A wonderful AFD from the boys regarding our next system. I have not had to post one of these in a while.
.SHORT TERM /6 AM TUESDAY MORNING THROUGH TUESDAY NIGHT/...
A mid level low will pivot down out of the upper Mississippi
Valley and toward the Ohio Valley Tuesday into Tuesday night.
An initial mid level vort lobe rotating around the low will
lift northeast across our area Tuesday afternoon into Tuesday
evening. As it does, scattered showers and a few thunderstorms
will be possible, mainly across northwest portions of our area
through the afternoon. A better chance for showers and a few
thunderstorms will then overspread our area from the northwest
later Tuesday night as better forcing develops in association with
the actual low. Highs on Tuesday will range from the mid 70s
northeast to the lower 80s across our far southwest. Lows
Tuesday night will be in the upper 50s to lower 60s.
&&
.LONG TERM /WEDNESDAY THROUGH MONDAY/...
Focus in this timeframe will most certainly be on Wednesday-Thursday
as there is a high confidence signal of dry/quiet/warm weather for
Friday-Monday. In fact, temperatures into the 80s will be possible
under sunny skies Saturday-Monday.
Back to Wednesday-Thursday...on Wednesday morning a
persistent/compact/closed low will be near Chicago and arcs/bands of
showers and storms will likely be ongoing on the east/southeast side
of this circulation where thickness diffluence and moist
southwesterly inflow will combine with the approaching upper trough
to produce slowly migrating bands of showers and storms. Confidence
is not overly high amidst a higher background signal of strong
forcing and ample moisture - on how this activity and residual cloud
cover in the morning will limit/redistribute instability through the
day. Forecast soundings valid 27.18Z - 28.00Z across most of the
ILN forecast area indicates an airmass that should quite readily
develop new shower/thunderstorm activity with any boundary layer
heating . Seasonably cool temperatures aloft underneath the upper
trough will steepen tropospheric lapse rates /both low and mid-
level/ and this...combined with plenty of moisture...should foster
the development of areas of showers and storms...peaking at the top
of the diurnal heating cycle in the afternoon/early evening.
By Thursday morning the closed low will only be near Toledo/Fort
Wayne, so it will be a very slow progression of forcing thus higher
rain chances are warranted into Thursday as well for the same
reasons, although with convective overturning and some weakening of
the compact closed low, it is expected that coverage on Thursday
will be further east/south and a little less than coverage on
Wednesday afternoon and evening. That being said, there is enough
ensemble certainty of the tempo of the upper low, and various storm
outflows and/or surface troughs attendant to the feature, that
makes refined higher or lower rain chance areas more difficult
at this time. So chances are a little broad right now, and
probably a little too high in some areas and too low in others.
For impacts...both afternoons will offer some limited potential for
a stronger updraft given the steep lapse rates/stronger forcing.
Given these cooler temps aloft, small hail will be possible in most
stronger updrafts, and a rogue severe hailer or two seems possible.
Additionally, if a weak surface trough can focus a line segment or
two at peak heating when lapse rates are the steepest, a few
damaging gusts will be possible as well, but it is noted that
beneath the upper trough low level flow and shear are awfully weak.
That being said, overall southeasterly component of the low level
flow underneath weak-moderate southwesterly flow in the mid levels,
does offer weak hodograph curvature and the opportunity for a
transient weak supercell structure or two for a more robust storm in
a higher-end instability setup (shown most possibly - as usual - in
NAM soundings on Wednesday on the west side of the ILN forecast
area).
But given the very slow progression of surface/upper air features,
and the potential for slow moving arcs/bands of storms, short term
runoff issues may also be on the table in isolated fashion despite
very dry antecedent conditions. Moist southwesterly low level inflow
into a persistent surface trough would be a setup where this could
occur, but confidence in any location/timing just isn`t there right
now. But it is noted that EPS/NBM probabilities for 1" rain/24 hours
keep going up over the ILN CWA which may be manifestation of more
localized heavier amounts and/or coverage of storms.
