Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Indianapolis in
719 PM EDT Thursday may 28 2015
the aviation section has been updated below.
issued at 218 PM EDT Thursday may 28 2015
High pressure across the Appalachians will allow for dry weather
tonight. However...thunderstorm chances will return for the weekend
with the best chances on Saturday as a cold front drops southeast
across central Indiana.
The warm weather will continue though Saturday before much cooler
air arrives on Sunday. Then...temperatures will moderate next week
as southerly flow returns. This should allow temperatures to return
to the 80s by Wednesday after highs Sunday only in the 60s.
Meanwhile...there small thunderstorm chances at times next week as
the frontal system lifts back to the north in the southerly flow and
ahead of a slow moving upper low over the lower Mississippi
Near term /tonight/...
Issued at 218 PM EDT Thursday may 28 2015
Main issue tonight will be whether or not to introduce probability of precipitation. Models
in good agreement that an weak upper ridge...currently over the
area...will migrate east to the Appalachians as a quasi-stationary
frontal boundary remains draped over south central Indiana.
Southerly weak low level flow will also be in place ahead of
frontal from the upper Midwest to the Southern Plains.
All the 12z operational models and the 15z high res rapid refresh
suggest there could be some convection around late this afternoon
and tonight...especially south closer to the dew point boundary. The
12z NAM MOS has mostly 30-40% probability of precipitation...while the 12z GFS MOS is closer
to 10%. The NAM MOS seems to overdo dew points a tad and
consequently instability. With mixing this afternoon...unless we see
some convection in the hours leading up to the afternoon forecast
issuance...prefer the slightly drier GFS MOS and thus will keep it
dry tonight with probability of precipitation approaching 15%.
Should see some cumulus around tonight per model time sections...but
partly cloudy should do. MOS temperatures are close tonight and
mostly in the middle 60s which appears reasonable with cloud cover and
low level thermal profile considerations.
Short term /Friday through Sunday/...
Issued at 218 PM EDT Thursday may 28 2015
Unsettled weather is expected this weekend with much cooler
temperatures by Sunday.
Models in pretty good agreement as they bring a cold front southeast
across central Indiana on Saturday afternoon and evening. They also
bring a series of upper waves northeast over the area ahead of a
plains trough. Warm and unstable air will be in place for these
synoptic forcing features to interact with along with precipitable
water amounts to 1.7 inches or so. The result will be numerous
convective activity starting on Friday with the best chances and
most quantitative precipitation forecast Saturday into Saturday night...when 12z MOS likely to Cat
probability of precipitation look good.
In the wake of the front on Sunday...models move the plains upper
trough to the middle and lower Mississippi Valley. This is expected
to slow the cold front and perhaps bring it slightly further north
again. The 12z Gem seems to be an outlier with it further south
location of the front and associated deep moisture. The others
including the 00z European model (ecmwf)...12z GFS and 12z NAM all take a more
northern approach with quantitative precipitation forecast over central Indiana on Sunday...with the
highest amounts across our southern half. This matches up nicely
with the previous forecast with good chance probability of precipitation south tapering off
to slight chance northwest.
With mixed layer convective available potential energy over 1000 j/kg Friday afternoon and Saturday
afternoon...would not rule out an isolated severe storm per the Storm Prediction Center
marginal day2 risk. However...weak shear should prevent any sort of
widespread severe event either day. Best chances should be across
our northwestern counties...closer to slightly higher middle and upper
level winds. Also...confidence in thunderstorms is low on Sunday
except south per instability forecasts. Thus...will go no higher than
isolated regarding thunderstorms then.
12z MOS temperatures are pretty close and appear reasonable with
highs in the lower to middle 80s Friday and upper 70s to lower 80s on
Saturday. If anything...may cut a degree or two off with plenty of
clouds and storms around. By Sunday...with the front south of the
area...should see much cooler temperatures with blend suggesting
highs only in the 60s.
Long term /Sunday through Thursday/...
Issued at 154 PM EDT Thursday may 28 2015
The regional model initialization will be used with high confidence.
The GFS is saying about what it did yesterday for next weeks
forecast. The latest European has nearly converged on this. The
convergence is particularly evident in the handling of a trough over
the area that will be very important to our weather.
This is a strong indication the GFS has a good grip on what will
happen. Additional supporting evidence is small spread between the
various GFS ensemble runs.
Aviation /discussion for 290000z tafs/...
Issued at 708 PM EDT Thursday may 28 2015
Quiet night as diurnal cumulus dissipate. Some localized MVFR visibility
from may occur around daybreak. Dampening upper trough firing
Missouri and Illinois convection this evening will come into play
across Indiana Friday especially with diurnal heating. Expect
return of deeper layered moisture to bring cumulus formation
earlier in the morning...301300z-301500z...with more coverage and
with development into scattered afternoon thunderstorms.
Winds lie down overnight but with approaching weak upper trough
winds Friday will exceed 10kts by 301600z and gust to 20 kts in
Cloud decks should remain VFR through the period except brief MVFR during
any thunderstorm core passage.
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