Forecast Discussions mentioning any of "HRRR" "RAP" "RUC13" "RUC" "RR" received at GSD on 10/22/24

Area Forecast Discussion

National Weather Service Indianapolis IN

 555 PM EDT Mon Oct 21 2024

.KEY MESSAGES...

- Fire weather concerns will need monitoring through at least
  Thursday given low afternoon relative humidity values

- Dry weather with warm daytime temperatures will continue through
  Wednesday

- Cold fronts late Wednesday and Friday will bring temperature falls
  to the area

- Showers will be possible early Friday morning into early Saturday
  morning with the second cold front

&&

.SHORT TERM (This evening through Tuesday)...
Issued at 300 PM EDT Mon Oct 21 2024

Our primary forecast challenge in the short range is the approach of
a potent mid-level shortwave that is expected to arrive during the
day on Tuesday.

Until then, surface high pressure remains in control of our weather
and continued subsidence and weak MSLP gradients will result in
clear skies, light winds, and warm temperatures. Temperatures, dew
points, and winds today should largely be determined by boundary
layer processes. Deep mixing should lead to warmer temperatures than
guidance suggests along with lower dew points. Overnight lows will
be chilly, with substantial differences between rural and urban
areas. Patchy fog is possible along rivers and lakes as they are
still quite warm. Some fog in rural areas cannot be ruled out as dew
points are a bit higher than previous days.

The aforementioned shortwave trough arrives on Tuesday, and should
pass to our northwest. Moisture is lacking and large-scale forcing
should remain mostly to our north. As such, little in the way of
precipitation is expected...though a stray shower cannot be ruled
out. We will carry 20 percent PoPs at most across our northwest
where guidance shows the best chance of an isolated shower. Though
not mentioned in the forecast, a stray shower or sprinkle is also
possible further south.

FIRE WEATHER

The bigger concern regarding tomorrow`s weather is the potential for
elevated fire weather conditions. A very dry air mass is currently
in place across the region, and very little rainfall has occurred in
the past month or so. Surface fuels are therefore quite dry.

As the shortwave passes by to our northwest it should induce a tight
MSLP gradient that should generate breezy/gusty south southwesterly
winds Tuesday afternoon. Most guidance is on board with sustained
winds ranging from 10-20mph with gusts up around 25mph...perhaps as
high as 30mph. Model soundings show very dry air just above the
surface, with the driest layer occurring around 850mb. Our biggest
question tomorrow will concern how efficient boundary layer mixing
is and how much dry air can be mixed down to the surface. Some
models, like the HRRR, show dew points dropping into the 30s Tuesday
afternoon. Combined with temperatures in the high 60s to low 70s,
this corresponds to RH values in the 20-25 percent range.

While the HRRR may be a bit over-mixed, other models consistently
show dew points dipping into the upper 30s with RH values in the
high 20s percent. Lowest RH values tend to occur in our northwestern
counties which is also where the highest winds are found. Many of
these counties have burn bans in effect. Atmospheric conditions may
be close to Red Flag Warning criteria but surface fuel moisture is a
bit higher than required. Will hold off for now given marginal (but
still quite dry and windy) conditions. At the very least, elevated
fire weather potential is becoming increasingly likely.

&&

.LONG TERM (Tuesday night through Monday)...
Issued at 300 PM EDT Mon Oct 21 2024

Quiet and mostly dry weather will continue through much of the long
term period, with the possible exception of late in the week, when
some shower chances are present as a cold front moves through the
area. Temperatures will fluctuate somewhat and will feature fairly
large diurnal swings on some days, but should be near to a bit above
normal for highs and near to a bit below normal for lows for the
most part.

Two cold fronts are expected to pass during the period, the first of
which will likely pass dry as moisture return is effectively blocked
by broad high pressure over the southeast and lower Mississippi
Valley keeping the Gulf closed.

The second front will move through the region early Friday into
Friday night, and may  be able to access a source of slightly higher
theta-e air in the midlevels, allowing for top down moistening of
the column to occur, and necessitating low to middling shower
chances from early Friday morning into early Saturday morning.
Minimal upright instability should keep a lid on any thunder
concerns.

Otherwise, large areas of surface high pressure will keep the
remainder of the long term period dry.

Dry antecedent conditions will produce enhanced diurnal temperature
swings much of the period, with temperatures overall on a bit of a
rollercoaster given multiple frontal passages. Highs will generally
fluctuate between the 60s and 70s, but may struggle to get too far
out of the upper 50s to low 60s this weekend in the wake of the
second cold front. Lows will see a large spread as well, but will
likely drop as low as the 30s Wednesday night and Saturday night
post frontal passages.

Fire weather concerns will need to be monitored at least through
Thursday given low afternoon RH values.

&&

.AVIATION (00Z TAF Issuance)...
Issued at 555 PM EDT Mon Oct 21 2024

Impacts:

- Wind gusts Tuesday afternoon from 190 degrees at 22-26KT.

- Low level wind shear possible at IND after 00Z tomorrow evening.

Discussion:

VFR conditions are expected through the TAF period.

An approaching upper level low and surface boundary on Tuesday will
increase the pressure gradient across the area, and as a result,
expect southerly winds (around 190 degrees) to become stronger and
gustier from midday Tuesday into early Tuesday evening. Sustained
winds will be around 14-16KT with gusts 22-26KT.

As gusts subside in the evening, low level wind shear may develop.
This will be included in IND for the 30 hour period, and may be
needed in the other sites with the 06Z package.

Scattered high based cumulus are expected tomorrow, along with a few
high clouds tonight becoming broken tomorrow. A spotty shower cannot
be ruled out, primarily around LAF, but is far too low probability
for inclusion at this time, and would have little impact.

No obstructions to visibility are expected.

&&

.IND WATCHES/WARNINGS/ADVISORIES...None.

&&

$$

SHORT TERM...Eckhoff
LONG TERM...Nield
AVIATION...Nield