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Area forecast discussion 
National Weather Service Lincoln Illinois
900 PM CST sun Nov 29 2015

issued at 850 PM CST sun Nov 29 2015

A relatively quiet night continues to unfold weather wise across
central and southeast Illinois, and expect these trends to
continue for most (if not all) of the night. Model and radar
trends suggest that most of forecast area will be between two more
significant areas of precipitation. The first is south of the Ohio
River within a tropical moisture plume that parallels a frontal
boundary that passed through a few days ago. The second is a
developing area of precipitation, mainly west of Illinois, that is
associated with strengthening warm air advection/isentropic lift ahead of a
slowly approaching upper-level low centered near The Four Corners

Across the forecast area, a very dry airmass exist below about 800
mb, with the dry air being reinforced by persistent easterly low-
level flow. This is resulting in little more than sprinkles
falling across the area where the better radar returns exist
across the western portion of the forecast area. Warm air advection/isentropic
lift will increase across the forecast area overnight, which will
eventually help saturate the lower atmosphere. However, do not
expect this to happen much earlier than sunrise, or even later in
the morning for most of the area.

Plan to update forecast to delay pops a bit with the very low
moistening taking place. Otherwise, only a few minor forecast
tweaks are necessary at this time.


Short term...(tonight)
issued at 250 PM CST sun Nov 29 2015

High pressure ridge axis stretching from south of Hudson Bay, back
through the Great Lakes and into the upper Midwest. Although the
high is still building, it has lost ground today to the impacts of a
broad area of lift in the Missouri River valley. Precipitation has
slowly edged towards the region, overcoming a very dry layer in the
mid levels. The trend will continue with the moisture driving
precip in west central Illinois this evening. As the column
saturates a bit more, precip will spread across central Illinois
tonight. Precip is mainly an issue of scattered light showers
still...with little in the way of quantitative precipitation forecast. Current radar echoes are
mainly virga and it will be a while before that trend gives way
across much of the state. As a result, have held off until after
06z before the precip spreads into the eastern half of the state.
Lows tonight should remain above freezing with plenty of cloud cover
across the region.

Long term...(monday through sunday)
issued at 250 PM CST sun Nov 29 2015

The sprawling upper low that has been out over The Rockies for
several days is finally forecast by all models to start to push east
on Monday with surface low pressure developing ahead of it over
western Missouri. Stronger forcing ahead of the feature in the left
exit region of a 250 mb jet Max should increase the areal coverage
of rain across the forecast area on Monday. Initially, forecast
soundings across the east still exhibit some very dry air in the low
levels, similar to what we saw today which should keep the bulk of
the precip over western Illinois before shifting over the east during the
afternoon. Despite the better dynamics associated with the upper
wave, the deeper moisture will be to our southeast tied to the
stalled frontal boundary over parts of Tennessee and Kentucky and to
our northwest, closer to the track of the 500 mb low. As a result,
quantitative precipitation forecast amounts will only range from around a quarter of an inch to
nearly four tenths of an inch, with the higher totals across our

The aforementioned surface low will track north to western Iowa by
00z Tuesday with the higher pops maintained during the first half of
Monday night before a mid level dry slot shifts northeast into our
forecast area by 12z Tuesday. The storm system will then track
northeast into Wisconsin by Tuesday afternoon with a gusty southwest
wind across our area, with any lingering low chance pops confined to
the far north. Some fluctuations with the respect to the upper low
seen on the 12z ECMWF, compared to the GFS, Gem and sref which seem
more reasonable at this time. That would take the upper low east-
northeast into central lower Michigan by Wednesday afternoon with
our northern counties more favored for some scattered flurries
Tuesday night and possibly into Wednesday, especially if the European model (ecmwf)
turns out to be right. For now, will keep the slight chance of
flurries for Tuesday night and not have any precip in the forecast
for Wednesday based on the GFS, Gem and sref solutions.

Not a great deal of cold air poised to sweep across our area in the
wake of the storm on Wednesday and Thursday with seasonably cold
temperatures expected. That should be short lived though as upper
level heights build across the center of the nation as troffing once
again establishes itself over the southwest portion of the country.
However, medium range models showing some differences with respect
to shortwave energy tracking across the upper Great Lakes late Thu
into Friday. The European model (ecmwf) digs this feature sharply into the Midwest,
while the GFS and Canadian suggest more of a progressive track to
our north and northeast during this time frame. Whatever solution
works out, the overall affect in our area, at least precip-wise will
be minimal. A wide variation in temperatures were seen though as the
European model (ecmwf) solution would hold back the warming a couple of days, with
the more consistent GFS and Gem solutions pointing towards 50s for
afternoon highs starting on Friday in our west and south, and over
all but our northern counties on Saturday with little in the way of
significant rain or snow seen as we head into next weekend.


Aviation...(for the 00z tafs through 00z Monday evening)
issued at 554 PM CST sun Nov 29 2015

VFR mid-level ceilings will persist through most of the night as a
slow moving storm system approaches the Midwest. A very dry low-
level airmass will keep ceilings relatively high, although a few
light showers may break through from time to time. Then, the
lower layers of the atmosphere will finally saturate across the
central Illinois terminal area, a few hours either side of
sunrise. Once saturation occurs, expect conditions to rapidly
degrade to IFR, and periodic rain showers through the day Monday.


Ilx watches/warnings/advisories...


short term...hjs
long term...Smith

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