Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Indianapolis in
701 PM EDT Wednesday Mar 12 2014
the near term section has been updated below.
issued at 321 PM EDT Wednesday Mar 12 2014
Much colder air will return into the region tonight with high
pressure. The chilly weather will be short-lived...as the passage of
a warm front Thursday night will usher in milder temperatures
Friday. After a weak cold front brings a chance for light rain
Friday night...cooler weather will return into early next week.
Near term /tonight/...
issued at 701 PM EDT Wednesday Mar 12 2014
Did a quick update to add flurries past 00z across the far western
forecast area as radar shows snow showers and flurries across
eastern Illinois moving southeast. Kept mentionable probability of precipitation across the
far southwest who will have the best chance /but still low/ of
seeing measurable snow.
Previous discussion follows...
back edge of the light snow associated with the low pressure
moving across the eastern half of the forecast area as of 19z.
Another area of what appears to be convective snow showers over
northern/central Illinois associated with an upper level wave
continues to move slowly southeast. Temperatures have been generally
steady-state in the Lower/Middle 30s over the last few hours.
Went ahead and dropped both headlines /winter weather and wind
advisories/ as snow has lightened considerably and wind gusts have
dropped below 40mph. Additionally...with a generally heavy wet snow
in the locations where some accumulation was experienced...the
blowing snow concerns have been greatly diminished.
Initial issue focuses on the area of snow over Illinois. Most short
range model guidance struggling with this...but hrrr hints at
bringing the axis of snow across the lower Wabash valley into the
evening. Additionally...rap soundings showing subtle instability
present for the next few hours. Will put low chance probability of precipitation in through
middle evening from Terre Haute to Washington and points west. Even
though may have brief bursts of heavier snow showers within
convection...surface/ground temperatures are above freezing and expect
impacts to be minimal.
Remainder of the forecast is quiet for tonight as weak high pressure
builds in overnight. Expect skies to become clear tonight with wind
gusts diminishing during the evening as the surface pressure
Temperatures...cold night by March standards as clear skies with a fresh
snowpack over northern counties should enable temperatures to tumble into
the single digits. Even for the remainder of the forecast area that
is free of snow cover...expect temperatures to bottom out in the teens.
Generally went with warmer mavmos in snow-free areas...and much
closer to colder metmos over northern counties.
Short term /Thursday through Saturday/...
issued at 321 PM EDT Wednesday Mar 12 2014
Forecast challenges focus on precipitation chances with a weak front late
Friday...and temperatures throughout the forecast period.
The frenetic pace to weather across the region as of late will
continue through the period...with this brief return to winter
followed by yet another reminder by Friday that Spring is definitely
on the horizon. Cold but sunny start on Thursday with high pressure
ridge over the Ohio Valley. The surface ridge will shift east
quickly by Thursday afternoon with increasing low level warm
advection commencing by late day...followed by the passage of a dry
warm front Thursday night. Expect a period with increased middle/high
level clouds Thursday afternoon and evening immediately ahead of the
warm front...with skies clearing overnight.
Return of ridging aloft on Friday and southwest flow ahead of an
approaching weak cold front will set the stage for a mild yet windy
day. Low level jet will overspread the region by Friday afternoon
ahead of the front and with modest mixing levels up to about
3kft...should have no problem pulling some stronger gusts to
30-35mph down to the surface. The front will initially be lacking
moisture but as it moves into the Ohio Valley Friday evening...it
will tap into a slightly better fetch of Gulf moisture. Combined
with an axis of low level convergence along the front and a brief
yet subtle period of isentropic lift...expect a few light rain
showers to develop Friday evening...shifting south of the forecast
area prior to daybreak Saturday. The deeper moisture departs ahead
of the colder air...so anticipate precipitation to fall entirely as light
rain. Weak high pressure will then reestablish for Saturday with dry
weather and slightly cooler temperatures.
Temperatures...arrival of warm advection by afternoon should enable a nice
recovery Thursday with highs into the 30s. Mavmos in particular appears
to be a little too aggressive with the warmup...and undercut by a
few degrees in general. Followed the recent trend of going above MOS
for highs Friday as combination of warm advection...southwest flow
and compressional heating ahead of the front will allow temperatures to
approach 60 across all but northern counties. MOS looked reasonable
for highs Saturday generally in the upper 40s and 50s. Boundary
layer thermals supported MOS guidance for lows through the period.
Long term /Saturday night through Wednesday/...
Issued at 149 PM EDT Wednesday Mar 12 2014
High variability and low confidence during this period as models
suggest several weaker systems passing through the area albeit
with limited moisture. GFS pushes some upper support and a weak
trough through the area on Monday...however moisture remains
limited and confidence is low as the European model (ecmwf) pushes better support
through deep south.
A warming trend is noted through the period as strong warm air
advection is seen in the GFS on Tuesday through Wednesday. A thermal
ridge actually build across Illinois and Indiana with 850mb temperatures
rising toward 11c. This should produce some temperatures at or above
normals. Have included a chance for rain on Wednesday as a GFS suggest
a cold frontal passage...still very low confidence.
Aviation /discussion for the 130000z tafs/...
Issued at 646 PM EDT Wednesday Mar 12 2014
High pressure will build across the area tonight and subsidence
returns. Aside from some lingering mfvr for sites such as
kind/khuf/klaf through 02z...forecast soundings indicate a dry
column...thus will trend toward VFR overnight. Some good warm air
advection is expected on Thursday and this should result in some
middle to high clouds across the area....but still VFR conditions.
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