Here Comes Irene

I've decided to post a rare non-winter blog entry because of the impending hurricane here on the East Coast.  As usual, the post will be more concerned with effects in the DC area since that's where I live.

Irene is currently a Category 2 hurricane.  She has weakened a bit in the last day or so and has maximum sustained winds of 100 MPH placing it just inside Category 2 status (which begins at 96 MPH).  The intensity will likely not change significantly before landfall in extreme eastern North Carolina - probably in the vicinity of Cape Lookout tomorrow morning.

Two things to remember:

• This storm is large.  So is the extent of the rain and wind fields.  For this reason, its projected path offshore is still close enough to the coast to cause problems for us.
• However, the cut-off between significant rain and just a few showers is usually pretty sharp with hurricanes passing to our east.  If the storm follows the current forecast track, I'd guess that we'll get significant rain in the DC area - especially south and east of the city.

If this type of forecast sounds familiar it's because a hurricane's impact pattern is similar to the large snowstorms we get in the winter.  Track means a lot - although exactly where the center travels matters less...especially with a storm this large.  That's why the National Hurricane Center cautions people to concentrate less on the line that marks center track and just heed the warnings they issue.

Speaking of which...

We're under a Tropical Storm Warning here in the DC area - mostly for areas near water but including the following counties:

DISTRICT OF COLUMBIA...NORTHERN BALTIMORE...HARFORD...
MONTGOMERY...HOWARD...SOUTHERN BALTIMORE...PRINCE GEORGES...ANNE
ARUNDEL...CHARLES...CALVERT...PRINCE WILLIAM/MANASSAS/MANASSAS
PARK...FAIRFAX...ARLINGTON/FALLS CHURCH/ALEXANDRIA...STAFFORD...

SPOTSYLVANIA AND KING GEORGE.

And in St. Mary's county, the Tropical Storm Warning is now a Hurricane Warning.

So what should we expect?

The storm is likely to move fairly far off the coast so I expect the worst conditions on the Eastern Shore. 

Probably 6 inches of rain and winds gusting to 80+ there at times tomorrow.

Here, things start going downhill late tonight.  The first bands from Irene could arrive tomorrow morning

and the winds will be increasing all day.  Expect winds in the 20-30 MPH range by tomorrow afternoon,

increasing to 30-40 with higher gusts tomorrow night.  The worst should be tomorrow evening and 

overnight as the storm passes by to our east. 

Again, the cutoff for precipitation is expected to be fairly sharp.  I'd guess areas around I-95 could pick

up 1-3 inches of rain, more to the east, less to the west.  Places out in the Blue Ridge might just have a 

cloudy day.

This is a storm to take seriously but in DC, I also don't see the need to go out and buy all the bottled water

you can find.  Flooding is possible, power outages are probable and 50 MPH winds with saturated ground 

can make trees a problem.

The next official advisory from the National Hurricane Center comes out at 5 PM.

For now, here's an excellent graphic with the forecast storm track:

Boston Globe Irene Tracker

Quick Burst of Snow Tonight Into Tomorrow Morning

It's been a while since anything interesting (aside from some unusually warm weather) has happened around here but tonight we have a fairly unusual situation developing that could cause a quick burst of snow and some minor accumulations around the area.

First up is a cold front which will be moving across our area this afternoon.  Precipitation associated with the front will begin as some light rain which will increase a bit in intensity as the afternoon becomes the evening.  The cold air will take a little while to catch up but eventually it will move across the area from north to south bringing a changeover in the precipitation that's falling from rain to sleet and eventually over to all snow.

At the same time, a low pressure area will move out of the midwest and cross the region bringing the bulk of our precipitation - especially towards midnight and into the early morning hours Tuesday.

This should be a brief storm but it could be a bigger deal than it even looks now.  That "banding" effect we've talked about before could lead to locally heavy precipitation amounts and the model trends have been colder and wetter the last few runs.

Waiting for the afternoon models but I think 2 inches is a good possibility in the immediate DC area, more as you go north and west.  There is the potential for more - maybe 4 inches or more - but before we go there, I'd like to see what the afternoon models have to say.  But, just to put it out there, here's the GFS 24-hour total precipitation forecast:

You can see we're squarely in the .5-.75 liquid precipitation range.  Given that we may lose a bit to rainfall and some sleet, this argues for 4 inches or so.

One thing the precipitation will be fighting is the relatively warm ground temperatures that have developed after the past 5 days or so.  On the other hand, the precipitation could be quite heavy later tonight.

Stay tuned...

Snow Totals Currently From 2-6 Inches Around the Area - Snow Will Continue

Here in Arlington, I measure 2 inches of wet snow that is packing itself as it falls.  If the atmosphere were colder, we would probably have 3-4 inches at this point.

Snow will continue into the night ending from southwest to northeast around 10-11 PM.

A few of those small-scale bands have formed and are producing snowfall rates of 1-2 inches per hour.  I expect snow totals will vary greatly from place to place depending on those bands.

In the radar below, you can see the upper-level energy moving to the east where it will phase with the surface low offshore.  As it starts this process there's a "twisting" motion to the precipitation which will keep things going for a while - even though you can see the back edge of the snow around Charlottesville.

Enjoy!

More Data Coming In - Just Keeps Getting Better

The new data looks supportive of more snow - but a lot will depend on those small-scale conditions we've talked about before.  Get under one of the bands and you could see some hefty accumulations.

Right now 6-8 inches seems likely, 10 is within reach here in the immediate metro.

Interestingly, the bullseye is setting up where the models had it this morning - right over DC and up to the northeast.  We may end up with more snow in the immediate DC metro than in points to the west in Loudoun County.

Rain starting here in Arlington - a little windier.  Reports of lightning and thunder in Woodbridge - mostly rain for now...

Major Band of Activity Headed Into DC Area

A line of strong radar returns is moving through the Fredericksburg area and heading north towards us.  Reports with this line have included heavy rain changing to sleet and then to snow fairly rapidly.

This is an unusual situation where you'll probably be able to watch as the thermal profiles in the atmosphere change and support frozen precipitation.

To add to the fun, there are numerous lightning strikes associated with the line.

Shouldn't be long now before the activity starts around here.

Current Radar

You can see the precipitation in western Virginia and developing now to our south.  The low pressure center is clearly defined in the swirl that, at this moment, is located just along the border of Virginia and North Carolina.  I'm expecting snow to start falling - perhaps initially as a bit of rain which should rapidly change to snow - within the hour.

Models Continue to Show More Snow

The short-range models and radar trends continue to advocate more snow for our area and an unusually high probability of convection - thundersnow - this afternoon.

5 inches or more seems likely at this point with higher amounts out to the west.  Portions of Fairfax and Loudoun counties could see almost 10 inches.

The Winter Storm Warning has now been extended for all but the southern portion of Maryland.  The second round of precipitation is moving into the Charlottesville, VA area now.  The center of the upper-level energy seems to be in western North Carolina.

I worry that the worst of this will be right around rush hour and if you're reading this and you're in the DC/Baltimore Metro area (especially if you're to the north and west of the cities), you might consider leaving early.  The Federal Government just announced that they will close 2 hours early.

From the National Weather Service:

...RAPIDLY DETERIORATING CONDITIONS FOR THE MID TO LATE AFTERNOON
AND EVENING RUSH HOUR COMMUTE...

AFTER A BREAK IN PRECIPITATION THIS MORNING...ACCUMULATING SNOW
WILL OVERSPREAD THE AREA THIS AFTERNOON AND EVENING. A MIX OF RAIN
AND SNOW WILL QUICKLY CHANGE TO ALL SNOW DURING THE AFTERNOON AS
IT BECOMES HEAVY. THE PERIOD OF HEAVY SNOW WILL COINCIDE WITH THE
LATE AFTERNOON AND EVENING RUSH HOUR COMMUTE ACROSS MOST OF THE
REGION...INCLUDING THE WASHINGTON DC AND BALTIMORE METROPOLITAN
AREAS. CONDITIONS IN SOUTHERN MARYLAND WILL DETERIORATE AROUND
SUNSET AS PRECIPITATION CHANGES TO SNOW.

THE HEAVY SNOW WILL REDUCE VISIBILITIES BELOW A QUARTER MILE WITH
SNOWFALL ACCUMULATION RATES AT TIMES OF 2 INCHES PER HOUR. THIS
WILL PRODUCE DANGEROUS TRAVEL CONDITIONS DURING RUSH HOUR. SNOW
WILL MOVE OUT OF THE AREA BY MIDNIGHT.