A Super Forecast of Mid-Atlantic Excessive Rain on July 21 by the HRRR Model

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Discussion: As of 7 AM EDT Sunday July 22 Flood Warnings are in effect for the Washington, DC and Baltimore metropolitan (and vicinity) areas extending westward to Leesburg, VA and northeastward to Westminster, MD. The flood warnings are due to heavy rain produced by a rapidly intensifying low pressure system moving north through the eastern Mid-Atlantic region late yesterday. Official rainfall totals include daily records of 4.79 inches in Baltimore, MD and 4.00 in. in Washington, DC. Preliminary radar estimates rainfall totals of 4-7 inches occurring in-between Washington and Baltimore and points west.

The high-resolution rapid refresh (HRRR) model forecast an amazingly accurate depiction of what happened later yesterday (Fig. 1). Near 4 in. was forecast for both Washington and Baltimore with a swath of 6-7 in. between the two cities and westward to well northwest of DCA and west of BWI. By comparison the NOAA/WPC forecast issued yesterday morning focused an area of 2-3 in. of rainfall from Washington to Baltimore to Philadelphia and eastward (Fig. 2).

The preliminary explanation of this somewhat unexpected heavy rainfall was caused by a very unusual set of circumstances. At 2 PM EDT yesterday a subtropical low pressure area was centered just east of Cape Hatteras, NC. This system could have rapidly become a subtropical storm with sufficient distance from the coast. Forecast models were generally focused on this subtropical low as the rain-maker for the Mid-Atlantic region yesterday biasing the highest totals to the I-95 stretch (and eastward) as forecast by NOAA/WPC.

Fig. 1:  HRRR (model) rainfall forecast for yesterday’s Mid-Atlantic rainstorm.

Fig. 2:  NOAA/WPC rainfall forecast for yesterday’s Mid-Atlantic rainstorm.

Mid-afternoon Saturday a new low pressure area formed north-northwest of Norfolk, VA in response to an unusually strong upper trough (for July) over the Ohio Valley. The subtropical low was still present north-northeast of Cape Hatteras at that time. By late day/early evening the new low pressure system moved north into southern Chesapeake Bay while the subtropical low to the east became more of squall line offshore the Mid-Atlantic region. The new low pressure system entrained subtropical moisture from the offshore squall line and coupled with deep lapse rates over the Mid-Atlantic region caused by the strong trough to the west forced the heavy rainfall over Washington to Baltimore (and westward) with the excessive amount associated with the squall line staying just offshore.

The sequence described helps to explain the lack of Sunday morning flood warnings in the area expected to receive the most rainfall (DCA to BWI to PHL and eastward) from this event while the HRRR rainfall forecast biasing excessive rainfall farther west was on target where flooding continues to occur this morning (Washington to Baltimore and westward).

The “new” low pressure system described tracked north across the northern Chesapeake Bay early Sunday morning to north-central Pennsylvania at 7 AM EDT. This low pressure track is the storm produced by the upper trough yesterday afternoon and caused the excessive rainfall event. Forecast models were too focused on the subtropical low that formed on the North Carolina coast earlier yesterday expected to track northward but along the coast rather than farther inland.

The upper trough causing the farther west-biased storm in the Mid-Atlantic region yesterday caused 508 severe weather storm reports on Friday in the Mid-South to Ohio/Tennessee Valley and approximately 200 severe weather reports in the Southeast U.S. yesterday.

The unusually intense upper trough was induced by a blocking ridge pattern over eastern Canada to the east of New England causing the approaching upper level system to stall and strengthen. Since late winter/early spring and up to early July a persistent polar vortex has been present over eastern Canada/Greenland mirrored in the North Atlantic sea surface temperature anomaly (SSTA) pattern which has been cold east of Canada and south of Greenland. But! Just south of this cold SSTA region the ocean surface is much warmer than normal east of New England. There is a tendency for high pressure to be present across vast warmer than normal ocean surfaces.

Most likely due to seasonality the polar vortex across eastern Canada/Greenland through spring and into early summer retreated northward the past 7-10 days and high pressure shifted north to eastern Canada blocking progression of the upper trough causing the FRI/SAT severe weather and flooding rainfall in the Mid-Atlantic region.