Area forecast discussion
National Weather Service northern Indiana
615 am EDT Sat may 18 2013
Synopsis...
issued at 610 am EDT Sat may 18 2013
A front will remain over the region today...bringing a chance for
thunderstorms to mainly southern portions of the area. After a
break from the showers and storms Sunday... more rain is expected
Monday through Wednesday as moisture flow into the area increases
and as upper level disturbances move over the area. Temperatures
will be above normal from the middle 70s to the middle 80s and lows in
the 60s.
&&
Short term...(today and tonight)
issued at 416 am EDT Sat may 18 2013
A very tight moisture gradient continued over the area again
today...with dew points in the 30s north of the boundary to lower
60s farther south. Given very weak synoptic scale forcing and light
wind profiles...including wind generally around or under 20 knots
into the upper levels...current thinking showers and isolated storms
will be highly diurnal today...including late afternoon and early
evening. Cape values should remain very limited and convection
should be mainly confined to the much higher surface Theta-E areas along
and south of the front. Raised highs a little today given very large
diurnal temperature swings expected today...especially north of the
boundary. NCEP MOS guidance for highs has been running 2 to 4
degrees too cool.
&&
Long term...(sunday through friday)
issued at 416 am EDT Sat may 18 2013
Strong Pacific jet currently crashing into the western Continental U.S. Will
carve out a very large barotropic low pressure system east of The
Rockies by early next week. The gradual eastward progression of this
feature will bring increasing chances for showers and thunderstorms
to the local forecast area through midweek. Beginning with Sunday
though...midlevel ridge axis directly over the Great Lakes makes for
a somewhat precarious precipitation forecast. Low level winds begin to veer
to a more southerly direction during this time...advecting more
warm/moist air into the region. Latest MOS guidance supports highs
solidly in the middle 80s with dewpoints creeping into the low to middle
60s. NAM and GFS MLCAPE fields/forecast soundings continue to show a
moderately unstable and weakly capped environment...thanks to strong
solar insolation and increasing low level Theta-E advection.
However...with the lack of any synoptic scale lifting mechanism...
convection will be reliant on inefficient planetary boundary layer mixing to reach the
level of free convection. Also...given placement and evolution of ridging aloft...expect
the cap may be a bit stronger than what current bufr soundings
indicate. Q vector divergence fields show weakly subsident air mass
that will likely inhibit convection. Relatively dry tropospheric
column should also tend to inhibit expanding convection. All of that
being said...wouldnt completely rule out an isolated thunderstorm
but coverage will be very limited and decided to cut probability of precipitation to around
10 percent and remove blanket slight chance wording.
Southwesterly low level jet begins to ramp up Sunday night and Monday as the
approaching midlevel height anomaly forces a tightening of the low
level gradient. Warm front begins to really take shape and lift
north Sunday night as the ridge axis shifts east. Associated
isentropic ascent and moisture flux convergence in region of modest
elevated instability could spark off a few
showers/thunderstorms...but again coverage will be limited. County Warning Area will
be firmly in the warm sector by Monday afternoon. 850mb temperatures look
to climb to around 17c...supporting highs in the middle to upper 80s
with dewpoints in the middle to upper 60s. Precipitation chances will hinge on
exact timing of approaching trough and associated weakening of the
capping inversion. Latest 00z GFS is much slower than its NAM and
European model (ecmwf) counterparts and maintains a predominately dry forecast.
Sympathetic to a slower solution given overall pattern but
confidence too low to make significant adjustments to probability of precipitation at this
time and will go with low chance wording. If convection does
occur...severe weather is a distinct possibility given strong
instability and moderate shear profiles. As such...Storm Prediction Center has placed
our County Warning Area in day3 slight risk to cover this conditional threat.
Much better precipitation chances expected Tuesday and Wednesday as vertically
stacked low approaches and low level moisture advection continues.
Expect several rounds of showers and thunderstorms with exact timing
dependent on evolution of frontal features as well as potentially
meddlesome mesoscale convective system development. Severe weather remains possible during
this time but weakening dynamics aloft and increasingly stable
airmass does not Bode well. A few lingering showers are possible on
Thursday but 1030 mb Canadian surface high is expected to build over
the Great Lakes by the end of the week. This should result in a
pleasant...albeit cool...Memorial Day weekend.
&&
Aviation...(for the 12z tafs through 12z Sunday morning)
issued at 610 am EDT Sat may 18 2013
Drier air has spread back into the terminals from the east and
northeast. There is an outside chance for a mainly late afternoon
shower as tight moisture gradient moves north later
today and as daytime conditions destabilize...however...for now
kept rain out of the tafs with VFR conditions.
&&
Iwx watches/warnings/advisories...
in...none.
Michigan...none.
Ohio...none.
Lm...none.
&&
$$
Synopsis...skipper
short term...skipper
long term...agd
aviation...skipper
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