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NWS Forecast Discussion

FXUS63 KDTX 172349

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
749 PM EDT Fri Sep 17 2021


Our diurnal cu field is quickly eroding this evening leaving most
of the area with FEW to SKC. This will carry us through the first
couple hours of the forecast before a cold front will begin to
impact the area. Clouds mainly in the 5-7kft range will accompany the
front tonight moving from northwest to southeast in time. At this
time will leave the TAFs without a precip mention although there
area some showers, and even some thunderstorms over southern Lake
MI, at this time. Model guidance suggest an overall weakening trend
as the front reaches SE MI thus will highlight the front with an
enhanced cloud field and no showers. Even precip reaches, looks
isolated enough to amend a select TAF or two. Winds will veer
around to the north immediately behind the front and northeast by
Saturday. High pressure will then clear skies out again Saturday.


*   Low for ceilings below 5000ft tonight.


Issued at 357 PM EDT Fri Sep 17 2021


Ohio valley boundary layer moisture made good progress into southern
Lower MI during the morning which is now taking the form of a
cumulus field this afternoon. A few pockets of moderate cumulus
early in the afternoon produced a brief radar return as much as
possible in the cloud layer confined below a notable 700-600 mb
inversion. The mid afternoon cloud conditions otherwise constitute a
mid summer day allowing surface heating to boost temperatures into
the lower and mid 80s in a maturing SW low level wind pattern.

The inbound boundary layer moisture has lifted surface Td into the
mid 60s across the region and serves primarily as the low level
theta-e ridge ahead of the cold front advancing eastward from the
upper Midwest. Upstream observations indicated some weak surface
based convection in CAPE mostly less than 1000 J/kg along the front
which have since nearly dissipated despite some extra support from
the entrance region of the upper level jet. The jet forcing and
narrow band of fgen is shown nicely in afternoon satellite imagery
but remains projected to lift into northern Ontario this evening and
to Quebec overnight. The front is then left behind to produce what it
can in the absence of larger scale support and surface based
instability which is to say not much. Borderline 20-30% POPs are
maintained for a few nocturnal flare ups of elevated instability and
light showers late tonight into sunrise Saturday.

The surface front exits the Ohio border early Saturday morning
leaving some trailing higher based clouds south of I-69. These clear
to the south as well by afternoon thanks to surface high pressure
building in from the northern Great Lakes. The associated inbound
air mass is slightly below normal although full sun helps lift highs
into the lower to mid 70s. The cool and dry high pressure air is
then exposed a little more Saturday night as clear sky and light
wind lead to lows mostly in the 40s by Sunday morning.

Progression of larger scale features remains steady Sunday into
early next week leading first to the quick exit of high pressure
Sunday afternoon. The return to southerly flow is modest but just
strong enough to displace the west flank of the low level thermal
trough. Some lake aggregate boundary layer moisture could produce
scattered shallow cumulus although with little disruption to surface
heating contributions that lift highs back into the lower 80s.
Clouds become more of a factor Monday while southerly flow
strengthens in the gradient between systems. The key here for
temperatures is more dependent on showers inbound from the Ohio
valley which are still shown to hold off until Monday evening but
just barely. The deepening trough in the westerlies over the Plains
sweeps the remnant moisture from Nicholas northward from the Gulf
coast into the Great Lakes Monday night. Near tropical moisture
transport is expected to bring at least numerous coverage of showers
while fully building the moisture axis ahead of the cold front set
to sweep through during Tuesday.

The Tuesday cold front is associated with a full latitude upper
level trough and deep NW flow from central Canada. Extended model
solutions indicate a broad region of high pressure settling into the
Great Lakes for the Wednesday to Friday period. The large scale
pattern promises good predictability for dry weather but with highs
in the 60s and lows well down into the 40s each day.


South-Southwesterly winds will hold through this evening in advance
of a cold front now approaching Lake Michigan. The front will drop
southeastward across the eastern lakes late this evening into
tonight. Winds will flip around to the north immediately behind the
front around midnight veering more toward northeast by Saturday
morning. Though winds will remain below 25 knots, large waves will
affect the nearshore waters of Lake Huron along the Thumb resulting
in a period of Small Craft Advisory conditions tonight through
Saturday afternoon. High pressure will then expand across the Upper
Great Lakes from the west on Saturday and will influence conditions
into Sunday.


Lake Huron...Small Craft Advisory from 2 AM to 4 PM EDT Saturday for LHZ441>443.

     Small Craft Advisory from 2 AM to 10 AM EDT Saturday for LHZ421.

Lake St Clair...NONE.
Michigan waters of Lake Erie...NONE.



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