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USNA News Center

Unsettled Weather On-tap for the Weekend (09 SEP 2015)

  POSTED ON: Wednesday, September 9, 2015 3:18 PM by A.R. Davies

The Mid-Atlantic region has been inundated by unseasonably hot and humid conditions over the last three days.  The normal high temperature at BWI Airport this time of year is only 81oF. However, on Labor Day the temperature climbed to 90 oF at BWI, and increased to 93oF on Tuesday.  Today (Wednesday) as of 1500 EDT, the temperature at BWI had already soared to 92 oF under partly sunny skies, and threatens to break the record high temperature of 94 oF set in 1985.

The cause of the unusually hot and humid conditions has been southerly flow ahead of an approaching cold front. The Figure below shows the precipitable water (inches) at the surface projected for Wednesday at 1900 EDT.  In the Figure, a cold front is clearly identified by the change in precipitable water across the axis just west of the Ohio River Valley.  Precipitable water is the amount of water that would fall if all the moisture in the atmosphere were to rain-out (i.e. the more precipitable water, the more moisture in the air).   Ahead of the front, southerly flows has entrenched us in a warm and moist air mass (associated with warmer colors on the map), however behind the front cooler and drier conditions prevail. 

Precipitable Weather (inches) for 1900 EDT on 09 September 2015

Over the next twenty-four hours, the cold front will gradually slide east.  In doing so, it will tap-into a moist, tropical air mass currently located across Florida, Georgia, South Carolina, and off the Southeast Coast of the United States.  As shown in the Figure above, the combination of mid-level atmospheric dynamics associated with the approaching cold front and the moist air being pulled up the coast will spin-up an area of low pressure along the Mid-Atlantic seaboard during the day on Thursday.  As the coastal system develops, it will draw energy from the cold front and become the dominate feature.

As a result, periods of rain are likely late Wednesday and through Thursday. The National Weather Service’s Weather Prediction Center (WPC) suggests 1.0-2.0” of rainfall is possible across Maryland through 1700 EDT on Thursday.  The Figure below (from the high resolution NCEP WRF model) projects isolated locations across could see 2.5-3.0” of rain.  Although this model likely will not resolve the exact statewide rainfall distribution or totals, it does highlight that the heavier rain showers will develop in a more isolated manner, and likely will not be widespread.  In addition, it suggests the best chance for heavier rainfall will be north and east of Annapolis, MD (which coincides with WPC forecasts).  

Accumulated Precipitation (inches) valid for 2300 EDT on 10 September 2015

Friday will gradually clear behind a northwest wind with less humidity and seasonable conditions (the high temperature will likely be near 80 oF).  A deep wave of mid-level atmospheric energy will develop to our west late Friday night and early Saturday.  The models suggest an associated area of surface low pressure will form across the Ohio River Valley on Saturday morning, and will slide to our north and west.  Showers and thunderstorms may overspread the region Saturday evening and could persist into Sunday, although the timing and extent of the unsettled weather remains uncertain at this time. Expect high temperatures in the upper 70’s over the weekend, and perhaps an even cooler air mass behind this system for the beginning of the work week.


Disclaimer: This is not an official U.S. Navy weather forecast and should not be confused with the official Navy weather forecast provided by the Fleet Weather Center in Norfolk, VA.  Furthermore, the official U.S. Government forecast for the greater Annapolis area is issued by the National Weather Service Baltimore/Washington Forecast Office in Sterling, VA.

Category: General Interest


Alexander R. Davies
Oceanography Department