Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 181837
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
237 PM EDT Tue Apr 18 2017

.DISCUSSION...

Surface low pressure associated with small, but vigorous, shortwave
disturbance over east central Minnesota will track quickly into the
northern Great Lakes this evening and on into Ontario late tonight.
This will draw a warm front north through the region with southeast
winds veering to the southwest. This change in airmass will lead to
a much milder night as temperatures will only drop in the lower 50s
(versus widespread 30s early this morning). This southwest flow will
also bring an increase in low level moisture to the area, which in
turn will aid in the development of scattered showers or storms by
late tonight as a cold front trailing from the aforementioned low
sags southeast into lower Michigan. Timing of this front suggests
most activity (pre-12z) will be confined to the northwest forecast
area.

This front will settle south through the remainder of the forecast
area on Wednesday eventually stalling near the Michigan and Ohio
state line during the afternoon and early evening. Scattered to
numerous showers and embedded thunderstorms will settle south
through the area with this front with the best chance of rainfall
setting up over the southern forecast area during the afternoon as
the front stalls nearby. With southwest flow persisting into the
early afternoon over far southern lower Michigan, expected highs to
climb into the lower 70s. Further north, an earlier frontal passage
will most likely limit temperatures to the mid/upper 60s (60ish near
the colder waters of Lake Huron).

A second vigorous shortwave (this one in the southern stream coming
onshore California this afternoon) will emerge over the central high
plains on Wednesday and shift into the Missouri Valley by evening.
Forcing in advance of this system will induce cyclogenesis and as
this surface low lifts into the mid/upper Mississippi Valley during
the night, the front will shift back north as a warm front. This
will maintain rather mild conditions once again, particularly south
of I-69 where lows will hold in the 50s once again. There will be a
tighter south to north temperature gradient with a cooler feed of
air on northeast flow from a surface ridge north of the boundary.
So, I-69 north will settle into the 40s early before temperatures
rise overnight.

The risk of thunderstorms increases notably Wednesday night into
Thursday as the warm frontal surge brings better quality moisture
into the area as well as a decent shot of warm air advection during
the night. While the best forcing along/north of the low track looks
to shift into northern lower Michigan, there still appears to be a
good shot at convection overnight into Thursday morning. A lot will
depend on what sort of convective complex develops to the west with
the low itself and then trails the lead warm air advection elevated
convection.

Models pretty much across the board continue to slow the progression
of this low which is now positioned by over Lake Michigan, or even
eastern WI at 18z Thursday. This later timing brings the diurnal
cycle into play for convection on Thursday afternoon/early evening
as this low continues on an east/northeast trajectory and drives a
cold front through area near peak heating. Convective chances most
definitely appear better late in the day than in previous model
runs. However, there is also decent agreement in surging a pre-
frontal trough well east through area by early/mid afternoon, which
would complicate the placement of afternoon convection initiation.

Temperatures should warm into the 70s in advance of this system on
Thursday, especially over the southern half of the forecast area.
With dew points forecast to climb into the upper 50s, this would
yield SBCAPE values of 1000-1500 j/kg. While deeper MLCAPE values
are much more in question, this low level instability, combined with
decent mid level flow of near 50 knots, do suggest the potential for
storm organization and possibly some severe storms. In the end, much
will hinge on the eventual positioning of the best convergence along
the pre-frontal trough.

Dry conditions will take over to round out the week and into the
weekend.  Canadian high pressure moves in behind the departing low
on Friday and while this will bring drier weather, cooler weather
will also accompany it.  High temperatures through the weekend are
expected to hold in the upper 50s to around 60 degrees.

&&

.MARINE...

An increase in southeast winds this afternoon as high pressure exits
to the east.  Strongest response noted across northern Lake
Huron, where a period of 20 knot sustained winds may gust in excess
of 25 knots through early tonight.  Winds ease again late
Tonight into Wednesday as a cold front slides through.  A period of
northerly winds immediately behind this front will remain modest
during the day.   The approach of strong low pressure will then
bring an increase in northeast winds Wednesday night and Thursday
across portions of lake Huron.  The wind and wave forecast still
carries some uncertainty during this time, but looks like low end
gales will likely occur across the northern third of Lake Huron
during Thursday, and a Gale Watch has been issued. In addition,
small craft conditions appear likely for higher waves along the
nearshore waters and Saginaw Bay.  An unsettled period
Wednesday through Thursday with several rounds of showers and
thunderstorms possible.

&&

.HYDROLOGY...

Unsettled weather will return Tomorrow and Thursday with periods of
showers and thunderstorms. Thursday is expected to be the wetter
day, as strong low tracks through southeast Michigan.
Rainfall amounts in excess of 1 inch are likely north of the low
track, with the Tri-Cities region looking to experience the brunt of
the rain. The potential for some minor flooding will exist,
particularly given the wetter than average conditions noted in
recent weeks.

&&

.PREV DISCUSSION...
Issued at 107 PM EDT Tue Apr 18 2017

AVIATION...

Expansive high pressure centered over Eastern Canada will continue
to maintain dry low levels over southeast Michigan through at
least the evening hours. Diurnal mixing this afternoon expected to
support southeast wind gusts around 20 knots. Low level moisture,
along with showers and possible embedded thunderstorms will attempt
to arrive/develop late Tonight into Tomorrow morning, but coverage
is uncertain and may end up being limited as there is little to no
upper level support. Wind shifting to westerly direction behind the
slow moving front tomorrow.

For DTW...There is a chance of showers during the entire day
tomorrow, but majority of the day should end up dry. Thunderstorm
chances appear to be too low to include in TAF at this time.

//DTW THRESHOLD PROBABILITIES...

* Low for cigs at or below 5000 ft late tonight with medium
  confidence on Wednesday.

* Low for thunderstorm occurrence Wednesday


&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...NONE.
Lake Huron...Gale Watch from Thursday morning through Thursday evening for
     LHZ361-362.

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

$$

DISCUSSION...DG/SS
MARINE.......SF
HYDROLOGY....SF
AVIATION.....SF


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