Area Forecast Discussion
Issued by NWS Chicago, IL

Home | Current Version | Previous Version | Graphics & Text | Print | Product List | Glossary Off
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

000
FXUS63 KLOT 170900
AFDLOT

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

.SHORT TERM...
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
best.

Deubelbeiss

&&

.LONG TERM...
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
Monday.

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
temperatures.

Deubelbeiss

&&

.AVIATION...
For the 06Z TAFs...

Main concerns through the period continue to focus on the
increasing potential for a short period of light to moderate
snowfall this afternoon across the terminals. Not much has
changed with the previous thinking.

High pressure will shift over the region overnight resulting in
light winds. As this high shifts east of the area today the winds
will turn southwesterly across the area. Overall, this will help
transport atmospheric moisture back northward over the area into
the afternoon in advance of an approaching mid-level disturbance
and setting the stage for a period of snow. Currently it looks
like this will be a quick hitting snowfall (only 2 to 4 hour)
event during the mid to late afternoon hours (roughly from 20z
through 00z at the Chicago area terminals). While snow amounts
are likely to only be an inch or less, it could briefly fall at a
moderate rate, which drive down visibilities to, or just below a
mile.

Expect the snow to quickly end by early this evening, with winds
turning westerly.

KJB

&&

.MARINE...
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.

Ratzer

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Gale Watch...LMZ777-LMZ779...noon Sunday to midnight Monday.

&&

$$

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.