Forecast Discussions mentioning any of "ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 09/20/19

Area Forecast Discussion

National Weather Service Denver/Boulder CO

 836 PM MDT Thu Sep 19 2019

.UPDATE...
Issued at 827 PM MDT Thu Sep 19 2019

Although a brief thunderstorm did develop in the high country this
evening, most of the activity has been in the form of showers.
Have updated to primarily showers in the high country the rest of
tonight. Strong thunderstorms east of Colorado have enhanced
southeast winds across the northeast and east central plains this
evening. Will keep a slight chance of thunderstorms over the far
northeast corner until 09z but nothing elsewhere. Main issue will
be breezy condtions across the plains especially this evening.
Adjusted the trends in the pops based off radar, otherwise the
grids look good.

&&

.SHORT TERM...(This evening through Friday)
Issued at 250 PM MDT Thu Sep 19 2019

An upper low over the Great Basin will be slowly lifting
northeast tonight and across the northern Rockies by Friday
evening. As it moves closer, southwesterly flow aloft will be
increasing, while a surface low remains over the northeastern
plains of Colorado. A ribbon of moisture evident on satellite is
currently pushing in over the mountains, helping to spark off some
showers and thunderstorms. CAPE values of 400-600 J/kg and PW of
0.5-0.6 will allow for scattered activity the rest of the
afternoon with gusty winds and potential for brief moderate rain.
An inversion present around 550mb on recent ACARS should limit
convection over the plains for most of the day. However, with a
surface cyclone south of Denver creating easterly upslope flow to
the north, and QG lift increasing due to the upper system getting
closer, we should see some convection activity mainly north of the
Denver metro area up to the Wyoming boarder late this afternoon
into the evening. Further east, better moisture exists but little
shear, and current high res model runs have all activity east of
the forecast area. Will keep just a slight chance of storms over
the northeast corner this evening, with a threat of brief heavy
rain along with strong winds.

Overnight, the QG lift will continue to increase, keeping a threat
of showers and thunderstorms over the mountains. Southerly surface
winds over the eastern plains will remain breezy as well. This
will contribute to mild overnight lows.

On Friday, drier and cooler air will push over the mountains,
so should see less thunderstorm activity as well as cooler
temperatures. Over the plains, a dryline will set up from the
surface low pulling in the drier air from the west and moisture
up from the south. Shear increases as the southwesterly jet moves
overhead. Therefore will see a chance for severe strength storms
over the far northeastern corner, with large hail, strong winds
and possibly a tornado.

.LONG TERM...(Friday night through Thursday)
Issued at 250 PM MDT Thu Sep 19 2019

A dryline will shift east across the far eastern plains through
the course of Friday evening, taking the threat of isolated severe
storms east into Nebraska and Kansas by late evening. A cold front
will then sweep south across the plains, ushering in a cooler and
drier airmass. Large scale subsidence will bring clearing skies as
well.

On Saturday, the main upper level trough is expected to shift east
across the area. There is good agreement with a shot of Q-G
support during the afternoon and early evening. However, the main
question is how much moisture can return across the plains. At
this time, the return flow does not look great, but the large
scale support would support a decent chance of showers and
isolated thunderstorms. The best opportunity for light rain
continues to support areas just east and southeast of Denver
toward Limon. Have cooled off temperatures a couple degrees to
account for the shallow but cool upslope across the plains.

Sunday will feature dry and warmer conditions as the upper level
ridge builds overhead. The dry and warmer weather will likely hold
through the middle of next week, despite the passage of an upper
level trough toward Tuesday. The reason for the dry weather
scenario is that there is pretty strong agreement the upper
trough and all the upper level support cuts off into the desert
southwest into Arizona or New Mexico. Temperatures will have
potential to be considerably cooler on any given day Tuesday to
Thursday, but very difficult to pinpoint which day would be cooler
given the pattern and dispersion in the ensemble data. We`ll keep
forecast temperatures closer to normal for now.

&&

.AVIATION...(For the 00Z TAFS through 00Z Friday evening)
Issued at 827 PM MDT Thu Sep 19 2019

VFR overnight and Friday. Strong southeast winds this evening at
KDEN a product of drainage and strong outflow from thunderstorms
well to the east over Kansas. Will continue with some gusts to
25-35 kts through 05z then should see the stronger gusts settle
down overnight.

&&

.FIRE WEATHER...
Issued at 250 PM MDT Thu Sep 19 2019

Strong south-southwesterly winds are expected tomorrow over the
plains, with gusts in the 25 to 35 mph range. However humidity
will be hovering in the mid to upper teens, with lowest values
found on the northern fringes of the Palmer Divide with help of a
downsloping southerly flow. Seems like RH will stay just above
criteria of 15 percent, but gusty winds will still help to
increase fire weather concerns. May see a couple 15 percent
readings for an hour, but not the 3 hour criteria, so will not
issue a highlight.

Saturday will feature dry but breezy conditions over the
mountains and high valleys. Despite cooler temperatures,
humidities will be approaching critical 15% threshold in Grand
County, the valleys of Summit County, and Park County. Winds will
be marginal for any highlights, with speeds generally at or under
15 mph with gusts up to 25 mph at times. Fire danger will still be
elevated, but we should stay just shy of Red Flag criteria.

&&

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

$$

UPDATE...Cooper
SHORT TERM...Kriederman
LONG TERM...Barjenbruch
AVIATION...Cooper
FIRE WEATHER...Kriederman/Barjenbruch



Area Forecast Discussion

National Weather Service Chicago/Romeoville, IL

 636 PM CDT Thu Sep 19 2019

.SHORT TERM...
251 PM CDT

Through Friday...

Atmospheric moistening is underway and will continue into
Saturday and Saturday night. While the weekend looks to be the
main show presenting a widespread heavy rain threat, the chance
for thunderstorms exists into tonight and Friday, some of which
would be capable of heavy rainfall overnight into Friday morning.

What is left of a loosely-defined MCS earlier is an arc of showers
with a few thunderstorms in north central Illinois. This looks
rooted just ahead of a weakening mesoscale convective vortex
(MCV). While the MCV will reach Chicago late this afternoon,
visible satellite and aircraft soundings over northeast Illinois
indicate a less favorable environment for thunderstorms. While
some may reach the western suburbs prior to 4 pm, it`s more likely
that it is only showers that make it into Chicago if the activity
can hold together. In the wake of this MCV, there looks to be a
lull in activity during a good part of the evening hours. While
there is elevated moist and unstable air, weak suppression and
limited isentropic ascent should keep any activity isolated and
probably more so showers.

During the overnight, the moist advection aloft renews slightly,
with 15-20 kt of low-level flow in part response to a weak short
wave disturbance currently over central Iowa. Also, residual
outflow boundaries/low-level air mass discontinuities exist from
the multiple rounds of showers over north central Illinois today,
and these could very well serve as a focus later tonight.
Convection allowing models (CAMs) generally indicate spotty to
even numerous coverage of activity developing. With the forcing
and warm advection lift not overly strong (such as the case last
night to our northwest), feel that the convective footprint may be
more limited. That said, if the outflow boundary(ies) serve as a
focus, the activity could be somewhat training. Some of these
storms are likely to linger into the morning as well due to the
slow movement of the short wave.

If convection can develop overnight and into Friday morning, it
should be slow moving with again some potential to train, and in
an atmosphere with high ambient moisture (i.e. precipitable water
of 1.6 inches, 850 mb dew points of 15+C). This provides concerns
for heavy rainfall worthy enough of at least some localized
flooding concerns. The threat will likely be modulated depending
on coverage.

Into Friday afternoon, confidence really lowers as the air mass
will still be plenty moist, but possibly somewhat tapped into
already if activity in the a.m. hours. Clouds may keep
temperatures subdued in places (like north central Illinois
today), and thus destabilizing from the low-levels also may be
muted. So any convective focus in the afternoon may well be
isolated. Otherwise highs in the lower to mid 80s are expected.

MTF

&&

.LONG TERM...
213 PM CDT

Friday Night through Thursday...

Forecast concerns during the long term period continue to focus
on the increasing threat of very heavy rainfall and possible flash
flooding across the area, particularly late Saturday through
Sunday.

An upper level ridge axis, currently in place across the area,
will be shunted to the east by Saturday, as a large upper trough
over the western CONUS shifts across the Rockies. As this occurs,
strong and deep south-southwesterly flow will be induced across
much of the Mississippi River Valley and western Great Lakes
region on Saturday. This will thus allow tropical moisture from
the remnants of Imelda (featuring precipitable water values
around 2`) to be transported northward into our area this weekend
setting up the stage for some very heavy rainfall, and possibly
some significant flooding.

While we will begin to see chances for some showers and storms
early on Saturday our best chances for more widespread showers
and storms looks to be Saturday night and especially Sunday. This
is the period in which we will see the large scale dynamic
forcing for ascent maximize as the upper trough and surface cold
front approaches the area. With deep tropical moisture over our
area ahead of this front, deep warm cloud depths will result in
some very efficient rainfall producing showers and storms. As a
result, it appears that at least localized rainfall rates
exceeding 1.5" per hour will be possible at times into Sunday.

Total rainfall amounts across much of northern IL and portions of
northwestern IN through Sunday evening could exceed 2", with
locally higher amounts in excess of 3" possible. While there does
continue to be some uncertainty with where the heaviest rainfall
axis will occur, trends continue to suggest that much of northern
IL could be near this heavy rainfall axis. For this reason, flash
flooding and river flooding will be a concern that we will need
to monitor as we get closer to this event. Since it is too early
to consider any type of watch, we will issue an ESF (Hydro
outlook) to increase awareness of this increasing potential for
very heavy rainfall.

Surface high pressure is expected to build across the area Monday
following Sunday evenings frontal passage. This should result in a
couple of quiet and pleasant weather days early next week before
the possibility for showers and storms returns to the area mid to
late next week.

KJB

&&

.AVIATION...
For the 00Z TAFs...

636 PM...Primary forecast concern is the potential for showers
and isolated thunderstorms.

Earlier showers this afternoon have dissipated with new activity
now developing northwest of rfd. Short term guidance is not really
handling this activity too well. However...there still remains an
instability axis across this area that weakens to the east and
across the Chicago terminals. Its possible this activity may
continue to move east across far northern IL this evening but in a
gradual weakening trend. So for now have remained dry this evening
for the Chicago terminals with mention only at rfd.

There is a better chance for showers and a few thunderstorms
toward sunrise and then through mid/late Friday morning but best
location appears to be mainly southwest of the terminals. Have
maintained previous prob mention for this time period but overall
confidence is low. There will be another chance of showers and
possibly a thunderstorm Friday afternoon but confidence is low and
have remained dry during this time period.

Easterly winds to 10kts behind a lake breeze will become light
southeast across the entire area this evening and possibly light
and variable. Some of the guidance does show winds shifting
northeasterly late this evening but still light speeds. Winds are
expected to turn southerly by morning and remain southerly Friday
with a possible lake breeze Friday afternoon. cms

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...None.
&&

$$

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