Scientific Forecaster Discussion
Area forecast discussion
National Weather Service northern Indiana
1243 PM EST Tuesday Dec 1 2015
issued at 1036 am EST Tuesday Dec 1 2015
Dry conditions and clearing skies are expected this afternoon with
steady temperatures of 45-50. Fair weather will continue tonight
with lows in the lower 30s. Another disturbance will bring a
likelihood of rain and snow showers on Wednesday.
Short term...(today and tonight)
issued at 345 am EST Tuesday Dec 1 2015
Deep middle tropospheric cutoff to track slowly eastward to southeastern WI by
daybreak Wednesday...well removed of high belted westerlies over Canada.
While some convection early this am across northestern Illinois within region
of meager 250-500 j/kg MUCAPE...presence of triple point
mesolow/cusp on leading edge of dry slot...is likely affording
enhancement of convergence within lowest 1km. This feature will
continue to surge north-northeastward this am...and while southern extension of
rap/nam12 MUCAPE axis to swing cyclonically through County Warning Area this
am...am reluctant at this moment to add thunderstorms and rain mention. County Warning Area largely
split between foci of dynamic focus west-northwest of County Warning Area and...well to southeast-S
the fervent isentropic Ascension i300k surface with collocated
rich moisture over middle Ohio Valley. In between well perturbed
0-5kft saturated layer with less remarkable ascent eliciting
deep/low stratus and br/fog intermixed with mix of r--/dz with a
bit more convective elements especially over western County Warning Area. Will have
prefirst period with higher chance measurable this am across western
County Warning Area...thereafter trending eroding probability of precipitation SW to NE as core of dry
slot oversweeps County Warning Area. Temperatures tricky with early onset cold air advection across far
western/northwestern cold air advection to more middle afternoon thermal ridge pivot over southeastern
County Warning Area. Modified 3 hrly probability of precipitation with extracted maximum temperature with slight southeastern
County Warning Area bump. By late tonight effects of laggard upper low with
secondary wrap around moisture/DCVA begins to approach northwestern County Warning Area.
Given marine layer moderation along with maximum tw upwards of 2c in
lowest 1km will maintain mix rain showers/shsn until full erosion of warm
layer occurs around f36.
Long term...(wednesday through monday)
issued at 345 am EST Tuesday Dec 1 2015
Primary focus in long term remains on precipitation types and coverage
on Wednesday with little sensible weather concerns thereafter.
Very few changes to inherited forecast. Middle/upper level low to
pass over the Great Lakes Wednesday with weak surface reflection.
Deep wraparound moisture and midlevel cva will support modest
ascent/precipitation chances...aided by steepening lapse rates/cold pool
aloft and some Theta-E flux from Lake Michigan. This will result
in a showery nature to the precipitation with the best chances for
meaningful precipitation rates in our north/northwest counties near the lake. Areas
further from the lake will still see periodic showers but less
chance of accumulation and quantitative precipitation forecast amounts of only a few hundredths. A
bit of a tricky ptype forecast given weak thermal trough pushing
from west to east during the day but also being at least partially
offset/slowed by filtered diurnal heating. Stuck close to previous
forecast which captured the trend well. Expect a rain/snow mix at
the onset for areas outside of our far northwest...becoming all
snow by late afternoon. Our northwest zones will have the best
chance of accumulating snow given faster arrival of cold air and
slightly better moisture/forcing. Still...marginal surface/ground
temperatures...poor diurnal timing...and convective nature of precipitation will
prevent efficient accumulation. Even in our northwest...expect an
inch or less in sporadic areas and primarily on grassy surfaces.
Areas south of US-30 and areas east of I-69 will see little to no
accumulation. Did expand probability of precipitation in the 00- 06z timeframe Wednesday
evening given residual moisture and slower exit of upper low.
Precipitation will be winding down after 00z though with little
additional accumulation and dry conditions by early Thursday
Rest of the long term very quiet as longwave ridge supports high
pressure over the Ohio Valley through the weekend. Temperatures will be
slightly above early December averages.
Aviation...(for the 18z tafs through 18z Wednesday afternoon)
issued at 1242 PM EST Tuesday Dec 1 2015
Back edge of MVFR cloud deck approaching ksbn by 18z with an
arrival at kfwa around 1930z. Expect any stratocu which may
develop this afternoon to remain scattered at best. Focus then
turns to the potential for some snow tomorrow. Initially some
rain may mix in with the snow...but once the boundary layer
cools...expect a quick transition to all snow. Kept mention out of
18z taf for kfwa with expected arrival near or after 18z and
better chances north of the terminal.
Lm...Small Craft Advisory until 7 PM EST this evening for lmz043-046.
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