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Area forecast discussion
National Weather Service northern Indiana
1229 am EDT Fri Sep 20 2019

issued at 730 PM EDT Thu Sep 19 2019

Dry and warm conditions will persist through the first part of the
weekend. Clouds will increase Saturday in advance of the next
system that will bring occasional showers and thunderstorms late
Saturday night through Sunday night. Locally heavy rainfall is
possible during this period, especially from Northwest Indiana
into southwest lower Michigan where the heaviest rainfall totals
of 1 to 2 inches are expected.


Short term...(this evening through Friday night)
issued at 353 PM EDT Thu Sep 19 2019

Relatively quiet conditions expected for the short term with main
forecast challenge continuing to be low probabilities of showers,
especially Friday and Friday night.

Convective complex across northern Illinois has decayed through
the day with residual outflow boundary allowing a few isolated
showers to develop across northeast Illinois. This outflow
boundary should wash out over the next few hours and thermodynamic
profiles become sharply less favorable with eastward extent
across Northwest Indiana and southwest lower Michigan for any
additional shower development. Will keep forecast dry tonight with
just some cirrus debris across the area. Otherwise, will see
continued northward progression of weak warm frontal feature into
this evening which is marked by some uptick in low level moisture
transport. This is manifested this afternoon in visible satellite
imagery as a pocket of better diurnal cu development entering far
Northeast Indiana as of 19z.

Going forward into tomorrow, main question will be on friday's
precip chances. A remnant mesoscale convective vortex from earlier today eastern
Iowa/northern Illinois convection will be stuck in very weak mid-
upper level steering flow pattern, with this mesoscale convective vortex expected to only
reach Northwest Ohio Friday afternoon. The combination of this
convectively enhanced disturbance and the uptick in low level
moisture next 12 hours could be enough to generate some isolated
showers. Given low predictability in timing/location and expected
limited coverage, will continue to keep the tonight-Friday night
period dry. Best chance of additional shower development tonight
may be back across north central Illinois where weak low level jet
may interact with old outflow boundary.

Lows tonight will not be as cool given increases in low level
moisture, with generally lower 60s mins expected. Friday will
feature much above normal temperatures in the mid 80s, but any
convective cloud debris may eventually need to be considered.


Long term...(saturday through thursday)
issued at 353 PM EDT Thu Sep 19 2019

Focus for the Saturday through early Monday period will be on
heavy rainfall potential. Longwave upper trough across western
Continental U.S. This afternoon will lift across the northern rockies through
Friday night. Guidance continues to show a bit better convergence
in solutions with evolution of this system as the western Continental U.S.
Lead wave lifts northeast and dampens across the upper MS valley
while partially phased south central Canadian trough digs across
the northern plains Sunday. This evolution would tend to favor the
slower frontal progression as advertised by ec over past several
runs. The 15z sref mean also appears to be a bit slower than the
operational runs with the frontal progression. Taking the above
into account, tried to focus on the Sunday/Sunday evening period
for the heaviest rainfall potential, although depending on how
system evolves, this potential may extend through late Sunday

The heaviest rainfall amounts are still expected for western
locations across Northwest Indiana/southwest lower Michigan where
low level moisture transport will be maximized for a longer
duration Saturday into Saturday night as the lead wave eventually
dampens across the upper MS valley. Highly anomalous precipitable waters will
accompany best pre-frontal moist advective surge late Saturday
night into Sunday with pwats by daybreak Sunday ranging from
1.80-2.00 inches across the northwest to around 1.25 inches
southeast. An upper jet streak rounding the base of the secondary
digging upper trough will finally allow this trough to eject
eastward more rapidly later Sunday night. Have not strayed far
from previous forecast thinking in front tracking across the area
Sunday night. Previous forecasted rainfall amounts also generally
remain intact with 1 to 2 inch amounts across the west, and lesser
amounts of 0.5 to 1 inch far east where forcing mechanisms will
be more progressive. Dry antecedent conditions should help to
overall lessen higher end flood concerns, but more prolonged
forcing across the west/northwest and highly anomalous pwats could
lead to some flood concerns Sunday/Sunday night. Severe threat
still looks to be a second order concern at this point. While
shear profiles become robust by Sunday, instability continues to
look like a limiting factor for this event.

In addition, windy conditions expected for Sunday in advance of
the front with peak gusts in excess of 30 mph possible. Above
normal temps will continue through Sunday afternoon before a
return to more seasonable conditions Monday and beyond. A
disjointed upper pattern is still forecasted to take shape by next
Tue/Wed with good deal of southern stream energy that could lead
to some additional chances of rain toward the end of the period
with potential renewed advective forcing.


Aviation...(for the 06z tafs through 06z Friday night)
issued at 1228 am EDT Fri Sep 20 2019

VFR conditions and light winds will persist through the taf cycle.
Mid-high level clouds will be on the increase today as a weak
upper wave tracks through the western/central Great Lakes. This
feature is helping to generate widely scattered showers across
northern Illinois near a low-mid level moisture gradient. This shower
activity should dry up with eastward progression as moisture axis
struggles to edge east. Will hold with a dry forecast as a result,
though cannot completely rule out an isolated sprinkle or light
shower surviving into the ksbn later this morning into the early
afternoon. No impact anticipated regardless.


Iwx watches/warnings/advisories...


short term...marsili
long term...marsili

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