IWX issues Area Forecast Discussion (AFD) at Apr 9, 1:22 PM EDT540
FXUS63 KIWX 091722
AFDIWX
Area Forecast Discussion
National Weather Service Northern Indiana
122 PM EDT Wed Apr 9 2025
.KEY MESSAGES...
- Rain will overspread the area this afternoon. The arrival
time of the precipitation has slowed somewhat from earlier
indications.
- Temperatures warm a little bit for Thursday and Friday, but still
below normal. Temperatures rise back above normal by Sunday.
- Any additional rain chances early next week should be
associated with light rainfall amounts.
&&
.DISCUSSION...
Issued at 345 AM EDT Wed Apr 9 2025
A very dry low level airmass persists across the southern Great
Lakes this morning with sfc dewpoints from the mid teens to
lower 20s. Ridge axis will shift east of the area early this
morning allowing for the onset of broad low level warm/moist
advection. Some subtle trends over time with this system will
have some forecast impact today. Dprog/Dt with bulk of guidance
indicates a tendency for a slightly weaker low level trough
reflection as the low/mid level trough approaches the western
Great Lakes today. This should have the impact of slightly
slowing stronger low level moisture transport into the southern
Great Lakes this morning. Given the pre-existing very dry low
level airmass in place, have continued trend of the previous
forecast in pushing back the onset time of the precipitation a
few hours later this morning into this afternoon from west to
east. This trend may need to be continued and will continue to
monitor observational trends this morning.
Along with the slightly weather low level trough solution
appears to come slight slightly weaker low level fgen forcing as
the zone of stronger isentropic upglide shifts across
east/northeast portions of the forecast area later this
afternoon/early this evening. Precip types are somewhat tricky
with this particular system. The slower evolution and later
arrival should support precip onset times with warmer sfc temps
in place, but low level dew points will likely be slower to
recover as mentioned. Wet bulb effects could still produce a
marginal rain vs snow setup, particularly at leading edge of
advection-induced precip shield. Not expecting any snow
accumulation today at this point however.
It still appears as though the more significant precip will come in
a few rounds, the first with the stronger 290-300K isentropic
upglide this afternoon into early this evening from west to east,
followed by a response to primary mid/upper level trough and
stronger DPVA this evening. Effects to storm total precip from the
slightly weaker low level trough evolution has resulted in a
forecast shift to the north/northeast of the heaviest axis of
forecasted precip (0.5 to 0.80 inches) mainly north of the US
Route 30 corridor. Some weak convective response is possible
across the far west/southwest this evening with the primary
mid/upper level trough as some fairly steep mid level lapse will
be in place, but instability may be too weak and too highly
elevated to realize thunder. Prospects for any minor snow
accumulation on grassy areas tonight still looks to be confined
to perhaps extreme NE IN/NW Ohio/SC Lower Michigan this evening
where near sfc wet bulbs may be slightly depressed. One other
item that may need watching tonight into Thursday morning is
the potential for fog. Sfc reflection of this system is expected
to track across east central IN late tonight, leaving a weaker,
baggier pressure gradient in its wake by daybreak Thursday.
Weak gradient combined with recent rain and lowering inversion
could promote some patchy fog across the area. There is still
uncertainty whether this would be manifested as stratus vs fog
however.
By Thursday morning, rain chances should taper substantially as
initial system shifts off to east and secondary upper trough across
the Mid MS Valley lags back west. The southern Great Lakes and
portions of Ohio Valley should be in a relative minimum of low
level moisture in this setup, with best chance of any afternoon
isolated/scattered showers likely along and south of US 24
where some limited low level moisture recover is expected in
association with this weaker upstream wave and secondary weak
low level frontal response.
Low level baroclinic zone should shift far enough south by Thursday
evening to end any additional rain chances, with an extended
period of dry weather setting up through the weekend. Upper wave
pattern will stagnate a bit for first part of weekend with
eastern CONUS negative upper height anomaly slow to breakdown.
By Sunday, low level warm advection should become more
pronounced with temps likely shooting back above normal into the
low-mid 60s by this time.
Medium range guidance still hints at a more potent upper level
trough early next week, but uncertainty remains in the latitudinal
extent and duration of better low level moisture transport preceding
this system. Thus, at this forecast distance, will maintain
broadbrush 20-30% PoPs for Monday. This system should be followed by
slightly cooler temps for Tuesday-Wednesday.
&&
.AVIATION /18Z TAFS THROUGH 18Z THURSDAY/...
Issued at 118 PM EDT Wed Apr 9 2025
Developing rain today and deteriorating cigs later this
afternoon into tonight will be the primary aviation weather
forecast issues. Initial airmass is still quite dry and will
take some time to moisten, so timing of rainfall has been pushed
back some. Latest thinking is now that rain shower chances will
increase at KSBN after 19Z and possibly closer to 21Z at KFWA.
A lull in rain is possible early this evening before primary
upper trough drops into the region with additional rain
expected. Will maintain idea of trend to IFR conditions this
evening as sfc low reflection tracks across central Indiana.
Combination of low clouds and even some fog north of this low
track could promote LIFR conditions later tonight. Visibilities
should improve late morning Thursday, but ceilings are expected
to remain IFR.
&&
.IWX WATCHES/WARNINGS/ADVISORIES...
IN...None.
OH...None.
MI...None.
MARINE...None.
&&
$$
DISCUSSION...Marsili
AVIATION...Marsili/Cobb
Source:
IWX issues Area Forecast Discussion (AFD) at Apr 9, 1:22 PM EDT---------------
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