National Weather Service United States Department of Commerce

Critique on the forecast issued at 1123 am edt thu jul 27 2017

Diurnal destabilization is allowing some convective redevelopment across the area. As of 18z, most
of this is occuring along a remnant mid level moist axis across the I-69 corridor. The mid level
moist axis will slowly sink south across the area late this afternoon and evening, suggesting
convective chances will decrease from north to the south. Mid level lapse rates across the region
are not terribly steep. This and ongoing weak low level dry air advection will limit the strength of
any late day convection. High pressure anchored to the northwest of the Great Lakes tonight will
sustain northeast winds across se Michigan. The rate of low level dry air advection will remain
weak. Model soundings suggest there will be enough remnant low level moisture to actually support
some late night stratus development over portions of the area. Some lingering fog/low stratus over
srn Lake Huron is lending support to this idea.

The northeasterly gradient will strengthen across the ern Great Lakes on Friday as high pressure
builds in from the northwest and sfc troughing holds across lower Michigan. The deepening daytime
mixed layer will lead to breezy conditions, especially downwind of Lake Huron and Saginaw Bay. There
is strong support from the 12z model suite in showing amplification of the mid level short wave
impulse now over nrn Minnesota into a compact upper low as it tracks across se Michigan ( 15-21z )
on Friday. Although there will be ongoing dry air advection, model soundings suggest there will be
enough moisture to generate weak to moderate instability as the mid level cold pool ( and associated
steep mid level lapse rates ) traverses se Michigan. These factors will support a chance of
showers/thunderstorms, with the exception of the northern Thumb which looks to remain displaced from
the colder mid level temps. Assuming instability will be able to achieve deep convection, the degree
of mid level dry air suggests gusty winds and hail would be possible with the strongest storms.
Subsidence in the wake of this feature and building mid level heights across the northern Great
Lakes will allow the sfc high to expand across lower Michigan on Saturday, with dry and relatively
cool conditions to start the weekend.

High pressure will remain stationary over the Great Lakes region for the beginning of next week
allowing for dry weather and clear skies. A cold front will then pass through the area during the
middle of the week, bringing the next chance for showers and thunderstorms. High temperatures will
be in the low to mid-80s and low temperatures in the 60s.

. . posted at 310 PM EDT Thu Jul 27 2017

13z surface/composite analysis shows a cold front sagging southward across southern Lower Michigan,
with a 1013mb frontal wave over far southwest lower. Surface winds have pretty much taken on a
northerly component across southeast lower this morning, low level theta-e gradient actually lies a
bit farther north. On the "cool" side of the boundary ( in a low level theta-e sense anyhow ) has
been a band of clouds and rain/drizzle which has been mostly along/north of the I-69 corridor though
some radar returns have spilled into northern Oakland county. Early visible imagery suggests some of
the northerly flow associated with cool outflow from these showers ( boundary/undular bore look to
clouds moving through southern Oakland/livingston counties and into jackson/calhoun counties ).
Meanwhile, clouds have thinned out over the southern third of the forecast area. 12z soundings show
moisture gradient from north- south across the state with precipitable water off DTX sounding of
1.41 inches, and about half that at APX ( and lots of sun across the northern part of the state to
boot ). Water vapor imagery also shows this dry air push from the north, with the tail end of a
short wave trough/PV anomaly clipping Georgian Bay.

Modified 12z DTX sounding for 86/67 yields around 800j/kg mlcape... With the best low level focus
for potential convection this afternoon mostly likely closer to the Ohio border where better
convergence should be ( synoptic and that associated with above mentioned outflow ). There are some
suggestions that enough lingering instability inland from Saginaw Bay, not totally sure about this
so for now will focus precip threat over the southern half of the area given above reasoning.

. . posted at Issued by National Weather Service GAYLORD MI