Scientific Forecaster Discussion
Area forecast discussion
National Weather Service northern Indiana
241 PM EST sun Nov 23 2014
issued at 241 PM EST sun Nov 23 2014
A storm system will bring widespread rainfall to the region from
late this afternoon through Monday morning. Rain showers will
change to snow showers Monday afternoon as colder air rushes into
the area behind this system. Some light snow accumulations will be
possible from Monday afternoon into Tuesday morning...mainly
across portions of northwest and North Central Indiana...along
with Southern Lower Michigan. Temperatures will not drop much
tonight...with lows from the upper 40s to lower 50s.
Short term...(today and tonight)
issued at 410 am EST sun Nov 23 2014
Forecast items to address for the short term will be evolution of
ongoing rain this morning...and then a more widespread rainfall
event for late this afternoon and tonight.
A relatively narrow zone of 900-750 hpa moisture convergence has
supported an area of rain that has affected primarily southeast half
of the area during most of the overnight hours. Short term NAM/rap
forecasts suggest that magnitude of this moisture convergence should
gradually wane over next few hours and lift to the northeast. This
idea is supported well in higher res arw/nmm output this morning.
Thus will carry categorical probability of precipitation over the next few hours across the
southeast...with a fairly steady downward trend in probability of precipitation toward
daybreak. Daybreak should also mark the approximate time weak vorticity
maximum evident in water vapor imagery across southwest lower Michigan
departs to the east. Thus...still expecting a sizable lull in
greater precipitation coverage from early this morning through at least
early afternoon with relatively weak forcing.
By middle to late afternoon...low level moisture transport should
strengthen once again as vigorous upper level short wave across the
lower MS valley begins to lift north-northeastward. Initial higher
probability of precipitation later this afternoon may be confined to a relatively narrow
zone and depend on exact positioning of strongest low level moisture
transport. However...toward evening...guidance is still in good
agreement in depicting synoptic support increasing as smaller scale
upper vorticity maximum tracks into Ohio Valley...and larger scale upper
trough works across central Continental U.S.. coupled upper jet structure also
should tend to aid in expanding rain shower coverage by early this
evening. No change to categorical probability of precipitation from late afternoon through
the evening with some respectable rainfall amounts of around an inch
possible by daybreak Monday. Most of this rain should occur in the
22z-06z timeframe as stronger uvm shifts northeast of the area
overnight with passage of initial short wave. Pocket of slightly
drier low/middle level air behind this feature will be short lived as
larger scale trough approaches from the west with more widespread
rain spreading back into the area toward daybreak Monday.
Given strong low level jet/upper dynamics/moisture transport...really
cannot rule out isolated thunder any time from late this afternoon
through tonight but probabilities still appear to be very low for
any given location and will not add any mention at this time.
Perhaps the best chance of isolated thunder will be very late tonight
across the west where differential temperature advection should setup
pocket of weak instability from northern Illinois into Northwest
Indiana. In terms of temperatures....highs today should reach into
the lower to middle 50s...with nearly steady temperatures tonight due
to continued warm air advection/llj/cloud cover.
Long term...(monday through saturday)
issued at 410 am EST sun Nov 23 2014
Main focus remains on ptype/strong winds/falling temperatures with dynamic
phased low lifting through the Great Lakes region Monday-Monday
evening. Unseasonably chilly/drier thereafter...
00z guidance overall picking up on stronger sheared shortwave energy
now into The Four Corners in left exit region of a 140 knots upper jet.
The result has been a trend toward a more robust deformation/fgen
axis lifting NE through the middle MS valley/western lakes Monday-Monday evening
as this jet coupling/shortwave energy rounds the base of an
amplifying central Continental U.S. Longwave trough. Strong cold advection
following the passage of the system cold front should allow rain to
change over to snow within this burgeoning deformation axis Monday
afternoon. Favored European model (ecmwf)/GFS guidance suggest that this feature will
pivot into our northwestern zones during this time with light
accumulations possible...especially SW lower Michigan thanks to lake
enhancement. Middle level dry slot should impinge on southeastern zones
supporting a downtrend in rain shower coverage during the day...with
later changeover to light snow showers/flurries (little to no accum)
delayed to late Monday evening here as primary height fall axis settles in.
Headline worthy wind gusts (>45 mph) also remains a possibility for
the entire County Warning Area by later Monday morning-aftn. Still considerable
differences noted in guidance on just how deep surface reflection will
be once reaching the northern lakes...resulting in some uncertainty.
Impressive pressure rises and slight deepening of mixed layer in
response to the cold advection process may be enough to tap into
40-50 knots 900 mb jet core (low confidence).
Cold/moist cyclonic flow within deep layered eastern Continental U.S. Trough
axis will keep clouds and sporadic snow showers/flurries around
later Monday night into Tuesday...especially northwest zones as 850
mb Delta t's drop into the middle teens. A gradual transition to lower
amplitude west-northwest flow is expected Wednesday through Sat...but expect the local
area to mainly remain on cold side of a plains to middle MS valley
baroclinic zone...resulting in an extended period of below normal
temperatures. Several moisture starved shortwaves will ripple through in
this flow...one later Wednesday-Thursday am and another possible toward end of
period...bringing a couple chances for light system snow and renewed
Aviation...(for the 18z tafs through 18z Monday afternoon)
issued at 1253 PM EST sun Nov 23 2014
An intense upper level trough will lift north across the area
tonight and become negatively tilted. The exit region of a very
strong upper level jet and a 65 knots low level jet will help aid in
the development of precipitation as moisture flux increases in the
low levels. Added tempo heavy rain at both sbn and FWA in this
regard. Also...windy conditions are expected after 12z as a very
tight surface gradient moves over the area.
Lm...gale watch from Monday morning through late Monday night for
Small Craft Advisory until 9 am EST Monday for lmz043-046.
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