February 19: A “Nostorm” for Baltimore

That’s right, we had a nostorm, not a snowstorm.   For days, the computer models went back and forth on this one.  Early on, it looked like DC and Baltimore would be ground zero for a significant Mid Atlantic snowfall.  But then the models began tracking the storm further to our south, and moving it quickly out to sea.   And the bulls eye for heavy snow turned out to be southwestern and central Virginia.

Let’s look at the factors that contributed to a “near miss” for Baltimore.

Not An Ideal “Snowmaker” Setup

To get heavy snow in Baltimore, we need three key ingredients:  (1) A rapidly intensifying coastal cyclone (nor’easter) moving slowly up the coast (tracking from south to north); (2) a deep layer of cold air, and (3) abundant oceanic moisture.

As the surface synoptic chart below shows ) (7 pm, February 19), a cyclone did develop along the Gulf of Mexico and track across the southeastern U.S.  However, the jet stream – which has been stuck in a zonal or “west to east” configuration most of this winter - never developed a deep loop or “trough” along the east coast.   These troughs tend to draw coastal cyclones northward along the coast.   The February 19 storm instead tracked nearly due east, emerging into the Atlantic south of the Outer Banks.   It moved quickly, meaning fewer hours for heavy snow to accumulate, nor did not undergo the rapid deepening that characterizes truly heavy snowfalls.

One of the key reasons why this low failed to rapidly deepen hinged on the configuration of the jet stream (see chart below).   This chart shows the winds at the 30,000 foot level.   The jet had split into two separate streams:  A weak northern stream draped across New England, and a vigorous southern branch with a core of 150 mph winds across Georgia.   This “split flow” pattern is often a precursor of big East Coast snowstorms.   However, the trough in the northern branch failed to link up or “phase” with the trough in the southern branch.   Had this occurred, a much deeper trough would have set up over the East Coast.  This would have promoted a rapidly intensifying storm that moved north along to the coast (the classic “snowmaker” type of nor’easter for the Mid Atlantic).

Cold Enough For Snow

The second main ingredient, a fairly deep layer of subfreezing air from surface through at least 5,000 feet, was present in this case.   In fact, one of the key elements is called cold air damming, in which a cold, dense air mass gets wedged up against the eastern slopes of the Blue Ridge.  The figure below shows the characteristic surface pressure ridge east of the mountains when damming is in place.   Note how the isobars (black solid lines) sag to the south east of the mountains, indicating dense, cold air flowing southward.  Superimposed on the isobars is the distribution of precipitation as revealed by regional radar.  While the precipitation fell as rain over North Carolina, the heavy batch over Kentucky, southwest Virginia and streaming east across Central Virginia is snow, forming within a deep, cold air mass.

The figure below shows the actual air temperatures at 5,000 feet, the critical snow-forming layer in the clouds.   The closed vortex of the coastal low is centered over North Carolina’s Outer Banks.  To its west, a tongue of very cold air is being drawn southward (blue and purple colors), over the Appalachians and into the cold damming region east of the mountains.   Temperatures at 5,000 feet were -4 to -8 °C, plenty cold for vigorous snow formation.

On A Knife Edge:  Insufficient Moisture Over Baltimore

The southern track of this storm put the Baltimore region on the extreme northern fringe of the storm system, where clouds were being undercut by very dry air in the lowest several thousand feet.   This resulted in a sharply delineated border or northern edge to the heavy precipitation, which remained mainly south of DC.   The sharp cutoff in snow is shown on the regional radar image below.   Superimposed on the radar image is the amount of moisture contained in the air (solid lines and green shaded regions) – a quantity called “precipitable water”.   You will note over an 1” of liquid available to the storm along the Outer Banks, but the precip water drops off rapidly moving north toward Baltimore.  Because the air was so dry over us, any snowflakes that managed to develop completely sublimated before reaching the surface. 

Major Impacts Over Appalachia

The snow that did fall north and west of the low’s center was heavy and wet, thanks to large precipitable water content close to the storm’s center.  Snow totals ranging from 6” to 10” were fairly widespread across southern WV and southwest VA, with locally higher amounts at the highest elevations.    The snow accumulation map below reveals a fairly compact region of heavy snow:

 

Over 1000 traffic accidents were reported across the snow region, and as predicted, the blanket of heavy snow brought down tree limbs onto electric utilities.  More than 60,000 power outages led to an extended period of power restoration.  The map below shows “ground zero” for these outages on the day after the storm:

Lucky This Time…

In a nutshell, Baltimore dodged a bullet, thanks to a weaker storm that moved rapidly and too far to our south.   But I find it instructive to do a careful post-mortem of these “near misses” because there are always important meteorological lessons to be learned!

 

Baltimore’s Only Brush With Winter: February 11, 2012 Arctic Front and Snowfall

This has been an unusual, but not unheard of, mild and snow-free winter.   Heavy snow in Baltimore can best be described as “episodic” in nature, a feast-or-famine in terms of total seasonal snowfall.   Recall the record-setting 2009-2010 winter in Baltimore, with a season total snowfall of 77 inches.   This season, cold air masses have sequestered well to the north of the Mid Atlantic and the jet stream has been largely zonal, running flat in a west-to-east flow.   Without big, cold meanders in the jet building down the east coast (these are called jet stream troughs), storminess and winter air outbreaks have been minimal.    Whether we can blame this behavior on La Nina, and/or the North Atlantic Oscillation, remains to be carefully assessed once the season ends.

Baltimore endured a brief winter blast on February 11-12, when a chunk of that cold air mass over Canada pushed south into the Plains and Great Lakes.   The anticyclone that propelled this cold air south and east was intense, with a surface pressure of 1046 mb (fewer than 5% of all anticyclones over CONUS ever reach this intensity).   The leading edge of this cold surge invaded the Mid Atlantic in the form of an arctic front – essentially a cold front on steroids.   The synoptic weather map at 7 AM on February 11 (below) shows the front (blue line running through West Virginia) poised to blast through Baltimore:

Ahead of the front, there was sufficient moisture and an unstable (overturning) air mass.   As this air was lifted along the front, a narrow band of convective snow showers erupted east of the Blue Ridge Mountains.   Around 2 pm, this sudden squall slammed through the area.   Visibility plummeted as a heavy fall of snowflakes and graupel (tiny white icy spheres) fell in a volley, accompanied by gusty winds, rapidly falling temperatures and even lightning.   Many areas picked up a quick ½”-1” of accumulation in a matter of 20 minutes.   The regional weather chart (below) shows this squall as detected by weather radar, along with snow streamers blowing off the Great Lakes and a patch of upslope snow along the Allegheny Plateau.

In this image, isobars (lines of constant pressure) are drawn at 2 mb pressure intervals.  The very compact squeezing of isobars implies an intense pressure gradient, which drives the wind.   Winds from the northwest rapidly increased trough the late afternoon as the heart of the cold air mass began to filter in.  The arrival of cold air from the northwest was very swift.  The diagram below shows this cold air advection at about 5,000 feet above the surface.  Cold air advection is a calculation of how rapidly the temperature changes at a location, due to the influx of cold air by the wind.  Dark blue shades in this diagram show where the chilling effect was most pronounced.   Note that the air temperature at 5,000 feet above Baltimore was 18° F!  

But really cold air was only part of the story that night.   Wind speed steadily increased through the evening, as surface pressure surged in Baltimore.    The winds were also very gusty, peaking at 49 mph.  The diagram below, called a time series, plots hourly observations of wind gust and air temperature at Dulles airport.  Note the arrival of the arctic front at 3 pm on February 11, followed by the precipitous drop in air temperature and surging winds.

 

Intermittent snow squalls continued through the evening, exemplified by the patch of moderate snow captured by weather radar just before midnight on the 11^th:

The air temperature never climbed above 32° F the next day!

What’s interesting is that none of this extreme weather – arctic chill, thundersnow, high winds – were actually produced by an area of low pressure, but rather by an exceptionally strong anticyclone.    We normally think of low pressure regions or cyclones as bringing disturbed and stormy conditions, while anticyclones or regions of high pressure herald fair weather.  But in the case, the tables were turned!

 

Dec 7, 2011: Floods, High Winds, Power Out For Thousands

It's just another stormy, early winter day in the Mid Atlantic and New England!

The culprit, as is typical for this region and this time of year, was a compact and powerful extratropical cyclone that developed and amplified along a stationary front draped through the Mid Atlantic.

Synoptic surface chart at 7 pm, December 7 (unisys.com)

Yes, that is a severe thunderstorm watch box (yellow quadrilateral) over southeast Virginia.  Although no tornadoes developed, there were about 40 reports of wind damage due to severe thunderstorms.

The storm had a very sharply demarcated warm and cold side.  Washington-Baltimore was fortunate to remain in the warm sector of this system throughout the duration.  The flip side is that rainfall in excess of three inches fell across most of our region, setting new all-time December 1-day rainfall records at the area's three airports.  Despite the threat of 1"-3" of heavy wet snow, the moisture and energy exited our region faster than sub-freezing air could arrive from the northwest.  Heavy rain totals were due to five factors:  (1) the overall slow movement of the stationary front;  (2) training (repeated passage) of storm cells over the same locations;  (3) high precipitable water content (abundant Atlantic moisture feeding into the storm);  (4) very energetic conditions in the upper atmosphere that favored vigorous ascent of air;  and (5) strongly "frontogenetic" conditions i.e. a rapidly intensifying front as the wave of low pressure deepened.

Radar view of the storm, showing a plume of heavy rain extending across the region.  Embedded reds indicate thunderstorms.  Blue colors indicate moderate snowfall (weathertap.com)

Radar-derived rain accumulation for the December 7 storm (NOAA)

Once the precipitation exited (around 11 pm), the next hazard was high wind.  Winds gusted to 46 mph at BWI at 3 AM.   Saturated soils combined with these strong winds led to numerous treefalls through the region.   Many thousands lost power in the early morning hours.    Power outages numbered in the tens of thousands from Virginia through New England.  The high winds were the consequence of an intense pressure gradient between the departing storm and an approaching cell of high pressure.   This created a pressure surge during the early morning hours:

Pattern of isobars (black solid lines) - notice the extremely large pressure gradient over western Maryland and West Virginia.  Red solid lines indicate regions where the surface pressure is abruptly rising (NWS).

The final image (below) shows that great variety of weather watches and warnings that were posted for our region, at the height of the storm.  It is quite clear that Washington-Baltimore was on the thin dividing line between cold and warm weather hazards.  Whenever you see lots of different colors such as this, look out!

Nov 23, 2011: Autumn Cyclone Brings Flooding, High Winds To Mid Atlantic

As is typical for late November, strong disturbances called extratropical cyclones frequently pass through our region.  On November 22-23, periods of moderate rain combined with dense fog, followed by unseasonably mild temperatures and then a period of brisk winds as skies cleared.  This sequence of weather describes the classic passage of a wave cyclone through the Baltimore region.  Thirty of more of these cyclones traverse our region in the course of a typical year, although most are not especially intense.

The sequence of three surface weather charts below (courtesy of Intellicast) shows a warm front to the south of Baltimore on the evening of November 22 (red scalloped line).   An extensive canopy of moderately heavy rain had overspread the region, in advance of the front, during the daytime.  Dense patches of fog developed during the evening as the warm front approached from the south.  This type of frontal fog is a common feature along warm fronts. 

The warm front moved north through Baltimore after midnight, and quite paradoxically, temperatures began to rise into the lower 60's after midnight!   This occurred as winds turned from the south and ushered in a warm air mass from the south, within the cyclone's warm sector.  A band of heavy showers swept through Baltimore in the early morning hours, within the warm sector and ahead of the cold front.  

Then, around Noon on November 23, the cold front pushed through from the west.   This marked the arrival of a dry air mass, clearing skies, and strong winds.  Paradoxically again, even with the early afternoon Sun, temperatures rapidly fell through mid-afternoon as a cold Canadian air mass arrived from the west and north.

 
The meteogram below (from Unisys) shows an atypical flip-flop between nightly minimum and daily maximum temperatures.   This meteogram, from Dulles airport, shows temperature (green trace, upper panel) on November 22 and part of the 23rd.  Note the steadily warming temperatures after midnight (05 UTC);  the warmest temperature on November 23 actually came during the morning (61 F at 9 AM).  The rapid drop in temperature through the afternoon (down to 51 F at 1 pm) followed the cold front.  Wave cyclones, with their strongly contrasting temperature fields, frequently modulate the expected diurnal cycle of afternoon high temperature and nighttime low temperature.

Prolonged moderate rainfall across central Maryland on Nov 22-Nov 23 lead to 2"-3" of accumulated rain and moderate flooding of the Monocacy River near Frederick, MD (below):

The other weather hazard of note was strong wind behind the cold front, particularly at the high elevations west and south of Baltimore.  At the area airports (DCA, BWI, IAD) winds gusted to nearly 40 mph as the cold front swept through the region.   These high winds were the result of a strong pressure gradient (pressure surge) behind the cold front (isobars are shown as solid black lines):

 The NWS issued a Wind Advisory for counties immediately west of Baltimore, and a High Wind Warning for counties in the mountains of central Virginia:

Since friction reduces wind speed at Earth's surface, the high elevations of the Appalachians (3000'-4000') frequently experience stronger winds when the pressure gradient is intense.  A High Wind Warning is issued when gusts are expected to exceed 50 mph.  In the NWS maximum gust forecast for the afternoon,  windy ridge tops show up quite dramatically.  Also note the high wind gusts (nearly 50 mph) predicted for the middle of the Chesapeake Bay - a location where the wind accelerates across the extensive, flat water surface.

October 29-30 Snowstorm Ranked a NESIS Category 1

The Northeast Snowfall Impact Scale (NESIS) is a recently developed metric for assessing the geographical impact of large East Coast snowstorms.  Statistically, it is computed by examining the correlation between population density and snow footprint (areal coverage, depth of snow).  Widespread snowstorms of significance in the Northeast Corridor are assigned a NESIS score, and ranked on a scale from 1 to 5 in terms of overall impact.  This enables individual snowstorms to be placed in a historical context.   The NESIS is computed in the weeks following a snowstorm by NOAA.   NESIS categories range from 1, Notable, to 5, Extreme. 

The extremely early season Nor'easter of October 29-30 was determined to be a NESIS Category 1.   This score is based strictly on population and snowfall.  NESID does not provide an an assessment of infrastructure disruption, nor of societal disruption and hardship, as was generated by 3+ million power outages from downed tree limbs. 

In spite of being rated near the "bottom of the heap", in terms of all-time great snowstorms, what is remarkable is that no snowstorm in the NESIS Top 45 (Northeast urban corridor) has ever occurred earlier than December!   The large majority of historical snowstorms occur in January-February. 

Here is the official snow accumulation map from October 29-30 and the NESIS score as provided by NOAA:

October 29, 2011: October Nor’easter, Trick or Treat?

This blog was written by Alexandra St. Pe with contributions from Professor Storm.

Winter storm headlines came earlier than many expected this year and transformed Halloween traditions of spooky sights into winter wonderlands across portions of the Mid-Atlantic and Northeast states. Forecasters first became apprehensive about what could unfold during the holiday weekend when weather forecast models indicated the formation of a rapidly intensifying low pressure system tracking northeast along the East coast. This system had similar characteristics of a classic “Nor’easter” weather system that typically occur in the late winter season (January/February). A Nor’easter or coastal low is infamous for ushering in an exceptionally cold Arctic air mass on northeasterly winds, and is often accompanied by powerful winds and heavy snow as cold and warm air masses converge. With tree foliage still turning colors, a Nor’easter in late October would not bode well for tree limbs and widespread power outages became a major concern.

Critical Temperature Profiles

As a low pressure system approached from the Carolinas, a second system began to spin off the Atlantic coast. Even though the synoptic conditions were aligning for the development of a Nor’easter, there was still much uncertainty about the characteristics of the air masses involved. Weather forecast models disagreed on the chilliness of the air mass moving towards the coast and made it extremely difficult for forecasters to predict where snow verses rain would fall.   In addition, slight variations in storm track by as little as 20-30 miles would make a big difference in who received heavy rain, and who received heavy snow.

During the morning of the event, the 8am Dulles International Airport  (IAD) temperature profile from weather balloon data hugged the freezing line. This profile suggested that the atmosphere east of the Blue Ridge Mountains was only slightly cold enough to support a snow event. The red arrow in the figure below points to the temperature (red) and dew point temperature (green) vertical profiles and the black dashed-line highlights the freezing line. A warmer air mass being ushered in the lower levels of the atmosphere is also shown in the black box.

Baltimore was spared heavy snow because in October, the Atlantic water temperature is still quite warm, and kept temperatures just above freezing in the critical snow-making layer in the clouds at 5,000 ft.  The image below shows the region of warming (called warm air advection, red colors) in the lower atmosphere, as southeasterly winds moved warm Atlantic air in over the Baltimore region:

 

Cold enough for snow?  Forecast Models Help Solve The Dilemma

Due to widespread forecast disparity for snow accumulations east of the Blue Ridge, (NAM model forecasting > 10 inches, GFS model forecasting a < 1 inch), forecasters relied on the High Resolution Rapid Refresh (HRRR) model guidance. This experimental model has better temporal and spatial resolution than the aforementioned operational models. After forecasters analyzed the HRRR’s forecast for a swath of heavy snow accumulating over the next 15 hours along a southwest-northeast axis, confidence grew that this would be a significant snow event to the north of the Mason-Dixon Line.

By mid-afternoon, the coastal low pressure system began to strengthen as it tracked northeasterly, parallel to the Mid-Atlantic coast.  Cold air wrapped around the backside of the counterclockwise spinning system and collided with relatively warm air over the Atlantic coast. The converging air masses induced a nebulous rain-snow transition line and therefore heavy bands of mixed precipitation along the I-95 corridor.  

This weather radar snapshot captures the very narrow rain-snow line that set up just to the north and west of Baltimore:

Periods of moderate rain during the morning hours gave way to intense episodes of sleet during the afternoon, and finally moderately heavy snow that tapered to flurries by late afternoon.  The transition occurred as the low moved to the northeast, wrapping in a cold air mass from the north on its back side.

Gusty Winds

The increasing pressure gradient between this system and an approaching high pressure system from the west kicked off gusty winds throughout the day as well.   Because of exceptionally vigorous jet stream energy above the storm, the Nor’easter rapidly deepened, becoming what is called a “meteorological bomb”.  The powerful winds reduced visibility conditions to less than 3 miles at BWI airport and held the “feels-like” or wind chill temperature below freezing for most of the day! (black box = temperature, blue box = wind chill temperature, red box = wind gusts and visibility).  

 

Winds were much more significant to the northeast of Baltimore, where gusts in the 50-60 mph were commonplace along the New England coast:

 

First Freeze of the Season

After the winter storm headlines expired, strong northwest flow behind the system ushered in the chilliest air mass of the season and prompted NWS Sterling forecasters to issue a Freeze Warning for the Baltimore-Washington corridor Saturday night. The Dulles weather balloon data at 8pm indicated a much cooler temperature profile compared to the morning profile. In the figure below, the red arrow points to the temperature profile well below the black-dashed freezing line and the chilly northerly winds in the lower atmosphere are shown in the black box.

 

Accumulated Snowfall

According to unofficial observations submitted to the NWS office in Sterling, storm total snowfall varied from a measured trace in eastern Maryland to nearly 12 inches across west and northern portions of the state. The HRRR 15 hour snow accumulation forecast listed above verified quite well with the heaviest snow accumulation along a southwest-northeast axis.  The heavy snowfall across the W. Va panhandle and western Maryland reflects mainly below freezing temperatures prevalent at the higher elevations.  The heavy snow across New England reflects a combination of higher elevation temperatures, and below-freezing air transported toward the coast by the low.

 

Snow from Space

Finally we see a snapshot from the MODIS sensor onboard NASA’s Aqua satellite. This image was captured Sunday, the day after the event, and shows only the remnants of the rare October Nor’easter.

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: