Area forecast discussion
National Weather Service northern Indiana
229 am EDT Tuesday may 21 2013
Aviation...(for the 06z tafs through 00z Tuesday evening) issued
at 225 am EDT Tuesday may 21 2013
Made some adjustments to tafs to accommodate accelerating line of
storms moving out of NE Illinois into Northwest Indiana. Storm Prediction Center mesoscale analysis
shows 3 hour surface based convective available potential energy changes 200 to 800 j/kg lower...so
expect storms to gradually weaken the rest of the night. Also...
some support from the right entrance region of an upper level jet
will wane as the jet axis translates back to the west given the
response to an upstream trough moving into the Central Plains. So
kept thunder out after 12z. A fairly tight pressure gradient will
persist today...with daytime mixing of higher momentum air causing
southwest gusts to around 25 knots.
&&
Synopsis...
issued at 532 PM EDT Monday may 20 2013
Shower and thunderstorm chances will continue to increase tonight
through Wednesday...with a few showers possibly lingering into
Thursday. There is a slight risk for severe thunderstorms this
evening through Tuesday night. Drier and cooler weather is
anticipated late this week into next weekend.
&&
Short term...(this evening through tuesday)
issued at 305 PM EDT Monday may 20 2013
Remnant Gravity wave from overnight convection and
associated with a ~10f degree dewpoint drop in its wake has washed
out as it moved into our forecast area. This has kept moderately
unstable airmass across the eastern two-thirds of our forecast area.
Added slight chance probability of precipitation for the east this evening as convection
could fire along or in front of this boundary where the airmass is
characterized by SBCAPE values in excess of 3000 j/kg. The greatest
limiting factor is the lack of significant forcing...although
convection across Michigan and additional convection firing along
the boundary across eastern Illinois at 1830z indicates this
boundary may provide just enough forced ascent for isolated storm
development.
Still some uncertainty regarding convective evolution and impacts
across our area tonight. Decaying storm cluster across western
Illinois should continue its weakening trend as it moves into a
moisture void area across Illinois with surface dewpoints in the
upper 50s. With that being said...some short term model guidance is
indicating a quick recovery of the moisture profile in that area as
850 moisture transport increases this afternoon. If the airmass
ahead of these storms can destabilize over the next few hours...as
the hrrr suggests...upscale growth of that cluster as well as
additional storm development may track across the northwest portion
of our forecast area during the late evening. If storms can
develop...severe potential does exist as 0-6 km shear of 35-45 knots
overspreads the northwest portion of our forecast area with the main
threats being severe hail and wind.
Remain more confident in precipitation chances after 06z when
remnant convection from Missouri tracks towards the area. Severe
threat will be minimal with this convection due to its timing during
the early morning hours.
Tuesday looks to be cooler than today as little sun will be seen
across our forecast area due to cloud debris from from overnight
convection as well as additional dynamic lift from 40-50 knots low level jet.
Severe threat does exist if a few breaks in the clouds can provide
significant surface heating for even marginal surface based
instability. The main threat will be large hail and damaging winds
as unidirectional 35-40 knot shear overspreads the area.
&&
Long term...(tuesday night through monday)
issued at 305 PM EDT Monday may 20 2013
One or two more rounds of convection to begin the forecast period.
Models in pretty good agreement in short wave coming out of base of
closed low and impacting the area Tuesday night. Good support will
accompany this feature and 50-60kt middle level speed maximum will combine
with influx of low level moisture with at least sufficient
instability to initiate or maintain convection. Unidirectional wind
profile with around 30kt of speed shear will have damaging winds as
the main severe threat. Closed low finally opens up and will move
eastward into the Great Lakes region Wednesday night...bringing surface
low and main cold front with it. Models offering varying
timing/placement of these features but one more round of convection
possible Wednesday...mainly for the eastern half of the County Warning Area. Upper
trough axis rotates through the area Wednesday night into Thursday
keeping chance for a few showers before cooler drier Canadian high
pressure begins to build into the Great Lakes region on Friday. This
will provide a nice start to the Holiday weekend...questions remain
as to whether it will last through Monday. Surface ridge axis will
progress eastward Sunday and Monday...placing our area on the
western periphery of the high while medium range models attempting
to bring short wave energy down in northwest flow. Getting better model
agreement with this solution but lots of variation in timing and
placement. Ensemble guidance near or mostly below climatology for probability of precipitation as
well so will still hold out for dry forecast for at least one more
issuance. Temperatures trending seasonably cooler with at or below normal for most
&&
Update...
issued at 857 PM EDT Monday may 20 2013
Difficult forecast with respect to convective probabilities through
the forecast period and dismal early failure. Amid a strongly
divergent boundary layer axis per visible Sat imagery lying across
northestern Illinois/northwestern in past few hours...has proven to be strongly
detrimental. This despite moderate surface based instability pool
situated across northern Indiana/Southern Lower Michigan this
evening. A well scoured/deeply mixed environment exists across
northern/central Illinois with putrid middle/upper 50s dps this
evening...removes any immediate concern for initiation next few
hours. This immediate upstream environ was within wake of early
afternoon shortwave...that initiated strong/severe convection
midday across yoop/northern lower Michigan. Eastern fringe of dry
slotted environment with 7.5-8.25 c/km middle level lapse rates to
likely keep updrafts in check/isolated remainder of the evening.
Will maintain that any sig chance for convection to be associated
with well upstream convection presently across east central
MO...with several hour wait until a more vigored upstream 925-800 mb
wind field with present focus into the Ozarks/northern Arkansas
shifts northeastward overnight into southern/central Illinois while
jetlet veers/ramps to 45-55kts...finally nosing into klaf vicinity
around 09-12 UTC per rap/arw...and a significantly poor/laggard
hrrr3km that recently came in line to reality. Sufficient
preparatory conditioning/recovery of environment via moisture
flux convergence surface-900mb along/north of strong isodrosothermal
gradient with upper 60s/near 70 dps source tapped with
time....pushing north of i70 to maintain MUCAPE pool of 1200-2000
j/kg across forecast area. This should allow decent maintenance of
convective complex with across Missouri to reach northern Indiana
well after midnight into ksbn and near daybreak at kfwa.
Disfavored timing/parcel ascent becoming less surface based with
time and modest deep layer shear to largely preclude severe
potential...though have continued to ack that strongest cells may
approach low end severe limits in severe weather potential statement.
&&
Iwx watches/warnings/advisories...
in...none.
Michigan...none.
Ohio...none.
Lm...none.
&&
$$
Synopsis...steinwedel/kg
aviation...skipper
short term...Bentley
long term...Jal
update...Murphy
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