Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Indianapolis in
215 PM EDT Sat Apr 18 2015
the long term section has been updated below.
issued at 159 PM EDT Sat Apr 18 2015
As high pressure departs the area tonight...an active weather
pattern is ready to set up as the new week begins. Low pressure
over lower Mississippi River valley is expected to push across
Indiana on Sunday...bringing rain and perhaps a thunderstorm.
As the low pushes northeast to the Great Lakes on Sunday night...a
broad upper level low pressure system is set to remain in place
over the Great Lakes. Weak weather disturbances along with cooler
air will push across Indiana from time to time through
Tuesday...bringing daily chances for showers and perhaps a
Cooler weather is expected through the end of the work week as
northwest flow from Canada is expected in the wake of the
Near term /tonight/...
Issued at 159 PM EDT Sat Apr 18 2015
Surface analysis early this afternoon shows high pressure centered
over West Virginia. Low pressure had developed over northern
Louisiana. Water vapor continued to show an upper level low over
Colorado with a stream of tropical moisture ahead of this
system...streaming north through the Mississippi and into the Ohio
Valley. Dew point temperatures across central Indiana remained in the dry
40s...while more moist air with dew points in the 60s was not far
away over western Kentucky.
Main forecast challenge tonight will be probability of precipitation and temperatures.
Much of this forecast period will be dry as best forcing
associated with the approaching low pressure system from the
southwest fails to arrive until after 06z. Dry air remains in
place across the region through the evening. 300k GFS isentropic
surface shows good upglide and a surge of specific humidities in
excess of 7 g/kg near 12z...with an even better surge after 12z
Sunday.; Forecast soundings show increasing saturation by
12z...with best saturation across the southern area where moisture
is expected first. With excellent forcing expected as with the
arrival of upper dynamics...will trend probability of precipitation higher across the
southwest where moist should arrive prior to 12z...but stick close
to guidance elsewhere. The evening should remain dry.
Given the expected clouds and precipitation will trend lows at or above
Short term /Saturday through Tuesday/...
Issued at 159 PM EDT Sat Apr 18 2015
A wet day is a given on Sunday. GFS and NAM both show deeply
saturated columns on Sunday through the middle of the day as
strong forcing works across the area associated with the passing
surface low. As stated above...the GFS 300k isentropic surface
shows excellent upglide along with very moist specific humidities
in excess of 7 g/kg. Thus will use 100 probability of precipitation on Sunday...along with
temperatures cooler than mavmos given the expected clouds and rain.
GFS and NAM show the first wave of forcing departing the area
after 00z...however broad cyclonic upper flow appears to develop
as the upper low reaches the Great Lakes and begins to merge and
deepen with another upper low sinking out of northern Canada. An
additional short wave embedded with the flow looks to pass across
central Indiana on Sunday night and into Monday. Again good
moisture and forcing appears in place. Forecast soundings show
deep saturation again on Monday morning as this wave passes. Thus
will again trend probability of precipitation higher on on late Sunday night and again on
Monday. Given the expected clouds and cold air advection and
possible rain showers will trend lows warmer than mavmos and highs
A brief break appears on Monday night as the previous second
short wave departs across the area and some subsidence is seen
within the broad cyclonic flow. Good middle level dry air is seen as
well as within forecast soundings. Will trend toward a drier forecast
on Monday night...with lows at or below mavmos.
NAM and GFS both suggest a weaker and poorly defined short wave
pushes into the area late on Tuesday. Best forcing appears to be
across the southern parts of the forecast area. Forecast soundings
continue to show a rather dry column with convective temperatures
reachable in the lower 50s. However plenty of dry air aloft will
hamper shower development. Overall confidence is low for that
time. Thus will trend to probability of precipitation at or below mexmos...and stick close
Long term /Tuesday night through Saturday/...
Issued at 215 PM EDT Sat Apr 18 2015
Models in general agreement that the extended will be cool with the
potential for a few showers mainly Tuesday night through Wednesday
night and again next weekend. With good model agreement...made
little changes to regional blend probability of precipitation. 5400 meter 1000-500 millibar
thickness line across the area through the period strongly supports
the below normal blend temperatures with afternoon highs in the middle
50s to lower 60s and lows in the middle 30s to lower 40s.
Aviation /discussion for the 18/1800z taf issuance/...
Issued at 1230 PM EDT Sat Apr 18 2015
Good confidence that VFR conditions will dominate through 9z with
just cirrus and perhaps some diurnal cumulus. Backdoor cold front will
result in winds shifting to the east less than 10 knots this
afternoon. The front will stall across the area tonight with winds
increasing to 10 knots or more. Of more importance...moisture will
be increasing markedly after 09z as an upper wave approaches from
the southwest. This will result in widespread showers and likely IFR
or worse conditions. Would not rule out thunder...but chances too
low to mention...especially that far out.
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