National Weather Service United States Department of Commerce

Forecaster thinking

The risk of strong/severe storms is now east of se Michigan as the prefrontal instabily is quickly
exiting the area. The cold front now tracking across se Michigan will also quickly pass to the east
by early evening. Some of the stronger wind gusts today ( around 40 mph ) have been within the
prefrontal warm sector and immediately along the sfc cold front. The better mass
adjustments/subsidence push associated with the wave now traversing nrn Ontario will largely remain
across the nrn Great Lakes this evening. This is were some of the stronger wind gusts will remain. A
secondary push of low level cold air will arrive this evening, at which time the low level wind
field will weaken. Based on latest sfc obs, wind gusts in the advisory are still near 40 mph, so
will hold onto the advisory through 6 pm. Despite the better push of cold air this evening, the
decrease in the low level wind field will support a gradual weakening of the winds during the course
of the evening.

The airmass advecting into se Michigan tonight will bring a real touch of autumn for this weekend.
925mb temps will plunge into the single digits by Saturday morning. Low level anticyclonic flow
amidst weakly confluent mid levels will allow sfc high pressure to expand into the region from the
west tonight and become centered over Lower Michigan on Saturday. Sfc-925mb wind fields will veer
toward the north across Lake Huron tonight, becoming northeasterly across the southern portion of
the lake by Saturday morning. The degree of cold air coming across the relatively warm lake ( water
temps in the 60s ) will support a healthy lake strato cu field. A building subsidence inversion will
limit over-lake instability, a limiting factor for carrying anything higher than a slight chance of
lake effect rain showers in the Thumb.

Model solutions seem rather aggressive in clearing skies out early Saturday. Given the warm lake
temps and depth of the low level inversion, suspect the strato cu field will be rather persistent.
The circulation around the low level anticyclone will switch the wind field off shore late in the
day. This and the potential to mix into very dry air atop the inversion will offer at least better
chances for clearing late in the day. Considering the airmass and expected cloud cover, high temps
in the low 60s at best seems reasonable on Saturday. High pressure will hold across se Michigan
through the end of the weekend. Optimal radiational cooling sat night ( assuming skies clear ) will
support min temps down into the 40s. Some modification to the airmass and potential for a fair
amount of sunshine will boost high temps toward 70 on Sunday.

A tightening pressure gradient across the Great Lakes in response to a strong surface high located
just east of maine and low pressure over western Ontario will help funnel in warmer air across our
region through Monday. This warm front will return temperatures back into the lower 70s for a
daytime high and will increase cloud cover from south to north throughout the day as the front

Waa will increase late Monday into Tuesday as noted by an increase in theta-e values, which will
help aid likely rain and thunderstorm chances across se Michigan. Given the increasing deep layer
shear ( 40 kt by 18z, 50 kt by 00z Wednesday ) and unstable air mass ahead of an approaching cold
front, will have to keep an eye out for severe weather chances with additional model runs. The cold
front is expected to push through late Tuesday into Wednesday and will bring additional rain and
thunderstorm chances along with the additional possibility of severe weather, before cool and dry
air filters in behind the front by Wednesday.

A strong thermal trough and associated surface high pressure system will bring slightly below-normal
temperatures with highs peaking in the 60s and lows in the 40s to lower 50s Wednesday through

. . posted at 328 PM EDT Fri Sep 21 2018

A prefrontal low-mid level moist axis/convergence has been responsible for the region of showers
this morning across much of the northwestern portion of the forecast area. These showers have been
weakening over the last couple hours. This trend will continue through the rest of the morning.
Diurnal heating and the resultant destabilization will however support convective redevelopment this
afternoon ( mainly east of a Sandusky to hillsdale line ) as this moist plume/convergence axis
continues on an eastward path. The main point of uncertainty at this stage is whether convective
initiation fires over the eastern counties or east of se Michigan ( in Ontario ). The eastward
movement of an outflow boundary from the morning showers ( noted on radar and satellite ) will be
east of the forecast area by afternoon and may very well prove to be the focus for convective
initiation this afternoon. Can not however ignore the clear skies over the eastern third of the
forecast area late this morning. This will afford an opportunity for weak destabilization by early
afternoon. This window of destabilization will be brief ( 1pm to 4 pm ) before the moist axis and
instability heads east of the region. Even with weak instability ( 0-1Km ml cape just under 1K j/kg )
a strongly sheared environment ( 50-60 knots deep layer shear ) and some degree of sub cloud layer
dry air will make strong/damaging wind gusts possible with any convection that is able to develop
early this afternoon.

The period of clear skies in the east this morning may push a few peak gusts over 40 mph in the warm
sector today within the gusty southwest flow. The longer duration 40+ mph winds are still expected
across the Saginaw valley and Thumb region in the post frontal westerly flow. So no adjustments to
the wind headlines will be made this morning.

. . posted at 1027 AM EDT Fri Sep 21 2018