Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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829
FXUS63 KDTX 240710
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
310 AM EDT Mon Sep 24 2018

.DISCUSSION...

Ideal late September day yesterday with max temperatures right where
they should be for this time year, around 70 degrees.

Moisture pooled along the Ohio River early this morning will be
lifting north today as sheared out shortwave/upper level PV over
Western Ohio Valley tracks northeast.  PW Values increasing to
around 1.5 inches over southeast Michigan toward 00Z Tuesday. Waves
of isentropic ascent/moisture advection will saturate the low
levels, and should be able to generate light rain this
evening/tonight, per local SREF weighted probabilistic guidance.
Even marginal instability (showalter index slightly negative) to
potentially support a few thunderstorms, although the low level jet
core tonight looks to be tracking just to our east.

What happens to the low clouds on Tuesday will go a long way in
determining out severe chances as cold front arrives during the
evening hours. Raw 00z nam output suggests surface dew pts rising
into the upper 60s, with MLcape of 1000-1300 J/kg north of M-59,
closer to the mid level cold air (500 MB temps -10 to -13 C). If
that amount of instability verifies, 0-6 km bulk shear of 50+ knots
will support a severe wind threat, as large scale ascent arrives
from the strong upper level trough entering the western Great Lakes.
SPC day 2 indicates a slight chance of severe, as increasing 0-1 KM
bulk shear (30+ knots) coming up from the south and arriving during
the evening hours also suggests a low chance of tornadoes with
sufficient instability/low level cape, which remains highly
uncertain at this time.

Cold front clearing the State by sunrise Wednesday, with pretty good
low level cold advection during the morning hours, offset by
sunshine underneath mid level dry slot. Although, a decent CU up is
expected, along with breezing conditions. 850 MB temps of 3 to 5 C
at 21z suggests max temps holding in the 60s, setting us up for cool
night as mins fall into 40s Wednesday night. Should be just enough
gradient/southwest winds to keep mins from cratering within the dry
airmass (PW values around half an inch), but locations near the
southern Michigan border stand the best chance of going calm, and
potentially sneaking into upper 30s, and bringing a chance of fog,
especially around the warm inland lakes.

Another potent upper level trough will rotate through the Western
Great lakes Thursday evening, with the cold front on schedule to
slip through southeast Michigan by sunrise Friday. Tight baroclinic
zone at 850 MB then looks to be draped across southern Lower
Michigan through the weekend, and subtle upper level shortwaves
riding along the zonal flow may be enough to get mid level FGEN
showers going, and low chance pops will be carried. On Monday, a
stronger ripple/low pressure forecasted to ride along the front (per
00z Euro) to support a much higher chance of rain.

&&

.MARINE...

A moderate early day easterly flow will gradually veer to
southeasterly while strengthening through the day. Winds gusts will
peak near 30 knots at times over portions of central Lake Huron
early tonight. The onshore flow will maintain significant wave
action over the next 24 hours, with maximum wave heights reaching 10
to 15 feet over much of central Lake Huron. Wind will then diminish
on Tuesday as low pressure moves over the waters. However, moderate
to fresh northwest flow will quickly redevelop on Wednesday as a
large Canadian high builds from the west.

&&

.HYDROLOGY...

Numerous showers are expected this evening and overnight as a warm
front lifts across the state. Rainfall totals of one quarter to one
half-inch are anticipated with this round of rainfall. The chance
for additional showers, along with isolated thunderstorms, will
exist Tuesday and Tuesday night. A cold front will sweep through the
region Tuesday night, bringing drier conditions for the late week
period. No flooding is expected through the upcoming week.

&&

.PREV DISCUSSION...
Issued at 1153 PM EDT Sun Sep 23 2018

AVIATION...

Low level dry air and midlevel anticyclonic flow will keep
conditions quiet throughout the overnight. There remains some
potential for very shallow ground based radiation fog around
sunrise, but light easterly gradient flow will exist. A substantial
amount of low level moisture advection will then ramp up on Monday
as flow turns southerly ahead of the approaching Plains trough.
Model soundings show a significant amount of low level saturation
developing during the midday period for all of the southern Taf
sites. Generally, went slightly slower with onset timing of MVFR cig
heights and hydrometeor timing. Given the lack of midlevel
saturation in the soundings, preference was to just carry a
prevailing mention of -dz during the afternoon. Best potential for
light rainfall will lift into far Southeast Michigan after 03Z
Tuesday.

DTW THRESHOLD PROBABILITIES...

*  High for cigs aob 5kft after 15Z Monday.


&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...NONE.
Lake Huron...Small Craft Advisory until 10 AM EDT Tuesday for LHZ441>443.

     Small Craft Advisory until 4 AM EDT Tuesday for LHZ421.

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

$$

DISCUSSION...SF
MARINE.......MR
HYDROLOGY....MR
AVIATION.....CB


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