Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service Indianapolis in
920 PM EST Thursday Nov 26 2015

the near term section has been updated below.


issued at 305 PM EST Thursday Nov 26 2015

Mild temperatures will continue through Friday ahead of an
approaching cold front. Rain will overspread the region late tonight
and early Friday as the front nears...with unsettled weather
persisting through much of the upcoming weekend as the front drifts
slowly into the lower Ohio Valley. After a brief respite as high
pressure passes by to the north late weekend...unsettled and
progressively cooler weather will return for the first half of next
week as a strong upper low tracks through the Midwest.


Near term /tonight/...
issued at 920 PM EST Thursday Nov 26 2015

Short term model guidance suggests cold front off to the northwest probably
won't reach the far northwest zones until towards or after daybreak Friday.
Given most of the lift/precipitation looks to be in the Post frontal
zone...will trim the probability of precipitation in space back to the northwest zones only and in
time to the pre dawn hours.

Sustained warm advection tonight should help to keep temperatures falling to
much overnight. Will bump up the lows a category.

Previous discussion follows.

Getting treated to a mild Thanksgiving afternoon across the Hoosier
state...with the sun even making an appearance as drier air
advecting back into the region. Breezy conditions ongoing with
peak gusts around 30mph so far. 20z temperatures ranging from the upper 50s
to middle 60s across the forecast area.

Main focus for the near term is to better define the onset of
rainfall...which is looking more and more likely to impact the area
in the predawn hours as the cold front approaches slowly from the

Model consensus has slowed down precipitation arrival with the evening now
expected to be dry over the entire forecast area as forcing is
refocused closer to the front to the West. Pocket of weak isentropic
lift that was responsible for the sprinkles earlier has shifted away
to the northeast with partly cloudy skies left across central
Indiana for the rest of the afternoon. Model soundings and relative humidity forecasts
indicate clouds will thicken back up across the entire forecast area
as the night progresses.

Deeper moisture and isentropic lift will arrive as the low level jet
sags into the region late. Hi-res model suite makes an argument that
most of the forecast area remains dry through daybreak Friday but
with the arrival of more pronounced isentropic lift into the
northern Wabash valley around 06z...will introduce low probability of precipitation after
midnight and ramp them up over the northwest half of the forecast
area towards daybreak. The southeast half of the forecast area will
remain dry overnight...protected by the high pressure ridge off to
our east. Gusts should drop off with sunset but the presence of a
tight surface gradient will keep winds in the 10-15mph range all

Temperatures...considering the warmth this afternoon courtesy of the bonus
sunshine and the continued warm advection overnight...will be
trending towards the warmer mav guidance for lows. Will remain mild
with lows in the 50s.


Short term /Friday through Sunday/...
issued at 305 PM EST Thursday Nov 26 2015

Forecast challenges focus on rain chances throughout the period.
Confidence remains high in wet conditions Friday everywhere and
Saturday for much of the area. Differences in location of where the
frontal boundary ends up combined with the strength of high pressure
to the northwest lends to greater uncertainty in the precipitation
forecast for the second half of the weekend.

A strong upper low will retrograde slightly across the intermountain
west through the period...keeping a moist southwest flow aloft
pointed directly towards the Ohio Valley through Saturday. At the
surface...the aforementioned cold front will drift slowly south
through the region before stalling in the Ohio Valley late weekend
as the boundary becomes near parallel to the upper flow pattern. The
combination of this setup aloft and at the surface will maintain a
dual moisture flow off both the Pacific and Gulf with precipitation water
values near November climatological maximum levels in the 1.3 to 1.4
inch range focused across the region from late Friday into early

Steadier rainfall will overspread the entire forecast area Friday
morning as strong isentropic lift and deep moisture. Expect the most
widespread and persistent rainfall from Friday afternoon through
early Saturday as the highest precipitation water values coincide with the
period of best forcing and diffluence aloft as the region aligns
into the right rear quadrant of a strong upper jet passing through the
Great Lakes. At the same time however...the low level jet will
weaken as it gradually becomes stretched out along the frontal
boundary. A weaker low level jet will not diminish the rainfall
threat at all...but it will limit the efficiency of the moisture
within the boundary layer which will cut down on potential precipitation
totals somewhat.

The frontal boundary will settle along the Ohio River Saturday as a
secondary wave develops and tracks through the lower Ohio Valley.
This will keep plenty of moisture and isentropic lift over central
Indiana with rain continuing through the day...especially focused
over the southern half of the forecast area in closer proximity to
the surface wave and boundary. Rainfall should gradually diminish
and shift south Saturday night and early Sunday as strong high
pressure moving into the Great Lakes begins to exert its influence
and the deeper moisture is forced south into the Tennessee and lower
Ohio valleys. Will maintain low probability of precipitation over southern counties Sunday
afternoon as the drier airmass to the north briefly makes inroads.
Relative humidity forecasts suggest clouds may be somewhat more stubborn to shift south
on Sunday despite the drier air arriving...although do think some
sunshine is possible north of I-70.

Overall rainfall from late tonight through early Sunday will
generally be 1 to 2 inches with the highest amounts over southern

Temperatures...trended towards warmer mavmos for highs Friday ahead of the
approaching front. Trended towards high end of guidance for both
Saturday and Sunday as overall MOS looks rather chilly...especially
considering linger clouds and for some rain showers all the way into
Sunday. Generally went at or above MOS for lows through the period
for the same reason. Both the mav and met look too aggressive with
the drier and cooler air Saturday night in particular.


Long term /Sunday night through Thursday/...
issued at 223 PM EST Thursday Nov 26 2015

Models are fairly close that the superblend initialization was
accepted for most items.

An upper level low in The Rockies Sunday night will move into
the Great Lakes by Wednesday and exit Thursday.

After a dry start in northern counties Sunday night...threat of
precipitation returns/continues in south/ through Tuesday. Colder
upper low cyclonic flow to keep clouds into Wednesday before high
pressure takes over.

At the moment it appears all precipitation will be in the form of
rain though a mix of snow showers may be possible Tuesday night in
our northeast counties with temperatures near freezing.

Temperatures will be near to above average through Tuesday...then
cool to below average...and in line with previous forecast and
current MOS data.


Aviation /discussion for the 27/00z tafs/...
issued at 615 PM EST Thursday Nov 26 2015

VFR much of the night. MVFR likely to develop near or just after
precipitation onset near daybreak Friday...deteriorating to IFR as
the day wears on.

Frontal system very slowly approaching the area. Expect this
boundary to get into the area tomorrow...with prefrontal widespread
rainfall much of the day. Conditions appear likely to be MVFR or
worse most of the latter half of the taf period.

Winds will be out of the south/southwest overnight around 10-15kt.
Winds will remain fairly strong throughout the period...turning
northerly as the boundary sags through the area. Cannot exclude an
occasional gust but appears unlikely to be prevailing.

This may cause low level wind shear to be a borderline concern at
times...but again too low to mention at the moment.


Ind watches/warnings/advisories...none.



near term...Ryan/jas
short term...Ryan
long term...tucek

National Weather Service Glossary of Abbreviations