Wednesday, August 31, 2011

Further Analysis of Irene's Impacts On The Mid-Atlantic

Here are a couple of interesting analyses post-mortem on Irene.  The first was sent to me by UMBC Ph.D. student Aaron Poyer, who is in my research group studying severe storms.   This is the 7-day rain accumulation that brackets the storm's trek up the Eastern Seaboard.  The swath of red and purple tells the story - with 15"-20" across eastern North Carolina.  Keep in mind that these are early radar estimates and these maps will be refined over time.  I will post analyses that use rain gauge data as they become available.  Click on this image to see more detail.

Here is a more regionally-focused rain map released by NWS Washington-Baltimore:

For comparison, here is the rain accumulation map for Floyd (1999), for which readers of this blog know I have made a rainfall analogy:
However, while the track and rain are very similar, it is important to bear in mind that Floyd hit as a Cat 2, and evolved in a different manner than Irene.  Floyd underwent a classic extratropical transition across the Mid Atlantic, while Irene's was delayed until reaching New England.  But I find it very interesting that the rain patterns and intensities came out quite similar.

This next figure was sent to my by Ray Hoff, UMBC Physics Professor, who received it from a colleague of his at Johns Hopkins.   The graph shows a dramatic time series of pressure and wind, based on 1-minute surface observations, collected at NOAA's Chesapeake Bay Lighthouse. The lighthouse had a very close encounter with Irene's center.  The data have not been quality controlled, but the impacts are quite apparent.

The pressure trace is classic hurricane, featuring an exponential decrease and rise.   The storm's central region shows a sharp drop in wind speed at the time of minimum surface pressure.   Peak eyewall winds are symmetric before and after center passage...keep in mind that 60 kts equates to about 70 mph.   The fall off in wind (second wind) as the storm retreated unfolded much faster than the wind rise (first wind) as the storm approached.  This is interesting because the pressure trace rise and fall is perfectly symmetric.  

Since this is a water station, we wouldn't expect to observe a large gust factor (increase in gust speed over 1-min sustained winds).  For the Irene trace,  this is generally the case, except during eyewall passage, when gust factor became more significant.  As the sustained winds increased, so did the turbulence.   I wonder if the lighthouse structure itself generated the observed turbulence?  This suggests just how hard it is to get clean measurements of a meteorological phenomenon.

Here is a final figure, provided by NWS Washington/Baltimore, that maps the most extreme wind gusts across our region.  There is a gradient in winds, from NW to SE, with the strongest gusts (65-70 mph) over Calvert and St. Marys Counties, which were closer to the track of the storm than Baltimore County.  The pocket of higher winds through Montgomery, Frederick and Carroll Counties (50-55 mph) is interesting;  I would not have anticipated this ahead of time, given the more inland location of this region.

Sunday, August 28, 2011

August 28, 2011 After The Storm: Analysis of Wind and Rain

Surface Analysis, 8 AM EDT:   The storm's peak winds and rain were exiting Baltimore at this time.   The heavy rain shield remained highly asymmetric, concentrated west and north of is typical for tropical systems undergoing extratropical transition.  Dry air off the U.S. mainland was being swept into the southern half of Irene, evaporating rain and clouds there.  The stationary front - initially over the Appalachians - began merging with the large wind circulation.
Regional Scale Surface Analysis:  This short movie loop shows the extremely large footprint of the pressure field, and intense pressure gradient.   The radar is superimposed on the pressure field, so you can watch both features co-evolve.   The loop shows hourly fields from 10 pm EDT on 27 August through 6 AM 28 August.
Rain Accumulation:  Here are two scales of charts:  Regional scale and local scale.  Both maps are derived from radar estimates.   These totals have yet to be compared with gauge estimates.   Extreme point estimates may not be accurately measured by radar.  But in general, the rain accumulation agrees quite well with the predictions for our region.
Winds:  Shown here are several time series analyses of hourly winds.   The graph below shows the sustained winds, in mph, at three stations:  Dulles (IAD), Baltimore (BWI) and Washington Reagan (DCA).   DCA was the winner with highest sustained wind, which peaked above 40 mph at 1 AM.  I constructed these graphs very quickly...the number "1" across the bottom starts the graph at 9 AM on August 27.  The observations are hourly and end at 11 AM this morning.
Here are similar plots but comparing sustained wind vs. peak gust (mph) at each of the three airports.  The time convention is the same as described above.  At BWI, the peak gust was 51 mph at midnight.  In the BWI plot, you can see evidence of higher gusts when multiple rain bands moved through.  The highest gust was at DCA - 57 mph at 1 AM.
Wind Damage:  Here are utility outage maps I grabbed off the web at 9 AM this morning.  Outages were most extensive along and east of I-95...closer to Irene's center of circulation.


Saturday, August 27, 2011

Aug 27, 2011 6 PM: Irene Weakens to 80 mph But Moving Slowly

The 5 PM update from the Hurricane Center has the storm moving slowly N-NE at about 14 mph with sustained winds of 80 mph.   Irene is predicted to be directly over Ocean City at 2 AM, also the time when maximum winds (sustained near 40 mph) will be impacting the Baltimore region.
Hurricane Center Irene position at 2 AM Saturday

Here is the surface weather map at 2 PM today, showing the large, asymmetric and mainly inland rain shield.  Note that while Irene is likely pulling in dry air off the U.S. along its southern side (which is helping to weaken the storm), it is not interacting directly with the stationary front (along the Appalachians), as I thought might happen.   It is also not fully embedded in the jet stream trough coming through the Great Lakes.  Thus, extratropical transition - a process by which tropical cyclones acquire weather fronts and morph into an extratropical cyclone - may not occur until the storm is over New England.

Surface weather map at 2 pm Saturday

The final image from late this afternoon shows the rainfall overlaid on top of the detailed surface pressure distribution.   Note the intense pressure gradient, and the enormous size of the pressure field.   A note about the winds:  The surface winds to the left of track, over land, are significantly weaker than right of track, over water.   This large asymmetry is a consequence of land friction, which slows the winds, and the motion of the storm.   However, just a few thousand feet above the surface, the winds are not slowed by friction.  These strong winds are brought down in occasional pulses to the surface within spiral rain bands.  Corridors of intense wind gusts tend to occur within the strongest rain bands.   This process contributes to the localized nature of most intense wind damage in a hurricane over land, which tends to be streaky in nature.

Rainfall and pressure field at 5 pm Saturday

Aug 27, 2011 11 AM: Irene's Predicted Baltimore Impacts Are On Target

First, let's look at the 10 AM radar image of the storm.   The eye is making contact with North Carolina's Outer Banks.   The distribution of heavy rain is asymmetric, with heaviest amounts on the northwest side.  This distribution is typical of storms that move into the Mid Atlantic.  The heavy rain shield will continue to move north and west through the morning and afternoon, enveloping the major cities (DC, Baltimore, Philadelphia, NYC).   Periods of heavy rain will alternate with lulls as individual rain bands sweep through.  Maximum rains of 10"-12" are expected along a narrow stripe that extends up the center of the Delmarva, southern Maryland, Delaware, New Jersey and NYC - essentially along the projected storm track.   Several inches are anticipated locally in Baltimore.

10 AM radar (Weathertap)

Hazards:  These maps from NWS Washington-Baltimore office are shown below.  The reasoning has not changed much since yesterday's post.  Note that hazards are not shown east of the Chesapeake Bay as this region is not covered by NWS Washington-Baltimore.   Timing is such that the most intense wind and rain are expected overnight.  Conditions improve steadily through the day on Sunday.

11 AM Hurricane Center Storm Update and Track:   The storm is down to 85 mph sustained winds - a low end Cat 1 storm.  Continued slow weakening is expected.  The storm is feeling impacts of wind shear and dry air off the mainland.   As the storm passes east of Baltimore tonight, the intensity should decrease down to 75 mph, then weaken to tropical storm Sunday morning.  Below is the track and radius of expected tropical storm wind at 8 pm tonight (Stormpulse):

Friday, August 26, 2011

Aug 26, 2011 5:30 PM Irene Impacts To Come A Bit Sooner

At 5 pm, Irene has weakened to 100 mph.   This is weaker than what was forecast yesterday and the storm is now rated a low-end Category 2.   The Hurricane Center reports that the storm's inner core has lost its structure, and strong winds are impinging on the storm at upper levels.  Thus, the storm is likely experiencing the effects of increased wind shear, cooler water, and perhaps dry air entering along its west side.  Any opportunity for re-intensification seems to be dwindling.

However, Irene remains a very large storm, with tropical storm winds extending 250 miles east of center, and a large core of hurricane-force winds.  Irene is still a very potent, serious system, and will remain so, for at least the next 48-72 hours.   In fact, the Hurricane Center cautions us that whether the storm is rated a weak hurricane, or an intense tropical storm...the hazards will remain the same.

The storm is expected to begin accelerating to the north and northeast tomorrow, which will usher in its effects a few hours sooner for the Baltimore region.   The worst weather will come Saturday night, and the effects will begin to diminish Sunday morning.   The rains will likely end early Sunday, but the winds may take longer to calm down.  In fact, even though the storm will be moving away from the region on Sunday afternoon, some of the faster winds at higher levels may mix down to the surface, creating gusty conditions.   These winds mix down because of turbulence, which is caused when the Sun warms the surface and the air begins to percolate.

The first image below is a radar snapshot of the storm from late this afternoon, showing outer spiral rain bands scraping North Carolina (courtesy of Weathertap).  The second is a special satellite image, which shows the amount of moisture (water vapor) in the atmosphere (courtesy of Weathertap).  Notice the very dry air across the southeast U.S., right up along the west side of Irene.  The final image shows the Hurricane Center's 5 pm track prediction (Courtesy of Stormpulse).  The storm's position is shown at 2 AM Sunday, when Baltimore's weather deteriorates.   The storm is predicted to be a weak Cat 1 at this time (80 mph).  The outer white semicircle is the predicted radius of tropical storm force wind.  Note that Baltimore is right on the edge.

August 26, 2011 Baltimore's Impacts From Irene Remain On Course

Here is the current surface map (from Intellicast).  Irene is most conspicuous, and notice the weather front the storm will interact with:

But there will be other interactions with Irene, including the jet stream level;  the storm will become embedded in a trough (region of low pressure) moving towards us from the Great Lakes.  Both the front and the trough will energize the storm, even as it is moving over cooler water, and this effect could sustain the storm as it moves into New England, and enhance the rainfall.   Here is the forecast map for tomorrow (Saturday) evening:

Notice how the weather front begins to reorient along the coastline.  As tropical humidity is lifted along the front, very heavy rains will develop.

All of this is part of the complex process called extratropical transition - a tropical storm begins to lose some of its tropical identity, and take on characteristics of a mid-latitude weather systems.

Now let's look at the likely impacts - using NWS graphics.

Official Watches and Warnings - As of 10 AM today:

Winds:  As of 10 AM today:

Baltimore is on the edge of low and moderate wind impacts...therefore a slight shift in storm track or change in storm intensity will change the impact level.   We are expecting sustained winds 30-40 mph over Baltimore's metro west, 35-45 mph sustained over metro east, with higher gusts.

Rainfall:  We've recently had considerable rain, so soils are moist.  Given the intense rain rates in hurricane rain bands, and frequent rain band passage, it won't take much to cause local flash flooding.  I expect 2"-4" broadly over Baltimore, more to the east (3"-6"), much more over the Delmarva (12" or more) - click on the map below to zoom in:

Storm Surge on the Bay:   This is complicated somewhat by the fact that wind direction during the strong wind period will be shifting throughout the event.  Initially, northeasterly winds may push water along the Bay's western shore, through the day on Saturday.  Saturday night and early Sunday morning, the strongest winds blow from the north, pushing water down and out of the Bay.  During the day on Sunday, the winds switch to westerly and begin to slacken, but this may lead to flooding of the Bay's eastern shore for a time.

Timing of Impacts:   Irene will likely be a Cat 1 storm as the core passes up Maryland's coastline tomorrow night and early Sunday.  It has a large circulation, so the weather impacts will take time to unfold, and time to wind down.  Rains will begin tomorrow afternoon.   The most intense period of rain and wind will be after midnight Saturday.   But the storm is progressive i.e. it will not stall, as it will get pushed along by south-southwesterly winds in the jet stream;  in fact, it will begin to accelerate toward New England.

Thursday, August 25, 2011

August 25, 2011: Irene's Threat Intensifies Across Baltimore Region

Overview:   Prediction models have trended toward pulling Irene's track back westward, bringing Irene's core close to the Maryland coastline.   While the coast is 125 miles to our east, Irene has a very large wind circulation, larger than average.   Presently a major category hurricane, Irene is expected to weaken to Cat 2 or 1 by the time the storm passes through eastern Maryland.  However, Irene may be slow to wind down because of its high initial intensity, and the fact that much of the circulation will still be over warm Gulf Stream water.  And, as hurricanes weaken, their wind circulation tends to while winds may weaken in the core, winds on the fringe (i.e. central Maryland) may intensify. 

An additional factor is that this storm will likely undergo extratropical transition as it moves through Maryland.   The prediction models are swinging a jet stream trough through the Great Lakes on Saturday.  This is a ripple or disturbance that creates rising motion in the atmosphere.   It is expected to merge with Irene's circulation over eastern Maryland and New Jersey.   The additional energy source that this trough provides may help to re-energize Irene, countering some of the weakening due to cooler ocean water.   The additional uplift of air will squeeze additional tropical moisture out of the storm, creating heavier rains.   And Irene is also expected to interact with a weather front over southern Maryland, from which it can draw additional energy.   There is precedent for this type of extratropical transition, namely, Hurricane Floyd of 1999.

Let's talk about likely impacts in the Baltimore region.  The main time frame for these impacts is Saturday afternoon through Sunday afternoon, with the most dangerous conditions early Sunday morning.

Storm Surge on the Bay:  Initially, strong winds from the north will blow water down and out of the Bay.  However, on Sunday afternoon, the winds will shift direction, blowing strongly from the west and southwest.  This may cause flooding along the Bay's eastern shore - particularly since this weekend we are experiencing spring tides (larger high tides than normal).

Heavy Rainfall:  These rains will likely start falling well before the actual storm center passes by.  These "rains in advance" are part of the extratropical transition process.   Light rain may start to fall Saturday morning.  It will intensify through the day, and will be showery in nature.  The heaviest rains will fall overnight Saturday into Sunday, and may taper early Sunday afternoon.  Below is the latest NWS guidance on rain accumulation.  Amounts in the 3"-6" range are possible in and around Baltimore.  Notice the heavy axis of 13"+ rains along the Delmarva:

Winds:  With the storm potentially tracking closer to the coastline, and because of the storm's large size and intensity, the wind threat across Baltimore has increased.   Being on the right side of the track diminishes the winds somewhat, but winds of tropical storm force (40 mph and greater) are possible across Baltimore.  Below is a sequence of wind intensity forecasts from the NWS, showing the maximum expected wind gusts:

Wind Gust Forecast, Knots, 2 PM Saturday

Wind Gust Forecast, Knots, 2 AM Sunday

Wind Gust Forecast, Knots, 8 AM Sunday

Wind Gust Forecast, Knots, 2 Pm Sunday
Notice the enhancement of winds along the Bay.  Peak winds may gust to 70 mph in and around Baltimore early Sunday morning.   Hurricane force wind gusts (75-80 mph) may occur along the Bay.  Given saturated soils from recent rains, and trees in full foliage, all area utility companies (BGE, PEPCO) are advising customers to prepare for possible widespread, multi-day outages.   The last time our region experienced power outages of this magnitude was during Hurricane Isabel of 2003.

Wednesday, August 24, 2011

August 24, 2011: What Will Irene Bring To Baltimore? Epic Rainstorm Possible Across New England

We're getting to the point where we can begin contemplating some local effects of Irene.

Winds:  Here is an image from NWS Sterling that is very interesting.  It is this morning's prediction of peak sustained winds across our region on Sunday as Irene slides by to the east.   This shows the very tight gradient in wind.  According to this forecast, those in Baltimore won't have to go very far east to experience tropical storm force winds (in this image, 43 kts = 50 mph).  This is a great example of how the Bay enhances the winds in a hurricane.  On the west side of the storm, Irene's winds will be northerly, and notice how the winds really scream down the Bay.  This is in contrast to Isabel, which slid by to our west, sending strong winds and a surge moving up the Bay from the south.

Could Baltimore still experience tropical storm force winds (> 39 mph)?  Possibly.  This depends critically on track.  It also depends on Irene's intensity when she moves by.  It also depends on the size of the storm, which is actually linked to intensity:  When a hurricane weakens, the winds near the center diminish, but the radius of tropical storm force winds actually expand outwards. depends on the storm's speed of forward motion.   To the east of the storm's track, you add the wind speed to the speed of movement (both effects reinforce).  To the west of the storm track, you subtract the speed of movement from the wind speed (the effects tend to cancel).  So to minimize the wind speeds over Baltimore, we actually want Irene to move faster!

Regional prediction of winds on Sunday morning due to Irene's passage to our east.

Rainfall:  I still expect this could be a concern, starting as early as Saturday morning.  Big storms like Irene, even when along the coast or offshore, can circulate Atlantic moisture inland well in advance of the actual storm center. Moderate to heavy rain may begin spreading up the East Coast 24-36 hours ahead of the storm. The models are certainly presenting this scenario:

GFS model forecast for Sunday morning showing moderate-heavy rain spreading along the East Coast hundreds of miles north of Irene's center.
But the other aspect to note is the very tight gradient in rainfall.  Just a few miles can make all the difference between drizzle and a real soaker.   This is typical of hurricanes, which tend to generate very compact regions of hazardous weather.

Now slightly further north and east of Baltimore, the ingredients may come together for a monstrous rain storm - from NJ northward.   By this time, Irene will likely be undergoing a process called extratropical transition, whereby the storm's energy and tropical moisture begin interacting with mid-latitude weather systems.  The GFS model (below) suggests that Irene will be absorbed into a trough in the jet stream swinging across the Great Lakes.  This adds a lot of energy to the system.  Additionally, a jet streak over New England will enhance the ascent of moist air.  And...Irene will likely join forces with a stalled cold front over the Mid Atlantic.  Vigorous uplift along the front, which may become reconfigured as a coastal front, will further enhance the rain. 

GFS model forecast for Sunday morning, showing Irene linking up with a jet stream trough and jet streak.
Below is a figure from the Hydrometeorological Prediction Center that shows the predicted rain accumulation for the next five days...the swath of rain in excess of one foot will pose serious problems for New England:

HPC heavy rain prediction during Irene's expected extratropical transition.
You might recall what Floyd did in 1999 - massive flooding (12"-15") along the Eastern Seaboard from North Carolina northward.  As you can see below, Floyd's (1999) track is virtually identical to that predicted for Irene:

Storm Surge:  Not a concern for the Bay;  the strong northerly winds will actually push the water out of the Bay, creating an abnormally low tide.   By the way, it's very important to consider the height of the tides when predicting storm surge.  This Sunday, with the new moon, Earth will experience a spring tide, which pushes high tides even higher.  This could exacerbate ocean inundation along the coast.

Tornadoes:  Not a concern for Baltimore.  Most hurricane-spawned tornadoes develop only when the right front quadrant lies over land. 

Bottom Line:  With the storm predicted to be tracking so close to the coastline, heading toward the beach this weekend is a very bad idea!

Tuesday, August 23, 2011

A Further Jog Eastward For Irene

August 22, 2011:  The 5 pm Hurricane Center official forecast track nudges the storm further east.  Brief landfall may be expected along the N.C. Outer Banks early Sunday morning. 

Always bear in mind the size of typical errors in a forecast that is 4-5 days out, and this track hinges on a large-scale flow pattern that has yet to fully materialize i.e. a weakness in the subtropical ridge and a trough approaching the east coast of the U.S.  Don't be surprised to see the official track wobble west-east over the next several days.

Much can still change, but the trend of an eastward shift in track continues.  Even as the track shifts further east, the storm circulation is large and will potentially spread its weather impacts well inland.

August 23: Virginia Earthquake Might Be Tied To Deep, Ancient Fault Zone

At 1:51 pm EDT today, a moderately intense earthquake (magnitude 5.8) shook the ground for 30 seconds, centered on the town of Mineral, Virginia.  Mineral is located along the Piedmont about halfway between Richmond and Charlottesville.   The USGS estimates that the quake's epicenter was buried 3.7 miles deep.  The quake was felt from New York to North Carolina.  One thing to note about east coast earthquakes is that they are felt over much larger areas than west coast quakes, due to the unique geology of the east.  Additionally, the very shallow nature of the quake contributed to its widespread footprint.  The quake was felt in 22 states, from Maine to Florida, from Virginia to Illinois.

When you plot Mineral on a geological map of Virginia, you note that it sits within the metamorphic rock province of Virginia.   The region encompasses the Central Virginia Seismic Zone.  The rock units are extensively fractured and deformed, from ancient compressive forces that raised the Appalachian Mountains.  Mineral, Virginia sits exactly over an extensive fault zone, called the Spotsylvania Lineament, that cuts northeast to southwest across the state.  To geologists, the lineament is somewhat of a mystery.  Some believe that it developed hundreds of millions of years ago, during the Paleozoic Era, as part of tectonic collisions between small crustal plates called terranes.  

If the quake was indeed associated with this ancient, deeply buried fault zone, it is interesting to speculate that a geological feature this old still occasionally must relieve some strain (stored energy) - restless Earth at its best!

August 23, 2011: Irene To Become Major Storm, Landfall Likely Over North Carolina, Baltimore May Experience Effects

Irene is moving over very warm waters of the Sargasso Sea with little wind shear to impede continued deepening.  Intensification to Cat 3 or Cat 4 seems more certain.   The prediction models also remain fairly tightly clustered in terms of track, and the model consensus has trended toward an eastward shift in track.  The Hurricane Prediction Center at 11 pm has nudged its official track eastward a bit, with landfall most likely over eastern North Carolina.   The storm center may cross the mouth of the Chesapeake Bay early Sunday.  Given these trends, what is the likely impact on the Baltimore region?

The storm's predicted intensity makes this a particularly dangerous system, even a hundred or more miles from center.   Certainly, the eastward-trending track is a good thing for Baltimore.   But if part of the storm's circulation remains over the warm Gulf Stream, it could maintain hurricane status as it moves by Maryland.   A more intense storm means stronger winds, across the board.   Also, the Maryland impact is still 5 days out.   The track error at 5 days is 250 miles.   That's a huge swath of uncertainty...ranging from the storm passing hundreds of miles to our east, to tracking up the spine of the Appalachians.   But the most likely scenario puts Baltimore in the left semicircle, a fair distance from the center.  Winds are usually 20-30 mph weaker on the left side, are further reduced by surface friction compared to open water, and diminish rapidly as distance increases from the center.  But as these tropical cyclones interact with mid-latitude weather systems and the Appalachians, more often than not the location of heaviest rainfall shifts to the left of track.   So what we miss in terms of wind, we gain in terms of heavy rain. 

If track trends continue eastward, the worst case scenario would involve the storm moving up the western side of the Bay, with the center passing close to Baltimore.  This would potentially generate a powerful storm surge moving up the Bay, and threaten the Tidal Potomac, Baltimore Inner Harbor, and Annapolis...a la Isabel in 2003.   Let's hope this isn't the case.

As for now:  Sunday and Sunday night looks like a rainy, breezy day in Baltimore. 
The image above (from Stormpulse) shows the 11 pm official track and cone of uncertainty, superimposed on a dozen predictions of storm track made by the various models.  When the impact is still 4-5 days out, it's important not to get "hooked" on the solution of any one model...but rather, look for trends that remain consistent from day to day.  That one trend thus far has been to shift the storm's track progressively further and further east.

Monday, August 22, 2011

August 22: Irene Predicted To Become A Major Hurricane

The Tropical Prediction Center forecasts Irene to move up the Eastern Seaboard east of Florida and intensify to a Category 3 (Strong) hurricane.   The latest landfall guidance at 5 pm today brings the storm inland along the NC/SC border 2 pm this Saturday. 

It's still too early to discern the potential impacts on Baltimore.  When post-tropical remnants come inland, and combine with pre-existing weather systems in the Mid Atlantic, we call this process extratropical transition (ET).  How ET plays out in terms of who gets what type of severe
weather (flash flood, river flooding, high winds, tornadoes, damaging surf) depends on myriad factors,
including:  (1) storm intensity; (2) storm size;  (3) storm speed;  (4) storm track;  (5) interaction with the Appalachians; and (6) interaction with weather systems such as fronts, areas of low pressure and jet stream features.   Every post-landfall scenario is unique, and many are poorly forecast.   Just look at the different outcomes for Hazel (1954 - damaging wind storm), Camille (1969 - 27" rain in 8 hours in the Blue Ridge), Agnes (1972 - widespread river flood in the Susquehanna Basin), Isabel (2003 - widespread wind damage and storm surge in the northern Chesapeake Bay), Gaston (2004 - localized flash flood killing 13 in Richmond) and Ivan (2004 - tornado outbreak across Northern Virginia).

But I am becoming increasingly confident that Baltimore will experience periods of heavy rain and gusty winds for some portion of this weekend.

August 21, 2011: Another Round Of Storms Pummels D.C.-Baltimore

Today's storms fired ahead of a cold front approaching from the Ohio Valley.   Because of intense late morning afternoon heating, the storms fired early, initially over the Appalachians, then consolidated into a narrow line over the I-95 corridor.  The Storm Prediction Center anticipated severe activity and issued several watches over the Mid Atlantic (yellow boxes on the surface weather map, below).  Ahead of the cold front, a tongue of very unstable air resided over the Virginia and Maryland Piedmont and Coastal Plain.  This, combined with modest wind shear, caused several storms to become severe.  The other maps below show the regional radar, reports of damaging weather, lightning activity and rain accumulation.  Wind damage was fairly isolated but rain amounts locally exceeding 2" fell along the I-95 corridor.    Over 20,000 power outages resulted from wind and lightning damage.                                               t
Surface Weather Map 8 PM EDT August 21, 2011

Regional Radar View 3 PM EDT August 21, 2011

Severe Storm Reports for August 21, 2011

Lightning Strikes August 21, 2011

Rain Accumulation August 21, 2011