Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 200751

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
351 AM EDT Sat May 20 2017


Closed mid level circulation currently centered over Kansas will
wobble northeast over the next 24 hours, settling to a position over
northern Minnesota by Sunday.  Increasing mid level southwesterly
flow as the associated height falls begin to expand eastward will
subsequently promote a corridor of deep layer northward moisture
transport from the mid MS valley into the great lakes through this
time.  Retention of a drier low level easterly flow tucked beneath
shortwave upper ridging will dictate conditions locally through at
least the first half of Saturday.  Plenty of mid cloud around under
this pattern, but shower potential appears minimal until mid
afternoon given no real source of ascent outside of the lingering
850 mb frontal slope.  The noted increase in mean thicknesses under
the higher upper height field will translate into some moderation of
the existing air mass relative to yesterday.  The diurnal response
will still be muted given the easterly flow and existing cloud
cover, but readings of low-mid 60s are attainable outside of the
immediate shoreline locations.

Mixed signal yet across the model spectrum in defining possible
rainfall chances this afternoon.  Potentially looking at a repeat of
yesterday, with strong dependence on the downstream evolution/track
of convection and/or the remnant MCV from thunderstorm activity now
lifting across Missouri.  Possible that one or more impulses embed
within the southwest flow, driving a weak axis of moist isentropic
ascent up the warm frontal slope and translate into an expanding
region of showers by mid-late afternoon.  Lower confidence in terms
of both possible coverage and timing for the latter half of the
daylight period, certainly justifying a conservative approach to the
defined precipitation chances.

Linear ribbon of higher theta-e will accompany the stronger mid
level and warm frontal ascent into the region tonight.  The overall
setup continues to point toward a solid window for several hours of
rainfall during the overnight period.  Diminishing mid level
stability will also favor a good component of elevated thunder with
this activity.  Low level stability concerns enhanced by the
nocturnal/late night timing minimizes the severe risk, although the
degree of mid level instability/lapse rates could generate heavy
rainfall within more organized clusters.

Deeper moisture axis shifts to the east by mid morning Sunday, with
a corresponding downturn in precip coverage during this time.
However window exists for greater low level moisture advection to
commence immediately in advance of the inbound cold front during the
midday period.  Some uncertainty yet in the exact frontal timing,
but a slower passage enhances the potential for enough air mass
recovery to occur to potentially support a secondary period of
showers/thunderstorms Sunday afternoon.  Dewpoints briefly inching
into the lower 60s, as temperatures recover into the lower and
middle 70s will yield at least a weakly unstable profile - MLCAPE
800-1200 J/KG.

Dry slot works in by Sunday evening, as the sprawling lower height
field associated with the closed system now over Minnesota arrives.
Main moisture plume tied to the attendant cyclonic flow will largely
remain north and west of the region through Monday.  This will
support a cooler but dry day, with late day temperature readings
simply a shade below normal.  Becoming increasingly unsettled during
the Tue - Wed periods as a piece of this closed system rotates
southeast and into the region.



A low pressure system will be lifting northward through the western
Great Lake Saturday night and Sunday. This system will pull a warm
front through the eastern lakes Saturday night with a cold front
quickly following on Sunday. This will result in several wind shifts
through the weekend which will lead to varying wave heights through
that period as well. forecast starts off with northeasterly flow
today gusting to around 20 knots with waves building along the
western Huron basin. Winds turn more easterly as the warm front
approaches shifting the higher waves slightly northward.  This will
lead to marginal small craft conditions through most of the day.
Winds will veer to the southeast while still gusting to near 20
knots on Sunday reducing the wave heights along the nearshore waters
thus small craft advisories are less likely.



Scattered showers may lift back into the area this afternoon but
better chances of precipitation will come overnight into Sunday.
Total rainfall looks to average 0.5 to 1.0 inch. Locally higher
amounts will be possible for those locations affected by
thunderstorms. No flooding is anticipated at this time.


Issued at 1207 AM EDT Sat May 20 2017


Shower activity tied to synoptic scale stretching deformation has
all but peeled east of the area. Will continue to see the 850mb
frontal boundary remain in place over southeastern Michigan tonight.
This will keep midcloud in the picture particularly south of the Tri
Cities tonight. Amplifying shortwave ridging will keep precipitation
out of the region through the overnight. Question is on timing of
precipitation onset tomorrow. There is enough signal that suggests
convection from MO will lift northward overnight, sliding northward
within the crux of the main theta e ridge and moisture transport.
Larger scale ridging will be dominant which should work in dampening
out prior precipitation as it arrives, however, midlevel lapse rates
appear steep enough to allow for shower potential. Decided to go
with a tempo grouping for showers in the 17-19Z time window. Strong
midlevel trough and associated surface low pressure system advancing
out into the central Plains this evening and tonight will set the
stages for developing and strengthening easterly flow. Expectations
are for a stiff breeze Saturday between 10 and 20 knots.


* Low confidence in cigs aob 5000 feet tonight medium confidence
  Saturday afternoon Saturday.


Lake Huron...Small Craft Advisory until 10 AM EDT this morning for LHZ442-443.

     Small Craft Advisory until 4 PM EDT this afternoon for LHZ421-422-

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



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