Area Forecast Discussion


345 
FXUS63 KDTX 200350
AFDDTX
Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
1150 PM EDT Thu Jul 19 2018
.AVIATION...
High pressure to the east of the region will continue to dominate 
conditions across Se Mi through Friday morning, providing light 
southeast winds and clear skies below 15k feet. The approaching 
upper low will shed an axis of deep moisture into Se Mi on Friday. 
During the morning, much of this convection will be weakening as it 
approaches Se Mi with a more stable environment in place across 
the area. Diurnal destabilization within this moisture and a 
increasingly favorable upper level lift will warrant showers and a 
good chance of thunderstorms late Fri afternoon through the evening. 
For DTW...Daytime heating within the moisture plume that will lift 
across the area will support a chance of TSRA basically anytime 
Friday afternoon. Latest Hi Res model suite suggest the most 
probable time period for convection will be late (after 21Z).
.DTW THRESHOLD PROBABILITIES...  
* Low in cigs aob 5000 feet Friday afternoon and evening.
* Moderate in thunderstorms late Friday afternoon and evening.
&&
.PREV DISCUSSION...
Issued at 344 PM EDT Thu Jul 19 2018 
DISCUSSION...
Anticyclonic flow regime of the last few days now exiting east as 
height falls associated with the upstream closed low begin to spread 
into the area. Temperatures in the low 80s and dewpoints in the 50s 
this afternoon are characteristic of the resident dry airmass that 
extends westward into much of Wisconsin, Indiana, and far eastern 
IL. Areas of shower and thunderstorm activity are ongoing within the 
1.5"+ pwat plume that demarcates the airmass boundary which extends 
from southeast Minnesota to the lower portions of the Ohio River 
Valley. Isentropic ascent and increased convergence along the lead 
edge of this moisture axis will support either continued or 
redeveloping convection over Illinois and Iowa overnight, 
particularly as upper PV advection increases. Loss of deeper forcing 
for ascent as forward progress of the low almost stops altogether by 
Friday morning warrants a reduction to chc pops as any band of 
showers lifting through the CWA should be both transient and 
weakening. Deep layer cyclonic flow and ongoing moist advection will 
force dewpoints well into the mid or even upper 60s by late Friday, 
posing a threat for thunderstorm development within the weakly 
sheared, but very moist environment. 12z model progs suggest 
activity will be focused along a second wave embedded within the 
parent low over SW Lower Michigan late Fri afternoon before 
spreading east through 00z. Cooler mid-level temps open the door for 
moderate destabilization late in the day, but late arrival of 
forcing and the likelihood of extensive cloud cover casts doubt on 
potential. Instead, the more likely outcome is a relatively quick-
moving heavy rain threat late eve/first half of the night with any 
stronger convection capable of producing small hail and/or wind 
gusts to around 40 mph. Shower/tstorm chances will be pervasive 
heading into the first half of the weekend. Lack of steering flow on 
Saturday will favor some degree of a heavy rain threat depending on 
degree of destabilization beneath the upper low.
By Sunday, the low pressure system that had been situated over the 
are will weaken and slide southward away from the region. Scattered 
showers/storms will come to an end by late Sunday as high pressure 
builds in.  For the most part, the area will then remain dry going 
into next week with only slight chances for some shower activity. As 
upper level riding builds in next week, temps will rebound back into 
the mid 80s.
MARINE...
Southeast winds gradually increase with a minor wave response 
tonight and Friday as low pressure organizes and moves toward the 
south end of Lake Michigan. Southeast winds look to peak around 20 
knots on Saturday, with low pressure then sinking southeast into the 
Ohio Valley late in the weekend. This will allow for winds to become 
northeast, but will be light into early next week.
HYDROLOGY...
Increasingly moist and modestly unstable conditions as low pressure 
slowly tracks through the region will lead to periodic bouts of 
showers and thunderstorms Friday into the upcoming weekend. Highly 
variable rainfall amounts are likely depending on the eventual track 
and pace of this system. Latest forecast calls for rainfall amounts 
ranging from a half an inch to an inch and a quarter during this 
period. Some localized amounts in excess of this are certainly 
possible in thunderstorms, leading to poor drainage flooding.
&&
.DTX WATCHES/WARNINGS/ADVISORIES...
MI...NONE.
Lake Huron...NONE.
Lake St Clair...NONE.
Michigan waters of Lake Erie...NONE.
&&
$$
AVIATION.....SC
DISCUSSION...JVC/SP
MARINE.......SF
HYDROLOGY....SF
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