A volatile 48 hours: Severe thunderstorm watch today, flash flood threat tomorrow

July 2

5:05 p.m. update: We’re still tracking two areas of storms: 1) In our western suburbs (central Fauquier through western Fairfax County and into parts of central Montgomery county) and 2) in Southern Maryland. Pockets of strong winds are possible in these storms. For more on the evolving storms and our forecast into tomorrow, see our PM Update, just published. This is the last update in this post.

4:45 p.m. update: Radar shows really well-defined outflow boundary out ahead of main cluster of storms in north central Va. An outflow boundary is effectively the exhaust or cool sinking air from those storms. It can act as a mini-cold front along which additional storms form. We’re seeing that around North Potomac and Rockville. It’s possible isolated storms will form along it further south as it pushes through the region. When it passes your location, expect winds to pickup and temperatures to mercifully drop about 10 degrees.

(Weather Underground, from 4:35 p.m.)
(Weather Underground, from 4:35 p.m.)

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

4:25 p.m .update: Two new warnings: 1) Severe thunderstorm warning issued for central Fauquier and northwest Prince William County until 5:15 p.m.   Storm headed in general direction of Warrenton and The Plains over next 45 minutes could pack damaging winds.  2) Severe thunderstorm warning for isolated cell in southeast Calvert, central St. Mary’s and southeast Charles counties until 5 p.m.  Storm headed into Leonardtown area may also generate very strong winds.

4:15 p.m. update: Not storm related but D.C.’s 4 p.m. temp was 98 (hottest of the summer) with a heat index of 109.  Ouch.

3:55 p.m. update: Considering overall radar evolution and best atmospheric ingredients, it appears D.C.’s north and west suburbs are most likely to experience  thunderstorm activity this evening.  This includes Loudoun, Frederick, northern and west Montgomery, Howard, Carroll, and Baltimore counties.   From near the Beltway and points south, storms cannot be ruled out but will likely be more isolated.  The Nats may be able to get their game in tonight.

3:45 p.m. update: Severe thunderstorm warning posted for central Loudoun and western Montgomery County until 4:30 p.m.  A storm to the west and southwest of Leesburg is intensifying and may produce damaging winds gusts as it heads northeast towards Leesburg and eventually Poolesville (not currently under warning).

3:00 p.m. update: A nasty storm with a history of wind damage will move into Frederick, Md. over the next 30 minutes.  A severe thunderstorm warning is in effect for central Frederick County until 3:30 p.m.

2:35 p.m. update: Some storms are already impacting our far western areas (Frederick and Loudoun County).  A severe thunderstorm warning has been issued for northwest Loudoun and southwestern Frederick  counties until 3 p.m. for the possibility of damaging winds over 60 mph.

2:15 p.m. update: A line of storms has developed along I-81 and looks like it may want to move through the metro region – from west to east – between roughly 4 and 8 p.m.  So far, they are not severe but are moving into very unstable air where they could intensify.  We’ll keep you posted on the progress of these storms.

From 12:59 p.m.: Today’s heat and humidity climbs to oppressive levels, fueling a chance for severe thunderstorms this afternoon and evening.   Tomorrow, tropical moisture from Arthur interacts with a slow-moving cold front, setting up a possible flash flooding scenario.   We lay out all the details in the discussion that follows.

Today’s severe thunderstorm threat

Area covered by severe thunderstorm watch. Click to enlarge. (National Weather Service Storm Prediction Center)
Area covered by severe thunderstorm watch. Click to enlarge. (National Weather Service Storm Prediction Center)

A severe thunderstorm watch covers the entire region through 8 p.m. tonight.  Very heavy rain and lightning are likely in most storms.  Some storms may contain damaging winds as high as 70 mph and large hail (to 1 inch in diameter).  Isolated storms could begin popping as soon as mid-afternoon in the region, with more widespread activity concentrated in the evening – from roughly 6 to 10 p.m.

With an afternoon high expected to reach the mid-90s and dewpoint temperature in the low 70s, the buoyant energy (convective available potential energy, or CAPE) powering thunderstorms is expected to reach high values.    Anytime CAPE exceeds 2000, strong thunderstorms are possible.  Today’s numbers may soar to 3500-4000, a very unstable atmosphere.

The figure below shows the severe thunderstorm outlook published by the Storm Prediction Center (SPC), highlighting a widespread 15% probability of severe thunderstorms, for both damaging wind and large (golfball size) hail.

 


Figure 1. SPC outlook for severe thunderstorm potential, Wednesday afternoon and evening. (NWS SPC)

In preparing a forecast of severe thunderstorms, instability or CAPE is just one factor to consider.  The other is wind shear (the change in speed and/or direction of winds with altitude) that organizes ordinary thunderstorms into longer-lived, stronger systems.  A trough in the upper level westerly flow will approach our region through the day.  This will boost upper-level southwest winds to 30-35 knots.   This is a moderate value of wind shear, enough to organize storms into multicell clusters and line segments.   Some of these segments could develop bowing structures, indicative of downburst winds.

The high resolution (mesoscale) models predict that these clusters will initially form along the eastern mountain slopes, then move over the adjoining Piedmont, beginning mid-afternoon.   These storms may regenerate over the greater D.C. region late afternoon through the evening.    At jet stream level, there is a flow pattern called diffluence (a spreading apart of airflow streamlines) across our region, which draws up air from below…and this may promote widespread coverage of storms.  One possible scenario is shown by the high resolution NAM model in Figure 2, below.


Figure 2. NAM 4-km grid simulated reflectivity valid 8 p.m. this evening.  Tstorm coverage is widespread, consisting of strong-severe clusters and bowing line segments.    (StormVistaWxModels).

You may recall the severe weather event from June 18, when clusters of severe thunderstorms erupted over our region – producing extreme lightning, torrential rain and spotty microburst damage.   Local power outages were significant.  This is the type of scenario I envision for later today – not necessarily in the same locations as June 18, nor at the same time (those storms blossomed late in the evening).    But patches of storms could become quite vigorous, starting around 3 p.m. today and continuing until after dark, perhaps until midnight. SPC will likely consider issuing a severe thunderstorm watch area-wide sometime during the afternoon.

Thursday’s heavy rain threat

Tropical cyclone Arthur begins approaching the region on Thursday, as a cold front slowly moves in from the west.   The combination of (1) the front, (2) tropical moisture surging ahead of it, (3) an unstable atmosphere and (4) jet stream dynamics portends an active weather day…with the chief threat being locally heavy rain and flash flooding.

First, let’s clear any misperceptions about Arthur’s direct impacts.   Arthur is forecast by the Tropical Prediction Center (TPC) to become a marginal Cat 1 hurricane.   This is a small storm with an offshore trajectory, with the center passing a few hundred miles east of D.C.   The storm is far enough away that winds will not be an issue for the D.C. region, aside from a southwesterly breeze Thursday night becoming northwesterly through Friday.   Arthur’s main impact will be showery rains and thunderstorms, some heavy, as its moisture field interacts with the slow-moving cold front.

Figures 3 and 4 show the forecast synoptic charts, for Thursday evening (8 pm) and Friday morning (8 am).   Note the slowly advancing cold front, with Arthur’s circulation moving parallel to and close to the boundary.    Dark green shading indicates heavy rain potential.  Bands of heavy rain may set up from the Carolinas to Maine.  In fact, northern portions of the East Coast could experience a type of rain storm called a Predecessor Rain Event (PRE) – in which tropical moisture interacts with a jet streak entrance region hundreds of miles from the actual tropical cyclone.

 


Figure 3. Surface forecast for 8 p.m. Thursday, showing interaction of Arthur’s tropical moisture and a cold front. (NWS)

 


Figure 4. As in Figure 3, but valid 8 a.m. Friday.   Note how elements creating heavy rain have shifted north and east of our region.  (NWS)

 

On Thursday, winds aloft favor the diffluent type of pattern that promotes widespread uplift and heavy rain.   Total precipitable water (TPW) will be very high, two inches or more.  The winds aloft will also stream from the south, parallel to the frontal boundary.    This is a setup for cell training – when convective cells develop and move over the same locations, repeatedly.   It’s too early to say where these rain corridors might set up.   Instability may be limited somewhat by cloud cover, but moderate values of CAPE could develop;  combined with moderate wind shear, some of the thunderstorm activity could be strong to locally severe.  SPC foresees only a 5% risk of severe thunderstorms for Thursday, but this could be revised upward.

Figure 5, below, shows a corridor of heavy rain-producing cells setting up over the Maryland and Virginia Piedmont, according to the high-resolution NAM simulation, valid at 7 p.m.

 


Figure 5. NAM 4-km grid simulated reflectivity valid 7 p.m. Thursday.  Storm cells are likely training from south to north along stalled frontal boundary.    StormVistaWxModels.

How much rain will fall?     I expect a widespread 1-2”, with pockets of heavier amounts, up to 3-4”.  The timing of heaviest rains is most likely during the late afternoon through overnight.   The figure below is the NWS Weather Prediction Center’s (WPC) latest prediction of area-wide rain totals, through Friday morning.  The main event is well offshore, close to the core of Arthur – but in this figure, you will note the band of enhanced rain inland, along the front, through the D.C. region.

It’s worth noting that the combination of a tropical weather system and a frontal boundary, historically, has caused flooding problems across the Mid-Atlantic.   However, I believe there are mitigating factors that will limit the more extreme rain amounts.   These include fast movement of the cyclone and its increasingly offshore trajectory – that is, the storm will approach then move away from our region, rather than merge with the boundary and track up the Piedmont.   There is thus a limited “time window” during which the front and tropical system will interact across the region.

One consideration, for those who have basements prone to flooding:    Today’s round of storms could knock out power for some, and these outages could extend through the day Thursday.    If flash floods develop tomorrow, those without battery back up or a generator to power sump pumps may be in a spot of trouble.

 


Figure 6. WPC’s forecast of rain accumulation, for the period 8 a.m. Thursday through 8 am Friday.    This is only a generalized map that does not reveal pockets of locally higher rain amount. (NWS WPC).

The good news is that the rain exits our region Friday morning, and by afternoon, a push of dry air will arrive from the west.  Arthur begins to undergo extratropical transition, moving rapidly off to the northeast.    The elements of our rain-making machine will rapidly disperse, leaving the region in good shape for Friday evening’s July 4 festivities.

Jason is currently the Washington Post’s weather editor. A native Washingtonian, Jason has been a weather enthusiast since age 10.
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