Monday, October 17, 2011

October 13 2011: Mini Severe Weather Outbreak in the Mid Atlantic

On Thursday, October 13, a small outbreak of tornadoes and severe thunderstorms developed during the afternoon hours.  There were several sightings of funnel clouds and tornadoes across Northern Virginia.  The image below shows reports of tornadoes received by the NOAA Storm Prediction Center (tornado locations are shown as small red triangles):
This was a rapidly developing severe weather situation, and unusual for its small geographic coverage.  The surface weather map (below) shows several classic elements came together to produce rotating, supercell thunderstorms.   Warm, humid air was being lifted along a warm front draped from west to east across Northern Virginia, assisted by air converging into a region of low pressure approaching from over West Virginia.   The small pocket of warm, humid air - a moderately unstable air mass - is shown by the shaded magenta region.  This pocket was located just along and to the south of the warm front.   Warm and humid air was being transported toward the warm front by low-level southeasterly flow (thin orange arrow).
The region was thus primed for the development of thunderstorms.  However, supercell thunderstorms, which contain a vertical core of rotating wind, acquire their rotation because of wind shear.  Wind shear is the change in wind speed and direction with increasing altitude.   Tornadoes develop when low-level winds both increase in speed, and veer - that is, turn clockwise - with height.  The large yellow arrow shows the wind direction at 18,000 ft, from the southwest, and stronger than at the surface.   A more detailed view of these veering and strengthening winds is provided by an instrument called a wind profiler, located at Beltsville, MD:
Warm fronts are common breeding grounds for tornadoes in the Mid Atlantic.  The winds are often east-southeasterly close to the frontal boundary, along the surface, but rapidly veer to southwesterly winds overhead. 

Additional, vigorous uplift of air was generated by an approaching trough in the jet stream.  The trough took on a "negative tilt", which means that its axis (shown below by the dotted magenta arrow) is oriented from NW to SE, as opposed to N-S.   When a trough becomes negatively tilted, the upward flow of air intensifies downstream of the trough (in the case, across the Mid Atlantic).  This helps to invigorate thunderstorms erupting upward from the unstable air mass.

A radar loop of the heavy thunderstorms moving through the DC-Baltimore region is shown below:

But tornadoes were not the only severe weather story this day.  Intense rains accompanied the strong thunderstorms.  In some cases, repeated movement of cells over the same location, called "echo training", dumped 2"-3" in some locales.   To generate flash flooding, the air mass must be very moist.  The map of "total precipitable water" (a measure of the amount of water vapor in the air column) below indicates a plume of 1.5"-2.0" values south of the warm front.  This very humid Atlantic air was streaming toward the warm front at low levels, where it became lifted into narrow corridors.

A radar loop showing these narrow corridors of heavy rain - the "rain train" - is shown below:

October 12, 2011: Late Fall-Like Clouds and Rain Blanket The Mid-Atlantic

Post written by Alexandra St. Pe.

After a lovely start to the week with clear skies and northeast winds funneling in a cool and dry air mass, the strong high pressure system responsible for the pleasant weather conditions began to exit the Mid-Atlantic region. As the high pressure system traversed the New England states, a low pressure system developed off the Mid-Atlantic coast, locking in an all too familiar southeasterly wind flow infamous for ushering in copious amounts of moisture. 
This process is known as Warm Air Advection (WAA) and is often associated with stratiform cloud decks and widespread showers. The weather map below depicts the warmer air (red shaded region) being transported inland on southeasterly winds off the Atlantic.  Stratiform rain refers to light-moderate rain or rain showers that fall over a large area from layered, horizontally extensive cloud layers.  (Convective rain refers to heavy showers and/or thunderstorms forming from deep, isolated clouds).

The development of this low pressure system and co-existing WAA set the stage for an overcast and wet middle of the week as the moist-laden air was forced to lift over a cooler dome of air already in place. This process is often referred to as overrunning - meaning warm, humid air will gently ascend along a sloping layer of cool air at the surface.  The cool air was flowing down the east side of the Appalachians on northeasterly winds at the surface.  The diagram below illustrates this process of warm (less dense) air overriding the cooler (more dense) air.

Overrunning often denotes the beginning of lengthy rain periods as showers can stream along a narrow axis as long as there is a sufficient moist airflow. The 12:00 pm analysis below from the Storm Prediction Center provides evidence of a generous tropical-like plume of moisture with dewpoint temperatures ≥ 64 degrees Fahrenheit stretching along the east coast of the US.

The upper levels of the atmosphere (~ 27,000’) were also conducive for maintaining widespread showers as divergence aloft was occurring over the shower genesis area. Divergence aloft serves as another ‘lifting’ mechanism in the atmosphere and is marked by fast moving winds spreading apart.  When winds spread apart high in the atmosphere, air is drawn up from below to “fill the void”.  The pocket of diverging air over the Mid-Atlantic is outlined by the purple lines below.

NWS Radar Base Reflectivity imagery indicated widespread showers across the Mid-Atlantic by 12:15pm. The light to moderate showers continued throughout the evening until the surface low pressure system finally began to retreat northeast.

Due to the blanket of clouds and showers that persisted Wednesday, temperatures didn’t budge at BWI airport- holding steady at 63 degrees Fahrenheit during the entire day! Note also the light and steady rain that fell during the event.  Accumulations in the immediate DC-Baltimore metro region were few tenths of an inch, but over an inch fell in Central Virginia.

Tuesday, October 4, 2011

October 4, 2011: Never Trust A Cutoff Low!

This piece was written by Ms. Alex St. Pe, Professor Storm's grad student conducting research on severe storms in the UMBC GES program.
Star filled skies were visible for the first time in awhile Thursday evening (September 29, 2011) after persistent cloudy conditions and scattered showers exited the region behind a cold front. Unfortunately the clear skies were only temporary, as an upper-level disturbance vigorously dove south into the Ohio River Valley. Forecasters watched closely as the weather models predicted this wouldn’t be an ordinary low pressure system, and likely develop into a “cut-off low” feature. Confidence in a forecast involving a cut-off low is typically low amongst many meteorologists due to the system’s capricious movement and the unreliability of weather forecasting models.

Cut-off lows occur when a trough in the jet stream becomes so highly amplified that it literally pinches off, leaving the spinning low pressure system behind. A cut-off low has the characteristics of an upper-level cold-core center. Because the low is no longer embedded within the jet stream, the spinning whirlpool meanders slowly or becomes stationary.  Associated showers typically form around the center of the low.

This past weekend, the Mid-Atlantic region experienced unseasonably cool temperatures, dreary overcast sky conditions and persistent showers in response to a cut-off low meandering towards the east coastline. In order to gain a better understanding of this past weekend’s relentless weather pattern, I will investigate the infamous cut-off low.


On Thursday evening (September 29, 2011) the 300mb pressure (~ 27,000 ft) map indicated strong north/northwesterly winds of approximately 100-120 knots ushering chilly Canadian air southward towards the Great Lakes Region. The pink contours on the map denote regions of “divergence aloft”. Areas of diverging winds at this level in the atmosphere create favorable conditions for the genesis of showers and storms at the surface. 

This digging trough within the jet stream was forecast to become a closed upper-level low, or “cut-off low” during the day Friday, before dropping south into the Mid-Atlantic Friday night into Saturday. For this reason, forecasters across the Mid-Atlantic anticipated weekend weather conditions of noticeably cooler temperatures, and scattered showers.

Friday – Saturday:

Friday evening, a surface low-pressure system developed off the Delmarva Peninsula and created light showers ahead of the anticipated main shower event for the weekend.   The surface low helped reinforce cold air arriving from the northwest.

On Saturday morning a cooler Canadian type air mass had successfully reached the Mid-Atlantic, with temperatures dropping the lowest for the season yet - the mid-40s. At 8am EDT atmospheric weather balloon data from Dulles International Airport (IAD) suggested a cold, deeply saturated vertical profile as the cut-off low approached the region. The tropopause, which is typically found at much higher altitudes (~12km),  dropped down to an astoundingly low 7.5 km within the cold core of the cutoff vortex.

Notice the surface temperature within the orange circle. The surface temperature at 8am was 51 degrees Fahrenheit and actually dropped throughout the morning (the day’s high temperature, 61 F, occurred just after midnight!) before topping off in the lower 50s by 4pm.

By Saturday afternoon (October 01, 2011 3pm EDT) the 500mb (~15,000ft) pressure map indicated a closed circulation around the center of low pressure. The two height contours within the red oval provide evidence of an intense cut-off low over the Mid- Atlantic.

Once displaced from the jet stream’s steering winds, the movement of the cut-off low became very unpredictable. The system began to slowly meander eastward, producing a thick deck of stratus clouds, persistent precipitation, and chilly temperatures. At greater elevations near the Potomac Highlands, even cooler temperatures coupled with persistent precipitation generated a wintry mix along the western slopes of Allegheny Mountains.


  As the cut-off low continued to spin across the northern Mid-Atlantic Sunday, light precipitation and dense cloud cover held temperatures steady in the mid-upper 40s for most of the day again. The weather system surprised some forecasters when it refused to exit off the coast Sunday, and extended its stay through the beginning of this week. According to the Storm Prediction Center’s 500mb pressure map, strong northwesterly winds  (shaded blue) continued to filter cool air into the Mid-Atlantic area Monday morning. A strong mid-level temperature gradient is also noticeable from the closely spaced red-dashed temperature lines.  Also worth noting is the difference in temperatures over Maryland/Virginia in comparison to Mississippi/Alabama. The cold-core of the cut-off low is clearly visible off the coast of Virginia, defined by the -24 C isotherm – nearly 14 C colder than air over Mississippi.

The NWS’s radar loop from the Sterling, Virginia site captured the system’s counter-clockwise flow as it initiated precipitation around its center on Monday. Patches of light-moderate rain were observed to pinwheel counterclockwise around the low’s center all weekend and into the Monday.

The meteogram below (BWI) shows that high temperatures over the weekend were about 20 F colder than average for this time of year – weather more typical of late November – thanks to a very stubborn cutoff low!