Area forecast discussion
National Weather Service northern Indiana
145 am EDT Tuesday Jun 18 2013
Synopsis...
issued at 425 PM EDT Monday Jun 17 2013
A weak cold front will approach the region this evening and bring
a chance for showers and thunderstorms overnight and again Tuesday
evening. High pressure will then build over the Great Lakes by
midweek with a return to dry conditions.
&&
Update...
issued at 812 PM EDT Monday Jun 17 2013
Updated mainly to freshen zone forecast product wording... but also bumped up probability of precipitation a
bit northwestern areas this evening due to scattered showers over and approaching this
area. As mentioned in aviation discussion... instability currently
limited over our County Warning Area and Prospect for destabilization slim with
mesoscale convective system over the lower Ohio Valley preventing much Theta-E advection ahead
of the front... so thunderstorm coverage only expected to be isolated to scattered
in our area. Steep low level lapse rates and fairly dry low levels
will support gusty winds and possibly small hail if thunderstorms do
occur through the evening before diurnal cooling stabilizes low
levels.
&&
Short term...(this evening through tuesday)
issued at 350 PM EDT Monday Jun 17 2013
Perturbed west-northwest flow aloft will persist this period between an eastern
Canada negative height anomaly and an intermountain west/Southern
Plains upper ridge. A sheared pv filament embedded in this flow
over Wisconsin will translate through the central Great Lakes this
evening...forcing a backdoor cold front southeast through the area
tonight. This cold front will get a little extra boost from Cool
Lake Michigan waters...forcing it into far northwest zones by 00z. Isolated
to scattered convection may fire by the way 23-02z along this boundary and
then slowly work southeastward overnight. Kept coverage/probability
of convection low (20-30 percent) with deeper moisture locked up
south of the area and stronger support aloft passing north and east
of the iwx County Warning Area. Moderate deep layer shear (40-45 knots) and weak
instability (500-1000 j/kg of mlcape) may support an isolated
wind/hail threat if convection fires early this evening across
northern zones.
Frontal slope will sink just south of the Highway 24 corridor by 12z
Tuesday...and then clearing our southeastern zones by midday. Low
level cool/dry advection will be more impressive over the northern half
of the forecast area...especially downwind of Lake Michigan into Northwest Indiana and
SW lower Michigan...with guidance backing off somewhat on this
cool/dry push into southern areas. 12z guidance also came in a little
stronger with a more pronounced pv anomaly (across the northern
plains as of this writing) prognosticated to drop southeast through the
Northern Ohio valley Tuesday afternoon. This upper level support per
DCVA/7-3h q vector convergence forecasts...coupled with modest bl
destabilization (maybe 1000-1500 j/kg of cape to work with) and a
possible low level focus along a leftover inverted-like trough may
combine to help generate scattered showers/storms Tuesday
afternoon...best chances south of Highway 30 in Indiana. Bumped up probability of precipitation
a bit but held shy of likely with deeper moisture and more unstable
air suppressed south of the area along the primary baroclinic
zone. There does appear to be a marginal hail/wind threat with
sustainable updrafts given prognosticated weak to moderate instability and
near 40 knots of effective deep shear.
&&
Long term...(tuesday night through monday)
issued at 350 PM EDT Monday Jun 17 2013
A weak upper level trough will be moving across the southern part of
the area early in the period. NCEP models have been trending up the
chances of precipitation over southeast areas as the upper level
trough combines with a weak front and moves southeast. Added a chance
for showers and thunder before midnight Tuesday night. Bulk shear
and cape should not be sufficient for severe storms. This system
should move out of the area fairly quickly...with dry weather and
moderating temperatures through the end of the work week as an upper
level ridge builds into the area. Have trended temperatures warmer
over the last several days of the this period. CR allblend looks too
cool and appears to have ingested some of the cooler model data.
Overall...favor the GFS with an upper ridge extending into the
forecast area over the weekend. 850 mb temperatures over 20c support
highs around or topping 90 degrees. CR allblend has trended
shower/storm chances lower and appears to be on track as middle level
cap should become fairly substantial. Subsidence from the upper
level ridge should also inhibit storm activity.
&&
Aviation...(for the 06z tafs through 06z Tuesday night)
issued at 145 am EDT Tuesday Jun 18 2013
Surface cold front is currently pushing south across the area and
remaining showers/storms will continue to gradually dissipate with
the loss of diurnal instability. Light winds and still relatively
high surface dewpoints may result in some patchy fog before
sunrise...especially around ksbn where there was a greater
coverage of showers. No visible reduction seen in area observation yet so will
maintain tempo MVFR group as anything below that level will likely
be fairly brief. A potent shortwave will then cross the area later
today and there may be enough moisture/instability to support a
few isolated showers and thunderstorms around kfwa. Confidence is
still very low that this will occur though as the better
instability remains well south of the terminal.
&&
Iwx watches/warnings/advisories...
in...beach hazards statement from this afternoon through late tonight
for inz003.
Michigan...Beach hazards statement from this afternoon through late tonight
for miz077.
Ohio...none.
Lm...none.
&&
$$
Update...jt
synopsis...ng/kg
short term...steinwedel
long term...skipper
aviation...agd
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