Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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538
FXUS63 KDTX 201046
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
546 AM EST Tue Feb 20 2018


.AVIATION...

Constant stream of very moist air will continue to stream northeast
through the area into tonight. This will maintain fairly steady
rainfall throughout the day into tonight. However, the I-94
terminals may only get brushed by the southern edge of this plume at
times. Ceilings/visibilities have been quite variable and will most
likely continue to be. In general, expect MVFR to lower VFR cigs and
vsbys with occasional IFR in heaviest pockets of rain (and also in
patchy fog early in the forecast). Embedded thunder remains a
concern, but timing is too uncertain to pin down (and probably
better handled by shorter term amendments).

For DTW...Cigs/Vsbys will fluctuate today into tonight as terminal
goes in and out of rain, but generally expect MVFR/lower VFR. The
lowest cigs will most likely come tonight as plume of deep moisture
and rain settles through terminals as cold front begins to sink
southeast through lower Michigan.

/DTW THRESHOLD PROBABILITIES...

* Medium for ceilings 5000 feet today, high tonight.

* Low for thunderstorms impacting terminal today

&&

.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.....DG
DISCUSSION...DRK
MARINE.......DG
HYDROLOGY....DG
CLIMATE......DRK


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at www.weather.gov/detroit.



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