Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Lincoln Illinois
1145 PM CDT Tuesday Sep 1 2015
issued at 904 PM CDT Tuesday Sep 1 2015
Thunderstorms that developed from near Decatur to Bloomington to
the Champaign area earlier this evening are gradually weakening
as nighttime cooling begins to stabilize the atmosphere. Have
already done a few updates for thunderstorm coverage through the
evening but given current radar trends not anticipating any
further updates. Locally heavy rain of up to around 1 inch fell
with several of these storms due to their slow movement...and one
storm over central Piatt County accumulated a radar estimated 4
inches just north of I-72 and southwest of Lodge. For
tonight...skies expected to clear as upper level high pressure
settles over the area. Could see some fog overnight especially
toward eastern portions of the state where the lightest winds are
expected and in areas where rain fell this evening. Some
adjustments for fog coverage will be sent shortly.
issued at 223 PM CDT Tuesday Sep 1 2015
Last of the morning thunderstorms east of I-57 has finally faded
away. Latest water vapor imagery shows the upper low near
Indianapolis has moved only slightly northeast...so some deeper
moisture remains in place across the eastern County Warning Area. Nice area of
subsidence shown on visible imagery expanding across northeast
Illinois and Northwest Indiana...associated with the outflow from
the morning convection...with some diurnal cumulus developing to the
west. Radar showed a couple cells trying to fire along the
boundary... but these faded off quickly.
Currently think that any residual showers that do form will fade
with sunset...so will maintain the dry forecast for tonight. Have
also kept mention of patchy fog along and east of I-57...as there is
now quite a bit of moisture from the earlier heavy rains and a weak
wind flow at the surface. Winds just off the deck prognosticated to 15-20
knot range...so any fog should be rather shallow. Temperatures
expected to be fairly uniform overnight...mainly 67-70 degrees over
the forecast area.
Long term...(wednesday through tuesday)
issued at 223 PM CDT Tuesday Sep 1 2015
High pressure ridge will build into the plains tomorrow and continue
the hot and humid conditions through the week and into the weekend.
With all models forecast 850 temperatures around +18 to +20c...am expecting
highs to be around 90s into the lower 90s through the rest of the
week and through the weekend...and likely into Monday...Labor Day.
Conditions through the period should be dry until late in the
weekend and early next week. However...there is one period where a
short wave trough is forecast to come over the top of the ridge Wednesday
and Wednesday night through the Great Lakes region into Indiana. This
could bring some quantitative precipitation forecast to the northeastern parts of the state...which
could effect northeastern and eastern parts of the County Warning Area. NAM
continues to have precipitation in the area but varies with the
amounts...while GFS brings some in but also seems to overdo the
amount of quantitative precipitation forecast. 12z European model (ecmwf) seems most consistent with keeping precipitation to
the north. For now will keep precipitation out of the County Warning Area Wednesday night and Thursday
and let overnight shift take a look after another model run.
The next chance of precipitation will then be late in the weekend and the
beginning of next week and a stronger middle level low pressure system
flattens the ridge out and trys to drag a cold front into the area.
Models differ on the surface solution through the period but GFS and
European model (ecmwf) does appear to be in better agreement with some quantitative precipitation forecast moving
into the area Monday. However...they do not agree with amounts as
the GFS weakens the system as it moves into the area. Given that the
precipitation will be moving into a strong ridge...am inclined to limit quantitative precipitation forecast
and have gone with slight chance for Monday through Monday night. This
boundary does look to sag south enough that Monday night and Tuesday it
looks to develop more precipitation that moves into the area for Tuesday.
Temperatures will finally cool as well with most of the County Warning Area being north of
the boundary and effected by clouds and precipitation. Highs Tuesday look to
have cooled down into the lower to middle 80s...with upper 80s close
to the front.
Aviation...(for the 06z tafs through 06z Wednesday night)
issued at 1145 PM CDT Tuesday Sep 1 2015
An upper level high pressure ridge will move in for the night
bringing clearing skies after the evening thunderstorms that
affected portions of east central and southeast Illinois. The main
weather feature overnight will be the possibility of increasing
fog development...especially east central Illinois where there will be a
combination of lighter winds and low level moisture following the
evening rainfall. 06z tafs include periods of IFR-MVFR visibility
at kcmi-kdec-kbmi. VFR conditions developing by 13z. Winds for
the period generally S-SW up to 10 kts.