Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 211410 AAA

Area Forecast Discussion...UPDATED
National Weather Service Detroit/Pontiac MI
1010 AM EDT Mon May 21 2018


As of 1010 AM EDT...Ongoing forecast continues to be on track, and
have made some tweaks to PoPs for today based on the latest trends in
hi-res guidance that slows onset, but a little more widespread in
coverage mid-late afternoon. Locally dense fog that occurred this
morning in the Tri-Cities region and portions of the Thumb has all
but dissipated as morning heating continues. Cloud cover continues to
increase out ahead of a decaying MCS currently moving into lower
Lake Michigan. A few streamer showers east of this feature have
attempted to move into lower Michigan, but stronger than anticipated
surface high pressure and associated low-level dry air will preclude
any of this precipitation from reaching the ground.

It will take the remainder of the morning hours to saturate the
column as the MCS approaches. Onset of showers now looks to be in the
2-4 PM timeframe west of US 23/I-75, and mid/late afternoon across
the rest of southeast Michigan. Surface-based instability will be
limited as lower Michigan will reside on the northern side of low
pressure approaching from northern Illinois/Indiana. There will be
some elevated instability moving in however and cannot rule out
embedded thunder. Where thunderstorms and heavier showers occur,
downpours will be possible with ponding of water on roadways and
possible poor drainage flooding.

Temperatures will only make it into the 60s early this afternoon,
then gradually fall towards evening into the upper 50s/lower 60s with
thick cloud cover and rain-cooled air. Dewpoints rising into the 50s
will lead to abundant near surface moisture, and with easterly flow
advecting additional moisture off Lakes Erie, St. Clair, and
southern Huron, have added patchy fog to the forecast for general
reduced visibilities late this afternoon and through the evening.
Continue to also closely monitor lakeshore flooding potential with
developing moderate easterly flow this afternoon and evening for the
shores of western Lake Erie.


Issued at 647 AM EDT Mon May 21 2018


The fog now impacting FNT and MBS is rather shallow and will erode
quickly following the TAF issuance (likely by 13Z). An influx of
moisture from the southwest will allow rain to overspread the area
during the course of the afternoon. East/southeasterly winds will
hold cigs relatively high (above 6k) during the bulk of the
afternoon as the rain slowly advances across the area. During the
course of the evening, an influx of low level moisture combined with
the rain will support a steady lowering of cigs to MVFR, with some
IFR possible overnight.

For DTW...The intensity of showers will increase during the late
afternoon and evening with additional moisture advection. Elevated
instability will increase during the night with an additional push
of low level moisture along a sfc low. So thunderstorms will be
possible at any time from late evening into the overnight.


* Low in ceilings at or below 5000 this afternoon. High tonight.

* Moderate in thunderstorms impacting the airport tonight.

Issued at 300 AM EDT Mon May 21 2018


A wet start to the week as two distant upper waves tracks through
Lower Michigan. The first wave will exit the midwest and reach
southern Lake Michigan by early evening, sending a good deal of
moisture into the State as 850 MB dew pts push toward 13 C (per 00z
NAM), with low pressure tracking along the southern Michigan border
this evening providing the focus for rainfall.  Being on the north
side of the low, no surface based instability to speak off, but the
showalter index does go slightly negative (mainly along/south of M-
59) with strong low level jet working through, so chance/scattered
thunderstorms appear fine, which will only act to enhance rainfall.
Detroit will easily surge into the top 10 wettest May`s on record,
as the climo site currently sits at 5.60 inches/11th place. Cool
start this morning, light easterly flow, coupled with clouds
thickening up and rain showers arriving later in the day should hold
high temps in the 60s.

Trailing upper wave over central Plains to track through northern
Lower Michigan Tuesday morning, which may allow for a few more
showers, but mid level dry slot look to encompass much of the CWA
during the day, before low level convergence/secondary cold front
slips south of the border. Pronounced upper level northwest
confluent flow Tuesday night will allow surface high (1020 MB) to
build over Central Great Lakes. Could be enough surface/near surface
moisture to support radiation fog.

Upper level ridge axis over the Mississippi River Valley will slide
east for the end of the work week, allowing for a significant warmup
(80s). However, the warm front lifting through could become active,
and derail the temperature climb if it gets hung up.

Upper level low/trough coming out of the northern Rockies looks to
be moving through the Great Lakes region over the Weekend,
potentially bringing unsettled conditions with a good chance of
showers and thunderstorms, but the amount of moisture around will
need to be sorted out.


High pressure will remain across Lake Huron today, generally
supporting light and variable winds. A frontal boundary located
across the Ohio Valley will lift north as a warm front into Southern
Michigan and Lake Erie tonight. Numerous waves of low pressure will
track along this front, crossing the southern lakes region tonight
and Tuesday. There will be an enhancement to the easterly winds on
Lake Erie today. At this point, winds and waves are expected to hold
below small craft conditions. The winds on Lake Erie will become
more variable tonight with the passage of the sfc low. There will
however be a developing northeasterly gradient on southern Lake
Huron tonight, which will persist into Tuesday, as low pressure
systems slide across the southern Great Lakes. At this point, the
gradient is not expected to be strong enough to warrant small craft
conditions. High pressure will expand back across the Great Lakes by
Wednesday, resulting in light winds.


Showers will overspread southeast Michigan late this afternoon
and will be widespread this evening, tapering off overnight.
Rainfall totals around half an inch are expected across most
locations, but a few thunderstorms are also likely, especially south
of I-69, which will tend to enhance the rainfall, sending isolated
amounts around an inch, which could lead to poor drainage flooding of
low lying and urban areas.


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



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