Update, 3:15 p.m.: Severe thunderstorm watch issued for region until 10 p.m.

It’s been several weeks since the D.C. metro region has heard mention of intense thunderstorms – at least the possibility – in the forecast.  For three days, hot and humid conditions associated with an unusually unstable, mid-September air mass are about to culminate as a strong cold front passes through.

This morning, the front is rolling through the Ohio Valley, spreading showers and thunderstorms along a narrow band as it advances eastward.

The Storm Prediction Center (SPC) says the D.C. metro region has a 15% risk of severe thunderstorms, which is the lowest of its three tiers of risk category (slight risk).  The threat zone extends toward the northeast, through New England, where chances of severe weather are a bit greater.

SPC’s 8 AM outlook for probability of severe thunderstorms.

Over the past two days, the air mass has been unstable enough for strong thunderstorms, but we have lacked a lifting mechanism.   The approaching cold front will change that.

Additionally, an upper-air disturbance will cause winds through the deep atmosphere to speed up, creating “wind shear”.   Combined with fairly high instability, the wind shear causes thunderstorms to become better organized, longer-lived and intensifies the storm updrafts and downdrafts.

Predicting the timing and coverage of today’s Storms

To get a handle on the local time and area impacted by significant weather, meteorologists are increasingly using “mesoscale models”, which make short-term predictions over relatively small geographic regions, such as the Mid-Atlantic.    They are generally more accurate at pinpointing specific weather impacts than general, short- to medium-range, synoptic predictive models. This is because they include the details of terrain elevation and use a fine “grid spacing” between mathematical calculation points.   They can resolve locally-generated circulations and boundaries such as fronts better than our general synoptic models.

Shown below are predictions for thunderstorm intensity and coverage over the D.C. metro region, based on the output of this morning’s mesoscale model runs.   Two runs are showed, the WRF-ARW model (top image) and HRRR mesoscale model (bottom image), both for the same time (valid 7 p.m. this evening).  In both, the colors correspond to “simulated radar reflectivity”, as though you were viewing an actual weather radar display.

WRF-ARW mesoscale model simulation, valid 7 p.m.

HRRR mesoscale model simulation, valid 7 p.m.

Both models reveal a fairly robust line of broken convection will sweep across our region.  The timing is a bit different between the two models, with about a 2-3 hour difference in time of peak impact.

The WRF-ARW places the line on our doorstep at 7 p.m., while the HRRR moves the heavy activity over our eastern counties at 7 p.m.  Regardless of timing, the line is narrow, so it will be a quick hit of heavy weather, with perhaps a 1-2 hour trailing period of light rain or drizzle.

Intensity of today’s storms

What type of severe weather could these storms produce?  SPC feels that locally damaging wind gusts are the likeliest type of severe weather (15 percent probability).  Damaging hail probability is 3-5 percent, and tornadoes are close to zero chance.  However, rainfall could be heavy, and lightning intense.   Because the front is progressive and we do not expect upper level winds to align along (parallel to) the front, the probability of flash flooding from “echo training” is low.

Neither the models nor SPC highlight the possibility of particularly widespread severe forms of thunderstorms, such as a bow echo or intense squall line.  Nor is there concern for supercell-type severe storms.   The most likely form of convection will be a broken line of multicell storms, which could generate locally strong to severe winds.

A few isolated showers and thunderstorms could also pop up anywhere across the metro region today, in advance of the main line expected early this evening.  Because of stronger wind shear over New England, that region stands a greater chance of experiencing isolated supercells or bowing line segments.

We’ll provide additional information later this afternoon (see our PM Update, to be published between 3:30 and 4:30 p.m.) as we monitor how the atmosphere is destabilizing, and review the latest radar imagery and mesoscale model runs.