IWX issues Area Forecast Discussion (AFD) at Apr 20, 8:58 PM EDT505
FXUS63 KIWX 210058
AFDIWX
Area Forecast Discussion
National Weather Service Northern Indiana
858 PM EDT Sun Apr 20 2025
.KEY MESSAGES...
- Showers and a few thunderstorms will be possible tonight, but
severe weather is not expected.
- Chances for showers and perhaps some thunderstorms returns mid
to late week this week.
- Highs in the 70s to perhaps 80 degrees will be possible
Wednesday and Thursday.
&&
.UPDATE...
Issued at 858 PM EDT Sun Apr 20 2025
Convective chances will continue to be the primary short term
forecast issue to resolve through the overnight hours.
A potent upper level trough across northwest Missouri will
continue to lift northeast and into northwest Wisconsin by
morning. Additional deepening of low level reflection is
anticipated this evening through the early overnight via
primarily diabatic tendencies. This should allow stronger and
more veered low level wind profiles to take shape with best
low/mid level moisture transport through the early overnight
hours. 00Z sounding from KILX (Central Illinois) indicates
fairly impressive surface based instability around 1000 J/kg via
strong 725-600 mb lapse rates on the order of 8 deg C/km. Near
term RAP progs do rapidly advect these steeper mid level lapse
rates northward across the western/southern Great Lakes through
early evening which should trend to diminish pre-frontal
instability across much of the area by 06Z. Slight boundary
layer cooling will also act to create some more CIN for sfc
based parcels which may only leave approximately 200-500 J/kg
MUCAPEs for elevated parcels. IR cloud tops have shown a recent
warming trend with upstream convection across IL and would
expect this trend to continue through 04Z as downstream
instability gradually weakens. Given strongly forced nature to
setup, will make minimal changes to previous forecast with
likely PoPs overspreading the area from west to east overnight
along with slight chance/low chance thunder PoPs given the
elevated instability. Severe weather is currently not expected,
but cannot completely rule out a stray 40-50 mph gust across far
western portions of the forecast area during the early
overnight hours.
&&
.DISCUSSION...
Issued at 256 PM EDT Sun Apr 20 2025
The northern stream and southern stream jets are disconnected as the
northern stream trough evacuates to the east while an upper low that
slowed down in the southwest yesterday is picked up by a trough in
the west and carried towards the Great Lakes between today and
Monday. In between the squeeze play of these pieces, high pressure
nosed in last night to allow 30 degree dew points to sink into the
area. As precipitation attempts to enter from our southwest, this
high dew point depression airmass will act to limit rainfall output.
Later this afternoon, the NAM brings through a CAPE axis as a warm
front lifts north, but that's only got around 300 J/kg in the moist-
bias NAM model. Once the warm front lifts north of an area, the NAM
brings in MUCAPE topping 1000 J/kg after 00z tonight, but it'll
likely take until better forcing (associated with the occluded
front) can arrive (just after 6z) to lift the cap. It may be too
late to be able to tap into the better instability and is likely why
the convective schemes are quick to make the convection along the
front be outflow dominant. If the instability is slower to depart
the area or the arriving convection with the occluded front has more
momentum behind it or comes in earlier, then perhaps it could tap
into the better instability. It's harder to see any stronger cells
erupt through the cap. At this point, will side with the forecast
non-severe thunderstorms as the more likely outcome. Otherwise,
could see gusty to strong winds and hail from stronger storms
overnight.
Dry air moves in behind the occluded front allowing rain to
dissipate in the morning and clouds to scattered out in the
afternoon Monday. As CAA takes over, mixing just below the departing
45 kt LLJ jet may be able to occur. For the most part, think 25 to
35 mph wind gusts will be possible as mixing starts to take hold,
but could see some 40 mph gusts along the front as it pushes
through. Also, for areas west of IN-31, the high temperature will
likely be in the morning before CAA comes in to cool the area down.
High pressure continues to keep the area dry on Tuesday and, while
it may start cooler than Monday starts, a warm front arriving late
in the day may help temps push into the 60s to around 70 degrees
before the day is done.
The aforementioned warm front stalls in Lower MI Tuesday night, but
there is some discrepancy on where it stalls out. The ECMWF appears
to be a southern outlier with the NAM more of a northern outlier.
This is important because guidance has an MCS-like system moving
along the front and causing some showers into Wednesday. Instability
appears to be lacking so thunder may be fewer and further between.
Then, a slow northward movement of energy from the south as a trough
approaches from the west brings a chance for showers/storms to end
the work week. The GFS is still much earlier with the arrival of the
weak energy from the south than the ECWMF and therefore brings its
rain chances in much earlier. The connection to the Gulf is present
in the ECMWF just not as strong. The ECMWF fills in much more of the
area on Friday with rain/storm chances whereas the GFS prefers
Thursday for the rain chances.
Either way, still expect high pressure to come in from the north for
next weekend bringing a return to cooler/drier conditions as an
upper low deepens over eastern Canada.
&&
.AVIATION /00Z TAFS THROUGH 00Z TUESDAY/...
Issued at 800 PM EDT Sun Apr 20 2025
A potent upper level trough across northern Missouri early this
evening will continue to take more of a north-northeast track
into this evening. Best low/mid level moisture transport
northward into northern Indiana will occur in the 01Z-05Z
timeframe in advance of an associated cold front. This advective
forcing could be enough when combined with weak elevated
instability for a few showers to develop. Attention after 06Z
will turn to more strongly forced area of showers/embedded
thunderstorms current across MS River from northwest Illinois
into southeast Missouri. This line of showers/embedded storms
will begin to outrun a weakening instability axis by the 06Z
timeframe across extreme northwest Indiana. Confidence remains
high in a period of showers at terminals in pre-frontal zone
overnight, with lower confidence on thunder given weak nature to
elevated instability. Will maintain trend to MVFR cigs around
2k feet post-frontal late tonight into Monday morning before
likely improving to MVFR Monday afternoon. Good post-frontal
mixing/steep low level lapse rates will support gusts to around
30 knots on Monday.
&&
.IWX WATCHES/WARNINGS/ADVISORIES...
IN...None.
OH...None.
MI...None.
MARINE...Small Craft Advisory until 5 AM EDT Tuesday for LMZ043-046.
&&
$$
UPDATE...Marsili
DISCUSSION...Roller
AVIATION...Marsili
Source:
IWX issues Area Forecast Discussion (AFD) at Apr 20, 8:58 PM EDT---------------
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