Area Forecast Discussion
Issued by NWS Chicago, IL

Home | Current Version | Previous Version | Graphics & Text | Print | Product List | Glossary On
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50
260
FXUS63 KLOT 262026 AAA
AFDLOT

Area Forecast Discussion...UPDATED
National Weather Service Chicago/Romeoville, IL
326 PM CDT Fri May 26 2017

.MESOSCALE DISCUSSION...
325 PM CDT

The severe threat across the far southern CWA continues for the
rest of this afternoon, including Pontiac, Gibson City, Watseka,
and Fowler. A tornado watch is in effect for these areas until 8
pm, but likely will be able to end earlier from west-to-east for
these locations.

The mesoscale convective vortex (MCV) is centered over northwest
Illinois, with mainly stratiform rain with sporadic cloud-to-
ground lightning north of I-80. This will be moving over the
Chicago metro area during a busier commute day prior to the
holiday weekend and some temporary heavy rain rates are likely.

Further south, particularly along/south of U.S. Highway 24,
storms have more potency as they tap instability. Via the ILX
radar, storms over the I-39 corridor north of Bloomington have
shown good signatures for severe wind. With a current trajectory,
this will move into southern Livingston and southern Ford County
near 4 pm. North of the small scale apex, some strengthening is
possible mainly for hail, but the severe weather threat probably
likely drops off fairly quickly. The southern part of this
segment/line has shown some recent supercell characteristics
(focus low-level inflow, some rotation, and streaming spotter
video of a well- defined updraft base), and this storm will
approach the Gibson City area again shortly after 4 pm.

Latest hand analysis indicates the warm front has some northward
bulge to near Rantoul/Gibson City. The cumulus does not look
particularly impressive via satellite or webcams, but observations
indicate that there should be a pretty tight surface-based CAPE
gradient near that southern CWA edge. This will be the area most
primed for severe winds/hail and have some tornado threat with the
backed low-level inflow. Storms could show an acceleration across
the southern CWA especially if they become more linear/quasi-
linear.

MTF/Deubelbeiss

&&

.SHORT TERM...
300 PM CDT

Tonight through Saturday...

For near term convective/severe thunderstorm trends, check for the
latest Mesoscale Discussions above. The weather will quiet down
tonight with the effective warm front shunted well south behind
the MCV. Showers and thunderstorms should exit quickly by the
early to mid evening hours. This will set the stage for a quiet
and pleasant start to the Memorial Day weekend, at least for the
daylight period. Strong heating will take place away from lake
influence on Saturday with plenty of sun followed by increasing
mid and high clouds during the afternoon. Low level thermal progs
support highs in the mid 70s to around 80 away from the lakeshore
and few miles inland, which will be kept in the 60s.

Castro

&&

.LONG TERM...
300 PM CDT

Saturday night through Friday...

The effective warm front will gradually lift north in advance of a
likely robust MCS, but mainly stay confined to the southern CWA or
points south. There is uncertainty on timing and trajectory of
this MCS, with the an increasing likelihood for most significant
impacts to remain well south of I-80 or even south of the CWA
counties. Portions of the area are likely to remain for outdoor
activities during the evening hours with likely PoPs for most
widespread shower and thunder coverage after midnight (and highest
thunder coverage along/south of I-80). Deep layer shear and MUCAPE
may be sufficient to support an isolated hail threat and perhaps
localized strong to damaging winds.

A surface low will take shape and drag a cold front across the
area on Sunday with perhaps an isolated risk for gusty to damaging
thunderstorm winds and marginally severe hail southeast of I-55.
There is plenty of uncertainty with the convective scenario on
Sunday and much of the afternoon could certainly be dry northwest
of I-55 or I-57. Expecting breezy west-northwest winds in the
afternoon behind the front, with only modest cool advection so
highs should reach the lower to mid 70s.

Memorial Day Monday could be fairly similar to Sunday temperature
wise. Main feature of note is a large upper low over the northern
Great Lakes, from which some isolated diurnally driven showers
and possibly thunderstorms could form during the afternoon. Again,
most of the day should be dry, so no need to cancel outdoor
plans- just keep an eye on the forecast and the radar. The upper
level low pressure will only slowly slide east through the week,
keeping area under northwest flow aloft and generally slightly
below normal temperatures for the end of May and beginning of
June. Can`t completely rule out any shower activity in the mid to
late week, but it looks to be an overall drier regime.

Castro

&&

.AVIATION...
For the 18Z TAFs...

A convective complex is crossing the Mississippi River and will
impact the terminals from the early/mid aftn through the early
evening. The best chance for strong to severe storms will be south
of the terminals. Have low confidence in thunder being reported
at RFD so only have a VCTS there, but have higher confidence in
thunder at the eastern terminals so they each have a 2 hour TEMPO
for thunder.

Cigs under the convective complex are mainly VFR right now, but
guidance continues to suggest that MVFR cigs will form as the
complex pushes east. Since guidance is doing such a good job
capturing the complex and current cigs, I decided to follow their
lead and keep lower cigs in the forecast. Dry conditions return
in the early evening. MVFR cigs scatter out and east winds become
north-northeast. East winds less than 10 kt are expected Saturday
along with dry, VFR conditions.

JEE

&&

.MARINE...
227 PM CDT

Will keep the marine fog advisory going as web cams indicate
haze/fog remains over the open waters.

A high pressure ridge over the lake will continue east. A weak
surface low over the plains will pass south of the lake tonight and
continue across the Ohio Valley Saturday. The low may cause wind
speeds to increase to 10-20 kt over the far southern end of the lake
this evening. One model would suggest gusts up to 25 kt, but do not
have enough confidence to go that high.  High pressure passes over
the lake early Saturday morning and light winds will vary in
direction across the lake.  Variable winds are expected over the
southern end and southerly winds over the northern end.

The next low forms over the plains Saturday and reaches the Great
Lakes region Sunday morning.  The low moves over Michigan Sunday
afternoon and winds become west 10-20 kt behind the low.  The low
over Michigan merges with another Canadian low to the north over
Ontario Sunday night. The low is forecast to remain relatively
stationary over Ontario early next week which may lead to a an
extended period of westerly winds of 15-25 kt. High pressure moves
in behind the low mid to late next week.

JEE

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...None.
&&

$$

VISIT US AT HTTP://WEATHER.GOV/CHICAGO (ALL LOWERCASE)

FOLLOW US ON FACEBOOK...TWITTER...AND YOUTUBE AT:
WWW.FACEBOOK.COM/NWSCHICAGO
WWW.TWITTER.COM/NWSCHICAGO
WWW.YOUTUBE.COM/NWSCHICAGO



USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.