Rockford Scanner™: Severe Weather Possible Saturday, Including The Possibility Of Tornadoes





Through Sunday…

Multiple concerns in the short term period, most notably the risk
for strong to severe thunderstorms tonight and especially on
Saturday, when a severe weather outbreak including all severe
hazards is possible for portions of the region. The risk tonight
is primarily for large hail. Localized flooding may be a threat
tonight into Saturday morning, especially in thunderstorms. In
addition to these primary concerns, fog will be a threat at
times, starting with this morning, with visibility under 1 mile
near Lake Michigan and several miles inland, and then this
evening/tonight into Saturday as a pronounced warm front lifts
north. Finally, late Saturday night into Sunday, the focus shifts
to strong southwest to west winds with gusts up to 40-45 mph or
higher possible.

This morning, with yesterday’s front analyzed near the Ohio River,
the local area is entrenched in modified marine air with minimal
to nil dew point spreads. The focus in the near term is with
areas of fog spilling south from Lake Michigan, with 1/2 to 3/4
mile visibility noted at several sites in the vicinity of the lake
and several miles inland. Not anticipating needing a dense fog
advisory, but will monitor obs and webcams for possible SPS
issuance. Patchy reduced visibility will also be possible inland.
Convective activity is well south of the CWA across downstate
Illinois. Some lead lighter scattered showers may spread into
areas primarily south of I-80 through the morning. Shower activity
should gradually uptick in coverage into the afternoon, but could
remain mostly dry later into the day for the north. East to east-
northeast flow will keep temperatures in check despite a warm and
moist air mass being advected in aloft, especially lakeside with
low-mid 40s (50s inland).

Turning to this evening/tonight and risk for elevated hailers and
heavy rain/flooding, latest mesoanalysis indicates a plume of
anomalous 1-1.5″ PWATs from the central Ohio River back west
across downstate Illinois and to the lower Missouri Valley. In
addition, broad southwest mid-level flow emanating from Mexico,
the southwest and High Plains, has transported a plume of steep
mid-level lapse rates across the Midwest. In the early evening,
there will be a lead short-wave exiting east-northeastward, with
perhaps a lull in shower coverage immediately to the south and
west of this, with increasing ascent from another approaching wave
from west-central Illinois.

This stronger forcing will induce an uptick in showers and
embedded thunderstorms late evening into the overnight, perhaps
overlapping into Saturday morning. The main question is the
magnitude of elevated instability (MUCAPE), with the NAM and to a
slightly lesser extent the RAP more aggressive in drawing higher
MUCAPE (1000-1500+ J/kg) farther northward. Meanwhile, strong
west- southwest flow aloft will yield strong effective bulk shear
values in excess of 50 kt. Thus, with the steep lapse rates and
moderate elevated instability is the concern for elevated
supercells capable of producing large hail. As such, SPC has
included areas along and south of I-80 in the day 1 slight risk,
which appears reasonable given uncertainty in northward extent of
higher instability. We will certainly need to monitor trends for
this. In addition, with the near top of the charts column
moisture, the risk is there for localized flooding and perhaps a
non-zero flash flooding threat for the southern CWA, though it
hasn’t been all that wet of late, so not anticipating a
significant flash flooding risk.

In addition to the above items of note, saturated boundary layer
conditions with the area still north of the sharpening warm
frontal zone to our south, could be conducive for fog development,
particularly during lulls in precipitation. Opted to broad brush
patchy fog mention this evening/tonight and then focus that I-80
and north Saturday morning, with areas of fog possible near the
lake.

Turning to Saturday’s potentially higher end severe risk, there
are some lingering uncertainties to consider. Continued strong
isentropic ascent across the warm frontal zone could keep elevated
showers and some embedded thunder threat going into the morning,
though likely less t-storm coverage than Friday night-early
Saturday. The 00z guidance continued to offer a rather classic
synoptic pattern for upper echelon severe weather potential, with
a sub 990 mb low occluding into the evening, warm front with rich
boundary layer moisture lifting northward, and an arcing Pacific
front/dryline. These surface features will be topped by extremely
impressive wind fields, including south-southwesterly 40-50+ kt
at 850 mb, southwesterly 90-100+ kt at 500 mb, and 150+ kt 250-300
mb upper level jet core, along with still steep mid- level lapse
rates.

The main uncertainties with this set-up are the northward progress
of the warm front, owing to the possibly slowing progression
east-northeastward of the surface low, threat for continued
convection (non-zero severe hail risk) north of the warm front
that could temporarily reinforce the front, and likelihood of
extensive cloud cover that hindering insolation and perhaps
destabilization. While there is overall agreement in the big
picture in the guidance, any further slowing of the warm front
and then Pac front/dry line advancing from the psuedo-triple
point could shift severe risk a bit farther west and threat into
the CWA a bit later, likely into the evening. The ECMWF and UKMET
indicated this sort of possibility. That all being said, with the
inherent uncertainties aside, the set-up has the hallmarks of a
classic midwest severe weather producer. Advection and kinematics
will play a large role in compensating for likely lacking
insolation, with advancing warm sector conditions featuring temps
around 70/low 70s, with dew points into the 60s, and
aforementioned steep lapse rates driving moderate instability
(1000-1500 j/kg MLCAPE).

The impressive wind fields will support extreme deep layer bulk
shear and favorable sufficiently orthogonal orientation to likely
support discrete or semi-discrete supercells erupting from the
Pacific front. Supercells with time to mature south of the warm
front will pose greatest risk into the CWA, with strong low level
shear (ample speed shear) and sufficient veering from southerly
at the surface to south-southwest at 850 mb for hodograph
curvature that could support tornadoes, some possibly strong,
along with large hail and damaging wind risks inherent to
supercells. Storm motions will be very fast off to the northeast,
with right moving vectors over 60 mph. At this time, do have
concerns with how far north and east into the CWA the more ominous
severe risk will be realized given aforementioned uncertainties.
However, given the available data and recent trends, placement of
SPC Day 2 Moderate Risk into the western CWA appears reasonable
given the upper level jet core intersecting the advancing low
level theta-e axis favorably in western Illinois and into north
central Illinois. ENH extending east-southeast marks possible
warm front progress uncertainty, keeping higher probabilities
south of city of Chicago for now.

Exact timing of cold front will determine how long into the
early evening higher coverage of numerous storms extends, with
severe risk then quickly ending with frontal passage. While
discussion focused on potential for slower/farther west trend with
convective initiation, certainly can’t rule out a faster outcome,
such as was depicted on the 00z HREF CAM suite. Will continue to
highlight given the range in possible outcomes, a timing of severe
threat being roughly 4-10pm.

Attention into Sunday then turns to the risk for strong southwest
to west winds behind cold frontal passage, a concern for any
temporary tent structures that have been set up in recent weeks.
Depending on the exact low track, a corridor of stout isallobaric
pressure rises will wrap into the area into Sunday morning, with
the GFS and ECMWF (and ECMWF ensemble) most noteworthy in this and
thus more concerning for headline criteria type winds (higher end
advisory potential 50-55 mph, perhaps isol’d near 60 mph gusts on
high end). The other guidance is not quite as impressive, but
with 925 mb winds 35-40 kt even on these, we could be heading for
needing a Wind Advisory for at least portions of the CWA either
way. For now, with the synoptic wind portion of the event still a
few days away, have gusts up to 40 mph in the grids. The winds
will then gradually subside Sunday afternoon. Outside of the
strong winds, wraparound showers will be possible, especially
north during the morning. Temperatures will be much cooler than
Saturday in the low to perhaps mid 50s, made to feel cooler by the
winds and mostly cloudy skies.

 

 


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Rickie

I enjoy bringing the community the news on what is going on in the area. Be sure to check out my personal website as well! http://RickieTraeger.com RockfordScanner@Gmail.com