Sunday, June 3, 2012

Northwest flow to bring thunderstorm chances to the Mid-South

A significant weather pattern shift has taken place across the eastern U.S. as a large upper level low pressure area has set up over southeastern Canada.  Initially, this was evident as a very nice cool down the past couple of days as surface high pressure built in behind a strong cold front. Now, "northwest flow" has set up over the area, which means our mid-level wind (the level that typically steers storm systems) is out of the northwest between high pressure to the west and the low to the northeast (see graphic below).

Mid-level (~18,000 feet) setup at mid-day on Sunday.
The Mid-South is on the periphery of a classic northwest flow regime.

Mesoscale Convective Systems (MCS's)

Typically, summertime northwest flow means somewhat unpredictable storm systems that tend to form during the overnight hours and move along frontal boundaries that are draped from the plains into the southeast.  These storm systems are often referred to as "mesoscale convective systems" (MCS's) and the current setup favors just that type of scenario.  The areas affected by these storm systems vary, but the threats posed generally are damaging straight-line wind and hail.

MCS's in a northwest flow typically develop from clusters of storms that fire after dark as low-level (but above the surface) wind picks up after dark, then organize and follow the mid-level northwest flow downstream.  Many times the storms begin to weaken and dissipate during the mid-morning hours, but occasionally they will continue through the next day.  More often, as the storms dissipate during the morning hours, an outflow boundary from the storms will linger, allowing for the development of additional storms during afternoon heating.

Sunday mid-day surface setup with a warm front just north of  the metro, ample low-level moisture (green shading indicating high dewpoints) and a resultant Slight Risk for the Plains into the Mid-South.

The Forecast

All this talk about northwest flow and MCS activity leads us to the forecast for the next couple of days.  A warm front has moved over the metro, providing southerly flow and warm, very humid air to the region - a far cry from the past couple of days. This morning's scattered storms were the result of the warm front lifting north.  Scattered storms are likely to fire late this afternoon into the evening in the humid airmass near and behind the front.  These should end shortly after sundown.

However, the first mid-level impulse in the northwest flow will provide for the formation of storms after dark not too far to our northwest, likely in northern AR.  As the storms follow the flow in the first graphic above and the warm front, they will likely affect the metro area overnight.  A Slight Risk of severe weather has been identified by the Storm Prediction Center as indicated in the graphic above. The primary risk with any storms today and tonight will be large hail and damaging wind, though storms that cross the warm front this afternoon could have enough low-level wind shear to spin up a brief tornado. The graphic below indicates the NAM model's interpretation of where storms might be early Monday morning (between 4-7am).

As indicated above, northwest flow events are very difficult to forecast precisely.  After tonight's likely storms scenario, there will be additional chances for storms Monday afternoon and evening on any outflow boundaries produced by tonight's storms.  However, it appears that, beginning Tuesday, the warm front may be pushed back to our south and become stationary southwest of the metro.  If this is the case, further northwest flow storms would likely remain to our south for the mid-week and later period.

MWN will be monitoring the situation carefully and provide nowcasting of any adverse weather on our social media channels (Facebook and Twitter links below).  The best way to stay on top of the latest developments is via our MWN mobile app for Android and iPhone, which you can download at the link below.  Be sure to upgrade to include StormWatch+ so that severe weather won't catch you off guard anytime of the day or night.

For weather information for Memphis and the Mid-South, where and when you need it, visit on the web, on your mobile phone, download our iPhone or Android apps, or visit us on Facebook or Twitter.

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