Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service northern Indiana
351 PM EDT Sat Aug 1 2015

issued at 345 PM EDT Sat Aug 1 2015

There is a slight chance for a shower this evening but most areas
will be dry and partly cloudy. Low temperatures will be in the low
to middle 60s. It will be warm and more humid on Sunday with
afternoon highs in the middle 80s. There is a chance for
thunderstorms through the day.


Short term...(this evening through sunday)
issued at 348 PM EDT Sat Aug 1 2015

Difficult short term forecast with convective trends the main
issue. Surface dewpoints have mixed substantially this afternoon with
readings into the upper 40s across lower Michigan and lower to
middle 50s over North Central Indiana and Northwest Ohio. This
problematic initially as these dry low levels will inhibit shower
development this evening as weak Theta-E ridge and some elevated
instability move across the area. Inherited low chance probability of precipitation for
this evening and hires models still hinting at a few possible
light showers. For consistency opted to keep a slight chance pop
for light rain showers and dropped thunder mention.

Models have slowed approach of cold front with main area of
convection expected Sunday night. Mesoscale convective system expected to develop tonight
upstream to our northwest and likely move east. Models hinting that
part of this system could clip our north Sunday morning. Otherwise
residual cloud cover will once again dictate how unstable we get
tomorrow afternoon and whether any convection develops in warm
sector. Nam12 showing MUCAPES in excess of 3k j/kg with increasing
deep layer shear in afternoon. Eml will be moving over the region
with steep middle level lapse rates approaching 9c/km. Inhibiting
factors for afternoon convection will be cap...residual cloud cover
and lack of strong forcing ahead of cold front. Theta-E min also
remains over the region until late afternoon. Have trimmed probability of precipitation
back...especially south...but did leave probability of precipitation in all areas for


Long term...(sunday night through saturday)
issued at 348 PM EDT Sat Aug 1 2015

On Sunday evening/00z Monday a 500mb closed low lingers over James
Bay...with the corresponding surface low across northern Ontario and
trailing trough across the Great Lakes into Missouri. The surface low
forecast to develop over Minnesota/WI earlier Sunday morning is situated
over Northern Lake Michigan and eventually wraps in with surface low to the
north as it progresses eastward through Monday. Precipitable waters around 1.3
increase to near 1.9 inches as a 850mb 45 knots jet pumps in
moisture/instability to the region through Sunday evening within the
warm sector. First shortwave rounding the base of the trough is
forecast to swing through Sunday afternoon...then the second is
associated with the cold front approaching the west around 3-6z. Middle
level lapse rates are decent as well...reaching around 6.5-7 c/km by
21z Sunday afternoon and remaining until the cold front passes
Sunday evening. Thinking there may be a brief break/lull in
precipitation around 00z in the wake of the first shortwave...but
left in chance-slight chance probability of precipitation due to uncertainty in
timing/development of precipitation Sunday afternoon with the first

As stronger forcing approaches with coupled 300 mb
jet/500 mb shortwave/surface cold front around 3-6z...expect showers/T-
storms. Models still suggest around 1-2k j/g of ml cape in the west
by this time...with about 30-40 knots of 0-6 km shear. This would
still suggest chances for severe weather...though with the
unfavorable late evening/overnight timing have lower confidence in
this. If any will be in the far northwest near Lake Michigan as
the front approaches. For now...bumped up probability of precipitation to likely during the
3-9z time frame west and central...and towards 12z east.

The cold front will continue to sink southeastward through the
morning hours Monday...clearing the region by about 15z. In its wake
gusty northwest winds of 15-25 knots. This will lead to waves of 3
to 6 feet and dangerous currents along Lake Michigan beaches. Have
issued a beach hazards statement for hazarous conditions Sunday
lasting into early Monday to account for this.

Otherwise...a few isolated showers are possible Monday afternoon in
the eastern County Warning Area with a weak shortwave rounding the base of the
low...but have low confidence in this. Kept in slight chance probability of precipitation.
Monday night into Tuesday looks dry...with zonal flow aloft and weak
high pressure at the surface. Temperatures will be cooler...with highs
in the 70s.

Wednesday the upper level low over northern Ontario begins to shift
eastward...reaching the New England states by Thursday-Friday. Model
discrepancies become larger at this point...with the GFS bringing in
a surface low from the south/having a weaker high to the north with
precipitation over the County Warning Area...and the European model (ecmwf) bringing in a stronger high from
the north and thus keeping the surface low far enough south to keep
US dry. Will keep consensus probability of precipitation for now. Either way...temperatures looking
cooler through the mainly in the 70s for highs.


Aviation...(for the 18z tafs through 18z Sunday afternoon)
issued at 203 PM EDT Sat Aug 1 2015

VFR conditions expected this period. Very subtle moisture
advection and weak short wave late this afternoon and early
evening to pass through taf sites. Not overly impressive but a few
hires models still show possibility of some isolated rain showers
developing. Opted to replace thunderstorms in the vicinity with vcsh given very low chance
of occurrence and lack of good forcing and moisture. Otherwise
expect dry conditions overnight with light and variable winds.
Surface winds will increase to between 10 and 20 knots Sunday morning
well ahead of a strong cold front. Slight chance for thunderstorms and rain before
18z but not enough chance for inclusion at terminals at this time.


Iwx watches/warnings/advisories...


short term...Lashley
long term...mesoscale discussion

Visit US at www.Weather.Gov/iwx (all lower case)

Follow US on facebook...twitter...and youtube at:

National Weather Service Glossary of Abbreviations