Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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199
FXUS63 KDTX 210649
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
249 AM EDT Fri Jul 21 2017

.DISCUSSION...

Very warm and increasingly humid conditions are expected over the
area today into Saturday. Highs today will range from 85 to 90 today
and 80-85 Saturday, but heat indices will climb into the lower 90s
in some locations both days given the steady increase in low level
moisture over the next 30-36 hours. This building heat/humidity will
play a part in the main forecast challenge over the next few days.

This main issue will be both the timing and location (and severity)
of potential MCS late tonight into Saturday morning. Convection is
expected to ignite over the upper Mississippi Valley from this
afternoon into this evening within area of forcing in advance of a
shortwave which will track from along the US/Canadian border. As
previously elaborated, this area of convection will also have jet
support within entrance region of a rather large jet streak which
shifts off to the east/northeast with time late today/tonight.

While models do offer differing solutions, especially in terms of
timing this system on its trek from the upper midwest into the
southern/central Great Lakes, all do have a rather well organized
convective system tracking into the vicinity somewhere in between
the 06z-08z and 12z-14z time frame. Hires models of late have,
however, had a great deal of difficulty handling MCS development and
subsequent evolution. Case in point would be a general underplaying
of ongoing convection from Iowa into northern Illinois. While this
deficiency may impact model performance somewhat on late day storms,
recent HRRR runs, which do simulate the current midwest convection
pretty well, suggest the development and expansion of convection
form Minnesota into Wisconsin this afternoon/evening (well within
consensus of 00z Hires model solutions).

With this in mind, still feel pretty good about the going likely
precipitation chances late tonight into Saturday morning. Will
remove pops from the 00z-06z window this evening as timing does not
seem to support this, but leave the 06z-18z time frame unchanged at
this point. Area remains in a marginal/slight risk of severe weather
during this period, which will depend a great deal on the ability of
an MCS to remain well organized during a diurnal convective minimum.
Think the more likely issue may be locally torrential rains/flooding
as PWATS increase to 2.00 inches. This will depend largely on how
well convective scale forcing is able to fire additional storms along
the frontal boundary draped in the vicinity. In theory, this idea
makes some sense given the location of boundary and upper jet
support which should allow for convection to remain fairly far north
on a E/ESE trajectory as opposed to propagating into higher CAPE
environment to the south.

The remainder of the forecast shows a return to a more moderate
environment as cold front associated with the aforementioned wave
along the US/Canadian border ushers in cool (and especially less
humid air from late Sunday on into next week. While scattered
showers or storms will be possible along this front Sunday, the
activity should be minor compared to any activity in the vicinity
tonight/Saturday. WIth the passage of this front the humid airmass
with high temperatures into the 85-90 degree range will edge back
into the upper 70s/lower 80s on average.

&&

.MARINE...

Quiet weather with light winds is expected today as broad high
pressure encompasses the area. Low pressure tracking into the Great
Lakes will provide a very good chance for showers and thunderstorms
late tonight into Saturday, especially across western Lake Erie,
Lake St Clair and southern Lake Huron. Some of these storms could be
strong to severe. Additional rounds of thunderstorms are possible
late Saturday through Sunday as the low tracks overhead. Winds
outside of thunderstorms will be fairly light through tonight,
before easterly winds over Lake Huron increase 10 to 20 knots
Saturday as the low tracks across Michigan. Winds will decrease for
Saturday night and Sunday, before a resurgence of gusty winds from
the northeast occurs over Lake Huron Sunday night into Monday on the
backside of the low. Wave heights could potentially grow to more
than 4 feet over southern Lake Huron on Monday, and may necessitate
Small Craft Advisories.

&&

.PREV DISCUSSION...
Issued at 1157 PM EDT Thu Jul 20 2017

AVIATION...

High pressure will build into the region through Friday, yielding
generally benign weather conditions.  A warm but stable environment
will support mostly clear skies both tonight and during the daylight
period Friday.  This environment will support a greater inland
penetration of the lake breeze off both Lakes Huron and Erie,
prompting a noted wind shift at the favored terminals in Detroit and
MBS.

For DTW...Late afternoon lake breeze passage will shift a
light/variable wind to southeasterly with a slight increase in
speed.

//DTW THRESHOLD PROBABILITIES...

* None


&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...NONE.
Lake Huron...NONE.
Lake St Clair...NONE.
Michigan waters of Lake Erie...NONE.
&&

$$

DISCUSSION...DG
MARINE.......HLO
AVIATION.....MR


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at www.weather.gov/detroit.



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