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Area forecast discussion 
National Weather Service northern Indiana
345 am EDT Monday Apr 21 2014

issued at 345 am EDT Monday Apr 21 2014

One more warm day in the middle to upper 70s expected before a
cold front crosses the region tonight. This front will also bring
numerous showers and perhaps a few isolated thunderstorms to the
area. Highs tomorrow will likely remain in the 50s as cool and dry
Canadian high pressure settles into the region.


Short term...(today and tonight)
issued at 345 am EDT Monday Apr 21 2014

Pair of pv anomalies currently traversing southern
with a well defined "swirl" on morning water vapor...will be the
main items to note for today's short term forecast. There also
exists a loosely organized midlevel trough over the Southern
Plains...muddled by convective elements...that may aid precipitation
chances somewhat later today as it shears northeast. Northern
stream wave will be the main feature to watch though as a
strengthening middle/upper level jet streak is expected to carve out
a decent trough over the Great Lakes by late tonight due to
amplifying pattern aloft over much of the Continental U.S.. this will support
cyclogenesis over the central Great Lakes with a corresponding
cold frontal passage for our County Warning Area around midnight tonight. Best
upper jet dynamics pass just to our north but decent midlevel
height falls and corresponding low level convergence should be
able to support at least scattered showers overnight given
adequate moisture/instability. Southwest flow/Theta-E advection
finally begins to increase this afternoon and forecast
soundings/relative humidity fields show a healthy region of deep layer moisture
convergence right ahead of the front. Surface based instability
does increase but will be very limited with only a few hundred
j/kg given largely moist adiabatic lapse rates. This will likely
aid the response to overall modest synoptic forcing but could also
support an isolated thunderstorm during the evening hours. Doubt
precipitation coverage will be widespread enough to warrant categorical
probability of precipitation though and will stick with likely wording. Also expect precipitation
to hold off for much of the day given late arrival of convergent
boundary and sufficient low level idea supported by
latest hi-res guidance. This should allow for another day at least
in the middle 70s...if not upper 70s...given increasing warm air advection and much
deeper planetary boundary layer mixing. Note this will be in spite of increasing cloud
cover. Expect a sharp gradient in overnight lows depending on
exact timing of cold front. Airmass immediately behind the front
is not particularly cold but steady cold air advection and northwest flow off
Lake Michigan could allow our northwest zones to drop into the
upper 30s/lower 40s around 12z Tuesday.


Long term...(tuesday through sunday)
issued at 345 am EDT Monday Apr 21 2014

Tuesday will begin the period with cold air advection and a return
to below normal temperatures. Cold front will be east of the area
with northwest winds in its wake. Lake Shadow should spread well
inland and keep temperatures across northwest in and SW Michigan in the upper 40s to
lower 50s at best. Remainder of the area likely to rebound into middle
and upper 50s with afternoon insolation but breezy conditions will
make it feel even cooler. High pressure will nose in Tuesday night and
still expect a good radiational cooling night with lows near or just
below freezing and widespread frost likely.

High slides east on Wednesday with return flow developing Wednesday afternoon
and night. Models developing some light quantitative precipitation forecast in warm air advection as
low level jet increases Wednesday night. Sounding profiles are very dry in
the lower levels and it will once again be difficult to saturate
lower levels. Likely see virga event with some possible sprinkles
over the northwest. Not ready to include the very low probability of precipitation given by
allblend looking at the significant dew point depressions.

Better chance for showers and a few thunderstorms arrive late
Thursday afternoon in west and in the evening most areas. Model
timing different by a few hours but still worthy of likely probability of precipitation with
good forcing along occluded front lifting through the area.

Significant model differences continue in wake of this boundary for
Friday and the upcoming weekend with secondary cold front. GFS
continues to show much colder air wrapping around a strong low in
the western lakes while European model (ecmwf) is not as cold and further north. 00z
GFS is very cold at 850mb for this weekend with temperatures around -7c to
-9c Saturday over our area while European model (ecmwf) is in the +2c to +7c range.
Given the differences and uncertainty have opted for the allblend
which is still quite cool and similar to mean model ensembles.
Further adjustments will obviously be necessary as deterministic
model solutions converge.


Aviation...(for the 06z tafs through 06z Monday night)
issued at 130 am EDT Monday Apr 21 2014

Mainly VFR conditions expected today. Shortwave trough will
eventually dig into the Great Lakes tonight and send a cold front
through the region. This cold front will intersect an increasingly
moist and weakly unstable environment advected north by southwest
flow. Numerous showers are expected by tonight and brief
reductions into MVFR category are possible. An isolated
thunderstorm may also be possible but confidence is too low for
inclusion in the taf at this time.


Iwx watches/warnings/advisories...


short term...agd
long term...Lashley

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