A Slowly Evolving Pattern

September 23rd:  A pattern that can be described as slow-moving, or even stagnant, exists across North America at present.  This type of pattern, which is not infrequent around the time of changing seasons near the spring or fall equinox, features a high amplitude trough of low pressure over the inter-mountain west and a ridge of high pressure aloft over the central US.  The pattern can be described as ‘blocky’ – that is, eastward movement of weather systems is ‘blocked,’ or at least very much slowed down. The current charts show features of two types of classic upper level blocking patterns – the Omega Block and Rex Block.   These are described in more detail below.

The high Saturday at the Lawrence Airport was 91 and the low 67.  The dew point was very slightly lower than the previous two days, averaging 69 degrees.  This may be a product of vegetation and surface ground moisture beginning to dry out across the region more than anything else.

A very slow-moving surface front associated with the western upper trough started to impinge on far northwest Kansas this afternoon.  The town of Goodland reported a third of an inch of rain at the airport up to 7 PM, with a high of 73 and low of 56.  That was the coolest and wettest in the state.  Salina was the warm spot across Kansas today with a high of 95 and low of 74.  Manhattan was runner up with a high of 94.


3:15 PM CDT:  Upper trough of low pressure appears centered near the Utah/Nevada border with enough progress eastward to generate thunderstorms from the southern high plains across far northwest Kansas, central and western Nebraska, South Dakota and central Minnesota.  This frontal zone appears to a have tapped into some subtropical moisture from Mexico.  Hurricane Maria is still moving north-northwest at the rate of about 200 miles or less per day, and is now expected to make closest approach to the outer banks of North Carolina on Wednesday.  Post tropical system Jose is now basically a rotating batch of low clouds centered well southeast of Cape Cod.  An afternoon cumulous field covers our region but the air aloft is stable above the low-level cloud layer.


250 MB Chart at 7 PM today:  A slow moving deep upper trough continues over the intermountain west with the upper ridge position right over the Kansas Missouri border.  An upper low is evolving over Alabama with a ridge to the north over the Great Lakes – an extension of the ridge over our area.  The west looks like like an ‘Omega Block’ while over the east, the pattern more resembles a ‘Rex Block’ type of blocking pattern.


Dropping down to the more commonly seen 500 MB level, an upper low looks to be trying to close off over the Nevada-Utah border area – a version of an Omega Block with slow-moving, high amplitude waves.   Over the eastern US, an upper high is more clearly seen, centered about over Michigan.  South of that is an area of cyclonic flow, or a developing upper low, over southern Alabama and the Florida panhandle.  This is a Rex Block pattern.   That system is starting to develop a few thunderstorms over the Southeast as seen on the satellite image above.   Blocking patterns can be difficult to forecast, as these patterns tend to evolve more slowly than numerical models tend to want to change them.

Below are descriptions of the above mentioned blocking patterns, courtesy of the National Weather Service at this link:  http://www.srh.noaa.gov/jetstream/constant/basic.html


Omega Block
Omega blocks get their name because the upper air pattern looks like the Greek letter omega (Ω). Omega blocks are a combination of two cutoff lows with one blocking high sandwiched between them.Because of their size, Omega blocks are often quite persistent and can lead to flooding and drought conditions depending upon ones location under the pattern. Cooler temperatures and precipitation accompany the lows while warm and clear conditions prevail under the high.


Rex Block
Rex blocks are characterized by a high pressure system located pole-ward of a low pressure system. The Rex block will remain nearly stationary until one of the height centers changes intensity, unbalancing the high-over-low pattern.Unsettled, stormy weather is usually found near the low pressure while dry conditions are typical with the high pressure. Strong, particularly persistent Rex blocks can cause flooding near the low pressure part of the block and short-term drought under the high pressure part.


There was enough wind today for sailing on Clinton Lake among the plentiful gulls.  Sometimes the sound of the calling gulls makes the place sound almost like being at the sea shore.


Zooming in on a point of land across the lake with a very low and unobstructed sun.   Here there are interesting snags with many gulls, both perched and swimming, as well as a turkey vulture perched here and there.


The sun sets through some distant cirrus, probably the left over tops of thunderstorms from the high plains of eastern Colorado and New Mexico.  These storms brushed far northwest Kansas late today.


Just after the sunset, a gull is silhouetted against a backdrop of still illuminated cirrus.  Satellite imagery showed the cirrus to be over the western half Kansas at this time, the near edge around 150 miles distant.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google+ photo

You are commenting using your Google+ account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )


Connecting to %s