June 21 weekend weather

[Don Grillo]

Here is the outlook for our deeply wet and stagnant weather pattern this weekend and continuing thereafter.

 

The prog chart for Saturday shows what has become a typically complicated series of low pressure systems linked together and not really going anywhere.  In this case the low pressure system over southern Canada is linked by a northwest-southeast cold from to another low over western Iowa. The Iowa low appears poised to drive a warm front north across central Illinois this weekend.  This is a pretty good structure for rain and perhaps thunderstorms, which are shown as a possibility.

 

 

Below is the prog chart for 24 hours later — Sunday morning.  What is most significant is how little has changed.

 

The prog chart for Sunday shows relatively little movement by the Iowa low, despite the passage of 24 hours.  The bigger message from this sequence is the absence of the sort of progression that makes for weather changes that generates soaring conditions — namely that a Canadian polar airmass that pushes south, clears out the moisture and generates soaring conditions.  Or, alternatively, we would happily accept the second best option: A west to east zonal flow with a north-south cold front pushed to the east across Illinois by a pacific zonal flow.  The soaring conditions won’t be as strong as generated by the Canadian polar airmass, but will generate usable soaring conditions.

 

We have neither of these typical weather patterns ahead in the foreseeable future.  Instead, we have what looks a lot like a cut off low parked over southern Canada or the upper midwest, and linked by a cold front that is not moving to another cut off cold front over western Iowa.  Cut off lows are always bad news for soaring.  We seem to be in a pattern in which they are the norm, not the exception.

 

The NWS dicsussion mostly focuses on what is of concern to the general public: the likelihood of rain and T-storms:

Friday night: The overall pattern combined with guidance suggests
the first round of convection will lift NE toward the CWA Friday
night into Saturday morning in response to substantial WAA and
moisture transport on a 45 kt low-level jet over northern MO into
southwest IL. Impressive steepening of mid-level lapse rates over
this forcing will support the development of nocturnal convection
southwest of the CWA during the evening. Modest 1-6 km effective
shear around 30 kts along with decent MUCAPE within the steep
lapse rates will support a risk for organized severe convection.
As has been the trend for the last day, most of the significant
convection should remain south and west of the CWA where
convergence is maximized on the nose of the LLJ. But as the LLJ
veers through the night, some severe convection may reach roughly
the southwest 2/3 of the CWA through daybreak Saturday. Again,
severe convection, mainly in the form of large hail, will be
possible with any of these storms.

Uncertainty increases beyond Saturday morning as Friday night`s
convection may slow the northeastward expansion of the warm sector
through the CWA until late in the day and ultimately change where
and when additional convection develops on Saturday. The forecast
trend continues toward a slightly cooler (but still warmer than
as of late) Saturday with a decent amount of convective debris
clouds around as a convectively enhanced warm front remains south
of the CWA. This should limit precip chances through the day to
some degree given an overall absence of notable forcing. Have
therefore continued the gradual trend toward a drier forecast. LLJ
activity may then focus convection west of the CWA Saturday
evening before veering and shifting storms into northern IL
Saturday night. Additional rounds of storms will be possible
through Monday morning until a trough axis clears the CWA during
the day Monday.

* * * * * *

While its not necessary to double check this dismal forecast, here is the 3-degree spread as depicted for Saturday morning by Dr. Jack.  There is no meaningful 3-degrees spread in our area.