Forecast Discussions mentioning any of "ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 08/06/23

Area Forecast Discussion

National Weather Service Denver/Boulder CO

 824 PM MDT Sat Aug 5 2023

.UPDATE...
Issued at 819 PM MDT Sat Aug 5 2023

Radar shows a few storms around the area this evening. A couple of
these are still strong to severe, with a 53 mph gust at a CDOT
sensor near Loveland, in addition to the stronger storm near the
Elbert/Lincoln county line. The Denver ACARS soundings still show
as much as 1500 SBCAPE in the area, and these storms appear to be
using all of it. With a continued cool push of air with the
frontal passage (embedded within a series of outflow boundaries)
along with the loss of daytime heating, instability should wane
through midnight with a decrease in both storm coverage and
intensity likely beginning within the next couple of hours.

Our previous forecast grids, while imperfect, were in good enough
shape that it was difficult to make many improvements to it. Have
tweaked the PoPs and made minor adjustments to other grids.

&&

.SHORT TERM...(This afternoon through Sunday)
Issued at 232 PM MDT Sat Aug 5 2023

Convection continues developing at this time, with the strongest
cells over the Palmer Divide.

Forecast pops are in fairly good shape. The southern half of the CWA
still has 1000-2000 J/Kg of MLCAPE. It is under 1000 J/kg north of
there. The downdraft CAPE is around 1000 J/Kg for all the CWA.  The
high resolution models keep the best convective coverage over the
southern CWA with a less batch to run across the CWA northern
border.  So will go with the highest pops in those areas.  Most of
the convection will be done by 06Z.

On Sunday, the CAPE is best over the western and southern CWA.
The northeast looks pretty stable with the cooler airmass in
place.  For temperatures, both tonight`s lows and Sunday`s highs
should be 5-12 F below seasonal normals; at least for the plains
and foothills.

.LONG TERM...(Sunday night through Friday)
Issued at 230 PM MDT Sat Aug 5 2023

The eastern plains will be more stable into Sunday evening as
cooler air continue to push in from the northeast. However, there
is a weak wave in west/northwest flow aloft as noted in the QG
fields, and we`ll have a continuation of shallow upslope flow into
the Front Range. As a result, we`d expect at least scattered
showers and a few storms to be able to spread off the Front Range
and across the adjacent plains Sunday evening. Intensity should be
limited by the lack of instability, but still some brief heavy
rain and small hail from the stronger storms.

On Monday, temperatures will be warming as low level flow turns
south/southeast. There will be plentiful moisture on the plains
and building instability and a theta-e ridge nosing northward.
MLCAPE of 800-1800 J/kg, with the highest values expected to set
up on the eastern plains. We`ll start the day strongly capped, but
another speed max/upper level disturbance is forecast to approach
the area from the northwest which combined with daytime heating
should be enough to break the cap. Bulk shear is an impressive
35-50 knots, so a few severe storms will be possible on the plains
along/east of the theta-e/instability axis.

Tuesday could still be an interesting/impactful weather day as one
last and potentially stronger shortwave arrives. At this point, it
appears models have slowed sufficiently to allow continued upward
forcing, weakening cap, strong shear, and enough instability for
one last day of severe storms for awhile. The highest coverage and
threat should stay east of the I-25 Corridor, but something to
keep en eye on should that disturbance slow any more.

Drier weather arrives for Wednesday and will stick around through
the end of the work week. Temperatures will also be turning warmer
during this period, with highs rebounding into the lower 90s on
the plains.

&&

.AVIATION...(For the 00Z TAFS through 00Z Sunday evening)
Issued at 549 PM MDT Sat Aug 5 2023

The stronger north/northeasterlies at the terminals should fade
over the next couple of hours, but light northerly flow is
expected to continue for most of the night with this ongoing
frontal push. Some weak convection has developed over the Denver
metro in the last hour and this may bring some brief rain and
outflow winds, especially further south towards APA. Any
convective activity should fade after 02-03z, with an isolated
shower or two left through midnight.

Afternoon convection is forecast again tomorrow, though there is
uncertainty in coverage/intensity. Will maintain just a VCTS in
there for now but may need to eventually go with some sort of
TEMPO for gusty winds/rain in the afternoon or evening hours.


&&

.HYDROLOGY...
Issued at 240 PM MDT Sat Aug 5 2023

Scattered showers and thunderstorms are expected over the burn
areas the rest of this afternoon into early evening. The storms
will be moving fairly fast so there will only be a limited threat
of flash flooding.

The flash flood threat remains limited Sunday, Monday, and
Tuesday as scattered showers and storms are possible each
afternoon. They will move rather quickly, however, so despite
brief heavy rain the threat should remain relatively low.

Drier weather will prevail Wednesday to Friday with no threat of
flash flooding.

&&

.BOU WATCHES/WARNINGS/ADVISORIES...
None.
&&

$$

UPDATE...Hiris
SHORT TERM...RJK
LONG TERM....Barjenbruch
AVIATION...Hiris
HYDROLOGY....RJK/Barjenbruch



Mesoscale Update forecast discussions.



 Rest of This Afternoon into Mid Evening...

An MCV continues to move slowly east across northern Illinois this
afternoon. A surface low was across central Illinois, with a front
extending east into the southern half of central Indiana. Some
breaks have developed in the cloud cover across the area.

Near and south of the front, temperatures were in the lower to
middle 80s with dewpoints from near 70 into the mid 70s. North of
the front, temperatures were around 80, with dewpoints in the mid to
upper 60s.

Instability will spread east during the remainder of this afternoon
as the system slowly moves. Highest instability will be across the
southern half of the area, south of the front. Wind shear will
increase as the surface low gets closer. Instability and shear are
in place for severe storms, which could produce tornadoes in
addition to damaging winds (thanks to strong low level shear and
lower LCLs due to plentiful low level moisture).

In the northwest forecast area, north of the front and closer to the
MCV, heavy rain will be a threat with the slow moving nature of the
system and some additional frontogenetical forcing. HREF Local
Probability Matched Mean (LPMM) shows highest rainfall likely to
stay just north of the forecast area and looks reasonable, so do not
plan on a flood watch at this time. Will continue to monitor closely.

Given the above, will have high PoPs across the northern forecast
area, likely or higher PoPs central forecast forecast area, and high
chance to low likely PoPs south, farthest from best forcing. These
will occur at some point into mid-evening based on location.

Mid-evening through the Overnight...

The MCV will weaken some during the night. With loss of heating and
the atmosphere being worked over from previous convection, the
forcing will have less to work with. Will keep some higher PoPs
early in this period in the north, closer to the MCV. Otherwise,
will go with chance PoPs or lower.

With moisture still hanging around and low cloud cover developing,
low temperatures will be in the mid to upper 60s.

Sunday...

Central Indiana looks to be in between significant forcing on
Sunday, which should keep much of the day dry. However, leftover
forcing from the previous system could linger Sunday morning, and
some weak upper energy will move through Sunday afternoon. Thus,
will keep some low PoPs around, but with low confidence that
anything will happen.

Low clouds will be around during the morning, but these should lift
and mix out some during the day. These will keep temperatures down,
especially north where they should mix out last. Highs will be in
the upper 70s north to mid 80s south.

Low level moisture will linger, so it will be humid. Dewpoints will
be in the upper 60s into the 70s.

&&

.Long Term...(Sunday night through Saturday)
Issued at 306 PM EDT Sat Aug 5 2023

* More strong thunderstorms with a damaging wind threat possible
  Sunday night
* Additional rain possible on Monday, tapering off from SW to NE as
  the day progresses
* Another chance of storms in the mid to latter portions of the
  workweek

Sunday Night through Tuesday:

Forecast guidance continues to depict the slower progression of a
seasonably potent upper level trough into the early part of next
week. This will yield in rather active - and perhaps wet - weather
across much of central Indiana within this time frame. Earlier in
the day on Sunday, a vigorous upper level shortwave disturbance is
expected to enhance convective initiation upstream in Illinois. As
this feature dives south and east within the mean troughing,
convection appears likely to congeal into linear segments given
shear parameter spacing. This storm mode is most likely to yield in
a damaging wind threat, especially if the system can organize and
produce a self-enhancing, mature cold pool. However, it is important
to note by the time these storms approach our area late Sunday
evening, a few conflicting factors may limit their intensity. These
storms will be working against the diurnal heating curve, and may
become elevated over a nocturnal temperature inversion as they
approach. Models remain conflicted as to whether or not this
happened, as some soundings still show surface based parcels into
the overnight hours. This is in part due to the similarly timed
strengthening (albeit marginal) of the nocturnal jet, which may
continue surface instability advection into our area. Likewise, the
synoptic forcing for thunderstorms is there; our area will be
positioned in the enhanced lift of the right rear exit of an upper
level jet. Any storms that form will propagate SE through the area,
and we will accordingly monitor radar and surface observations for
the potential weakening trend tomorrow evening.

In the wake of this initial MCS-type passage on Sunday night, models
continue to portray sufficient moisture in the column and enough
upper level support for further precipitation Monday. As dry air
wraps back around the southwest side of the parent low, this
precipitation (and cloud coverage) will taper off from SW to NE
through Monday night. Any areas that saw repeated periods of heavy
rain this weekend will need to be monitored for localized flooding
concerns if precipitation is prolonged there. The severe weather
threat for Monday proper looks low; this appears to be more wrap-
around type precipitation with plenty of stratiform cloud coverage.
Monday`s forecast is thus not the classic early August summertime
weather that one would expect in central Indiana. Afternoon highs
will be relegated to below the 80 degree threshold in most areas,
and winds could become quite blustery. Sustained speeds up to 15mph
appear likely, with additional gusts up to 30 mph possible, amidst a
tightening pressure gradient. The features responsible for these
atypical summer conditions will slowly push out of the area and
yield to clearer skies by Tuesday, but high temperatures will remain
muted in the 80s compared to climatological norms thanks to mean
troughing and zonal surface winds.

Wednesday Through Friday:

Model spread remains high for next week, but the midweek prevailing
weather pattern in the proximity of a surface high pressure pattern
will be drier than earlier in the week. Another upper level
disturbance will approach the area somewhere in the temporal
vicinity of Wednesday evening into Thursday and reintroduce the
chance of thunderstorms to the forecast. It remains too early to
forecast exact spatial and hazard details regarding said storm
chances, but they will be supported by a return of southerly flow by
Wednesday on the backside of the departing sfc high. This will pump
some combination of warm air and lower level moisture back into the
atmospheric column. Once again, depending on the eventual evolution
of mesoscale features and upstream activity, strong to severe storms
may be possible. Stay tuned to future forecasts, but otherwise,
temperatures will return to near normal values for the latter
half of the week.

&&

.Aviation...(00Z TAF Issuance)
Issued at 736 PM EDT Sat Aug 5 2023

* Convection briefly impacting IND and perhaps BMG early in the
  period.

* Ceilings deteriorating overnight to IFR.

Discussion:

Broken line of showers and storms ahead of a remnant MCV is moving
eastward across central Indiana, and will impact IND and perhaps BMG
in the first 2-3 hours of the period. Will carry VCTS at BMG and
IND, with a TEMPO in the 00-02Z time frame at IND. Storms have not
been particularly strong, but have been occasionally rotating and
producing some heavy downpours.

After storms exit, deteriorating ceilings down into the IFR range
can be expected late tonight, continuing into Sunday. Gradual
improvement back to VFR is expected through the day into the mid
afternoon hours at all sites, save for perhaps LAF, which may hang
onto MVFR conditions a bit longer.

Winds will be southerly or southeasterly (easterly at LAF) early in
the period, generally becoming more southwesterly and westerly
throughout. A few gusts tomorrow are possible but appear a bit too
inconsistent at this time for inclusion. More clearing would promote
more gustiness, perhaps into the upper teen to near 20KT range.

&&

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

&&

$$

Mesoscale...Nield
Update...Nield
Short Term...50
Long Term...Marcus
Aviation...Nield