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000 
FXUS63 KDTX 202308
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
708 PM EDT Fri Sep 20 2019

.AVIATION...

SE Michigan remains between high pressure over the mid Atlantic 
coast and a low pressure system developing over the Plains tonight. 
Cloud moisture remains minimal to start the night but then gradually 
increases between the systems toward sunrise. Southwest surface wind 
helps lift humidity but remains just light enough to allow some MVFR 
fog during early morning before clouds become a factor. Southwest 
wind increases and accelerates moisture transport from the Gulf 
coast and Mississippi valley during Saturday helping to thicken and 
lower clouds with a few showers possible during the afternoon. 
Ceiling and visibility remain VFR but trending below 5000 ft toward 
Saturday evening. 

For DTW...Mixed coverage of mid and high clouds produce VFR above 
5000 ft tonight through Saturday morning. Southwest wind also 
remains light enough for some shallow fog in the MVFR range but 
increasing clouds toward sunrise likely to keep fog from slipping 
into IFR. 

DTW THRESHOLD PROBABILITIES...

* Low for ceiling at or below 5000 ft.

&&

.PREV DISCUSSION...
Issued at 343 PM EDT Fri Sep 20 2019 

DISCUSSION... 

Aside from a few brief sprinkles that survived through expansive low-
level dry air this morning between I-69 and US-10, high pressure 
still maintains control over the region a while longer. GOES-16 
water vapor imagery depicts a corridor of midlevel clouds along a 
subtle shortwave with varying coverage expected through tonight. 
Temperatures are approaching daytime highs, generally in the lower 
80s which continues a stretch of above normal warmth with overnight 
lows a bit higher than the previous night due to high cloud 
insolation effects.

Ridge axis aloft will begin translating eastward on Saturday as a 
longwave feature over the western CONUS deepens and tilts 
northeastward into the Midwest. Meanwhile, winds will reorient in 
the lower levels as a midlevel circulation sets up aloft. Moisture 
advection will also be ramping up extensively with isentropic lift 
becoming a significant factor along the southern half of the CWA 
early on Saturday. PW values will increase by nearly 75 percent come 
Saturday afternoon with some solutions suggesting PW values 
approaching 2.00 inches as a 0-3 km max ThetaE axis of 348 K works 
across the region. There is the potential for some Saturday 
afternoon thunderstorms, but given the limited shear environment the 
severe threat remains low with mostly just ordinary single-cell 
storms. Heavy downpours will be likely in this moisture rich 
environment, but the coverage and duration of this activity depends 
on how efficiently the low-level capping inversion erodes. Certainly 
worth noting that a plethora of forecast soundings are showing tall 
skinny CAPE profiles across the CWA while plan view guidance varies 
widely on the strength and distribution of the best MLCAPE 
environments.

Attention then turns to the approaching dynamic cold front beginning 
late Saturday night. This boundary will be quite the rainmaker with 
QPF around 1 inch for most of the CWA, but locally higher values 
cannot be ruled out. Given the impressive deep-layer saturation /950-
350 mb/, this moisture laden atmosphere will is placed precariously 
ahead of the front. Confidence is high for an extended period of 
showers and occasional embedded thunder as the front bisects SE MI 
late Sunday evening. 00-06Z is the most likely time period for the 
heaviest rainfall which aligns with the best LLJ enhancement 
/925 mb winds around 30 knots/ with multiple reinforcing shortwaves 
before and after the front triggering additional showers. Dewpoints 
will start to fall behind the front as dry air advection fills in, 
but cloud cover will be slow to erode.

Deformation showers should be quickly tapering off early Monday as 
mid level dry slot reaches at least as far north as the M-59 
corridor. Mid level circulation/cold pool (-2 to -3 C at 700 MB) 
will be tracking through the Northern Great Lakes during the day, 
with northwest upper level confluent flow building in for Monday 
night. 500 MB ridge axis moves over the Central great Lakes on 
Tuesday, helping to dry things out from the heavy rain during the 
second half of the weekend.

Digging upper level trough coming out of Western Canada will then 
bring another cold front and chance of showers on Wednesday. 
Somewhat cooler post frontal conditions behind the front on 
Thursday, but there is a wide disparity amongst the cooler Canadian 
(850 MB temps in mid single numbers) vs the much warmer European 
(850 mb temps around 15 C). The differences are clearly evident in 
the 500 MB 6 hr height changes as well, with the Canadian much more 
aggressive and farther south. The GFS is sort of in the middle, thus 
a forecast close to normal is probably best way to go for now.

MARINE...

High pressure over he eastern Great Lakes will give way to an 
approaching cold front later in the weekend. Southerly winds will 
steadily increase late on Saturday and especially into Sunday as the 
pressure gradient tightens over the region. 

Probabilities for wind gusts to exceed southwesterly 20 knots over 
northern marine areas are relatively small for the weekend ahead of 
the system. but increase to moderate or high levels in 
northwesterlies on Monday. Better probabilities for gusty 
southwesterlies exist for Saginaw Bay, far southern Huron, Lake St. 
Clair, and western Lake Erie ahead of the front later Saturday night 
and Sunday.  

There is also the possibility of heavy rain with thunderstorms over 
southern portions Lake Huron southward to western Lake Erie late 
Sunday afternoon to Monday morning.

HYDROLOGY...

A slow moving cold front combined with a region of relatively high 
moisture content that will lift into the region within the pre 
frontal southwest flow will provide increasing chances for rain late 
Saturday night through Sunday night. Total rainfall amounts between 
a half inch and an inch are highly probable. The pattern is also one 
that will be conducive to supportive locally heavy rainfall.

&&

.DTX WATCHES/WARNINGS/ADVISORIES...

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

&&

$$

AVIATION.....BT
DISCUSSION...KK/SF
MARINE.......05
HYDROLOGY....SC

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