550 
FXUS63 KIWX 131708
AFDIWX

Area Forecast Discussion
National Weather Service Northern Indiana
108 PM EDT Mon Sep 13 2021

.SYNOPSIS...
Issued at 356 AM EDT Mon Sep 13 2021

Warm conditions will continue today as high temperatures range 
from the mid 80s to lower 90s. There is a low chance for a 
thunderstorm into this evening across southern Lower Michigan, 
but thunderstorms will become likely late Tuesday afternoon into 
Tuesday night as a cold front approaches. Some of these storms may
be strong to severe with gusty winds and locally heavy rain. 

&&

.SHORT TERM...(Today and Tonight)
Issued at 356 AM EDT Mon Sep 13 2021

Short term forecast highlights will continue to center on the 
continuation of above normal temperatures and the potential of 
isolated convection across mainly southern Lower Michigan today and 
tonight.

Stalled sfc frontal boundary near MI/IN stateline will move very 
little today and may be focus for some additional low cloud 
development early this morning. These low clouds should mix out 
by mid morning. Already beginning to see some renewed convective 
development across southern Wisconsin in response to backing of 
low/mid level winds along this boundary across the MS River 
Valley downstream of next fast moving, low amplitude upper level 
short wave tracking across the Corn Belt. Based on orientation of
850-750 mb positive theta-e advection axis from northern Iowa to 
southwest Lower Michigan this morning, not completely out of the 
question an isolated shower could graze far southern Lower 
Michigan, but stubborn mid level inversion should keep best chance
of realizing this elevated instability just north of the local 
area. 

For this afternoon, the low amplitude short wave will reach the 
western Great Lakes allowing better northward push of the low level 
warm front, with expectation that better convective coverage will
remain across central Lower Michigan. Cap may be weak enough 
across far southern Lower Michigan for an isolated storm during 
peak heating, but confidence remains very low given weak capping 
likely persisting. If cap can be overcome, steep mid level lapse 
rates could allow for an isolated strong storm to develop. 
However, once again this potential appears maximized across 
central Lower Michigan.

A south-north temperature gradient is expected once again today as 
near sfc warm front not likely to completely clear southern Lower 
Michigan counties until this evening. Have not made many changes to 
previous forecast for high temps today ranging from low-mid 80s far 
north to around 90 south. 

For later tonight, attention will turn to evolution of a more 
vigorous upper level short wave trough which is currently 
progressing through the northern Rockies this morning. A 
downstream 40+ knot southerly low level jet will lead to 
strengthening low level moisture convergence across the Upper MS 
Valley with main convective threat across these locations tonight.
Track of this short wave and accompanying advective forcing will 
keep organized convective chances north of the area, and will just
continue to carry slight chance PoPs southern Lower MI until near
sfc front retreats well to the north late evening/overnight.

&&

.LONG TERM...(Tuesday through Sunday)
Issued at 356 AM EDT Mon Sep 13 2021

A broader, but disjointed eastern Pacific upper level trough will 
track across south central Canada/central Plains early Tuesday 
afternoon. with the mid/upper level trough axis approaching 
western Great Lakes early overnight Tuesday night. Positively 
tilted nature of this trough will increase frontal forcing Tuesday
afternoon, with main source of uncertainty in location and timing
of convective initiation being the influence of overnight upstream
convection on frontal placement/timing. Good upper support should
be in place by early Tuesday evening as frontal forcing allows 
for strengthening of broad right entrance region of a 110+ upper 
jet streak across the Great Lakes. With this positive tilt, better
shear profiles could slightly lag better instability axis from 
central Illinois to southwest Lower Michigan late afternoon. 
Steeper mid level lapse rates on the order of 7 deg C/km could 
compensate for this slightly lagging shear for isolated-scattered 
severe threat by late afternoon/early evening. An initial gusty 
wind/isolated hail threat should transition to a strong/severe 
wind threat in the evening as large scale lift gradually weakens 
the elevated lapse rates. 

While deep layer shear values will be on the marginal side, somewhat 
impressive 15-20 knot 0-1km shear values are expected late 
afternoon/early evening. Currently not expecting appreciable tornado 
threat as this better low level shear should be in place at a time 
of maximum LCL heights. Most of forecast area has been placed in SPC 
Day 2 Slight Risk, with greatest potential looking to be in the 21Z-
04Z window. In addition to severe potential, unidirectional boundary 
parallel flow could result in some training Tuesday evening that 
could produce some locally heavy rain amounts.

Cold front continues to slowly sag southeast into Wednesday as the 
upper level wave dampens across the eastern Great Lakes. Overall 
trend of past several cycles has been to a little more progression 
of the slow moving front, with perhaps a quicker reduction in PoPs 
from NW-SE on Wednesday. Temps should fall to more seasonable levels 
for Wed-Thu, with moderating trend back into the mid 80s for Friday 
into next weekend as large scale mid level height rises commence.

&&

.AVIATION...(For the 18Z TAFS through 18Z Tuesday Afternoon)
Issued at 100 PM EDT Mon Sep 13 2021

VFR conditions to prevail throughout the 18z TAF issuance. Models
do hint at the potential for shallow ground fog around 11z to 13z
Tuesday morning given increasing low level moisture. At this point
have decided not to include in prevailing conditions given low
confidence of occurrence. Gusty winds develop around midday 
Tuesday as stronger 850mb winds start to mix to the surface. Any 
convective concerns look to develop beyond this forecast window. 

&&

.IWX WATCHES/WARNINGS/ADVISORIES...
IN...NONE.
MI...NONE.
OH...NONE.
LM...Small Craft Advisory from 6 AM to 5 PM EDT Tuesday for LMZ043-
     046.

&&

$$

SYNOPSIS...Marsili/CM
SHORT TERM...Marsili
LONG TERM...Marsili
AVIATION...CM


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