National Weather Service United States Department of Commerce

Forecast commentary -

Today will likely be the fourth consecutive day of record high temperatures at Flint and Saginaw(
MBS ), and Detroit also has a chance to break the record of 89 set way back in 1891. The records for
Flint and Saginaw are 89/2007 and 90/2007 respectively.

The record early fall heat has been driven by a strongly amplified and stable upper air pattern over
north america that is producing a 591 dm ridge at 500 mb directly over Lower Michigan. Satellite
imagery and model analysis fields indicate the core of the ridge is still in place during the early
morning but simulations show with good agreement that it will drift eastward through the day. This
will still result in widespread high temperatures in the lower 90s but likely not as many mid 90s
readings compared to the last couple of days. A light southeast wind will also return to shoreline
areas and into the Detroit metro area from Lake Erie as the surface high follows the upper ridge
slightly eastward. Mid afternoon onset of this flow pattern was effective at preventing both DTW and
DET from reaching 90 Thursday and Friday and adds some uncertainty there today despite full sunshine
across the region.

The persistent nature of this stretch of heat keeps advisory headlines in consideration, however
there has been several hours of relief during each of the last few nights as low temperatures
dropped into the lower to mid 60s most locations. Surface dewpoint showed some vulnerability to
boundary layer mixing yesterday and guidance today points to readings in the lower to mid 60s which
will only produce a degree or two of heat index enhancement. These factors suggest advisory
headlines are not needed while heat precautions remain part of other messaging.

High temperatures Monday and Tuesday are expected to come off the weekend peak by a degree or two
each day. The record at Flint is only 86/1935 while readings will be near 91/1900 at Saginaw but are
expected to be short of 93/1891 at Detroit. This time period is the beginning of the upper air
pattern shift that has the Rockies trough lifting into the Northern Plains while eroding the ridge
in the east. This process and the added cloud cover will produce the slight cooling trend before the
associated cold front arrives. Usually temperatures overachieve prior to a frontal passage but in
this case low level advective processes are very weak.

The latest global/long range models are settling on Tuesday night and Wednesday timing for the front
and also depicting a rather anemic moisture pattern along it while passing through the Great Lakes.
A chance of showers and thunderstorms does seem warranted as the front continues to show enough
temperature contrast to drop highs back into the 70s by Wednesday and then into the 60s after a
reinforcing front by Thursday into Friday. Guidance then shows high temps possibly not warmer than
the 50s to start next weekend.

. . posted at 345 AM EDT Sun Sep 24 2017