Area Forecast Discussion


740 
FXUS63 KIND 110848
AFDIND
Area Forecast Discussion
National Weather Service Indianapolis IN
348 AM EST Mon Dec 11 2017
.UPDATE...
The SYNOPSIS...NEAR TERM and SHORT TERM Sections have been updated
below. 
&&
.SYNOPSIS...
Issued at 348 AM EST Mon Dec 11 2017
After a milder day today...the passage of a cold front tonight will 
bring another shot of much colder air for Tuesday into the middle of 
the week. Snow showers and flurries are possible late tonight into 
Tuesday with even a short period of patchy freezing drizzle in spots 
prior to daybreak Tuesday. Another quick moving system will bring a 
few snow showers Wednesday night and Thursday before high pressure 
builds in with dry weather and a warming trend for the weekend. 
&&
.NEAR TERM /Today/...
Issued at 348 AM EST Mon Dec 11 2017
Skies were mainly clear early this morning with the exception of far 
northeast portions of the forecast area as the back edge of stratus 
associated with a departing wave aloft linger. 08Z temperatures were 
in the 20s.
Another relatively quiet day for central Indiana as focus shifts to 
vigorous upper wave diving into the upper Midwest early this 
morning. This feature will track southeast through the day and into 
the region by early this evening. Snow had developed out ahead of 
the wave over Minnesota and northern Wisconsin and should move into 
the Great Lakes after daybreak with CAMs indicating potential for 
the southern fringe of the the lighter snow showers to make it into 
northern Indiana. This activity should remain north of the forecast 
area...but expect mid level clouds associated with the snow to 
expand into much of the northern half of the forecast area later 
this morning. 
The moisture will shift east by early afternoon with some thinning 
of the cloud cover as a result. Southerly flow will develop and 
strengthen as warm advection develops ahead of an attendant surface 
wave and cold front which will dive into the Wabash Valley by early 
evening. Could see gusts approach 20mph by late day as the front 
approaches.
Temps...a brief but noticeable warmup coming today with warm 
advection ahead of the cold front. Preferred the cooler METMOS for 
highs with a 10-12 degree gradient from northeast to southwest across 
the region. Still will be the warmest day in the last 5...and for 
the rest of the work week.
&&
.SHORT TERM /Tonight through Wednesday Night/...
Issued at 348 AM EST Mon Dec 11 2017
Forecast challenges focus on precip chances and types as the surface 
low and cold front sweep across the area tonight...followed by 
another round of lake effect snows Tuesday that will clip northeast 
portions of the forecast area. 
The upper wave with surface low and cold front will move across 
central Indiana this evening. Vigorous vort lobe aloft should 
provide the necessary upper forcing but moisture is lacking and 
largely confined to the boundary layer. Low level thermals will 
crash with the passage of the front...and any light precipitation 
that may begin as a mix of rain and snow or all rain should 
transition to light snow showers over most of the area by midnight 
or shortly thereafter. Any light snow accumulation prior to 12Z will 
be a dusting at best over eastern portions of the forecast area. 
Something that cannot be ignored any longer is the presence of much 
drier air flooding the column above the boundary layer and creating 
a short window where ice saturation is lost. With the near surface 
layer falling below freezing during the overnight across the 
region...a few hours with heightened potential for patchy freezing 
drizzle to mix in with any light snow showers/flurries is a real 
possibility. Model guidance has hinted at this potential for the 
last few runs and while confidence is not high in any substantial 
impact...feel it would be prudent to include a mention for a 3-4 
hour period predawn Tuesday. Certainly could create some slick spots 
on roads/overpasses ahead of the Tuesday morning commute.
The lake effect potential will once again crank up early Tuesday and 
take center stage through Tuesday night. While the focus for 
greatest lake effect snow impacts will be felt mainly north and 
northeast of the forecast area...northeast counties will again have 
some light accumulation potential from lake enhanced bands. Activity 
will begin to develop downwind of Lake Michigan after 06Z Tuesday as 
a second vigorous wave aloft dives south through the Great Lakes and 
serves to amplify the upper trough over the region. Boundary layer 
flow intensifies early Tuesday which should ensure that lake 
enhanced bands travel well inland as they dive southeast from Lake 
Michigan. Will introduce likely pops over northeast counties near 
daybreak for the expected arrival of the banding. 
Boundary layer flow should gradually back to a 300-310 degree 
direction by the afternoon which should focus the core of the 
stronger bands northeast of the forecast area. Will maintain likely 
pops all day in Randolph County which would be the most susceptible 
location for being impacted by lake bands even at a 300-310 degree 
wind direction. Outside of the lake enhanced banding impacts over 
northeast counties...models appear to be grossly underestimating 
light snow shower/flurry potential over much of the rest of the 
forecast area with strong forcing aloft present for much of the day. 
Moisture is limited but lapse rates of 8-9 C/km are present up to 
about 800mb into the afternoon hours. That combined with strong cold 
advection should be more than sufficient for flurries and perhaps 
even a few snow showers over much of the eastern 2/3 of the forecast 
area. Winds will be gusty throughout the day as well.
Low level wind flow will back further Tuesday evening and then 
diminish substantially as surface ridging builds in from the west. 
This will enable any lingering lake bands to shift further away from 
northeast counties Tuesday night. With the continued amplified flow 
aloft over the country...the ridging will be shortlived as yet 
another upper wave and associated surface low track into the Great 
lakes by late Wednesday. Moisture is once again a limiting factor as 
the system passes through the region Wednesday night...but enough 
forcing aloft is present to generate a chance for snow showers over 
northern counties. 
Temps...the airmass following in the wake of the frontal passage 
this evening looks arguably colder than the one that graced us on 
Saturday. Undercut guidance for highs on Tuesday as most of the 
forecast area remains in the mid and upper 20s. Generally utilized a 
model blend for temperatures through the rest of the period.
&&
.LONG TERM /Thursday through Sunday/...
Issued at 235 AM EST Mon Dec 11 2017
Ensembles in good agreement with the overall pattern during this
period. Long wave troughing over the eastern parts of the country
is expected to move off to the east, with the flow becoming more
zonal by the end of the week.
A short wave trough is expected to pass over the Great Lakes
around Thursday. Will go with small chance PoPs for mixed
precipitation on Thursday, mainly over the northern zones.
The rest of the extended looks dry at this time as heights rise
with the lifting out of the long wave trough. Ensembles suggest
the next system probably won't affect the area until after this
period.
&&
.AVIATION /Discussion for the 110900Z KIND TAF Update/...
Issued at 325 AM EST Mon Dec 11 2017
No significant changes made to the forecast at this time. Previous 
discussion follows.
Good confidence in VFR conditions through most of the TAF period. 
However, the SREF and GFS LAMP suggest there could be some MVFR 
stratocu in the wake of another cold front after 05z. 
Winds will be light to calm overnight. Then winds will veer from 
south to northwest 10 knots or less in the wake of the late night 
cold front. Winds will pick up at IND toward 12z Tuesday, however.
&&
.IND WATCHES/WARNINGS/ADVISORIES...NONE. 
&&
$$
SYNOPSIS...Ryan
NEAR TERM...Ryan
SHORT TERM...Ryan
LONG TERM....JAS
AVIATION...MK/JAS