Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 171853

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1253 PM CST Sat Feb 17 2018

.UPDATE...1249 PM CST...

Convective nature of incoming snow per visible satellite, 35 dBz
echoes on radar, and sounding analysis of elevated mid and higher
level lapse rates suggests a burst of moderate heavy snow this
afternoon. Upstream in Iowa near 1 inch per hour rates have been
observed. While snow will not last long, roads may quickly become
snow covered, particularly less traveled roads. Fortunately our
surface temperatures have warmed, but low dewpoints suggest as
soon as precip arrives wet bulbing will lower conditions. Be
prepared for a rapid deterioration with snow in the coming hours.
See our latest Graphical Nowcast being issued concurrently.



250 AM CST

Through tonight...

Early morning water vapor imagery shows a shortwave trough moving
east across Nebraska and guidance brings this wave across northern
Illinois this afternoon bringing a quick shot of snowfall to the
forecast area. Deep dry layer of air in place will delay the onset
of precipitation with isentropic ascent this morning only going
towards gradual top-down saturation. Precip should finally reach
the surface this afternoon but only expect a 3-5 hour window of
precipitation at any given location this afternoon before the main
upper wave passes overhead and forcing ends. Global and regional
models are in good agreement producing up to 0.05-0.06 QPF with
this system, though hi-res models slightly more bullish kicking
out around 0.10 inches. Surface temperatures will be marginal for
efficiently accumulating snowfall topping out above freezing much
of the CWA. Do anticipate some wet-bulbing to occur, and areas
north of I-80 where dew points should remain in the mid 20s this
afternoon will help maintain snow for those areas. Farther south,
dewpoints will be in the low 30s which coupled with temperatures
warming into the mid 30s resulting in a shallow near surface warm
layer will introduce the potential for rain mixing in with the
snow, and switching to all rain across the far southern tier of
counties. Expect snow totals around half an inch north of the I-88
corridor with amounts tapering off to just a dusting south
through the I-80 corridor. Southern tier of counties should
primarily see rain with no accumulating snowfall or a dusting at



250 AM CST

Sunday through Friday...

A lot going on within the extended timeframe including the
potential for thunderstorms & heavy rain that could produce
flooding as we melt any remaining snowpack early through the
middle of next week.

Sunday morning, ridge axis in place will begin to shift to our
east while first in a series of upper level disturbances tracks
across the Upper Midwest. Southerly flow ramps up ahead of the
wave locally as an elongated surface trough develops from the
Central Plains into the Upper Midwest. Looks to be a breezy day
out ahead of this feature with southerly wind gusts eventually
topping out near 30 kt. Strong flow will help drive up
temperatures, though, which are expected to top out in the low 40s
north to upper 40s south. Dew points also creep up through the
day and by Sunday evening and overnight should inch up above the
freezing mark for areas still with snowpack starting the melt-off
again in earnest.

Precipitation with this initial wave stays well to our north, but
West Coast longwave trough is progged to begin amplifying Sunday
into Monday with another shortwave ejecting out over the Central
Plains early on Monday. An attendant surface wave develops within
the aforementioned elongated trough and will move into eastern
Iowa and northwest Illinois Monday afternoon. A very spring-like
pattern develops across portions of the country with western CONUS
troughing and East Coast ridging resulting in an open Gulf with
unusually high PWats advecting north across the mid-Mississippi
Valley and Ohio Valley. NAM/GFS/SREF all indicate a corridor of
1.0+ PWats overspreading most if not all the CWA while an axis of
1.3-1.4 inches moves into the southeastern CWA by Monday evening.
Upper air climatology shows this to be 300-350+ percent above
normal highlighting the potential for a high-end rainfall event.

While rain appears likely area-wide on Monday, models are honing
in on the heaviest precipitation on Monday occurring over far
northern Illinois into southern Wisconsin near the surface trough
axis and within an area of strong moisture convergence on the
nose of a 50-60kt low level jet. GFS indicates MUCAPE of around
200 J/kg rooted around 800mb in this area supporting
thunderstorms. Given the strong kinematic field, cannot completely
rule out the potential for severe thunderstorms farther south in
the warm sector, but positively tilted shortwave doesn`t result
in strong surface low development and surface based instability
remains a big question mark. Still the surface low track is
favorable for severe weather in this area so will bear watching in
the coming days.

Tuesday and Tuesday night, another shortwave ejects from the main
longwave trough as it inches across the Intermountain West. Again,
expect another modest surface reflection to develop within the
elongated trough as it slowly pushes east across the CWA resulting
in another round of heavy rain and thunderstorms for the area.
Overall picture a little less clear for Tuesday, but in general
expect the axis of heaviest rain to be farther east than

Both Monday and Tuesday, deep layer flow aligns mostly parallel to
the surface trough which will be favorable for training rounds of
heavy rain and thunderstorms. Run-total QPF from the GFS and ECMWF
through Wednesday morning shows an inch across the entire CWA
with some corridors of 2-4 inches. This idea is well supported by
GEFS plumes. While there is a lot more variability in the members
across the western CWA, still come out with a mean run-total well
over an inch. There is a much stronger signal from GEFS along our
southeastern CWA boundary where the mean comes out to around 3.0
inches and shows reasonable clustering for the higher totals.
Given the frozen grounds and some lingering snowpack, there is a
heightened concern for rapid runoff and flooding from these storms
and will hoist an ESF (Hydrologic Outlook) to help highlight the
flood potential.

Model differences become much larger for the latter half of the
week. After a brief lull in precip Thursday, ECMWF and GEM lift
the baroclinic zone back into the region friday with the
potential for more precipitation. Meanwhile, GFS, which is much
faster ending precip early on Wednesday, maintains an area of high
pressure just to our north through the latter half of the week
into the weekend keeping the local area dry. Plenty to focus on in
the meantime while models work out their differences for the end
of the week, so for now just maintained a general model blend
calling for additional chances of rain along with mild



For the 18Z TAFs...

A period of moderate to heavy snowfall is on track for later this
afternoon. Expect LIFR to even brief VLIFR in the snow.

Upstream visibilities have generally been anywhere from 1/2SM to
1 1/2SM, and do feel we will see similar conditions for our
terminals. The snow is convective in nature, espeically south of
the terminals, but brief 1/4SM visibility is not out of the
question at ORD/MDW/RFD. At this time confidence is too low in
this lasting any long period of time to include yet in the TAF.
Main time frame is 20-22Z at RFD, and 21-23Z for ORD/MDW. Things
could start a tad sooner than in the TAF given current radar
trends. Accums to around 1 inch are possible, likely less for
most areas. Surface temps are climbing to at or above freezing
oustide of north central IL, but expect them to hold or drop once
snow arrives, and some mix is possible closer to central IL.

Precip will end quickly around 0z or maybe a tad later. Expect
initially gusty WNW winds behind the snow to ease and shift to
SSE toward daybreak Sunday. Some increase in the SSE wind will
occur by mid morning, with gusty S winds then expected Sunday



237 AM CST

Near term focus for Lake Michigan forecast is on a brief period
of marginal southerly gale force winds on the northern part of the
lake later this morning into mid-afternoon. Winds ramp up quickly
into the 30-35 kt range by mid-morning, diminishing nearly as
fast late this afternoon into this evening.

Winds decrease more significantly late tonight as a weak surface
high pressure ridge moves across the lake, then winds ramp up
quickly again from the south or south-southeast Sunday morning.
Model forecasts suggest another period of southerly 35-40 kt gales
Sunday afternoon and evening, and have issued a Gale Watch for
that time period.

A cold front sags across the lake late Sunday night, becoming
stationary Monday morning. A surface low pressure wave translating
along the front is expected to enhance winds a bit (southerly on
south end, and northerly on the north) to around 25 kts. North
winds around 25 kts then develop more fully late Tuesday as
frontal zone shifts to the south. Winds then diminish late
Wednesday night into early Thursday as another weak surface high
pressure ridge drifts across the lake.



LM...Gale Watch...LMZ777-LMZ779...noon Sunday to midnight Monday.




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