Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 202342

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
642 PM CDT Mon May 20 2019

240 PM CDT

Through Tuesday...

For the remainder of the afternoon and through tonight,
conditions will remain dry and quiet across the region. High
pressure extends across the Upper/Middle Mississippi Valley and
western Great Lakes and into the Lower Ohio Valley. Following the
passage of a cold front, northwest flow at the surface is becoming
more northerly. With the pressure gradient weakening as the high
builds south, a lake breeze or lake influence, to the sfc winds is
turning flow onshore along the lake front, keeping temperatures
generally in the upper 40s to around 50 F. Inland locations have
also been slow to warm up as extensive cloud cover has persisted
across the region, with highs topping out in the middle to upper

The period of dry weather will be brief as another round of rain is
expected for late tonight and into tomorrow morning.  A deep upper
low will move through the southern Rockies tonight and lift out into
the Central Plains.  In advance of the upper low, a series of
shortwave will lift newd, with the lead shortwave crossing the
Middle Mississippi Valley durg the pre-dawn hours spreading an area
of showers from southwest to northeast through the morning.  While
there are some timing differences among the models with the speed of
the main shortwave, there is still relatively high confidence in the
pcpn trends to carry likely to categorical PoPs for the morning
hours.  The region is expected to remain under easterly winds
through the day tomorrow, keeping the immediate lake front
relatively cool, with highs, once again, around 50 F along the
Illinois lake front.  Elsewhere, temperatures should range from the
middle 50s north to the lower 60s south.  Winds are expected to
strengthen through the day as low pressure deepens rapidly over the
Central Plains.  East winds should gust to arnd 30 mph in the


330 PM CDT

Tuesday Night through Monday...

The start of the period will have a fast moving line/complex of
showers and thunderstorms with a risk for a short period of
heavy/torrential downpours and a non-zero chance for isolated
strong/severe wind gusts in portions of the area. The other
concern will be strong easterly winds as mentioned in the short-
term discussion due to strong low pressure lifting northward over
the Central Plains. Easterly winds will gust to 35-40 mph during
the early to mid evening hours when pressure falls are most
pronounced, with the highest winds over the western half of the
area. Forecast soundings indicate that gusts could briefly
approach 45 mph advisory criteria. In addition, the stout east to
east-southeast winds could yield a risk for minor lakeshore
flooding along the Illinois shore.

For Tuesday evening-night`s showers and storms, the models are in
very good agreement in a linear/quasi-linear MCS quickly tracking
across the region from the mid-late evening through a portion of
the overnight. Despite unfavorable timing for maintenance of
convection, strong warm advection via an intense low level jet of
50-60 kt, will steepen lapse rates and maintain moderate MUCAPE of
near of over 1000 j/kg. In addition, there will be strong mid and
upper jet forcing and 50-70 kt of deep layer bulk shear. This MCS
will likely be a damaging wind and tornado producer southwest of
our area late Tuesday into the evening. However, the position of
the local area well north of the warm front means that there will
be a sharp low level inversion and fairly deep stable layer
overhead. This casts doubt on much/if any severe risk.
Nonetheless, if the strong kinematics can help transport the warm
sector north in tandem with the MCS and/or wind can punch through
the stable layer, then there may be an isolated strong/damaging
wind risk. Position of SPC Day 2 Marginal Risk seems reasonable
for this low probability (west central LaSalle county southeast to
southeast Iroquois County and points south and west).

The main threat aside from cloud to ground lightning with the MCS
thus looks to be a fairly short period of heavy to torrential
downpours. This is due to very strong moisture transport and PWAT
values in the 1.5 to near or over 1.75", which is near the maxima
for this time of year per ILX RAOB climo. It does not appear to
be a good setup for widespread flooding/flash flooding given the
expected very fast motion of the convective complex. Mean steering
flow is progged to be 50-60 kt to the east-northeast, which
should keep rainfall amounts in check somewhat with rain duration
in any location possibly only a couple hours. WPC does have a
Marginal Excessive Rainfall risk for about the western half of the
CWA, which also appears reasonable considering the ingredients
for heavy rainfall and still saturated soils. The combined
rainfall from Tuesday morning and Tuesday night could result in
rises on some area rivers.

The warm front will be able to fully lift north behind the MCS on
Wednesday, with breezy south-southwest winds and a rapid warm-up
after cool overnight conditions. Daytime hours will be dry and
trending mostly sunny. Progged 925 mb temps support highs around
80 to as high as the mid 80s in spots, with possible exception of
far northeast IL near the lake where southeast winds could linger
long enough to keep temps cooler there.

The next possible/probable round of convection will be later
Wednesday night into early Thursday with an approaching cold
front. The convection will likely weaken into our area with
unfavorable timing and then the front should wash out on Thursday
then lift back north as a warm front on Friday. A persistent
western troughing and eastern ridging pattern will persist through
the upcoming Memorial Day weekend. Our area will be on the
northern periphery of the mid/upper level ridging, which is a
favored area for periodic showers and thunderstorms. The air mass
aloft in this pattern will support generally above normal
temperatures and summer-like humidity at times, though the
presence and daily position of a quasi-stationary frontal
boundary nearby will help determine the daily trends, as well as
lake cooling potential. Mid-level flow will be strong enough that
depending on timing and evolution, thunderstorm organization will
be possible, so we`ll need to monitor for any strong/severe risk.
In general, there will likely be plenty of dry hours for outdoor
activities, but rainfall amounts through the period could be
sizable, especially considering convective rates at times and
high moisture availability.



For the 00Z TAFs...

The main forecast items are precipitation chances tonight followed
by increasing easterly winds Tuesday afternoon and then TSRA
timing Tuesday evening.

North to northeast winds will diminish somewhat this evening and
turn east then southeast late tonight and Tuesday morning.
Current lower level VFR cloud cover will dissipate as the sun sets
over the next few hours but mid and high level cloud cover has
moved in and will persist through the evening. Showers are
expected to move into the terminals late tonight and early Tuesday
morning. May see some minimal vsby reduction with isolated
heavier showers. Expect mainly VFR ceilings, though they will
lower through the morning and some brief MVFR could occur. Rain
showers should exit by midday or so.

East to southeast winds will increase Tuesday morning with gusts
to around 25 kt by early afternoon with a further increase to
around 30 kt during the late afternoon. Winds/gusts will stay
elevated through Tuesday evening and gradually ease Tuesday night.
A band of showers and thunderstorms is expected to cross the
terminals late Tuesday evening into the early overnight hours.
Good confidence that this will occur but may need to refine the
timing, perhaps adjusting it a little later.






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