Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 202050
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
350 PM EST Tue Feb 20 2018

.DISCUSSION...

A Flood Watch remains in effect for all of southeast Michigan today
and will continue through Wednesday afternoon. Many locations across
the area have seen 1 to 1.75 inches as of this morning with an
additional 1 to 1.5 expected through tomorrow morning. Locally
higher amounts remain possible in locations that experience higher
rainfall rates associated with stronger convective cells within this
system. Areas north of I-69 saw a break in persistent rain as the
main rainfall axis focused south of I-69 earlier today, but an area
associated with the low level jet is pushing northeastward through
the area now. The back edge of the precipitation is moving north of
Lenawee and Monroe counties as of 3:15 pm. Areas along I-94 and
northward to the Thumb and Tri-Cities will see additional rainfall
this afternoon and evening from this round of showers as they lift
north. The southern tier of counties including Metro, look to get a
break in precipitation for a few hours before the next round
arrives. Lack of strong instability across southeast Michigan has
kept embedded cells from overachieving with rainfall rates during
the early parts of the afternoon as they enter the CWA.

The warm front has lifted into northern Michigan leaving all of
southeast Michigan in the warm sector. Rich gulf moisture continues
to push northward with the aide of strong southwesterly winds. The
low level jet around 60 knots and 160+ knot upper level jet are both
aiding moisture advection and providing a source of lift for the
ongoing rain. Dewpoints are nearing 60 towards the Ohio border and
mid to upper 50s towards the Thumb. Forecast PWAT values streaming
in remain impressive for February with models consistently reaching
around 1.4 inches through this afternoon and evening. This will
continue to support the heavy rain potential through this evening.
Lapse rates and showalter indices continue to warrant a mention of
thunder, but most of the thunder has remained south of the Michigan
border so far today. No severe thunderstorms are expected, but there
remains potential for occasional lightning and a rumble of thunder.

The center of the surface low will lift north of the Great Lakes
tonight with a slow moving cold front trailing behind. This will
result in another concentrated round of rain ahead of this front
this evening and into tomorrow morning as shortwave impulses lift
along this boundary. Regional radar already shows this line
developing off the front across Wisconsin, Illinois and into western
Michigan. Current thought based on trends is that areas to the south
get a break in rainfall this evening and areas north of I-69 not
experiencing much of a break until the line passes due to the ongoing
rain lifting north. Have increased rainfall totals slightly across
the north to account for this. Will need to keep an eye on the urban
areas for flooding concerns with the rain moving in tonight ahead of
the front. Occasional lightning with a few rumbles of thunder remain
possible with the convection ahead of the cold front.

A high pressure will begin tracking into the northern plains and
will help push east of Michigan tomorrow. Rain will continue to move
through during the morning hours and likely clear out by the
afternoon and result in a drying trend into Thursday. Cold air will
arrive in the wake of the front bring temperatures down into the 40s
on Wednesday followed by temperatures in the 30s on Thursday.

An approaching warm front will bring precipitation chances across SE
MI throughout Friday morning and afternoon before an approaching
high pressure system from the Plains acts to diminish precipitation
chances late Friday into early Saturday. The biggest forecasting
challenge if precipitation does materialize on Fridays is
precipitation type. A wide variety of precipitation types will be
possible across SE MI as noted by internal top-down methodology,
with rain possible across the Metro region and rain/snow to snow
possible up towards Flint, Tri-Cities, and into the Thumb. While too
early to highlight freezing rain chances, the GFS and GEM model runs
hint at the possibility as above-freezing 850mb temperatures exist
above below to near-32 degree temperatures at the surface. Any
frozen precipitation that does materialize is expected to turn to an
all rain solution by Friday early afternoon as warmer air is pumped
into the area.

High pressure will build in across the northern Great Lakes late
Friday into Saturday which will bring a period of dry conditions,
before a second low pressure pushes northeast from KS/MO into MI
Saturday into Sunday. Similar to Friday`s forecast, a wintry mix to
snow could be possible with the onset of precipitation, especially
across the Tri-Cities into Thumb as temperatures hover at or below
the freezing mark, before warmer air converts precipitation to an
all rain solution for any lingering rain seen throughout Sunday late
morning and afternoon.

High pressure is then expected to push east across the Ohio Valley
Monday into Tuesday, which will keep conditions relatively dry. The
GEM and ECMWF produce weak low pressure systems and push them east
across the Great Lakes on Tuesday, but major divergence is seen
regarding strength, timing, and placement of the low. PoP values
will remain low at this time unless convergence is noted in future
model runs. Temperatures are projected to peak in the mid to upper-
40s late this week into early next week.

&&

.MARINE...

A warm front draped along the northern Lake Huron shoreline will
remain nearly stationary through this evening as low pressure tracks
along it. Moderate southerly winds south of the warm front will
continue across Lake Huron through the evening. Stable conditions
within this southerly flow regime will limit gust potential to below
25 knots. Low pressure tracking along the warm front will then pull
a cold front across Lake Huron tonight. In the wake of the cold
front, winds will shift to northwesterly late tonight before
continuing into Wednesday. Increased instability in the colder air
mass will allow gusts to approach 30 knots Wednesday morning across
northern Lake Huron. High pressure building into the region
Wednesday night will cause winds to decrease before slowly shifting
to northeast by Thursday.

&&

.HYDROLOGY...

Low pressure centered over the upper peninsula of Michigan will
continue advecting moisture across Southeast Michigan through
tonight. Precipitable water values will remain very high in this
airmass through this evening ranging from 1.3 to 1.5 inches (12Z DTX
sounding PW of 1.33 inches). Multiple rounds of rainfall starting on
Monday have dropped an average of 1.25 to 1.75 inches of rain on top
of a melting snowpack through early Tuesday afternoon.

An additional area of rainfall pushing northeast across Southeast
Michigan late this afternoon is expected to stall across areas along
and north of M-59 this evening. A final band of rainfall along a
cold front will then track eastward across all of the region
tonight. Additional rainfall amounts ranging from 0.75 to 1.50
inches will be possible late this afternoon through Wednesday
morning. This will allow storm total rainfall across Southeast
Michigan to range from 2 to 3 inches by Wednesday morning with some
areas exceeding 3 inches. Rainfall is then expected to end Wednesday
morning as high pressure brings a push of drier air to the region.

Rainfall of this magnitude will cause widespread ponding of water in
low-lying and poorly-drained areas. Additionally, minor to moderate
stream and river flooding is now occurring across Southeast Michigan
and will continue through late this week. The record warmth
accompanying high runoff into rivers will also lead to the potential
for ice breakup which could further exacerbate flooding in localized
spots.

&&

.PREV DISCUSSION...
Issued at 1221 PM EST Tue Feb 20 2018

AVIATION...

A persistent stream of deep moisture will continue moving across the
region through tonight. This will keep periods of rain in the
forecast through Wednesday morning at all sites. An area of light to
moderate rain across northern Indiana will first push across the
Metro Detroit terminals this afternoon. Expect mainly MVFR ceilings
with occasional IFR in heavier pockets of rain this afternoon for PTK
and sites to the south. This initial band of rain is then expected
to lift northeastward by evening while another final band of rain
pushes in from the west ahead of the main cold frontal boundary. A
break in steady rainfall is expected through the evening from PTK
southward with VFR ceilings while steadier rainfall and MVFR ceilings
overspread FNT and MBS. Still cannot rule out a rumble of thunder this
afternoon and tonight but potential continues to remain too low to
include in TAFs at this time. As the cold front pushes eastward late
tonight, expect a steady rain and MVFR/possibly IFR ceilings/visibilities
to overspread all sites. A wind shift from southwest to northwest is
will occur during the early morning timeframe before drier air
brings an end to rainfall and improving conditions Wednesday morning.

For DTW...Light to moderate rainfall is expected through much of the
afternoon while ceilings and visibilities continue to fluctuate from
VFR to MVFR during heavier pockets of rain. A break in rainfall is
expected this evening with VFR ceilings before a final band of rain
accompanying a cold front pushes through late tonight. Better chance
for MVFR/possibly IFR ceilings/visibilities will accompany the cold
frontal passage late tonight before drier air brings improving
conditions Wednesday morning.

/DTW THRESHOLD PROBABILITIES...

* High for ceilings aob 5000 feet this afternoon, medium this
  evening, high later tonight.

* Low for thunderstorms impacting the terminal this afternoon
  through tonight.


&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...Flood Watch through Wednesday afternoon for MIZ047>049-053>055-
     060>063-068>070-075-076-082-083.

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

$$

DISCUSSION...AA/AM
MARINE.......JD
HYDROLOGY....JD
AVIATION.....JD


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at www.weather.gov/detroit.


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