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Synoptic-scale weather is the king

Forecasters often rely on their understanding of synoptic-scale weather guidance to make a good forecast. At times local effects can have an important impact on the weather, but it is often that "big picture" that will dictate whether or not the sky is clear, cloudy or somewhere in between.

It's at the end of August and Charlotte, North Carolina should be basking in the upper 80s or low 90s. But the afternoon temperatures have been struggling to get out of the 60s making it feel more like an early autumn day. With this kind of anomaly there's no doubt this is related to the what's going on with the big weather picture. To get a view of the synoptic weather starts with a peek at the latest surface analysis chart.

Notice that there's a relative strong (for this time of year) area of high pressure centered over southeast Ontario, Canada. During your primary training you learned that low-level winds flow clockwise around an area of high pressure as depicted by the green arrow. Also unusual for this time of year, there is a frontal system that has managed to make it as far south as central Georgia. For the Mid-Atlantic and northern part of the Southeast that means the winds will be out of the east and northeast north of the analyzed front. This is a moist and generally cooler low-level flow from the Atlantic Ocean. This moist and dense air gets dammed up against the Appalachian Mountains.

Often this easterly or northeasterly flow will also be well represented very well at the 925 mb level or roughly 3,000 feet MSL. Here is the analysis from the High Resolution Rapid Refresh (HRRR) model which depicts that flow off of the Atlantic Ocean as highlighted by the green arrow.

One of the best resources to drill down and see the physical effect of this large scale synoptic flow is the Skew-T log (p) diagram. Below is the forecast sounding from the Rapid Refresh (RAP) model that nicely depicts the low-level winds from the east and northeast from the surface to about 5,000 feet MSL. Temperatures in this layer are nearly isothermal (not increasing or decreasing with height).

The overall extent of these IFR ceilings is depicted well by the High Resolution Ensemble Forecast (HREF) model. The large red area just to the north of the front shows a 90% or greater probability ceilings will be below 2,000 feet with much of this area covering the Charlotte region.

The surface observations for Charlotte Douglas Airport (KCLT) show the MVFR and IFR ceilings that were present during the afternoon.

KCLT 241952Z 02009KT 9SM OVC008 19/16 A3014 RMK AO2 CIG 007V011

KCLT 241936Z 01010KT 8SM OVC008 19/16 A3014 RMK AO2 CIG 007V011

KCLT 241917Z 01013G19KT 7SM OVC007 19/16 A3014 RMK AO2

KCLT 241902Z 36009KT 8SM BKN008 OVC015 19/17 A3014 RMK AO2

KCLT 241852Z 02012G16KT 9SM BKN010 OVC015 19/17 A3014 RMK AO2

KCLT 241752Z 01009KT 7SM -RA OVC011 19/17 A3015 RMK AO2

KCLT 241726Z 02011KT 10SM BKN008 OVC014 19/17 A3015 RMK AO2

KCLT 241717Z 02009KT 10SM BKN010 OVC014 19/17 A3015 RMK AO2

KCLT 241652Z 02008KT 8SM OVC009 19/17 A3015 RMK AO2

KCLT 241644Z 02009KT 8SM OVC009 19/17 A3015 RMK AO2

Certainly a rare day to go out and get some safe IFR practice during the summer months.

Most pilots are weatherwise, but some are otherwise™

Scott Dennstaedt

Weather Systems Engineer

CFI & former NWS meteorologist

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