Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 201721
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
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.

&&

.PREV DISCUSSION...
Issued at 1037 AM EST Tue Feb 20 2018

UPDATE...
Radar trends have shown rain easing up a bit across the northern
portions of the area with scattered less intense rain showers at the
moment. A stronger band of rain is now entering the southern tier of
counties and will be affecting the Metro and surrounding locations
through the rest of the morning and into portions of the afternoon.
Flooding reports received thus far have been localized. There remains
potential to see a break from the higher intensity rainfall this
evening after this round moves through. This will help alleviate
widespread flooding concerns. Will be keeping an eye on radar trends,
especially across the Metro, for any potential issuance of an Areal
Flood Warning for this afternoon.

PREV DISCUSSION...
Issued at 327 AM EST Tue Feb 20 2018

DISCUSSION...

The flood watch continues for all of southeast MI today through
Wednesday afternoon for the long duration rainfall event that is
ongoing. Rainfall totals on Monday were largely in the 0.25 to 0.75
inch range across the area which is very close to forecast. This
adds confidence going forward with the event totals around 1-3
inches by Wednesday afternoon. The one area I think may be overdone
is the far south (Lenawee, Monroe, and maybe into southern Wayne) as
bulk of the moisture today looks to be lining up to the north, which
would really cut into amounts. They will be left in the watch as
rain could easily fill back in or drift back south today, in
addition to all the rain upcoming Tuesday night into Wednesday
across the eastern CWA.

Models have changed little over the last 24 hours with the deep
trough over the western conus now releasing a strong but sheared
vort max up through the Midwest. This is driving the surface low to
lift up into Northern MI this morning which in turn is pulling the
surface warm front, that stalled across the Saginaw Valley
overnight, up north of the area today. The northward shift of the
warm sector will bring additional warmth and moisture into the
region today. The 00Z DTX sounding Monday evening already showed
pwats of 1.25 inches, with a further increase on the way. SPC
mesoscale analysis data supports model output showing 1.4 inch
readings on the way today. In addition, surface dewpoints as of 3am
were already in the mid 50s and will stay there through the day.
Strength of the low level jet (~60 knots) and upper levels (>150
knots) are both very potent and will aid in moisture advection and
dynamic forcing for the system. Marginal mid level lapse rates and
showalter indices between 0-2C will both warrant the mention of
thunder in the forecast.

Once the surface low tracks northeast this evening, we`ll be left
with the still active trailing cold front draped down through lower
MI. A series of shortwave impulses will lift up along the boundary
resulting in continued chances of heavy rainfall through the
overnight. A strong and cold arctic high building across the plains
and Midwest will then give the front a boost east allowing it to
finally work east of the region Wednesday. Some adjustments to
rainfall amounts remain possible and likely as the fgen and moisture
convergence shows itself better and how tight it may contract.
Current regional radar loop already shows some contraction of the
rain band further south and west which would pull the area of rain
to the north, possibly creating a longer period of light rain,
or lull, south of I94 today. Something to keep an eye on.

1040mb high will slide over the Great Lakes by Wednesday evening
bringing a much needed drying trend to the forecast through
Thursday. Temperatures will take a big hit as the cold front moves
through. Possible record highs today (low/mid 60s) will give way to
highs near 40 on Wednesday (higher across the SE with a later
frontal passage) and upper 30s on Thursday.  Still above normal for
late Feb but still a notable 20+ degree drop. Temps will then hover
in the low/mid 40s heading into the weekend. Precip chances will
return Friday as the western trough releases the next strong wave.
Mostly dry on Saturday before the next wave approaches for Sunday.

MARINE...

A warm front will resume its trek north today after stalling near
Saginaw Bay late Monday. Moderate easterly flow with gusts of 20 to
25 knots will persist early this morning over northern Lake Huron,
but expect the south to southwest winds in the wake of this front to
spread north over much of northern Lake Huron. Stable conditions
within this mild southerly surge will limit gusts to 20 to 25 knots
today into tonight. Low pressure will move along the front and north
of the Great Lakes tonight with a trailing cold front dropping south
through the area late tonight into Wednesday. Instability will
increase somewhat in this colder flow, so gusts will approach 30
knots over northern Lake Huron on Wednesday. High pressure will then
settle overhead by Wednesday night bringing a decrease in winds.

HYDROLOGY...

Low pressure will approach from the southwest with moist south to
southwest flow persisting in advance of this feature. Precipitable
water values will remain high, on the order of 1.25 inches, within
this ridge of high theta-e air. Numerous rounds of rain, some of
which will be heavy, can be expected as this low ejects into the
region today into tonight. Embedded thunderstorms will also bring
locally enhanced rainfall rates to some areas.

Once this low crosses northern lower Michigan into Ontario late
tonight into early Wednesday, a cold front will drop southeast
through the area, focusing additional rainfall into the first part
of Wednesday. Precipitation will then end by Wednesday afternoon.
All told, expect total rainfall in excess of 2 inches over a wide
swath of the region with some areas most likely topping 3 inches by
Wednesday morning.

This heavy rainfall will cause considerable ponding of water in low
lying and poorly drained areas. Sharp rises in area rivers and
streams will also occur with a number of rivers likely reaching
flood stage later today into Wednesday. The mild temperatures, along
with the substantial runoff into rivers, may also lead to some
degree of ice breakup which could potential exacerbate flooding in
localized spots.

CLIMATE...

Record high temperatures will be possible across much of southeast
Michigan today. Here is a look at record high temperatures for
Tuesday, February 20th:

Detroit     63 (2016)
Flint       61 (1930)
Saginaw     62 (1930)

With the high moisture content, there is also a good chance to break
the records for warmest lows February 20th:

Detroit     47 (1930)
Flint       45 (1930)
Saginaw     46 (1930)

&&

.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.....JD
UPDATE.......AA
DISCUSSION...DRK
MARINE.......DG
HYDROLOGY....DG
CLIMATE......DRK


You can obtain your latest National Weather Service forecasts online
at www.weather.gov/detroit.



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