Storms have moved out of D.C. metro; Low chance of more later this evening


Radar & lightning: Latest D.C. area radar shows movement of precipitation and lightning strikes over past two hours. Refresh page to update. Click here or on image to enlarge. Or see radar bigger on our Weather Wall.

Overview:

  • Severe thunderstorm and flash flood watches are in effect until 10 p.m. ET.
  • Timing: Storms are currently active around the area, likely through 6 p.m. ET, possibly lingering through 9 p.m. ET.
  • Hazards: Heavy rain is likely, and could approach two inches per hour. Strong winds are also possible. Tornadoes and large hail are possible but unlikely.
  • Power outages are possible. Get updates in our Outage Tracker.

4:30 p.m. update:  Rain has moved out of the D.C. area to the east, though moderately strong storms will likely continue in eastern Charles, Calvert, and northern St. Mary’s Counties for at least another 30 minutes. Storms have begun to train to the northeast in this region, and very little eastward movement remains. Radar-estimated rainfall rates are maxing out at 2 to 2.5 inches per hour in central St. Mary’s and southern Calvert counties.

This will be the last update for this blog — look for the P.M. Update at around 5 p.m. ET. There is a small possibility that we will see some weak storms move through the area later this evening, and we will update that post with any information.

3:55 p.m. update: The storms have mostly moved outside the Beltway to the east. Some light to moderate rain is still lingering in the eastern metro area. The storms have slowed down a bit, and there are were some regions of heavy precipitation building behind the main line of storms, east and south of the Beltway, although those are diminishing quickly now. Prince George’s, Calvert, and Charles counties could be seeing heavy rain for the next hour or so, until these storms either move off to the east or dissipate.

There’s a chance that another line of storms could move through between 5 and 8 p.m., though the atmosphere around D.C. has been mostly tapped from this afternoon’s bout of storms.

3:17 p.m. update:  The strongest storms, including the storm in central Anne Arundel that briefly had a tornado warning, have moved off to the northeast. Another line of strong storms with potentially heavy downpours and gusty winds have moved into the D.C. metro. These storms extend southwest from downtown D.C., along and east of I-95. A severe thunderstorm warning is in effect for a storm in Charles County, just south of St. Charles, and this storm has had a velocity couplet on it for a while, indicating the chance of a tornado. Reports of a funnel cloud have also come out of this storm while it was in King George County, Virginia.

(NWS)

(NWS)

2:55 p.m. update: A tornado warning has been issued for central Anne Arundel County until 3:30 p.m. ET. The potential tornado was detected by radar. Residents in Odenton, Millersville, South Gate, and Severna Park should be in their tornado shelters.

(NWS)

(NWS)

2:50 p.m. update:  Heavy rain has started within D.C., and it is pouring in Takoma Park again. This won’t be the only storm for downtown D.C. — more will approach from the south and west.

2:30 p.m. update:  A new severe thunderstorm warning has been issued for northern Prince Georges County and central Anne Arundel County until 3:30 p.m. ET.

(NWS)

(NWS)

Storms are approaching downtown, and could enter the area any time over the next hour. The storm in the northern metro area is building to the south, and is visible from downtown. There are also storms to moving in from the west, but new storms could pop ahead of that at any time.

Radar at 2:30 p.m. ET (Weather Underground, WunderMap)

Radar at 2:30 p.m. ET (Weather Underground, WunderMap)

2:15 p.m. update: A severe thunderstorm warning is in effect until 2:45 p.m. for northern D.C. metro, including cities of Rockville, Takoma Park, College Park, and Bethesda. These locations should expect heavy rain and strong winds over the next 20-30 minutes. Penny-size hail has already been reported in Bethesda.

(NWS)

(NWS)

1:57 p.m. update: A flash flood watch has also been issued for the area, in effect until 10 p.m. ET. The National Weather Service cautions that only two inches of additional rainfall will be needed to cause flash flooding in some areas, after last night’s storms, and that rainfall rates of two inches per hour are possible this afternoon and evening.

1:48 p.m. update: A severe thunderstorm watch has been issued for D.C. and the surrounding area. The watch spans an area from Connecticut to North Carolina. This watch is in effect until 9 p.m. ET.

(NWS SPC)

(NWS SPC)

1:35 p.m. update: A mesoscale discussion from the Storm Prediction Center puts our odds for a severe thunderstorm watch at 80%. This watch could come soon, given the storms that have already begun to pop in our area.

(NWS SPC)

(NWS SPC)

Original post:

“Storm fatigue” may be the operative word describing July thus far in Washington.   Today, the setup continues to favor widespread thunderstorm activity during the afternoon and evening.  Some of these storms may reach severe levels.   But a very pleasant stretch of days is just around the corner.

Heavy rain and frequent lightning are likely in storms that develop today. Pockets of damaging winds are possible, isolated cases of flash flooding cannot be ruled out (due to very moist air overhead) and there is a remote chance of large hail and/or an isolated tornado or two, mainly north and northeast of the District.  Storms may start to develop in our western areas around 1 p.m.  Like yesterday, we expect the greatest coverage of storms in the 3-6 p.m. window across the metro region (west to east); although it’s possible it takes until 9 p.m. or so for the storm risk to wane.

Technical discussion

A strong cold front arrives later today

We’ve been anticipating the slow approach of a genuine, mid-July cold front and its attendant dome of Canadian high pressure.   Ahead of this front, however, the Bermuda High continues to draw in a stream of hot, humid air (Figure 1).  Aloft, at jet stream level, a vigorous trough in the jet stream is on our doorstep.  The arrival of the jet stream core has increased wind shear conditions across the Mid-Atlantic (Figure 2).  Today’s shear is unidirectional – meaning the speed of winds increases with altitude, but all winds blow from the same direction – but it’s very significant, on the order of 45-55 knots.    This is a very large value for July!

 

Figure 1. Surface analysis valid 9:30 AM this morning.  (Intellicast.com)

Figure 1. Surface analysis valid 9:30 AM this morning.  (Intellicast.com)

 

Figure 2. Jet stream forecast valid 8 p.m. this evening.  (Unisys Weather)

Figure 2. Jet stream forecast valid 8 p.m. this evening.  (Unisys Weather)

In figure 2, note the strong U-shaped dip marking the jet stream trough.  Along its east side lies a pocket of fast wind (green shades).  In addition to creating shear, this so-called jet streak draws up air vigorously beneath its right-side entrance region.   This dynamic quadrant is parked right over the Mid Atlantic today.

So…to recap we have strong frontal uplift, dynamic uplift from the jet stream, and very significant shear.   Accordingly, the Storm Prediction Center has upgraded our severe thunderstorm probability, mainly for damaging wind, to 30% (Figure 3).  This is an upper-end Slight Risk outlook.    We should note, however, that the core of this higher risk zone lies to the northeast of the immediate D.C. region, with D.C. essentially on the edge.  Northeast Maryland including Baltimore, eastern Pennsylvania and New Jersey sit more squarely in the bullseye.

Figure 3. SPC’s Convective Outlook for Tuesday.  (NWS SPC)

Figure 3. SPC’s Convective Outlook for Tuesday.  (NWS SPC)

 

Wildcard:  Cloud cover

The dynamic elements described above would argue for a significant round of strong to severe convection, given a sufficiently unstable air mass.  An assessment of this afternoon’s instability based on the morning Dulles sounding implies about 1500 J/kg of buoyant energy (CAPE, or convective available potential energy).  This is a moderate value, less than what developed across the region yesterday afternoon.

Figure 4 illustrates a swath of dense cloud that has set up along the cold front, with embedded rain showers.  As of 11 a.m. this morning, the D.C. metro region was along the ragged edge of this band, with partly cloudy conditions.   Solar heating has been more sustained to the southeast of our region.   This band of cloud is slowly advancing towards us, and should limit heating over our western suburbs this afternoon…perhaps even overspreading the metro area.   The bottom line is that persistent cloudy or partly cloudy conditions could limit destabilization, particularly over the mountains, where convection often first initiates.

Figure 4. Visible satellite image from 10 a.m. this morning.   (WeatherTap)

Figure 4. Visible satellite image from 10 a.m. this morning.   (WeatherTap)

Model predictions of today’s thunderstorm organization

We turn to the high resolution models such as the NAM, RAP, HRRR and WRF-ARW to provide guidance on the anticipated areal coverage and mode of severe convection.   I show snapshots from three of these in the figures below, all valid for the 5-8 p.m. timeframe.   The influence of the cold front is unmistakable, with a squall line of strong convective cells.  The HRRR (figure 5) implies a more broken line of convection, whereas dense coverage is favored by the WRF-ARW and NAM-4 km (figures 6 and 7, respectively).

 

Figure 5. HRRR model prediction for 5 p.m. today.  (NOAA)

Figure 5. HRRR model prediction for 5 p.m. today.  (NOAA)

 

Figure 6. WRF-ARW model prediction for 6 p.m. today.  (NOAA)

Figure 6. WRF-ARW model prediction for 6 p.m. today.  (NOAA)

 

Figure 7. NAM-4km model prediction for 8 pm today. (StormVistaWxModels.com)

Figure 7. NAM-4km model prediction for 8 pm today. (StormVistaWxModels.com)

 

A squall line – either broken or solid – is the favored mode when we have widespread frontal and dynamic uplift, combined with unidirectional wind shear.   The leading edge of the line consists of strong multicells and line segments.  Given the intensity of today’s shear, the leading edge could also present with bowing segments (delivering severe wind gusts) and isolated supercells.   Behind the leading convection, a broader “trailing stratiform” rain may sets up, with moderate rain lasting an hour or so.

The intensity of this line will be strongly governed by the degree of surface heating through the afternoon – and this depends on the behavior of the frontal cloud band.  Will we get socked in, or will the D.C. region sit along the ragged, partly-sunny edge of this band through much of the afternoon?

As mentioned above, the greatest risk today will be severe wind gusts.  SPC portrays a low-end tornado threat (2%) to the northeast of our region.   Some models hint at a weak area of low pressure forming along the front later today.  That could induce a bit of low-level directional wind shear, which may increase the tornado threat.  Small hail is fair game, most likely in any supercells that fire up.

The cold front advances steadily through our region this evening, ushering a much milder and drier day on Wednesday…a welcome break to a very stormy stretch of days.

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