Monday, November 26, 2012

Nov 26, 2012: Weak Depression To Bring A Touch of Wintry Mix

10:30 AM

A weak low pressure system is expected to develop along the Delmarva Peninsula tonight.   Enough cold air will be in place at the surface and in the lowest 10,000 feet to bring wintry precipitation to parts of the Baltimore region.  However, there are several mitigating factors against significant accumulation:  (1)  the storm is weak (it is NOT a Nor'easter);  (2) the storm has limited moisture to work with;  (3) the storm will pass quickly to our south and east, exiting into the Atlantic on Tuesday.

The forecast map for tomorrow is shown below.  A stationary frontal boundary separates mild air to the south and east, and sub-freezing air to the north and west.   Thus, the further north and west from the front, the greater the probability of accumulating snow.   The highest amounts are expected across the Maryland mountains and along the Mason-Dixon Line (up to 3").   Within Baltimore, some surfaces may become slushy (< 1").  In Annapolis and along the Bay, just rain will fall.   So the temperature gradient is important in determining precipitation type, and this gradient straddles Washington-Baltimore, as is so often the case.


The NWS in Sterling, VA has issued their snow accumulation map.  It reflects the thinking that you must travel north and west of the D.C.-Baltimore metro to experience air cold enough for accumulating snow:

Timing of the event is critical.  The precipitation will develop overnight and continue through Tuesday morning.   In the cities, look mainly for a cold, light-moderate rain, with snowflakes mixing in at times.  However, the heaviest precip should be falling during the Tuesday AM rush.  This means slushy surfaces could create some delays.  The system clears out Tuesday afternoon.

Again, this is not a Nor'easter, and the storm is not intense enough to warrant a name given by The Weather Channel.

Wednesday, November 7, 2012

November 7, 2012: Winter Storm Athena Brushes Baltimore

What's In A Name?

Yes, the winter storm has received a name...courtesy of The Weather Channel (TWC).  While not officially sanctioned by the National Weather Service, TWC meteorologists took the unusual step of rolling out a naming convention for Nor'easters.   They have decided to name Nor'easters that generate impacts and cause disruption.   Personally I feel this is a good move and is long overdue.   We have been naming hurricanes since the 1950s and many Nor'easters are just as damaging and far-reaching as hurricanes.   I find it interesting that the NWS Buffalo has been officially naming Lake Effect snowstorms for many years now.  Time will tell if NWS, and other forecasting agencies, adopt TWC's somewhat daring naming convention.

A Brush With Athena

As of 2 pm today, Athena is a moderately intense Nor'easter, moving northward over the Atlantic about 150 miles east of Atlantic City, NJ.   The storm is rapidly deepening and expanding.   Moderate-heavy precipitation is occurring along the Delmarva, NJ and NY.   You can see the precipitation in relationship to the surface pressure field (isobars) in this image:


Note that the precipitation covers more area over the ocean than shown here, because weather radar along the coast has limited viewing range.

Not all the precipitation is rain.  A band of moderately heavy snowfall (1"/hr) has set up over NJ.  The snow band has developed in the "classic" location about 200 miles north and west of the storm's center, on the cold air side of the storm, in a region where a mid-level front is developing:

The light green colors represent intermittent light rain over north-central Maryland;  however, because the air beneath cloud base is dry, much of it is evaporating before reaching the surface.

Baltimore Region Impacts

Yesterday I was not calling for much, if any, snowfall in our region, and this appears to be verifying.  The NWS has scaled back their snow accumulation totals for the Baltimore region...down to a dusting around the city, up to an inch in the far northern suburbs.   Only the back edge of the precipitation shield is scraping by Baltimore, because the storm is so far out to sea.   The precip shield has expanded a bit southwestward through the day as the storm intensifies.  But there is not a lot of moisture to work with on the back edge.  And air cold enough for sustained snow has not filtered in from the north.

The storm will continue to intensify this evening, but will also continue moving away to the north.  The precipitation shield should retreat east of Baltimore after midnight.   Winds will remain breezy at times, with peak gusts in the 20-30 mph range.  Tomorrow will become partly cloudy as the storm moves further away, but the breeze will continue.   Tomorrow's the breeze will be sustained by a developing pressure gradient between the storm, and high pressure advancing in from the west.  Wind gusts should not exceed about 25-30 mph.

Tuesday, November 6, 2012

November 6, 2012, 4 pm: Backside of Nor'Easter to Clip Baltimore Region

Now less than 24 hours from the impacts, the forecast models are in good agreement that the center of this Nor'easter storm will track 200-300 miles east of Baltimore, putting us on the back edge of the precipitation shield and wind field.   The storm is forming now off the Georgia coast.  It will track N-NE over the Atlantic tomorrow, intensify tomorrow night, then slowly start to weaken on Thursday evening:

Model Forecast Positions - Nor'Easter Track, NWS
Inland, an unseasonably chilly air mass will be in place, and the storm's circulation may pull in even colder air from the north on Wednesday.   Thus, the "S word" (snow!) is added to the list of our potential impacts.

Overall, given the trend in the models, this should be a low-impact event lasting about 24 hours for the Baltimore region.

Specific impacts:

1.  Precipitation Amount:  Precip will fall intermittently from Wednesday evening into Thursday morning.  Amounts will be light i.e. just a couple tenths of an inch:

By far, the lion's share of rain is to our east, over the water.

2.  Precipitation Type:  A mix of rain and snow...ranging from a trace near D.C. to about an 1" north and east of Baltimore:


3.  Winds:  Strongest over the Atlantic and coastal zone - generally from the N-NW gusting to 20-30 mph, peaking on Thursday morning, and highest along the Bay:


Monday, November 5, 2012

November 5, 2012: Moderate Nor'easter Weather Impact for Baltimore

November 6, 2012, 4 PM:  Baltimore Nor'easter Impacts
Scenario and Timeframe
Our region is expected to pass through the back edge of a strong Nor'easter.
The various models portray the development of a Nor’easter off Cape Hatteras on Wednesday.   The storm will track N-NE and rapidly intensify off the NJ coast.   The storm may briefly stall off the NJ coast on Thursday, before moving NE away from the coast Thursday night.
The biggest impacts from this storm will be along the coastline from the Delmarva northward along NJ, NY and New England.
For Baltimore, the moderate weather impacts will occur during a 12-18 hr period from Wednesday night into Thursday morning.
Weather impacts will generally diminish moving inland from the coast (except at the higher elevations i.e. Catoctins, Appalachians – where winds will be higher and probability of snow is greater).
The following sequence of surface weather charts shows the general progression and intensification of the storm, at various times:
7 AM Wednesday. Storm develops, deepens, moves slowly north. NWS.

7 PM Wednesday.  Storm deepens, moving slowly north.  NWS.
7 AM Thursday. Storm reaches lowest central pressure.  NWS

7 PM Thursday.  Storm weakens, moves away toward northeast.  NWS

Another way to look at the evolution of the storm is to plot the likely storm center and intensity as a function of time:

Model Forecast Nor'easter Track and Intensity.  NWS

Up to an inch of precipitation will fall, with higher amounts along the coast, lesser amounts inland.  The rain may mix with snow Wednesday night.   Accumulation will be light, if any.   
This chart shows the liquid equivalent precipitation amounts:
Total liquid precipitation.  NOAA
This chart shows the probability of > 4” snow.  Note that there is only a slight probability of heavy snow well to our north, in the Poconos of PA.  
Probability of > 4" (Heavy) snowfall. NOAA
The most significant sustained and gusty winds will occur along the coast, from the Delmarva to Cape Cod.
Winds may gust 40-50+ mph along the Delmarva coast. 
Inland, including Baltimore:  Gusts 30-40 mph possible Wed night into Thursday morning.

Sunday, November 4, 2012

November 5, 2012: Strong Nor’easter Expected in Wednesday-Thursday Timeframe

In the wake of Superstorm Sandy, the prospect of another major East Coast storm is most unwelcome.   Yet, these intense coastal low pressure systems, called Nor’easters, are a staple of our region’s cool season.  Before we get into specifics about the storm expected here mid-week, let’s review a bit about Nor’easters.
What Is A Nor’Easter?
Like hurricanes, Nor’easters are powerful cyclonic vortices, producing high wind, damaging ocean waves and heavy precipitation.   Aside from similar sensible weather and oceanic impacts, there are big differences between these two types of storm.  I’ll point out these differences below.  Also referred to as coastal lows, Nor'easters can produce hurricane-category wind and water damage along the immediate coastline.
Nor’easters get their name from the direction of strong winds, blowing from the northeast, inward toward the core of low pressure.   Since these storms track along the coastline, the northeasterly winds sweep inland, pushing large amounts of water against the shoreline.   It’s also true that these storms track towards the northeast.  This odd opposition between wind direction and storm movement was first explicated by Ben Franklin in the 1700’s.
How and When Do Nor’easters Form?
Nor’easters are a cool-season phenomenon, impacting the Mid Atlantic and New England coastal regions between October-April, with a peak in the December-March timeframe.   During the coldest months of winter, these are the big “snowmaker” weather systems that occasionally bury East Coast cities under one or more feet of snow.  
Our region experiences a few of these coastal storms each year, but some years are more active than others.   For instance, the winter of 2011-2012 experienced very few of these storms, while the previous winter (Snowmagaddon) was visited by numerous, crippling Nor'easters.   A typical year may average a half dozen or so in our region;  in any given year, we experience more Nor’easter than tropical-type systems in the Mid Atlantic.
Nor’easters develop during the cool season because they require strong contrasts in air mass temperature to form and intensify.  This occurs when cold, Canadian air masses sweep over the relatively warm waters of the western Atlantic. The East Coast, particularly along the warm oceanic Gulf Stream off Cape Hatteras, is a preferred breeding ground.
Nor’easters are also creatures of the jet stream.  Their formation requires an intense trough of low pressure in the upper atmosphere.  The most intense Nor’easters develop when two separate troughs or “waves” collide and merge (phase) along the coastline.  Jet stream troughs are most numerous and intense during the winter months.
What Are The Differences Between Hurricanes and Nor’easters?
Recent hybrid-type storms, such as Superstorm Sandy, have blurred the distinction somewhat – but Sandy was highly atypical.   Hurricanes develop and intensify in the deep tropics during late summer and fall, while Nor’easters develop in the middle and high latitudes during late fall, winter and early spring. 
Nor’easters draw their energy principally from jet streams and fronts, while hurricanes are fueled by rings of intense tropical thunderstorms.  These thunderstorms are sustained by ocean waters that exceed 80 F.   However, note that the Gulf Stream's warm waters boost the energy level of a Nor’easter by about 20%-30%.
How Is A Nor’easter’s Impact Measured?
Many are familiar with the Safir-Simpson hurricane intensity scale, which rates hurricanes according to five categories of wind speed.   Nor’easters also have a rating system, called the Dolan-Davis scale, developed at the University of Virginia.  The scale ranks Nor’easters according to five categories of coastal damage, including storm surge and beach erosion.   If a Nor’easter produces heavy snowfall, the impact is assessed separately by the five-category NESIS (Northeastern Snowfall Impact Scale).
What Are The Weather Impacts of Nor’easters?
In coastal areas, a combination of high seas, freshwater flooding and strong winds can be as devastating as a single hurricane.  What’s worse, if several Nor’easters visit a region in a given season, their effects are cumulative.
The magnitude of impacts, particularly storm surge and winds, depends on several factors, including (1) storm intensity (measured by minimum central pressure), (2) pressure gradient, (3) speed of movement, and (4) track.   All of these factors are highly variable from storm to storm.   In terms of storm surge and wave inundation, the phase of the lunar tidal cycle is also critical.  Since these storms tend to be larger in size than hurricanes, their effects often linger across multiple high-tide cycles.
Strong high pressure to the north of the Nor’easter – often situated over Newfoundland and Greenland – plays an important role in creating damage.  For one, high pressure blocks the northeastward progress of the storm, slowing it down or stalling it offshore.  This prolongs the action of wind, rain and high seas.  Secondly, high pressure intensifies the pressure gradient along the north along the north side, increasing the strength of onshore winds.  Some of the most infamous and devastating Nor’easters – including the 1962 Ash Wednesday storm, 1950 Great Appalachian Storm, 1991 Perfect Storm, and even the 2012 Superstorm, all featured a strong, blocking high to the north.
So What’s Expected With This Week’s Nor’Easter?
The various forecast models have not yet come into agreement regarding intensity and track, as is typical for the 3-5 day time range.
- For the Baltimore region, in short, the period from Wednesday to Thursday will be raw, but nothing nearly as intense as Superstorm Sandy.
- Those along the coast will experience a 24-36 hour period of strong winds (gusts to perhaps 40-50 mph), several inches of rain and battering waves.
- Those of us lying inland (including Baltimore and D.C.) can expect 12-18 hours of moderate to heavy rain and gusty winds (perhaps up to 40 mph, approaching Wind Advisory criteria).  And it will be chilly – temps will probably not break 40 F.
- For the mountains to our west and north, there will be 12-18 hours of precipitation, which may include a period of heavy, wet snowfall…and gusty winds (to 40 mph).

- Factors that argue AGAINST measurable snowfall over Baltimore:  (1) the ocean waters are warm,  and the storm will draw significant amounts of warm air off the ocean, warming the lower layers of the atmosphere to temperatures above freezing;  (2) there is not a major source of deep, cold air inland associated with high pressure to our north.
In all, these are NOT hurricane-force impacts.   The Nor'easter's effects will be more of an inconvenience (likely including some local travel disruptions) but not a major, multi-day disruption.
Unfortunately, our concern and attention should shift further to the north and east, along the Jersey and New York coastlines, where much more significant impacts will be felt.