Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 210445
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
1145 PM EST Tue Feb 20 2018


.AVIATION...

Rain will continue overnight as wave of low pressure lifts northeast
along frontal surface. Once this low passes, the front (and rain)
will shift east of the terminals. This will occur in the 12z-15z
time frame. Moisture convergence along the front should produce
generally MVFR to occasional IFR ceilings with improvement midday
Wednesday as drier northwest flow increases. VFR conditions will be
prevalent to end the forecast period.

For DTW...More persistent rain will spread back into the terminal by
the beginning of the forecast period with occasional showers holding
on until roughly 15z Wednesday morning. Ceilings will drop to MVFR
(IFR at times) with this activity. Frontal passage will bring a wind
shift to west-northwest overnight veering to the north with time on
Wednesday.

/DTW THRESHOLD PROBABILITIES...


* High for ceilings AOB 5000 feet through Wednesday morning, low
  thereafter.

* Very low for thunderstorms impacting terminal overnight.

&&

.PREV DISCUSSION...
Issued at 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.

&&

.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.
&&

$$

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


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at www.weather.gov/detroit.



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