Area forecast discussion
National Weather Service northern Indiana
129 PM EDT Monday may 20 2013
Synopsis...
issued at 1052 am EDT Monday may 20 2013
A slow moving low pressure system will bring increasing chances
for showers and thunderstorms later today through Wednesday...with
a few showers possibly lingering into Thursday. There is a slight
risk for severe thunderstorms this afternoon through Tuesday
night. Drier and cooler weather is anticipated late this week
into next weekend.
&&
Short term...(today and tonight)
issued at 359 am EDT Monday may 20 2013
Deep Rex blocked cutoff low centered over northern NE this
morning will move little this period. Thus west/little change in jet
structure/positioning aloft expect convn will once again stay focused
westward in vicinity of MS valley.
Lg scale Gravity wave eminating from slowly decaying convn across western
Illinois southwestward into eastern MO will continue eastward this morning. However downstream
airmass of much poorer quality as evident in 00z radiosonde observations and
west/llj/mid level jet streak axis remaining anchored to the west see
little evidence of a sufficient synopic trigger for afternoon convn again
today especially in light of 00z model guidance surface moisture bias. Will retain
a lower bound chance mention northwest half in proximity to eastward edge of btr ll
mass flux this afternoon and in collaboration of western/northern neighbors but
some highres convn allowing guidance does capture Illinois/eastern MO
density current this morning and would appear to more correctly
suppress convn here locally this afternoon/evening within generally
subsident/capped airmass.
Otrws upstream convn expected to initiate and grow upscale this afternoon/evening
vicinity of the Ozark platue and lift northeastward in remnant fashion into at
least western areas lt tonight as low level jet focuses into the western lakes.
&&
Long term...(tuesday through sunday)
issued at 359 am EDT Monday may 20 2013
Convection plaguing the central Continental U.S. Over the past few nights will
finally fully infiltrate the southern Great Lakes by Tuesday as
closed upper low slowly migrates eastward. Lack of any distinct
synoptic forcing will lead to several rounds of showers and
thunderstorms similar to what has been experienced upstream. Exact
timing and location will depend on ambiguous midlevel waves rotating
through the mean flow and mesoscale subtleties difficult to pinpoint
this far out. Additionally...high Theta-E airmass will be firmly in
place across the region thanks to persistent south-southwest flow and surface
dewpoints should be hovering in the middle to upper 60s. Atmosphere
will be largely uncapped by Tuesday and this should support mass
convection in warm/humid airmass. Reasonably confident in precipitation
occurring at some point Tuesday but exact timing and the resulting
impact on both high temperatures and severe chances remains nebulous. Will
likely be a fair amount of nocturnal convection churning upstream
and latest hi-res guidance...and even the coarser NAM and
GFS...suggest this will move over our area by Tuesday morning. This
would severely limit our diurnal instability recovery and prevent US
from capitalizing on expected 30-40 knots of 0-6km bulk shear.
Severe weather chances will therefore depend on fortuitous breaks in
convection that allow instability to build...which may prove
difficult in an uncapped environment. Similarly...high temperatures will be
largely dependent on timing of convection. Thermal ridge will dampen
east a bit by Tuesday but 850mb temperatures still look to be around 15c.
Certainly wont be as warm as today but should still squeak out middle
80s in most places...upper 80s if any sun peaks through.
After another very mild night on Tuesday night...highs on Wednesday
will likely be a category cooler as airmass becomes increasingly
homogeneous and abundant clouds/precipitation temper diurnal response.
Associated weakening of lapse rates aloft will further limit our
severe chances by Wednesday. Cape values expected to be in the
500-1000 j/kg range...enough for thunder mention but no organization
expected. Widespread precipitation still expected to continue though with
similar setup to the prior day. Could see some locally heavy
rainfall totals when all is said and done given precipitable water values hovering
around 1.5 inches for much of the event. Weak surface low and
occluded front will pass through the southern Great Lakes by late
Wednesday and westerly flow will finally advect some drier air into
the area. Could still be some showers on Thursday as the old
midlevel vorticity maximum rotates through but heavier/widespread precipitation
should taper off Wednesday night. Tangible airmass change wont
arrive until Friday though when 1030mb Canadian surface high settles
over the Great Lakes. Expect highs only in the middle to upper 60s
Fri/Sat. Latter half of the Holiday weekend continues to be a
struggle for various deterministic nwp with potential intrusion of
precipitation on backside of aforementioned surface high. Placement of
relevant synoptic features and relatively Cold Lakes favor stronger
high and dry forecast but this will certainly have to be watched as
time GOES on.
&&
Aviation...(for the 18z tafs through 18z Tuesday afternoon)
issued at 128 PM EDT Monday may 20 2013
VFR conditions expected for the rest of today and into
the overnight hours. Slight chance that a stray shower or isolated
storm could pop up near kfwa...but confidence was too low for
inclusion in 18z taf issuance. Storms will fire across Missouri
today and will likely track into northern Indiana during the early
morning hours. This threat is greatest at ksbn. Will wait for later
taf issuances before including these storms as subtle timing
differences and exact location will not be known until the
convection has fired and begins tracking towards the area.
&&
Iwx watches/warnings/advisories...
in...none.
Michigan...none.
Ohio...none.
Lm...none.
&&
$$
Synopsis...steinwedel
short term...T
long term...agd
aviation...Bentley
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