NWS Forecast Discussion

FXUS63 KDTX 201943

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


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

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.



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.



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.


Issued at 104 PM EDT Fri Sep 20 2019


Minimal cloud cover, mostly at mid and high levels, will continue
over the region this afternoon and tonight. Gradually increasing low
level moisture and light winds will allow for the likelihood of some
light shallow fog over southeast Michigan airfields.  Not really
expecting anything much worse than MVFR visibilities as advancing
cloud cover decreases favorability for anything lower. A developing
low pressure system over the northern plains states will approach
the western Great Lakes Saturday... and spread more widespread cloud
debris over the region throughout the day. Again.. VFR conditions
are likely to be the case throughout saturday despite increasing
cloud cover. Could also see a gradual increase in southerly winds on
saturday as surface pressure gradient increases ahead of the
approaching front. Wouldn`t expect to see gusts over 20 knots though
until potentially on Sunday.

For DTW...winds overnight should be light enough for some shallow
radiation fog in the MVFR range, but increasing clouds likely to
keep fog from slipping into IFR.


* Low in ceilings at or below 5000 ft.


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



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at www.weather.gov/detroit.

NWS DTX Office Area Forecast Discussion