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Area forecast discussion 
National Weather Service northern Indiana
807 PM EDT Sat Jul 12 2014

issued at 544 PM EDT Sat Jul 12 2014

Low pressure and a cold front will be moving through the region
tonight. This will bring with it a chance for some showers and
thunderstorms. Low temperatures tonight will be in the lower 70s.
The front will still be in the area tomorrow continuing the shower
and thunderstorm chances. Highs tomorrow will be in the lower 80s.


issued at 747 PM EDT Sat Jul 12 2014

Convective initiation has occurred past across eastern
Iowa/northern Illinois where moderate to strong instability is
present. Large scale ascent has also been aided by potent upper
vorticity maximum tracking southeast Minnesota/west central Wisconsin. To
this point however...longevity/organization has been somewhat
minimal perhaps in part due to slightly sheared nature of upper
vorticity maximum...and some slight latitudinal displacement of strongest
upper forcing. Will need to monitor for scattered upstream storms
to congeal across northern Illinois which may pose some limited
wind threat across mainly northern half of the area late this
evening into the overnight...given supportive downstream deep
layer shear. Of some concern is robust low level shear that will
continue to develop into the overnight hours in association with
strong low level westerly jet. However...diminishing instability
magnitude with eastward extent into local area overnight should
keep any severe threat isolated at best...and mainly in the form
of an isolated wind threat.

Of greater concern will likely be the potential of locally heavy
rainfall amounts. Aforementioned ramping up of low level jet will
create fairly weak upwind propagation vectors overnight...and
potential upwind development. While middle/upper forcing may remain
strongest just to the north...strong low level forcing in terms of
low level jet/positive thetae advection will be supportive of localized
heavy rain. Given relatively dry antecedent conditions will
continue to hold off on any Hydro headlines at this time however.
Updated forecast sent to slightly push back timing of categorical
probability of precipitation to late evening...otherwise no major changes currently


Short term...(this evening through sunday)
issued at 235 PM EDT Sat Jul 12 2014

An area of showers with isolated thunder over Indiana and northeast
Illinois has been gradually weakening early this afternoon in the
vicinity of a weak upper level trough. These showers have also been
supported by weak low level Theta-E convergence. Expect this area to
continue to weaken as low level forcing becomes established west of
the area early tonight. Theta-E moisture flux will then increase
later tonight associated with a weak upper low and ahead of a weak
front. Not excited about severe chances given marginal parameters
and the large area of rain cooled air at the surface. However...
NAM/BUFKIT supports a strong signal for heavy rain given
precipitable water values over 2.3 inches at times exceeding the
99th percentile per 64 year upper air climatology. There will also
be a sufficient warm cloud depth and favorable net storm motion. As
a result...have raised rain chances and have also mentioned heavy
rain in the forecast/grids. Have also updated the hazardous weather
outlook in this regard. The best chances for thunder will be over
southeast areas Sunday. There is also a chance for severe storms
Sunday afternoon over these southeast area as the atmosphere
destabilizes and as upper level impulses track across the area.


Long term...(sunday night through saturday)
issued at 235 PM EDT Sat Jul 12 2014

Focus will be on effects of strong cold front which will sweep
across the region on Monday.

Piece of deep Canadian upper low will break off and quickly dive
south into the upper Midwest and Great Lakes reaching Wisconsin by
Monday evening. The upper low will quickly deepen in response to
110+ knots jet streak rounding the base of the trough. This will help
force a strong and generally fast moving cold front across the
region during the day Monday. While the better Theta-E air will have
been shunted southeast with the Sunday front...dewpoints along and ahead of
the front will still be in the lower 60s (pwats to 1.5 inches). This
combined with the strong forcing and somewhat steep lapse rates
should allow for at least scattered showers around the front. As the front
approaches southeast areas...somewhat better instability may exist and
given amount of shear with this system threat for isolated strong to
severe storms still looms. In terms of probability of precipitation...bit concerned about
dry forecast Monday morning given amount of moisture still around but
for now will keep chance probability of precipitation confined to afternoon/evening hours during
best time frame.

A cold middle week by July standards still looks on track with coldest
temperatures in the Tuesday to Wednesday night time frame as upper low
lingers across the Great Lakes. GFS has been thrown out with a must
faster trend of pushing out the upper low...which is a typical bias
in these situations. Highs Tuesday may not reach 70 in many
locations with Lake Shore areas possibly only in the lower 60s (or
even near 60). Overnight lows are forecast for the low 50s at this
point...but honestly wouldn't be surprised to see some 40s
especially north. With the cold air aloft...can't rule out a few
showers around the area but with majority of the moisture well south and
east not worthy of placing in forecast (which blend of models
guidance tries to do).

Finally the cold air mass will depart and allow slowly moderating
temperatures into the upcoming weekend as surface high pressure settles in the
east and keeps deeper moisture and most systems south of the area.


Aviation...(for the 00z tafs through 00z Sunday evening)
issued at 747 PM EDT Sat Jul 12 2014

An upper level short wave tracking across the middle/upper MS valley
will combine with associated frontal zone and copious moisture to
allow thunderstorm threat to expand eastward into northern Indiana
overnight. Timing of thunderstorms and rain with previous forecast still appears to
be the 05z-10z timeframe at ksbn and generally
from 08z-14z at kfwa. IFR conditions are possible with the heavier
showers and storms but will trend to lower MVFR conditions at this
forecast distance. Abundant low level moisture associated with
strong thetae ridge will likely keep some lower end MVFR ceilings in
place through much of Sunday morning before improving to VFR.
Sagging frontal boundary may pose some additional scattered thunderstorms and rain threat
at kfwa Sunday afternoon...however will keep tafs dry at this time
with most favored areas of thunderstorm redevelopment south of the


Iwx watches/warnings/advisories...


short term...skipper
long term...Fisher

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