch from the Tropical Carribean (see wind vectors on the southeast side of the image and the fetch of moisture elongated southwest to northeast in a squall line-type formation over the atlantic ocean) providing moisture to the system.

As you can see in the image to the right, the moisture fetch, strong dynamics associated with the strengthening low pressure off the Mid-Atlantic coast, and favorable omega values (widespread -15 values along the eastern seaboard) have led to very heavy precipitation values. In fact, the blue and light blue shading indicates QPF values of near an inch over the past six hours. However, despite the impressive precipitation rates, the most impressive facet of this solution is probably the dynamic cooling which occurs as a result of the heavy precipitation and aforementioned factors. We've included two images to the left, both showing the sounding from Morristown, New Jersey on the 18z NAM. The first sounding shows the temperature above freezing and much of the column aloft above freezing, but heavy precipitation falling. The temperature at the surface is 3.3 C, well too warm for snow. But lo and behold, just an hour later, the temperature has cooled to only 1.0 C, and the rest of the column has cooled dramatically. Heavy snow is now falling at the observed station.

It shouldn't be surprising that the process behind this is the omega values and dynamic precipitation/associated dynamic cooling we were mentioning. The image to your left shows the associated omega values (with the timeline from right to left). The high precipitation values associate with the strongly favorable negative omega values (-30 max right when the precipitation change is observed just after 12z Sunday) and you can see the precipitation change. Another interesting observation can be made: when the precipitation values decrease (meaning lighter precipitation) the dynamic cooling is less, and the column warms allowing for a changeover back to rain. When you have the column teetering on the edge of the freezing mark, the forecast really gets difficult. The 18z NAM today was really just a classic example of this. This is a medium range forecast, around 48-60 hours, so the confidence is rather low at this time that this specific guidance solution would verify. Neverthless, an interesting peak into a potential dynamic system later this weekend, and a process that will likely rear its ugly ahead again at some point during this upcoming winter. Take it easy and enjoy Game 1 of the ALCS.

John