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Isentropic Lift Results in Clouds and Rain on Monday and Tuesday (21 SEP 2015)

  POSTED ON: Monday, September 21, 2015 9:48 AM by A.R. Davies

Over the last six days, spanning September 15 to 20, the greater Annapolis region enjoyed seasonably warm conditions; the observed temperatures at BWI Airport each of those days surpassed the daily average high temperature.  However, the warm streak will end Monday as the high temperature will likely struggle to climb out of the 60’s.

A wave of mid-level atmospheric energy will dig south and across the Ohio River Valley on Monday. As shown in the Figure below, the mid-level flow will be out of the south ahead of the disturbance with most of the atmospheric dynamics located along the trough axis.  However, the mid-level system (and the southerly flow ahead of it) will run into a building area of surface high pressure to the north, which will be the dominant weather feature this week.  At the surface, northerly winds associated with high will result in seasonably cool conditions on Monday and Tuesday. As a reminder, the wind flows clockwise around high pressure systems and counter clockwise around low pressure systems in the northern hemisphere.

Mid- and Surface- Level Atmospheric Winds on 21 SEP

This pattern will produce clouds and spotty light rain on Monday and Tuesday as a result of isentropic lift or “overrunning.” As shown in the Figure below, isentropic lift occurs when cool and dry air at the surface, typically associated with northerly flow, is wedged below warm and moist air aloft that is typically associated with southerly flow (warm and moisture air is less dense than cold and dry air).  On Monday, mid-level southerly flow will collide with the wedge of cooler air at the surface.  As the air masses intersect, the less dense air (warm and moist) will be forced upward where it will condense to form clouds and rain.  Overrunning is common ahead of large weather systems that track to the west of Annapolis during the winter months, and often results in either sleet or freezing rain.

Isentropic Lift Example

As the mid-level short wave departs, high pressure will remain entrenched through the remainder of the work week.  As a result, expect mostly sunny skies and seasonable conditions on Wednesday and Thursday. An area of low pressure could develop off the Carolina coast late this week. This feature will be worth watching as a potential rainmaker on Saturday and Sunday.


 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