Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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818
FXUS63 KDTX 200000
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
700 PM EST Mon Feb 19 2018


.AVIATION...

Warm front slowly advancing northward across lower Michigan,
currently positioned between DTW and PTK early this evening.
Conditions north of this boundary will remain defined by substantial
near surface moisture content, translating into LIFR/VLIFR
conditions in both very low stratus and fog. Frontal passage timing
will dictate any ensuing improvement tonight at PTK and FNT, before
the boundary eventually stalls somewhere near or just south of MBS
overnight. To the south, conditions likely to remain more variable
with greater dependence on pockets of heavier rainfall to bring
intermittent reductions down into IFR/low MVFR. This episodic trends
will continue to be refined as necessary. Low potential for embedded
thunder throughout the period, but too limited in occurrence and
uncertain in timing to include.

For DTW...Greater fluctuations in cigs and vsby likely going forward
as pockets of heavier rain help dictate further reduction in
conditions /IFR to MVFR/. Winds firmly from the south-southwest
through the period.

/DTW THRESHOLD PROBABILITIES...

* High for ceiling 5000 feet or less for the entire forecast.

* Low for thunderstorms impacting terminal tonight and Tuesday.

&&

.PREV DISCUSSION...
Issued at 350 PM EST Mon Feb 19 2018

DISCUSSION...

The main story for the next few days will be the long duration
rainfall that began this morning and will last through Wednesday
afternoon. A Flood Watch will remain in effect for widespread
rainfall of 1-3 inches total over the course of this time frame. In
addition to the rainfall, remaining snow melt will add to runoff.
Frozen soils will not allow much absorption of rainfall and runoff
which may lead to flooding concerns. Ice break up on lakes, rivers
and streams will also be something to keep an eye on as temperatures
warm, which may exacerbate any local flooding concerns.

The set up for this rainfall is resulting from an amplifying
longwave pattern over the western US with a ridge across the
southeastern US. This pattern is leading to strong warm and moist
advection across all of Michigan ahead of a slow moving boundary
extending from the western Great Lakes southwest through the central
Plains. The digging western trough is allowing the strengthening low
level jet to pull moisture from the Gulf of Mexico into the Great
Lakes. Precipitable Water (PWAT) values of around 1.4 inches are
forecast to enter southeast Michigan by tonight. To put that value
into context if reached, it will likely break the highest PWAT value
observed in the month of February at DTX. Several waves along this
slow moving boundary will be able to tap into this high moisture
content to bring persistent rainfall and flooding potential.

Currently, the first wave is moving northward with a surface low
lifting out of the plains now approaching northwestern IL. The
associated warm front helping to drive organized convection in lower
Michigan this afternoon is forecast to lift northward between I-69
and M-59 by this evening. This will focus the higher rainfall
amounts this evening along and north of I-69. Strong theta-e
advection and isentropic lift will continue continue to support
rainfall through tonight as the low nears the Great Lakes. Mid level
lapse rates around 6-7 C/km for portions of the night along with
Showalter indices approaching zero would be enough to support some
embedded thunderstorms. Severe thunderstorms are not expected, but
any embedded thunderstorms tonight would come with more potential
for higher rainfall rates at those locations.

As the initial surface low pressure washes out moving northeastward
towards the Great Lakes overnight, another surface low develops and
takes a similar path as the previous low. By tomorrow morning, the
strongest push of warm and moist air enters southeast Michigan with
mid to upper 50 dewpoints and 850 mb temperatures around 10C. High
temperatures tomorrow should reach well above normal for February
into the mid 60s. The slow moving boundary will begin moving eastward
as a cold front tomorrow afternoon as a building surface high
pressure system moves into the northern plains behind it. The
atmospheric firehose of moisture will continue ahead of this cold
front as strong southwesterly winds associated with the southern
stream jet keep drawing Gulf moisture into the region through Tuesday
night. Highest rainfall totals for Tuesday morning into Tuesday
night will be areas south of I-69 as the low pressure moves through.
Thunderstorms will remain possible again tomorrow, but severe
thunderstorms again remain a low possibility.

The slow progression of the cold front will keep rainfall potential
into Wednesday morning. The front will eventually push east of
southeast Michigan by the afternoon and bring an end to the rainfall
as dry air and cooler temperatures move in for the evening. The
cooler air in the wake of the front will bring high temperatures
back down into the 40s for most locations on Wednesday.

The extended period starting on Thursday and lasting though the
weekend features periods of unsettled weather conditions.  Thursday
will remain dry before the next wave makes inroads into the Great
Lakes region overnight Thursday and into Friday.  Looking at a break
overnight on Friday before some more precipitation moves in early
Saturday.  The best chances for rain and/or snow as of now is Friday
morning and then again on Sunday morning.  Overall, temperatures
during the day are expected to remain above freezing with lows at
night hovering around the upper 20s to lower 30s.

MARINE...

A warm front will stall near Saginaw Bay and central Lake Huron
tonight as the next wave of low pressure organizes over the Midwest.
A moderate easterly flow gusting to 30 knots or less will occur in
the briefly stronger gradient north of the front tonight and this
air will be cold enough to support mixed precipitation over northern
Lake Huron. Rain will occur along and south of the front with a
chance of thunderstorms where milder air moves in on southerly flow.
Expect wind gusts will be under 30 knots in the warm air as well
especially considering high stability over open water and ice cover.
Low pressure will move along the front and north of the Great Lakes
Tuesday night and pull the associated cold front through marine
areas. Instability will increase in this colder flow, so gusts will
be around 30 knots with a brief gust to 35 knots possible Wednesday.
High pressure will then settle overhead by Wednesday night.

HYDROLOGY...

A warm front will continue to lift slowly through southern Lower
Michigan and stall through central Lower Michigan tonight. Abundant
moisture and very mild conditions will accompany the frontal
boundary leading to a complete melting of any remaining snowpack.
Rainfall totals of 1 to 3 inches can be expected during the time
period tonight through Wednesday with the potential for higher
amounts in areas of thunderstorms and training rain segments closest
to the frontal boundary.

While rain chances will be high area-wide through this period,
initial heavy rain potential will focus along and north of the I-69
corridor tonight along the stalled front. The focus will then shift
southward Tuesday into Tuesday night as the final low pressure
system tracks into area along the front and forces it to the
southeast with time.

Considering the rainfall will occur over a duration of two to three
days, flooding concerns will be greatest for area rivers and
streams, although ponding of water in poor drainage areas of roads
and other low lying areas will also be possible across the region.
Significant rises in area rivers and streams, including potential
sharp rises from ice breakup due to the very mild temperatures will
likely occur leading to potential minor flood stages being exceeded
on larger rivers and streams.

CLIMATE...

Record high temperatures will be possible across much of southeast
Michigan Tuesday. 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.....MR
DISCUSSION...AA/SP
MARINE.......BT
HYDROLOGY....BT
CLIMATE......AA/SP


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



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