National Weather Service United States Department of Commerce

Forecast commentary -

Morning satellite imagery shows some thin spots and breaks in the clouds over Lower Michigan that is
giving a boost to surface heating so far today. This will help convert elevated instability, that is
supporting the ongoing clusters of showers and storms over Lake Erie and Ohio, to surface based
instability. The magnitude of that instability is projected to approach 1000 j/kg during the
afternoon in the latest high res model data, which matches up with late morning hourly mesoanalysis
over Ohio. This will help showers and storms continue to expand over Southeast Michigan through the
day as deeper moisture flows into the region from both gulf and Atlantic trajectories in the low to
mid level southeast flow. The moisture transport will be enhanced by the low pressure system
deepening over the Ohio Valley through the day and into this evening and the pressure pattern on the
north flank of the system, extending into Lower Michigan, will also enhance development through
broad low level cyclonic flow/convergence. These trends support the steady increase in pops
advertised in the going forecast from scattered early to numerous and widespread through the

. . posted at 1118 AM EDT Wed May 24 2017

Large upper low continues to churn south-southeast across Iowa and will move into the oh and
Tennessee valleys by tonight. Then it slowly moves to the Mid Atlantic sea board by Friday. A
surface low will develop this afternoon over northern ky/southwest oh in response to wave rotating
around the east side of the upper low and the 150+ kt jet streak. These features will be the main
forecast issues over the next two days. While the models do exhibit some differences, the overall
flavor is the same for southeast Michigan.

Southeast Michigan is in a region of weak forcing this morning with weak moisture advection. This
will keep the scattered high based showers or sprinkles around this morning. Enough indication of
thin spots in the high clouds that there will be a few filtered rays of sun around daybreak before a
thicker cloud shield develops. Modest fgen and deformation will accompany a better theta-e advection
pattern from the southeast over much of the forecast area this afternoon. Model soundings indicate
possibly around 500 j/kg of ml cape this afternoon. This should be good enough for high likely or
low categorical pops for this afternoon with showers and a few storms. With the clouds and showers
around, will go toward the cooler met numbers for highs.

The best response with the strongest fgen, deformation and jet forcing occurs this afternoon and
early evening well south of the forecast area in Ohio and east central in. This area will pivot into
the forecast area from southeast to northwest during the overnight hours. While it will be weakening
as it does move into southeast mi, enough confidence of at least light QPF to have mostly
categorical pops tonight. Instability wanes tonight and will take out any thunder mention.

Weak deformation axis and surface troughing remain over southeast Michigan on Thursday with the
upper low centered over Ohio. With a little diurnal boost, expect scattered showers to develop
quickly on Thursday. Again will leave out any thunder mention with modest lapse rates and little if
any cape. Like Wednesday, clouds and showers should keep high temps close to the cooler met numbers.

Think we can squeeze a dry day on Friday as surface ridge builds over lower Michigan with rising 500
mb heights. Sun will be slow to return with plenty of low clouds in the morning and mid and high
clouds already returning ahead of the next weak wave in the afternoon. Speaking of that next weak
wave, the NAM and Canadian appear to be having some convective feedback issues. Meanwhile the GFS
and ECMWF keep the wave weaker and much further to the south. Will have just a chance of showers for
the far south late Friday night into Saturday morning. Saturday afternoon is expected to be dry and
warmer ahead of the next cold front. That front and the next wave still look to move through late
Saturday night and Sunday as another upper low settles over the Great Lakes bringing more cool and
unsettled weather for the beginning of next week.

. . posted at 257 AM EDT Wed May 24 2017